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Circuit Idea/Group 64a

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64a Group Student Page

We are students from Faculty of Computer Systems, Technical University of Sofia. Our 64 group is divided into two sub-groups; we constitute the first 64a one. Here are our names:

Andrey Rachev, Ivelin Ivov, Ivaylo Katrazanov, Rumyana Naydenova, Sasha Madov, Miodrag Hristich, Andon Radulov, Galin Gyurov, Kamen Avramov, Dimitrina Nedelcheva, Slavka Petrova, Zdravko Georgiev, PavlinPanev 64, Ivaylo Atanasov and Miglena Nesterova.

Lab 2: Computer-based Ohm's experiment
Thursday, March 20, 2008, 13.45 h

A wire itself, unlike resistors, has a length; that's why Ohm, the great scientist in electric circuits, has used it in his experiments. You can touch a wire to feel how it reacts on bigger current. In this particular exercise we have conducted experiments by investigating the voltage distribution along conductive foam as shown on the picture. As you can see, we have connected wires to two of the foam edges and put voltages from DAC1 and DAC2. We have made a lot of measurements with the 'probe' to investigate the voltage distribution along the conductive foam.

After using the computer-based system Microlab and analyzing the result, we found that we have made a parallel voltage summing circuit. If we put two voltages having the same magnitudes but different polarities (the one + and the other -), we will see that in the center point the voltage will be 0 volts.

The conductive foam is very interesting for investigation because we can see how the voltage is distributed across it. PavlinPanev 64

Lab 3: Building a full-wave bridge circuit
Thursday, April 03, 2008, 13.45 h

Participants of this lab: Andrey Rachev, Ivelin Ivov, Ivaylo Katrazanov, Rumyana Naydenova, Sasha Madov, Miodrag Hristich, Andon Radulov, Galin Gyurov, Kamen Avramov, Dimitrina Nedelcheva, Slavka Petrova, Zdravko Georgiev, PavlinPanev 64, Ivaylo Atanasov and Miglena Nesterova.

Investigating an LED

In this exercise, we have recreated the famous circuit of full-wave bridge. In order to visualize its operation (where currents flow), we have decided to replace the ordinary rectifying diodes by LEDs with different colors. Also, we have used another LED for a load. Firstly, we have investigated the very LED to make ourselves familiar with its behavior. We have found that its performance is good for our purposes, it would shine and work without problems. It was clear that we had to connect a protecting resistor in series with the bridge circuit. PavlinPanev 64

Building the circuit on the whiteboard

Step 1. We started building the circuit on the whiteboard by connecting the load (the tested red LED). Here we assumed that DAC 1 was producing a positive voltage from 0 up to 10 V (this was the positive half-wave). Note how we have outlined the inverse S shape of the "positive" part of the future bridge. PavlinPanev 64

Step 2. Then we assumed that DAC 1 was producing a negative voltage from 0 up to -10 V (this was the negative half-wave) and connected the true S shape of the rest "negative" part of the future bridge. Only, a short-circuit turned out:( What should we do then? We began thinking... We had already known that diodes can act as decoupling elements (switches); so, we put two red-colored LEDs in the two opposite "positive" bridge legs acting as rectifying and visualizing diodes for the positive half-wave. We expected that since all the three diodes (including the "load") were connected in the same direction they would enable the current to flow in the right direction and would shine during the positive half-wave.

Step 3. But it was not sufficient...:( To remove the conflict, we had to put further two (now, green-colored) LEDs in the two opposite "negative" bridge legs acting again as rectifying and visualizing diodes for the negative half-wave. We solved the short-circuit problem; we managed to "invent" the famous full-wave bridge rectifying circuit!

You can see on the picture how we made the full wave bridge – never mind if the current flows from –  to  +  or from + to -, our red load LED will always shine! Now we can carry out a real experiment to see if our circuit will really work. PavlinPanev 64

Mounting the circuit on a prototyping PCB

This was a true hands-on exercise where we mounted the real circuit on a real prototyping PCB; we soldered the two rectifying LEDs and the LED load on the board and connected them by wires. Thus we finally got a real full-wave bridge rectifier that didn't need any measuring devices. Imagine it should indicate its state (the current direction) by the very diodes! We were eager to get to know, if it would work really as we had assumed? It would be very interesting to test it. PavlinPanev 64

Investigating the circuit by Microlab

After we supplied the circuit by Microlab system we were ready to investigate the circuit operation.

Positive input voltage

It was working as we expected - the three red LEDs were lighting when we put a positive input voltage, the other two LEDs were extinguished. Everything was OK, as we wanted to be. User:PavlinPanev 64

Negative input voltage

Then we tested the circuit at negative input voltage; again the result was excellent as we had expected. We were content with ourselves as we did our job very good. The famous full-wave bridge was working not only in theory but in real! PavlinPanev 64

 * PavlinPanev 64, thank you for your participation. Your text is very useful and informative; what you write is exactly what we need for this page. Go on developing the page in this way. Place four tildes after your insertion (they will be replaced by your user name and the date of the creation). I have also placed some photos on your user page; you have only to say some words about yourself. Regards, Circuit-fantasist (talk) 05:16, 25 April 2008 (UTC)

Quick Introduction Of Op-Amp
Operational Amplifier or Op-Amp for short is one of the basic elements in the electronic circuits. In fact it is one abstraction. Under the word Op-Amp we understand an differential amplifier with very good properties like huge Ad - the differential-mode gain, Megaohm resistance and so long. On the picture below is one differential amplifier, which I have made with the OrCad instrument.

We have two bipolar transistors connected to the power source V1. The transistor Q2 has a constant base power source, while the transistor Q1 has a variable base power source. Let's see the time diagram generated by PSpice:

At time t=0 both the base source power are equal to 2V, and the Voltage of Q1 and Q2 are approximately equal. Then the V3 source is starting very slowly to decrease its voltage. As a result of this action the resistance of Q1 is increasing and Iq1 emitter is decreasing. According to first Kirchoff law the other Iq2 emitter current is increasing with the same value the Iq1 going down. In the generated time diagram you could see that when source power is decreasing the Uce of Q1 is increasing until saturation, and symmetrically Uce of Q2 is decreasing. When V3 is fall with 0.5V, the Vout is changed with 10V.

Now We replaced our discrete differential amplifier with Op-Amp:

And the time diagram:

The green line is for differential amp. Here we could see that op-amp is slightly different from my diff-amp. It has very improved dynamical characteristic from the usual amplifier, improved frequency characterist, and so long, all at the cost of more and more transistor, and high complexity. The conclusion is that op-amp for us is one abstraction. We don't concern what is in the op-amp, but the function which op-amp does. Like Prof. Anant Agarwal from MIT had said "...the analog of op-amp in programming is printf function..." very simple to use, but if we look at its implementation the things will change rapidly. --77.70.25.29 (talk) 14:46, 14 May 2008 (UTC)zdravko64

What negative feedback is

The idea of the negative feedback is to manage to keep the balance. The circuit with negative feedback actually takes the role of active regulating element – never mind what the resistance is it keeps this balance (what do you mean here? Circuit-fantasist (talk) 17:09, 27 April 2008 (UTC)). All that an op-amp tries to do is simply to act as a real thinking human being.

Look at the picture. The difference ∆V between the input voltage Vin and the output voltage Vout is applied to the op-amp differential input. The thing is that the op-amp tries to keep ∆V very close to 0 volts so that the op-amp compensates the difference. However it cannot be more than the voltage on the amplifier itself (what do you mean here? Circuit-fantasist (talk) 17:09, 27 April 2008 (UTC)). Hote that the op-amp does not take energy from the input voltage source to supply the load; instead, it uses its own power supply. The input currents are very low (nano- and pico-amperes); so, we can say they are voltage inputs.

The negative feedback follower just repeats the input value, no matter what the obstacles are. In this way, it acts as a real human being. PavlinPanev 64

Generic Negative Feedback Circuits
After Pavlin has introduced us the negative feedback concepts it is time to create and examine real circuits with negative feedback. I will use again the OrCad system to simulate two of the most popular kinds of feedback. The basic rule for this sort of schemes is:

"V- is less or equal to V+"

The first circuit is the ideal voltage divider (why it is ideal, what does it mean?) Circuit-fantasist (talk) 06:18, 19 May 2008 (UTC)

According to the rule mentioned above V-=0. Then the volage of R9 is V8-0. The current I1 is
 * $$I1=\frac{V8-0}{R8}$$

The voltage of R10 is:
 * $$I1*R10=-Vout$$

This lead us to:
 * $$Vout=-Vin*\frac{R10}{R8}$$

In this case Vout = -Vin, because R10=R8=1k. The time diagram is shown below:

--77.70.25.29 (zdravko64) 16:11, 14 May 2008 (UTC)

General idea
What happens, if we make a mistake when apply a feedback from the output to the input of the subtractor and, as a result, it becomes a summer? In life this means to fool someone to do things in the wrong direction (for example, to do something bad instead to do something useful and v.v.). He/she "thinks" that he/she reacts in a proper way but actually he/she does exactly the opposite!

