text:
Embedded Control Systems Design/Hostile Environment

Hostile environment is understood as every possible factor that prevents a system (of any complexity or at any level) from performing its function correctly.

As an embedded systems designer one should make sure that hostile environment is taken in account during the development stage. Although existing systems encountering hostile environments can perfectly be protected as well, it is helpful to take hostile environment into consideration already in the design stage, in order not to introduce hostile environment yourself by faulty system design. Apart from their positive effect on system performance, the countermeasures to hostile environments may have negative effects on other system parameters, e.g. rising economical cost, weight, power consumption, etc. But even if a design contains the necessary countermeasures to hostile environment, a failure of the system is not always inevitable. In case of failure, there are several manners in which a system fails. That is where failure modes turn up. When countermeasures against hostile environment fail, the failure modes need to be observed in a correct way.

In this section we will try to present some basic design rules to deal with hostile environment.

Examples of hostile environments
Below is an inexhaustive list of possible hostile environments

 * Extreme temperatures (aerospace: -200 °C to +150 °C)
 * Excessive current, overvoltage due to thunderstorm or due to starting electromotors
 * Electromagnetic radiation of other devices
 * Interference of the power grid (50 Hz or 60 Hz)
 * Electrostatic discharge
 * Robocup robots experience negative counteraction of the opposing football team
 * Software virusses and other malware
 * Adjustment of embedded control systems by end users in order to alter performance of a device (e.g. tuning the electronic control unit or engine control unit (ECU) of a car can devastate its exhaust emissions performance)
 * Communication signals (GPS, cell phone) are not picked up inside tunnels or elevators (Faraday’s cage)
 * Improper use by end user (unintentional generation of overflow)
 * Power blackout
 * Mechanical shock

In the following discussion we will take a closer look to some of the topics of the list above.

Consequences of a hostile environment
Possible consequences of an improperly functioning system are
 * System damage
 * Substantial economical costs because of production standstill
 * Life threats to the users of the particular system

Electromagnetic compatibility
EMC is broadly discussed on Wikipedia. Here we make a brief summary of the phenomenon in order to get as soon as possible to the design countermeasures. The proper functionality of an electrical device might be influenced by unwanted generation, propagation or reception of electromagnetic radiation. Electromagnetic radiation can thus form a hostile environment. In order not to get influenced by electromagnetic radiation a device needs to be electromagnetically compatible with its environment. EMC can be divided in two phenomena: emission and immunity. This subdivision means that a device shouldn’t emit to much radiation but on the other hand should be immune to the radiation emitted by others. It is a design requirement for embedded control systems to overcome these two phenomena. In the European Union it is advised by EU directive 2004/108/CE to check a device for EMC before placing a CE-label.

Sources of electromagnetic waves

 * FM-radio’s
 * Wifi
 * Anti-shoplifting devices
 * Electronic hardware: Inductive coupling between lines on a printed circuit board that are separated by less than a wavelength of their transmitted signals.
 * Lightning strikes can induce electromagnetic waves in telecommunication lines.
 * Interference of the power grid (50 Hz or 60 Hz)
 * Fluorescent lighting (100 Hz or 120 Hz)
 * Solar flares

Hazards

 * Electromagnetic radiation originated at the sun’s surface can reach earth’s atmosphere and interfere with terrestrial communication. Professor Paul Kintner Jr. en Alessandro Cerruti (Cornell University ) discovered that GPS signals were interrupted for several minutes on the dayside of the earth due to solar flares. There is only a minor inconvenience for automotive navigation, but it can be dangerous for aerospace navigation and stabilization of oil rigs. Aerospace industries have built in redundancy by using gyroscopes as a backup. In fact the gyro’s date from the pre GPS era. Also the more expensive automotive navigation systems can be equipped with gyro’s as a backup in tunnels where the signal is low. In future designs of aerospace embedded control systems the designer should concern about these solar flares as they are expected to cut the GPS signal for several hours in the years 2011-2012.
 * Medical applications exposed to electromagnetic waves can pose life threat, e.g. cardiac pacemakers.
 * Photodiodes experience a 100 Hz noise component (2 times net frequency) of fluorescent lighting tubes. Possibly causing faulty decision taking in an infra red communication device.
 * Data loss if a communication cable is influenced by a nearby power line (50 Hz or 60 Hz)