We can reproduce this situation in electronics, if we swap (usually, involuntarily) the op-amp's inputs. As a result, the op-amp is "disoriented"; like human beings, it "thinks" that it reacts in a proper way but actually it does exactly the opposite.

In this way, we can introduce the great idea of the positive feedback. It is extremely useful (for example, all the flip-flop RAMs are based on this powerful idea). PavlinPanev 64

Converting a non-inverting amplifier into an inverting comparator with hysteresis
Similarly, what happens, if we make a mistake when build a non-inverting amplifier swapping the inverting and non-inverting inputs? Let's try.

Schmitt Trigger
Returning from the world of abstractions, I would like to show you that there is no magic, live, or something similar in positive feedback, but only a pure physics based on very strong mathematical foundations. The positive feedback is classically explained with the equivalent model of the op-amp.

And now if we grounded V-, and connect V+ with the dependency source we are going to create something which in mathematics and programing is called recursion.

Meaning, V+=A*V+. Where is the end of recursion? This is when the op-amp get saturated. Knowing this, we could create a comparator, which compare the V+ value with zero and, if it is greater forms plus impulse otherwise minus impulse. With this comparator we could form a square signal from a sinus wave.

--77.70.25.29 (zdravko64) 17:26, 14 May 2008 (UTC)

Investigating the circuit

The concept behind diode
The first thing that a man is feeling, when he has heard an unfamiliar word is a confusion. He tries to understand what that word means, and after fails in this point he tries to connect this word with analogies. The word "diode" is coming from Greek and it means two electrode or two "ode". The first electrode is called an anode: in Greek ana- + hodos, a way up, and the second electrode is called a cathode: in Greek kathodos, descent. Ok, so we have one word, which actually means something like a hill or a rising ground, and if we have a bicycle we can descent from the way up without any additional efforts. But when we try to climb this hill it could be difficult to reach the end, and probably we would start walking with very low speed or going in the backward direction. Returning to the world of electricity the electrons are the villagers with bicycles, and the diode is the hill. The electric circuit with a diode could be presented as ski piste. The skiers are very happy when the lift is working in the direction from the bottom to the top, but if we change the directions of the lift all skiers are wondering what's going on, and nobody is down hill skiing.

The next questions are, "Why do we need such a part?" and "What were the electric circuits before the diodes?" These answers require some historical ideas to be introduced. Zdravko64

 * Your text is very interesting, intriguing and original!!! Your work deserves a reward! Please, create an account and log in before writing to go down in history:) Then write four tildes after your comments; wikimarkup will substitute them by your user name.

 * By the way, an accident has occurred with the battery of my solid-state recorder:( As a result, I have no record from our so interesting laboratory exercise. Please, help me to restore the so valuable data! Circuit-fantasist (talk) 20:54, 3 April 2008 (UTC)

Circuits before the invention of diode
Let me take you back in the beginning of the 19th century, and in this chapter I would like to enter in the minds of the great physicists, or as they called in that time the great nature philosophers of that time. In 1800 was made one great invention by the Venetian nature philosophers Count Alessandro Giuseppe and Antonio Anastasio Volta. All began with one dispute with his colleague Luigi Galvani. After a series of experiments with animals Galvani found out that muscle contractions occurred, if a metal scalpel was touched to a certain leg nerve during dissection. He thought that in the body is conducted some kind of "vital electricity" which is flowing through the nerves. Volta was not on the same opinion. In 1792 he wrote,

"I am persuaded that the electric fluid is never excited and moved by the proper action of the organs, or by any vital force, or extended to be brought from one part of the animal to another, but that it is determined and constrained by virtue of an impulse which it receives in the place where the metals join."

Isn't it incredible how they were thinking? Volta was the thought, that the electric current was some kind of a fluid. He didn't know that there is something like electron, further more he didn't know that the elements are consisted of atoms, because this was approved in 1827, when the botanist Robert Brown used a microscope to look at dust grains floating in water and discovered that they moved about erratically — a phenomenon that became known as "Brownian motion". The scientists suspected that this was not a usual fluid. Volta replace the legs of the frog with different chemical elements and discovered that the elements and aggregates (electrodes) which dissociate in electrolyte (galvanic element) gains a determined electrochemical potential (see Electrochemical_series). All Volta had to do is to connect two galvanic elements with different electrochemical potentials. The positive electrode of galvanic element was called cathode, and the negative - anode. So the reasonable question here is, "Is the battery a diode?" Zdravko64

 * Zdravko64, your question is proper for my educational purposes as I intend to ask my students during the next lecture on Wednesday, April 9, the reverse question: "Is a diode a battery?" I will be happy, if you answer this question in your essay about diodes and at the lecture as well. Circuit-fantasist (talk) 16:01, 7 April 2008 (UTC)

 * Circuit-fantasist, when I started to write this kind of article my intentions was to create a really helpful and serious work, similar to some kind of scientific work, but not so complicatedly expressed. Now I am realizing, that this has become more like an essay. I am expecting, that we are going to do some teamwork and you give me a directions how to improve my reasonings to achieve my point. I would be thankful, if someone told me how to add pictures here, and I could add it from my lectures notebook and my notes from the laboratories. Thanks in advance. Zdravko64 19:26, 14 April 2008

 * Hi Zdravko! I will be able to help you after Wednesday. Circuit-fantasist (talk) 18:10, 14 April 2008 (UTC)

We have the potential difference that creates our hill. But the next keyword - exhaustion, comes here. The main disadvantage of the Volta battery is irreversible moving of the charges. When we connect cathode with anode, we take Zn ion from the anode, which displace the weaker 2H ions from the sulfuric acid. The hydrogen ions give charges with cathode, turn into atoms, and fly away. The charge in cathode move to the anode (the direction is accepted to be from plus to minus by Benjamin Franklin), the anode become positive and more Zn negative ions are thrown. It is obvious that in some time either the anode will vanish or the acid will become salt. Anyway the process will stop. Then the battery will turn into dielectric. OK, but if the battery is ideal it is never going to exhaust. The battery has a fixed potential difference, and with the appropriate opposite high voltage we could manage to pass the hill. According to the superposition principle the two voltage would either add or substract, depending of the directions of the two sources. This means that we are descending from the first hill, and entering the second, if that hill is with the same raise I increase my speed and kinetic energy. Otherwise my kinetic energy is turning into potential one, if it is enough I'm passing or stop at the end, but if it is not enough I'm turning back, and pass through the first hill.

Volta has found out another interesting thing. He noticed that the battery has the ability to transfer the electric charges from anode to cathode. That force was called electromotive force (emf). In the handing case emf is provided by the process of dissociation. This force is missing in the diode. But in the voltaic battery, if we remove the emf, meaning remove the salt bridge or placing a wall between anode and cathode in the acid, the two electrode will equals their potential difference at once. As a result the electrode will become neutral. How we could prevent this happen? Zdravko64

In 1827 Ohm's law appeared in the famous book '"Die galvanische Kette, mathematisch bearbeitet"' (The Galvanic Circuit Investigated Mathematically) in which he gave his complete theory of electricity. Unfortunately the biggest part of his theory was completely ruined by greats mind like Fourier and Navier, for that Ohm believed that the communication of electricity occurred between "contiguous particles". Although, his experiments were certainly valid as well as his law. Eighteen years later Gustav Kirchhoff generalized work by Georg Ohm in his two laws. The circuits at this time was measurement bridges like the Wheatstone bridge in 1843, the Kelvin double bridge, and RC circuits. Two great discoveries occurred in 1832. The first was the electromagnetic induction discovering made independently by Faraday and Joseph Henry, and the second was the invention of telegraph by Baron Schilling von Canstatt. The discovering of electromagnetic induction show us, that the electric current is something slightly different from fluid. The electric current create a magnetic field around the wire through it flows, and also changing magnetic field produces an electric field:

 * $$ \mathcal{E} = - {{d\Phi_B} \over dt}$$.

'where $$\mathcal{E}$$ is the electromotive force in volts, $${Phi_B}$$ is the magnetic flux in werbs'. This law is telling, that "the magnetic fluid", which is also called a magnetic field produced an emf equal to minus the speed of flux change. Two type of inductance were found 'mutual inductance' and 'self inductance'. Not longer after that Hippolyte Pixii created the first dynamo machine.

The electric circuits start to include the element inductance, and RL circuits appears. The statement

 * $$\frac{d\Phi}{dt} = L \frac{di}{dt}$$

leads to:

 * $$i(t) = \frac{1}{L}  \int_0^tv(\tau) d\tau + i(0)$$.

That means we could use the inductance to slow the increasing rate of $$i(t)$$

 * The invention of telegraph is very interesting from engineering point. You can see that in 1898 there was a telegraph lines between Europe, America, Africa and Asia.[[Image:Telegraph Lines]]. And two questions were arising:
 * 1.How to support these thousand of kilometers telegraph wires.
 * 2.How to use our electric generator and power stations for telegraph supplying.

In other words could we use that new fluid, which Faraday was called a field for Morzse code transferring. Maxwell started to philosophize about the magnetic and electric phenomena in his articles "On Physical Lines of Force" (I recommend to read that remarkable work), and realized that electric and magnetic field are part of one electromagnetic wave. We don't need mechanical medium for this wave, because in this case the energy is oscillating. That is the beginning of RLC circuits.

The RLC circuit creates the magnetic and electric parts of the electromagnetic wave, and for that this circuit is call resonant circuit or tuned circuit. The natural frequency of em wave is:
 * $$\omega_o = {1 \over \sqrt{L C}}$$

The reason for development of diode. Evolution or genius revolution
A little of electron history. My point is to show, that there is nothing incidentally in the diode invention. In 1803 John Dalton use the forgotten theory of Democritus for explaining, why the elements are combined with a fixed mass, or in other words why we have SO2, CO2, Na2O, but never S01.8 or Na2.1O, which Dalaton had formulated like "Law of multiple proportions". After that in 1827 Robert Brown made his historical experiments, in order J. Desaulx to suggest fifty years later in 1877, that this phenomena was caused by the thermal motions of water molecules. . In 1883, Thomas Edison observed that a current flowed between the filament of an incandescent lamp and a plate in the vacuum near it [here I what to place a lamp figure], when the plate was connected to the positive end of the filament, but not when the plate was connected to the negative side. He was also rediscovering of thermionic diode principle. The idea is as following: if we heated the anode at a specific temperature some electrons could possible gain enough energy to split from the metal plate and reach the cathode through the vacuum. Here I want to place the thermionic diode figure

''If you wonder, how in this distance time, Edison was succeeded to isolate vacuum, and from where the world know about the electrons, years before the J.J.Thomson experiment follow this links:

1.[http:[//http://en.wikipedia.org/wiki/Cathode_ray]] 2.[http:[//http://en.wikipedia.org/wiki/Electron]]''

So the discovering of Edison was telling us something very important, and after it hystorics said, that electrical engineers was beginning to differentiate from the physicists, this think is: "The electrons are our soldiers, and we could control them". Of course, the process which is equivalent to operation of heated vacuum tube diode is the evaporation[here one picture of evaporation, and next to it picture of operation of thermionoc diode] In the first case the evaporation the fluid rise his temperature, which in macro level means that molecules rise their kinetic energy and fly away, when it reach an appropriate level. The second case has to work at the same principle, but Edison didn't know, what kind of a bond or energy is holding the electrons, surly it wasn't a chemical bond or gravity. The phenomena was called electron emission. Electron as a part of the atom was discovered in 1898 by Sir John Joseph Thomson [More about the three Thomson experiments [http:[//http://www.egglescliffe.org.uk/physics/particles/electron/electron.html]]]]. In his Nobel Lecture "Carries Of Negative Electricity", which I recommend every interested man to read (only 9 pages;)), he is talking about the abstraction of the diode operation principle, as we are going to see in the next section.

comments
I enjoyed reading this page. Alas, I see that an earlier edit deleted some pictures. Is there anything I can do to help? --DavidCary (talk) 03:00, 25 May 2008 (UTC)

 * Hi David! Thank you for your willingness to help. I decided to involve my students in Circuit idea two months ago somehow "in sport"; but it turned out so successful and exciting that I was entirely absorbed in this new web undertaking. I have told how it has begun in the discussion of the first completely finished page that my students and I have dedicated to the famous Ohm's experiment.

 * In general, you can second this "open student wikibooks project" by expressing your opinion about the concrete circuit phenomena considered in the student pages. I'm sorry I haven't invited you (see for example my invitation to Bill Beauty); but I hoped you would note the new initiative.

 * David, have you any idea how to restore the deleted pictures? They were created by students; probably, they haven't placed license tags. The problem is that the term has finished, the students have gone away and I have not copies of these pictures on my computer.

 * Regards, Circuit-fantasist (talk) 10:16, 25 May 2008 (UTC)

 * OK, I talked with WhiteKnight a bit ... what else needs to be done? --DavidCary (talk) 12:57, 23 June 2008 (UTC)

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title: Circuit Idea/Group 64a


text:
Circuit Idea/Group 68a

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68a Group Student Page

We are students from Faculty of Computer Systems, Technical University of Sofia. Our 68 group is divided into two sub-groups; we constitute the first 68a one. Here are our names:

Katerina Slavova, Eleonora Todorova, Radoslav Danchev, Hristina Malakova, Elena Hristova, Lubomir Nedyalkov, Dimitar Petkov, Ahmet Karaman, Alper Mutlu, Tamer Aydin and Kiril Keranov.

Lab 1: Investigating passive resistive circuits by Microlab system
In this laboratory exercise we were talking about how we can build a converters and how we may investigate them with MicroLab system. First, the instructor introduced to us what converter means.

Converter is a device that changes the continuous fluctuations in voltage from an analog device (such as a microphone) into digital information (?!?! Circuit-fantasist (talk) 06:29, 19 May 2008 (UTC)) that can be stored or processed in a sampler, digital signal processor, or digital recording device. Converters are generally two types:Voltage-to-current conversers and Current-to-voltage converters.

Hristina Malakova68 (talk)

Lab 2: The genuine Ohm's experiment
In this laboratory exercise we went centuries ago. We felt like in Ohm primitive laboratory. Georg Simon Ohm lived in 1789 - 1854. His inventions are amazing. Even nowadays we know him as one of the most excentric and most creative people. During one of his experiments he took a piece of conductor. He stretched it in two ends. He started measuring voltage and current in the ends. But in these years there was not modern instruments nor such comforts like these we can use - he made the history of the electricity with primitive things. So he took a compass to measure current and an electroscope to measure voltage.

Following his ideas we tried to repeat his experiment with laboratory conditions. We decided to make our scientific experiment more interesting and more attractive.Instead of conductor we used graphitе. Graphite has impedance consequently it has voltage. We found out that, if we measure two ends of conductor - one of sites has 0V and the another has max V. Hristina Malakova68 (talk) Thursday, March 20, 2008, 16.45 h

Lab 3: Building a transistor switch
Katerina Slavova, Tsarina Penkova 65gr, Eleonora Todorova, Radoslav Danchev, Hristina Malakova, Elena Hristova, Lubomir Nedyalkov, Pavlin Stoyanoff 64, Dimitar Petkov, George Gizdov 65, Ahmet Karaman 68, Alper Mutlu 68, Tamer Aydin 68, Maria Georgieva 65 and Kiril Keranov.

Thursday, April 03, 2008, 16.45 h

The history of the transistor
"The Transistor was probably the most important invention of the 20th Century, and the story behind the invention is one of clashing egos and top secret research."

The problem started in the 1907 when the AT&T (American Telephone and Telegraph) faced the competition erupting from the expiration of Alexander Graham Bell's telephone patents. And the solution was transcontinental telephone service. In 1906, the eccentric American inventor Lee De Forest developed a triode in a vacuum tube. It was a device that could amplify signals, including, it was hoped, signals on telephone lines as they were transferred across the country from one switch box to another. AT&T bought De Forest's patent and vastly improved the tube. It allowed the signal to be amplified regularly along the line, meaning that a telephone conversation could go on across any distance as long as there were amplifiers along the way. But the vacuum tubes that made that amplification possible were extremely unreliable, used too much power and produced too much heat.  After the end of World War II, a team of scientists was collected to develop a solid-state semiconductor switch to replace the problematic vacuum tube. This team included a mix of physicists, chemists and engineers. On November 17, 1947, Walter Brattain, an experimental physicist who could build or fix just about anything, made an experiment into a thermos of water. The silicon contraption he'd built was supposed to help him study how electrons acted on the surface of a semiconductor. The wet device created the largest amplification Brattain had seen so far. By turning on a positive voltage he increased the effect even more; turning it to negative could get rid of it completely. It seemed that whatever those electrons had been doing on the surface to block amplification had somehow been canceled out by the water - the greatest obstacle to building an amplifier had been overcome. Unfortunately this giant jump in amplification only worked for certain types of current - ones with very low frequencies. That wouldn't work for a phone line, which has to handle all the complex frequencies of a person's voice. So the next step was to get it to work at all kinds of frequencies.  In fact the device worked as if there was no oxide layer at all. And as Brattain poked the gold contact in again and again, he realized that's because there wasn't an oxide layer. He had washed it off by accident. Brattain was furious with himself, but decided to fiddle with the point contact anyway. To his surprise, he actually got some voltage amplification - and more importantly he could get it at all frequencies! The gold contact was putting holes into the germanium and these holes canceled out the effect of the electrons at the surface, the same way the water had. But this was much better than the version that used water, because now, the device was increasing the current at all frequencies.  Brattain had managed to get a large amplification at some frequencies and he'd gotten a small amplification for all frequencies - now he just had to combine the two. He knew that the key components were a slab of germanium and two gold point contacts just fractions of a millimeter apart. Walter Brattain put a ribbon of gold foil around a plastic triangle, and sliced it through at one of the points. By putting the point of the triangle gently down on the germanium, he saw a fantastic effect - signal came in through one gold contact and increased as as it raced out the other. The first point-contact transistor had been made.  He showed this little plastic triangle at a group meeting on December 23. It was official - this tiny bit of germanium, plastic and gold was the first working solid state amplifier. (The text is written by Radoslav Danchev)

Transistor as an amplifier

The first part of this laboratory exercise was to build a transistor switch on the whiteboard. And we started with a conversation about the behavior of the transistor. The lecturer asked us what are the real functions of a transistor and what a transistor consists of. Those who knew it were just few. What we knew from the previous year was that the transistor amplifies electrical current. But in the exercise we took a deep look in its structure and understood that the transistor is nothing more than electrically driven resistor. Actually there is no real amplification. The real function of a transistor is to regulate not to amplify. (The text is written by Radoslav Danchev)

In laboratory classes we imagined transistors the way they are normally explained in electronics - like a variable resistors. They are electrically-managed resistors, known as active transistors. Normally, the voltage across the transistor would increase its value, but actually it fades. The transistor decreases the supply voltage. It seems that the transistor can regulates it - this is one of the main functions of transistors.

Making transistor act as a switch
After discussion of what is transistor, we started talking about the meaning of the transistor switch. We concluded that this is an element with two states - OFF and ON. When the switch is off, no current flows through it, and therefore the transistor is off. The depletion region across the emitter-base junction is large. There is no potential differences between the base and emitter. The emitter-collector terminals act as open. The second state is ON - then the transistor turns on as hard as it can. A voltage is present across the base-emitter junction. The depletion region across the emitter-base junction shrinks as much as possible. Here is a presentation about transistor switch : http://www.wisc-online.com/objects/index_tj.asp?objID=SSE3703 It is very interesting! Take a look!

Eventually, we understood what exactly transistor and transistor switch are, along with their general purpose. So we started building one. The building scheme is in the photo. Hristina Malakova68 (talk)

Assembling a transistor switch by simpler converters

 * Here are some questions about the circuit of the transistor switch. What does the base resistor RB actually do in this circuit? Can we apply 1000 V or even more to the transistor switch input? What does the collector resistor RC actually do in this circuit? A tip: present the transistor switch as a circuit composed by three converters (a voltage-to-current, a current-to-current and a current-to-voltage converter). See the pictures below. Circuit-fantasist (talk) 14:32, 20 April 2008 (UTC)

Mounting the circuit on a prototyping PCB

The picture above shows how we have built a transistor switch. It was very interesting task for us. Some of my colleagues took part in this. I watched them with extreme attention. First, the lecturer found a prototyping printed circuit board (PCB), which he gave to us. After that we started building up our "invention". We soldered the supply voltage to the PCB. Then we put n-p-n transistor on the board. It was very difficult to solder its pins... but we made it!

Investigating the circuit
Then we put a lamp to use it like a collector load. The lamp had current charge of 100mA. We attached a button to control switching on and off the lamp. We soldered the button between the plus of the supply and the base of the transistor. When the lamp is off then the button is off, too. The transistor in this case is cut off. We attached also a contact point where we could measure the output (collector) voltage of our transistor switch. This was the final touch we made on our "invention". Our transistor switch worked very well and we tested it a couple of times. It was quite amazing. It was worth doing it.

Drawing conclusions
Finally we concluded that if we want our transistor switch to work properly, we have to observe some circumstances:

 * 1) The transistor must be saturated very well. Otherwise it would start warming up.
 * 2) If Rb isn't small enough, the transistor would be blocked and it would start warming up. Its behavior would be like an amplifier.
 * 3) The transistor must be in its two final states - saturated (fully ON state) or cut off (OFF state).

Hristina Malakova68 (talk)

Lab 4: Op-amp trigger circuits with positive feedback
Making the comparator with hysteresis memorize ("inventing" an elementary flip-flop)

A simple inverting comparator

A problem: false switching.

In this laboratory exercise we had the chance to talk about one of the most interesting electrical elements- amplifier and especially operating amplifier. The operating amplifier can be with or without back connection. Those amplifiers which do not have back connection are super sensitive. The lecture told us the simplest amplifier without back connection is comparator. This device function is to render an account of the current when it passes through the amplifier. Comparator is a logical signal which means: “+” corresponds to “1” and “-“corresponds to “0”. Hristina Malakova68 (talk)

Introducing a hysteresis into the circuit
A system with hysteresis can be summarised as a system that may be in any number of states, independent of the inputs to the system. Many physical systems naturally exhibit hysteresis. Human-designed systems will sometimes intentionally exhibit hysteresis. For example, consider a thermostat that controls a furnace. The furnace is either off or on, with nothing in between. The thermostat is a system; the input is the temperature, and the output is the furnace state. If we wish to maintain a temperature of 23 degrees, then we might set the thermostat to turn the furnace on when the temperature drops below 21 degrees, and turn it off when the temperature exceeds 25 degrees. This thermostat has hysteresis. Let us say that the temperature is 24 degrees. Given this information, we cannot predict whether the furnace will be on or off; it's not possible to predict the instantaneous output of the thermostat, knowing only its instantaneous input.(The text is written by Eleonora Todorova)

How we said earlier when the operating amplifier has no back connection it is super sensitive and its voltage-ampere characteristic varies. To ignore this variety it is used the phenomenon called histeresis. Histeresis actually is a positive back connection. It works like a pen-push down or push up. There is no average state-pen is either push-up or push-down and this is the mechanism of the histerezis. It enhances performance of the amplifier on the same direction and that is how the varies disappear. To use the histerezis  in the comparator scheme we drew it is necessary to change the kind of source. It must not to be a hard source but a source that the schema (operating amplifier) can use to control by itself in positive direction. We put e voltage divider to our schema. This allows the current to pass through the resistors R1 and R2 which values are equal. Hristina Malakova68 (talk)

Investigating the circuit on the whiteboard

This is a Schmidtt trigger, which is made by an operational amplifier. As you can see on the pictures, first we draw its transfer characteristic on the whiteboard. Then, one of my colleagues and the teacher made a demo at the whiteboard showing to us how exactly voltages were changing. It was like an animation. Hristina Malakova68 (talk)

Illustrating the circuit operation by a hysteresis curve

Hysteresis represent the history dependence of physical systems. If you push on something, it will yield: when you release, does it spring back completely? If it doesn't, it is exhibiting hysteresis, in some broad sense. The term is most commonly applied, as Webster implies, to magnetic materials: as the external field with the signal from the microphone is turned off, the little magnetic domains in the tape don't return to their original configuration (by design, otherwise your record of the music would disappear!) Hysteresis happens in lots of other systems: if you place a large force on your fork while cutting a tough piece of meat, it doesn't always return to its original shape: the shape of the fork depends on its history. Hysteresis can be used to filter signals so that the output reacts slowly by taking recent history into account. (The text is written by Eleonora Todorova)

What is Schmitt trigger?

The Schmitt trigger is a comparator application, which switches the output negative when the input passes upward through a positive reference voltage. It then uses negative feedback to prevent switching back to the other state until the input passes through a lower threshold voltage, thus stabilizing the switching against rapid triggering by noise as it passes the trigger point.

In the top diagram, the input voltage increases from zero, along the bottom horizontal line. The output voltage remains at zero on the vertical line. However, when the input voltage reaches 1.7 volts, the output shoots up from zero to 5 volts. Reducing the input voltage, as shown in the top horizontal line does not cause the output to drop to zero immediately. This only happens when the input voltage is reduced to 0.9 volts. The input level at which the output increases to maximum, and the level at which it drops to zero are different (the text is written by Radoslav Danchev).

 * Radoslav, I do not yet understand why the hysteresis curve is inclined to right. Please, elucidate me. Circuit-fantasist (talk) 13:44, 28 April 2008 (UTC)

 * Well, now I agree... Circuit-fantasist (talk) 06:31, 19 May 2008 (UTC)

In the lower diagram Schmitt Trigger action is demonstrated in another manner. The black graph represents a noisy logic signal. This is the input to the Schmitt Trigger. The green graph is the output signal. The output remains at zero until the input exceeds 1.7 volts. The output then shoots up to 5 volts and remains at 5 volts until the input drops to 0.9 volts. The output then drops to zero. An almost perfect output is recovered from a very noisy input (the text is written by Radoslav Danchev).

A problem: How do we make it memorize?
At the end of the exercise we were talking about a very interesting thing - how exactly we can make a device to memorize. This was very exciting for us because like a future computer engineers we had to get an idea about the problem how exactly a computer RAM is built in? The recipe of this creation is as follows:

The goal is to make a RS trigger (latch, flip-flop) that can memorize the signals applied to it. To get a trigger we have to get a 100% positive back connection (a feedback). Thus we can get an amplifier with back positive connection that means we have to create a hysteresis. To get a positive feedback we have to connect consecutively two invertors.

The secret of one trigger is to supply 10V for logical 1 and to supply -10V for logical 0 in the input and at definite moment trigger switch over one to another state. We have also to supply biasing voltage and that means that the incoming signal has to get a value which is in the middle of the hysteresis cycle. It said in another way this value has to be between two thresholds. Hristina Malakova68 (talk)

Considering the idea on the whiteboard

What happens, if we apply an initial input quantity whose magnitude is inside the hysteresis cycle?

"Inventing" a flip-flop
A tip: can we use here the powerful idea of biasing? Circuit-fantasist (talk) 13:49, 28 April 2008 (UTC)

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license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Circuit Idea/Group 68a


text:
Circuit Idea/Group 64b

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64b Group Student Page

We are students from Faculty of Computer Systems, Technical University of Sofia. Our 64 group is divided into two sub-groups; we constitute the first 64b one. Here are our names:

Vasil Tsanov, Martin Bochvarski, Selver Ismail, Georgi Drumev, Ivailo Kirov, Josif Hristov, Stoycho Barov, Maya Zhecheva, Vasil Tashev.

Lab 2: The genuine Ohm's experiment
Thursday, March 27, 2008, 13.45 h

Before the exercise our group is very excited to reproduce Ohm's experiments from the past.

Lab 3: Investigating transistor circuits with parallel negative feedback
(Building a transistor "zener" diode)

Thursday, April 10, 2008, 13.45 h

Tell this amazing story here and you will get more credits... Do you need some help? I am ready to assist you:) Circuit-fantasist (talk) 07:51, 28 April 2008 (UTC)

Is the real diode a perfect component?

Diodes are two-terminal semiconductor elements, characterized by allowing the current to flow in the forward direction when the voltage is positive but not in the reverse one when the voltage is negative. So diodes represent the action of a valve and so were they called in the past. However, the diode needs a little (approximately 0.5-0.7V) voltage-push in order the current to start flowing freely. The result is small but noticeable voltage drop - VF. What is this voltage drop - useful or harmful? Sometimes it is useful; other times it is harmful...

"Useful" examples. When we make voltage stabilizers, we need this voltage drop. In these cases we do all that is possible to create and increase this useful voltage drop appearing across various diode component: diodes, LEDs, zeners, multiple diodes connected in series...

"Harmful" examples. In other cases, when we use a diode as a switching element, we do all that is possible to remove and to remove this harmful voltage drop... Then we need an ideal diode without forward voltage drop VF...

Making the simplest series diode rectifier

A rectifier is an electrical element that converts alternating current to direct current. Rectifiers form the basis for electronic power supplies and battery-charging circuits. Furthermore, they are used in signal processing to demodulate radio signals and in the precision conversion of AC to DC voltage in electronic voltmeters. However, keeping in mind that the diode allows a current to flow in one (positive) direction but not in the other (negative), we can easily use the term "rectifier" to name a diode.

So let's make the diode perform as a rectifier and thus participating in a half-wave rectifier circuit (the circuit diagram is shown on the left). We have a sinusoidal source (for the experiment it is 1V) and a resistive load. Maya.zhecheva

When the source voltage is positive, the diode is in the so-called forward-bias region. If the diode was ideal the source voltage will appear across the load. However, the diode we have is not ideal; so it performs a voltage drop equal to the push-up voltage of the diode (approximately 0.5 - 0.7V). So during the positive wave the output voltage is less than the source voltage (for our example it will be VL = VS - VF = 1 - 0.7 = 0.3V. The result is shown in the diagram on the right.

During the negative wave of the sinusoidal voltage the diode is in its reverse-bias region and no current flows through the load. Thus only the positive half-cycles result on the load, which makes the diode suitable as a rectifier. Maya.zhecheva

Deriving the basic idea from our human routine
(describe here all the situations in your daily routine where you have removed all the obstacles standing in your way:) Circuit-fantasist (talk) 07:33, 27 April 2008 (UTC)

Making an almost ideal "op-amp" series diode rectifier

What can act as a varying voltage source in our electronic circuit? What can "help" the imperfect diode by adding so much voltage to the input voltage as it loses across the diode? At last Lab 3 we were using transistors for such a purpose; let's now, for the sake of change, use an op-amp...

Now the op-amp has to "insert" the "helping" voltage VF into the circuit; so, how to connect it?

Eureka! We have invented an (almost) ideal diode without (any) forward voltage VF! Let's investigate it by Microlab system.

Firstly, let's remind what an operational amplifier (op-amp) is: it's a device having a differential input (it may be current, voltage, mechanical motion?!?Circuit-fantasist (talk) 18:09, 7 May 2008 (UTC), etc.) and the output that varies according to the input but with a larger amplitude. Almost always they are used with negative feedback (the output signal is returned to the input in opposite direction to the source signal). However, they may be also used with a positive feedback (the output signal that is returned is summed with the input original signal) but much rarely. For ideal OA in a negative-feedback circuit, the network returns a fraction of the output to the inverting input and thus makes the differential input voltage toward zero.

So let's now go back to our experiment. What we are trying to do is to compensate the harmful forward voltage drop on a real diode with a operational amplifier. But isn't the result of this try an ideal operational amplifier?!

We have a source voltage Vin and at the point named "1" we will have that voltage. However, if we want to compensate the voltage drop across the diode, we should use an op-amp that will have on its output a voltage equal to the source voltage plus an additional amount that is equal to this voltage drop. This will be the voltage we will have at point "2". If this increased voltage is applied before the diode, the voltage after the diode will be the same as our initial source voltage and that is what we have at point "3". Which is our purpose!

...multiple diode voltage drops...

...battery voltage...

...resistor voltage drop
With the same success we can overcome the "harmful" voltage drop across a resistor. What a circuit can we "invent" in this way? Write its name here.... (reifilpma gnitrevni-non:)

Application. Now look at the old fashioned laboratory power supply that has helped us to carry out the famous Ohm's experiment. In addition to the positive and negative terminals it has also two mystic terminals named NFB. What are they? Why and when do we need them? Describe here and draw a picture about a possible application of this arrangement. By the way, when I was student and then young engineer, I have managed to burn out a few costly microchips when I was trying to supply them:((( What do you think, how have I managed to do that "heroism"? Circuit-fantasist (talk) 08:25, 27 April 2008 (UTC)

Making the harmful voltage drop act as an input

Resources
Strange things can be put into feedback loop a cheerful animated story (after Tom Hayes's Student manual for the art of electronics).

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title: Circuit Idea/Group 64b


text:
Circuit Idea/Group 68b

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68b Group Student Page

We are students from Faculty of Computer systems and Control, Technical University of Sofia. Our 68 group is divided into two sub-groups; we constitute the first 68b one. Here are our names:

Aleksander Kovachev, Antoniya Tasheva, Hristo Savov, Jordan Slavov, Kalina Mineva, Kristina Miteva, Lora Kirilova, Maria Lambova, Miko Stefanov, Petjo Dimitrov, Petyr Chorbadjiiski, Silvia, Spasen Tsenov, Stilyan Savov, Svetoslav Tsonev, Tanya Kostadinova.

Lab 2: The genuine Ohm's experiment
Thursday, March 27, 2008, 16.45 h

In the beginning, we had an "accident" with a $3 Chinese multimeter (VOM) - when we plugged its black test lead into the jack it just went down and "disappeared". Then we decided to take advantage of this opportunity and began applying a kind of reverse engineering to it:) Tell here what you have seen inside the multimeter...

Lab 3: "Inventing" a negative feedback BJT current source
Thursday, April 10, 2008, 16.45 h

Transistor as a current-stable resistor

Mounting the emitter follower on the PCB

Is the real diode a perfect component?

A forward voltage drop VF appears across a real diode when a current flows through it... What is this voltage drop - useful or harmful? Sometimes it is useful; other times it is harmful...

"Useful" examples. When we make voltage stabilizers, we need this voltage drop. In these cases we do all that is possible to create and increase this useful voltage drop appearing across various diode component: diodes, LEDs, zeners, multiple diodes connected in series...

"Harmful" examples. In other cases, when we use a diode as a switching element, we do all that is possible to decrease and even to remove this harmful voltage drop... Then we need an ideal diode without forvard voltage drop VF...

Making the simplest parallel diode limiter

Why do we connect a reasistor R between the input voltage source and the diode (load)?

What does the combination (VIN + R) represent? What is it?

Deriving the basic idea from our human routine
(write here all the situations in your daily routine where you removed all the obstacles standing in your way:) Circuit-fantasist 06:36, 26 April 2008 (UTC)

Implementing the powerful idea into an electrical circuit

What have we actually done here? How does it operate? What is the final result?

The answer is amazing: we have made just... a piece of wire...?!? What do you think about this speculation? Is it always right? Circuit-fantasist (talk) 06:59, 26 April 2008 (UTC)

An op-amp acting as a varying voltage source
What can act as a varying voltage source in our electronic circuit? What can "help" the imperfect diode by adding so much voltage as it loses across the diode? At last Lab 3 we were using transistors for such a purpose; let's now, for the sake of change, use an op-amp...

Now the op-amp has to "insert" the "helping" voltage VF into the circuit; so, how to connect it?

Finishing the op-amp circuit
Eureka! We have invented an (almost) ideal diode without (any) forvard voltage VF!

...multiple diode voltage drops...

...zener diode voltage drop...

...resistor voltage drop
With the same success we can destroy the "harmful" voltage drop across a resistor. What a circuit can we "invent" in this way? Write its name here....

Making the harmful voltage drop act as an input

Here we have connected a photoresistor in the feedback loop...

Generalizing the powerful "helping" idea

We are already true magicians as we can transform any imperfect component into an almost ideal one! In this lab, we have transmuted a real diode (ordinary, LED, zener, any combinations of them, etc.), a resistor and a photoresistor into... a piece of wire:) having no any voltage drop across it! But with the same success we can make a "botomless" capacitor (an op-amp inverting integrator)... For this purpose, we just incorporate a varying battery to the imperfect component that compensated the losses inside the component. All the op-amp inverting circuits exploit this clever trick.

Resources
Op-amp circuit builder shows how to build various op-amp inverting circuits (an animated flash movie). How do we create a virtual ground? is a circuit story about the great phenomenon. Op-amp inverting current-to-voltage converter reveals the basic idea behind the famous circuit. How do we build an op-amp ammeter? is a similar story about the active ammeter.

Lab 5: Investigating DAC and ADC
Thursday, May 8, 2008, 16.45 h

The pictures are made by Jordan Slavov.

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title: Circuit Idea/Group 68b


text:
Circuit Idea/Technical University of Sofia

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Joining Students from Technical University of Sofia to Circuit Idea Wikibook

(a teacher's story)

A dedication to my students
''I dedicate this exciting story to my students from Faculty of Computer Systems, Technical University of Sofia, spring 2008, that have inspired me to carry out this incredible experiment on the laboratory and to tell about it on Circuit idea wikibook. Cyril''

I have been teaching analog circuitry for more than 20 years. During this period, my students and I have been carrying out many and many times an attractive computer-based experiment (it is my favorite experiment): visualizing the voltage distribution along a linear resistive film that is supplied by two voltage sources connected to the two sides of the resistor. For this purpose, we use all kinds of "resistors", some of them quite odd: resistive wires, pencil's graphites, conductive foam and rubber, and of course, opened potentiometers and the good old rheostats. By sliding on the resistive surface and varying consecutively the two supply voltages, we "invent" various legendary resistive devices: movement sensors, voltage dividers, resistive summers, subtractors, Wheatstone bridge, op-amp inverting, non-inverting and differential amplifier, etc.

In addition, this marvelous experiment presents in an attractive non-traditional way the famous Ohm's law, virtual ground concept, negative resistance phenomenon, etc. to curious students. It even shows directly in a "geometrical" way classical relations between the electrical quantities (I = V/R, UOUT/VIN = -R2/R1, etc.) Furthermore, this experiment establishes a connection between the elementary passive resistive circuits (voltage divider, resistive summer, etc.) and the more sophisticated electronic op-amp circuits (non-inverting amplifier, inverting amplifier, etc.)

I have been having responsive, clever, curious and capable students that have been helping and inspiring me in my pursuit of revealing the truth about resistive circuits through these years. That is why, I have decided to place this experiment first on my site of circuit-fantasia.com and then on this wikibook.

Students

During this term I teach basic circuitry (both lectures and laboratory exercises) to 150 students from Faculty of Computer Systems, Technical University of Sofia. They are divided into five groups (64, 65, 66, 67, 68), each of them containing 30 students. Further, for the purposes of the laboratory exercises, these groups are divided into sub-groups of 15 students. Each laboratory cycle lasts two weeks: the first five sub-groups (64a, 65a, 66a, 67a, 68a) conduct laboratory exercises during every even week; the other five sub-groups (64b, 65b, 66b, 67b, 68b) conduct laboratory exercises during every odd week. My general idea is to carry out the same experiment in the laboratory consecutively with all the student groups during the present cycle (March 17–29, 2008).

Organization
Preparation. First, in the beginning of the week, I started this story on Circuit idea. For this purpose, I outlined briefly (about 25%) the future page: I traced out the contents by inserting the main subtitles; then, I placed semi-drawn pictures and a little explanatory text. By the way, it was very hard for me to refrain from making the whole page...

In the laboratory, I prepared all kinds of resistive elements (wires, pencil's graphites, conductive foam, etc.), power supplies, movements, VOM's, holders, etc. and arranged the laboratory equipment according to the future experiment's purposes.

I also prepared all kinds of recording equipment - a small solid state recorder (for quite a while I have recording my lessons, classes and laboratory exercises), camera and mobile phone. Maybe, next year I have to use a web camera:)?

I have also begun thinking who of famous web writers (mainly, university and college teachers) to invite to join this educational web initiative. For now, I have in mind Tony Kuphaldt (Lessons in electric circuits), Tom Hayes (Student manual for the art of electronics - great book!), William Beauty (Science hobbyist - great site!)...

Announcement. Then, in the lecture hall, I announced my intention to conduct such a web experiment to students and showed them how can we carry out it. I told them what is the great Wiki idea and how they can join the wikibook Circuit Idea. I suggested to students to choose meaningful user names consisting of student name and group number (thus I can observe their web achievements and add more credits to the rating).

During the laboratory exercises
In the student group pages, my students and I describe step-by-step how the laboratory exercises have passed. The great mass of the written is extracted from the records but there is also some fragments that is written directly to this page in the laboratory during the exercises. Note that students make their reports in real time at the laboratory; I make also my "report" on the white board (ten times during the cycle!), snap it from time to time and write it on this page. Students and I make a lot of photos during the exercises; they show drafts on the whiteboard, screen shots, laboratory setups and, of course, the very students.

The idea is first to write up here chronologically what is done in the laboratory; then to systematize and polish these raw materials and place them in the main story. We will use here (and why not in the main page?) a quite informal style. I will mark student's insertions by italic to distinguish them from my teacher's text. Well, let's start!


text:
Circuit Idea/Group 66b

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66b Group Student Page

We are students from Faculty of Computer Systems, Technical University of Sofia. Our 66 group is divided into two sub-groups; we constitute the second - 66b - one. Here are our names:

Borislav Bozhanov; Aydin Hafizoglu; Nicklay Lyubenov; Miroslav Otsedarski; Georgi Upalov; Zapyan Dimitrov; Borislav Borisov; Vanya Shiderova; Lyudmila Nenkova; Ana Ivanova; Yanita Petkova; Rumi Pasaoglu; Konstantina Gocheva; Petar Velichkov; Erol Ozturk; Lyubomir Todorov.

Lab 1: Investigating passive resistive devices
In this section we will explain and show some of our work during the Lab1:

The following picture presents the scheme that we work during the laboratory exercise. It has the following elements:

-	DAC1,DAC2 (DAC3,DAC4) : Digital-to-analog converters;

-	ADC1,AC2(ADC3,ADC4) : Analog-to-digital converters;

-	V1,V2,V3 : three voltmeters;

-	R = 4,7KΩ : A rheostat

-	A computer where we observe the diagrams

We have two input voltages that we are operating with (Vin1 and Vin2):

1.	Graphic presentation ot Potential Diagram.

'''2.	Moving the wiper of the rheostat (Vin1 = const, Vin2 = 0). '''

In this case we have a “moving-to-voltage” conversion. That means that while we move the wiper we change the output voltage.

3.	Changing Vin1 (the rheostat resistance is constant, Vin2 = const).

In this case we have a “voltage-to-voltage” conversion. While we are changing the voltage Vin1, we are changing Vout. In other words, this is a “voltage-to-voltage” converter that is controlled from the left hand side.

4.	Changing both Vin1 and Vin2.

In this case we have “invented” the voltage summer.

If these voltages have opposite polarities, our scheme becomes a comparing device. Somewhere on the rheostat we have a point with zero potential, that is named “virtual ground”.

Here, we have inserted some Wikipedia considerations about virtual ground point.

Virtual ground (or virtual earth) is a node of the circuit that is maintained at a steady reference potential, without being connected directly to the reference potential. In some cases the reference potential is considered to be the surface of the earth, and the reference node is called "ground" or "earth" as a consequence.

The virtual ground concept aids circuit analysis in operational amplifier and other circuits and provides useful practical circuit effects that would be difficult to achieve in other ways.

In circuit theory, a node may have any value of current or voltage but physical implementations of a virtual ground will have limitations of current handling ability and a non-zero impedance which may have practical side effects.

--Ludmila&Konstantin (talk) 12:15, 17 May 2008 (UTC)

Lab 2: Improving the genuine Ohm's experiment
Tuesday, March 18, 2008, 13.45 h

Ohm's law
1. Ohm's Law deals with the relationship between voltage and current in an ideal conductor. This relationship states that the potential difference (voltage) across an ideal conductor is proportional to the current through it. The constant of proportionality is called "resistance", R. Ohm's Law is given by: V = I.R where V is the potential difference between two points, which include a resistance R. I is the current flowing through the resistance. For biological work, it is often preferable to use the conductance, g = 1/R; in this form Ohm's Law is: I = g.V 2. Material that obeys Ohm's Law is called "ohmic" or "linear" because the potential difference across it varies linearly with the current.

3. Ohm's Law can be used to solve simple circuits. A complete circuit is one which is a closed loop. It contains at least one source of voltage (thus providing an increase of potential energy), and at least one potential drop i.e., a place where potential energy decreases. The sum of the voltages around a complete circuit is zero.

4. An increase of potential energy in a circuit causes a charge to move from a lower to a higher potential (i.e. voltage). Note the difference between potential energy and potential. Because of the electrostatic force, which tries to move a positive charge from a higher to a lower potential, there must be another 'force' to move charge from a lower potential to a higher inside the battery. This so-called force is called the electromotive force, or emf. The SI unit for the emf is a volt (and thus this is not really a force, despite its name). We will use a script E, the symbol, to represent the emf.

A decrease of potential energy can occur by various means. For example, heat lost in a circuit due to some electrical resistance could be one source of energy drop. Because energy is conserved, the potential difference across an emf must be equal to the potential difference across the rest of the circuit. That is, Ohm's Law will be satisfied: ε = I.R.

5. Here is a nice simulated experiment on Ohm's Law for you to test your understanding of this concept. Use the "back" button to return to this place. VanyaShiderova66b (talk)

Lab 3: Transistor circuits (trying to create a "diode" current mirror)
Tuesday, April 08, 2008, 13.45 h (extracted from the lab records made by a solid-state recorder) Circuit-fantasist 18:56, 29 May 2008 (UTC)

The idea of this lab is to transform the simplest transistor current source (that your colleagues from group 65b made an hour ago) into a current mirror circuit. You can see what they have discussed on the whiteboard and what they have made on the prototyping PCB. Well, let's first examine and discuss their creation and then try to metamorphose it to the desired circuit.

Look at the circuit on the whiteboard that is left from your colleagues. What do you see there? What is it? What is the idea of this circuit? What does the transistor do there? What is its behavior? What does the potentiometer do? What are these steady resistors? Why have they connected them in series with the varying ones? What does every component do in this circuit?

Well, let's begin reasoning to reveal the secret of this typical transistor circuit...

Understanding the transistor
We have to begin from somewhere; we need some idea to start the discussion... So, let's first consider the ubiquitous transistor. It is a three-terminal device... How have they connected it here? How do we connect three-terminal elements? They have connected one of its leads to the ground; so, the three-lead transistor is connected as a four-lead device. Also, in order to understand the more complex three-terminal transistor, we can break down it into simpler two-terminal parts. For example, we can first separate the output collector-emitter part as a two-terminal "element" and the input base-emitter part as another two-terminal "element"; then, to begin reasoning what does the transistor do.

Varying the input current
The simplest way to drive the transistor is by current. For this purpose, we have to connect only a base resistor Rb between the power supply and the transistor's base; for simplicity let's connect the same load resistor RL = Rb. Only, as a result, we get a current amplifier (Ic = βIb) instead the desired current mirror (Ic = Ib). How do we equalize the currents?

Varying the input voltage
We can connect a second resistor (actually, a potentiometer) in parallel to the base-emitter junction to divert the excessive current from the base to the ground. Where can we drive a transistor from? What is the input? The voltage between the base and the emitter drives the transistor. We can recognize in the left part of the circuit the ubiquitiuos potentiometer and a steady resistor connected in series. So, obviously the idea is to drive the transistor by a voltage. What is the mode of the transistor operation (saturated, active or cut off)? There are also measuring devices connected - two voltmeters and a (milli)ammeter.

What can we do with this circuit? We can vary its attributes one after another and observe its reaction to our intervention. Let's begin varying the input voltage. What happens, if we move the P's slider downwards to the end position? What is the input voltage then? Zero. Yes, there is no any input voltage. The output collector-emitter part of the transistor is in cut-off condition. The resistance between the collector and the emitter is infinite; no current flows in the output circuit. So, you can think of a transistor just as a varying resistor that has infinite resistance at these conditions (zero input voltage). Why the voltmeter V1 shows maximum voltage (equal to the supply voltage)? It is not so simple to explain why... Well, we have already discussed this phenomenon. We have a resistor whose upper end is connected to the positive rail of the power supply. What is the potential (the voltage regarding to the ground) of its lower end?

One possible viewpoint is to think of this circuit as a voltage divider with R1 = RL and R2 = ∞ (R2 >> R1; so, K ≈ 1. Another viewpoint is to see that the resistor R1 does not resist since there is nothing to resist (no current flows). Actually, there is no transistor connected in the circuit; we may erase it from the whiteboard...

The transistor is just a varying resistor
Now begin moving the potentiometer's slider up. At given moment the output voltage V2 begins lower; stop when it becomes somewhere between the supply rails (0 - 10 V), for instance in the middle (5 V). What is the mode of the transistor (saturated, active or cut off)? It is a "semi-opened"; so, it is in active mode. How does the transistor do this magic? Does it do something special? No, the transistor does not do anything supernatural in this region, it does the same what a tube, an FET, a rheostat, etc. would do, if they were at its place - it just changes its present resistance. So, the transistor is a varying resistor; only, it is an electrically controlled resistor. Well, if we have set a total load resistance RL = 10 kΩ and VCE = 5 V, what has the transistor done? It has set its present resistance RCE = 10 kΩ as well. It is just the same resistor as the collector one. Think of this circuit as a voltage divider with two identical resistors; its transfer ratio is K = 0.5.

This is the notion about a transistor that we need to understand how transistor circuits operate. All the active elements do the same; they are electrically controlled resistors. So, all the transistor circuits contain both ordinary resistors and electrically controlled resistors (active elements). They form the well-known voltage divider where transistors are put on the place of R1, R2 or the both.

If you continue moving up the slider, you will probably reach the saturation where V2 ∞ 0 V and the transistor is opened totally. What is the current now? It has a maximum value according to Ohm's law: I = VCC/RL.

Varying the load resistance

How have we arrived at a conclusion that the output part of the transistor behaves as a constant current source? We have somehow to check this speculation. For this purpose, we have to provoke the circuit and to see how it will react to our intervention. Well, what will we see if we vary the load resistance RL? The output transistor characteristic can give us the answer. Do you remember the subject of semiconductor devices? These characteristics are favorite topic for them. Only, they do not show how to use this transistor behavior. A student: the characteristic is horizontal; so, the current will not change, if the load varies. Yes, it is right. But what does the transistor do, what is its behavior to obtain such a characteristic?

Does the current change, if we vary the load resistance? A student: Yes, it does... But how do you explain it? Because the answer is exactly opposite - the current does not vary... Obviously, there are things that contradict our intuition and this is one of them... Well, look at the picture on the right (from Group 68b); it shows graphically how the transistor does this magic. Two IV characteristics are superimposed on a common coordinate system. The first curve is an IV characteristic of a real voltage source having a voltage VCC and an internal resistance RL (they name it "load line"). The slope of this curve depends on RL. If RL = 0, it is a vertical line; if RL > 0 it inclines proportionally to the resistance. The second curve represents the present output transistor resistance (RCE or shortly, RT).

The transistor does the following trick: if we try to decrease/increase the load resistance (to set upright the load line), it does the opposite - increases/decreases its present resistance (RCE or shortly, RT) so that the crossing (operating) point moves along the almost horizontal output characteristic of the transistor. As a result, the current stays almost constant. You can see how simple the idea of this dynamic constant current source is. It consists only of two elements: a voltage source and a dynamic resistor connected in series with the resistive load. The resistance of the dynamic resistor supplements the load resistance to a constant value (RL + RT = const); so, the current is constant - I = VCC/(RL + RT). Shortly, the dynamic resistor keeps up a constant total sum of the resistances.

A conclusion: ''If you connect in series a transistor and a resistor and begin varying the resistance of the resistor, the transistor will also begin changing its present resistance so that the sum of resistances to stay constant; as a result, the current stays constant. The transistor acts as a dynamic resistor that varies, as a current-stable resistor (an element that has the property to keep up a constant current)''.

And what about the voltage drop across the resistor? Does it vary? Move the slider and look at V2. If we lower RL, the voltage drop VL across the load lowers as well. But the transistor increases its present resistance RT and the voltage drop V2 across it increases as well. And vice versa; if we increase RL, the voltage drop VL across the load increases as well. Now, the transistor lowers its present resistance RT and the voltage drop V2 across it lowers as well.

A conclusion: If we vary the load resistance, the current stays constant but the voltages vary in wide limits.

However, at given moment the transistor depletes its reserve of resistance; it saturates and the voltage V2 across it becomes zero. The magic of dynamization ceases and if you continue changing the load resistance, the current will change according to Ohm's law.

Varying the supply voltage
Besides the load resistance, we can vary the supply voltage as well. Look at the picture on the whiteboard again and answer the question, "Will the current stay constant, if we vary the supply voltage VCC?" Well, try it on the lab set up. As we can see, the current varies. Why? What is the problem?

When we vary the supply voltage, the transistor would react to this new "intervention" as before: if we increase VCC, it would increase its present resistance RT and v.v. The transistor would keep up a constant ratio VCC/(RT + RL). But still the current varies... What is the reason? A student: The input voltage varies. Yes, this is the reason... The tragedy of this circuit is that they have supplied the input voltage divider by the same power supply. But we have not another power supply. What do we do then so that the current to stay constant when we vary the common supply voltage?

A student: ...to add another transistor... I don't know; tell where you will place this additional transistor. You have to have some idea to place it. What does a transistor stabilize? Do all colleagues understand that a transistor stabilizes current? A transistor is a current stabilizer while a diode is a voltage stabilizer. The characteristic of such a voltage-stable element is a vertical line. All the voltage-stable elements (diodes, zeners, LEDs, etc.) have such a characteristic; they behave as a voltage source.

A true diode sets the input current

Another student:...to place a diode... Yes, this is the right answer, to place a diode but now you have to say where. Please, write it on the whiteboard; here are whiteboard markers. Do not are confused that it looks not so pretty because we "invent" the circuit scribbling on the whiteboard. The other alternative is to show "cut-and-dried" pretty circuits on slides or Powerpoint presentations...

Well, your colleague begins drawing a diode; is she drawing it in the right direction? By the way, I know there are diodes that are usually connected in a reverse direction. What are they? Students: ...zener diodes... Right but let's use an ordinary diode. Only, let's connect it correctly... Is the circuit written correctly?

Another student: Let's erase the potentiometer... Go and draw what you mean. Maybe, it is better not to remove the potentiometer; instead, draw it higher up and connect it properly, as a rheostat (do not forget to connect the slider). Leave it as an element that allows us to regulate the input current. It is time to make some shots. This room is not so suitable to make photos as a Hollywood studio but your colleagues do something great; so, they deserve to be perpetuated:)

Don't you think it's time to discover the web for similar solutions? For this purpose, write "current mirror" in the Google window. As usual, the corresponding Wikipedia page (too formal and boring) stays at the first position.

Let's then see Tony Kuphaldt's page about current mirrors. In the beginning, he is reasoning about Ebers-Moll diode equation. We can see the load and the transistor and now we exclaim with surprise - here is the same diode! Tony Kuphaldt has proposed what our colleagues has proposed as well - to improve the circuit by connecting a forward biased diode in parallel to the base-emitter junction. Then, the author begins telling what the profit is in connecting an ordinary diode.

Let's scrutinize the Tony Kuphaldt's circuit since I have the following idea. As he is a very good teacher and an excellent web writer, I intend to write an email to him where to describe our Wikibooks "open student project" and to invite him to join it. More concretely, I will tell him how you and I, all together, have arrived at his idea; then, how we have checked it and have found out that it is right (or not, now we have to decide). That is why it is so important to judge his circuit.

Tony claims that the output current flowing through the load is equal to the input current flowing through the diode: "Current through resistor Rload is therefore a function of current set by the bias resistor, the two being nearly equal." But is it true? Let's try it.

Who can solder? Choose a diode, solder it on the prototyping PCB and measure the currents. As I can remember, your colleague has told that a transistor consists of two diodes. Here we need a diode whose IV curve is identical to the IV curve of the transistor driving the load. What an idea can arise then? Imagine we have not an ordinary diode; we have only transistors...

Replacing the true diode with a base-emitter transistor junction

Can we use the transistor as a diode? Yes, we can. In this case we can connect the base-emitter junction of an identical transistor (leaving the collector no connected) as a diode. A student: Yes, this was the lower circuit of Tony Kuphaldt...

They frequently use this "trick"... Even the transistor can be "semi-damaged"; we can use only one of its junctions (in this case we need exactly the base-emitter junction). Who will draw the new circuit? Let's the colleague to draw it. Leave the collector non-connected. Try to draw the circuit more symmetrically and turn it with the base to the other base; thus we will arrive easier to the great current mirror circuit. Finally, draw where currents flow by green current loops. All the currents begin from the positive lead of the supply voltage source and return to the negative one. First, an input current comes down from the positive rail, flows through the base resistor and arrived at the common point between the two bases. Here it strikes two p-n junctions connected in parallel (we have supposed they are identical). So, what will it do? The current will branch out into two directions - on the left and on the right; there are two base currents IB.

What is the idea of Tony Kuphaldt? It asserts that, if we create an input voltage by a stabilizer containing a resistor and a diode (or a base-emitter junction) acting as a voltage-stable element and apply this voltage to the base-emitter junction of another transistor, the two currents (the input and the output one) would be almost equal. Shortly, the two currents are equal since the two voltages are equal.

Then look for another but the same transistor and solder it. Who will show oneself? Only, let's connect another milliammeter in the input circuit. Can we expect that the two currents will be absolutely equal? No, we cannot since the two transistors are not absolutely identical; they are not selected. They made this structure exceptionally in integrated circuits. Who will solder? Look at the circuit on the whiteboard and implement it on the prototyping board. Do not remove the steady resistor connected in series to the potentiometer. Why is it there? It protects the base-emitter junctions of the transistors in a case when someone set zero resistance.

By the way, I have not ever carried out this experiment; so, I don't know what will happens. The result may be positive but maybe negative... And what do you think about the currents? Are they equal? Well, will check them later. Let's now tidy the measuring devices. Leave the voltmeter across the output transistor; we need it to know, if the transistor is in the active region. The two milliammeters will whow the two currents.

Now increase the input current by moving up the slider of the input rheostat. What about the output current? It does not "move"; it stays constant (about 2 mA). Why? Measure the input voltage (between the bases and emitters) to know why. As we can see, it is about 0.65 V. What does it mean? Maybe that the transistor is saturated... To make sure measure also the voltage VCE across the output resistor. You need a voltmeter; switch the ammeter as a voltmeter and connect it across the collector-emitter part of the second transistor. Only, close the loop by a piece of wire (use wires with alligators at the two ends)...

A problem appears: the transistor smokes!

Oh! The transistor begins smoking! It is warm! But why? What happens? It is impossible! If your circuit is the circuit that is drawn on the whiteboard, the transistor should not warm since there are "protecting" resistors of 3.9 kΩ connected in the base and in the collector circuits! There is another cause and you have to find it because it is very useful for us. IMO, there is some wrong connection... You can check the protecting resistors, if they are 3.9 kΩ. Yes, they are.

''A student: I have found the cause - we have bypassed all the resistors connected in the collector! Maybe, the transistor is not damaged?''

Well, let's check it. Alas! There is no voltage across the collector-emitter part; it seems the transistor is damaged. Or it is just saturated?

Only, our lab is finished. Your colleagues from the next Group 67b will continue your work.

(here, the record finishes) Circuit-fantasist 17:22, 2 June 2008 (UTC)

Lab 4: Op-amp inverting amplifier
Tuesday, April 22, 2008, 13.45 h

1.Principles of Negative Feedback (NFB)

The main idea of negative feedback is returning of the output (Y) back to the input (X) of the device (Image 1). In other words, we can think of this idea as a sort of active "tracing" or "following". Every device with negative feedback "solves" the equation X=Y. In order to do this, we need first energy (E). Also, we need a regulating element (R) to control the output quantity Y. The third element in our NFB device is a comparator (-), which will "analyse" the difference between X and Y and will give the result to the regulating element that have to reduce the difference to zero.

Devices and systems use NFB to compensate any disturbances that may occur during the working process.

Image 1:

2.Voltage Follower

This is the simplest operational amplifier configuration. It follows the voltage that is sent in. It is a very useful little circuit. You can use it to protect circuity of being overloaded. The voltage follower with an ideal op amp gives simply Vout = Vin. First, we have a Op-amp with NBF: Image 2: Then we add 2 resistors: Image 3: and now we have a Voltage Follower!!!

It's good to know that 2 resistors in series have the ability to subtrack 2 voltages: Image 4:. An interesting fact is, that while trying to compensate the disturbances the follower is turning into an op-amp.

Here we can see a resistive summer in ideal circuit:Image 5:

Yanita Petkova --shining*star (talk) 23:33, 17 May 2008 (UTC)

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title: Circuit Idea/Group 66b