Solutions
As explained before, EMC problems manifest at two domains: i.e. emission and immunity. It is self-explanatory that countermeasures can be made at both domains. Furthermore, a countermeasure to emission is often as effective as to the immunity of the device and the other way around.

Emission solutions

 * Avoid unnecessary operations. Necessary switching should be done as slowly as technically possible.
 * Noisy circuits (with a lot of switching activity) should be physically separated from the rest of the design.
 * Use harmonic wave filters.

Immunity solutions

 * Fuses, trip switches and circuit breakers.
 * Transient absorbers.

Mutual solutions

 * Decoupling capacitors ( 1 to 100 µF) (small localized energy reservoir; these supply the circuit with current during transient, high current demand periods, preventing the voltage on the power supply rail from being pulled down by the momentary current load) (Line filter, Signal filter)
 * RF chokes: Choke coils are inductances that isolate alternating current from certain areas of a radio circuit.
 * RC elements: RC-low-pass-filters can filter AC.
 * Shield Housings and lines: always exist out of conducting material. Although not all conducting materials are metals, shield housings mostly are made out of metal. Electromagnetic shields act as Faraday’s cages. Faraday’s cages shield electromagnetic radiation from the inside to the outside en the other way around, thus providing mutual protection. The mesh size of the cage should be smaller than the wavelengths to be blocked. Applications of Faraday’s cages are
 * A microwave oven is equipped with a grid in front of the window. The mesh size of this grid is smaller than the wavelength of typical microwaves (1mm – 1m). However, visible light (400-700 nm) can still pass trough in order to offer the user a clear view of the food being processed.
 * Plastic housings don’t act as a Faraday’s cage, but are aesthetically more suitable for consumer products than metal housings. Both aesthetics and EMC can be combined by coating a plastic housing with a metallic spray on the inside. Make sure that antenna’s have a proper access to the outside world of this housing.
 * Shoplifters often wrap the RFID chip in aluminium foil. That forms a Faraday’s cage as well, preventing the alarm from being triggered.
 * US passports are equipped with an RFID chip that can be read remotely at airport customs. However, to protect the owner's privacy, this passport shouldn’t be read at any time. That’s why these passports are provided in a shielding sleeve, acting as a Faraday’s cage.
 * Avoid antenna structures in PCB Design, such as loops of circulating current or unbalanced transmission lines.
 * Keep in mind that radio communication (GPS, etc.) can be interrupted by nature (solar flares) or by humans (military enemies, terrorists). Therefore in systems of vital importance, (e.g. airplanes, military vehicles) one should build in some redundancy. In the specific case of GPS navigation gyroscopes are often used to overcome dead signal periods.
 * Raise the transmitting power of the signal source. Note that this can be constrained by health regulations.
 * Implement weak signal-tracking algorithms in the receivers, so that they can detect a signal in worse circumstances such as solar winds. Mind that this measure can contradict other parameters, e.g. the economical cost, weight, power consumption, etc. of the receiver.

Power blackout
A power interruption can take from several milliseconds to several hours or even days. Long blackouts can be overcome by installation of an uninterruptible power supply (UPS). Al sorts and sizes of UPS’s are available. For small electronic devices a battery will do the job, for large plants, hospitals and systems of systems (telecommunication networks) diesel generators are commonly used. Selection and control of UPS’s is a discipline in itself and is widely available on the web and in literature.

Voltage Dips
When the supply voltage only reduces for a couple of milliseconds, it is referred to as a voltage dip or a dropout. These short interruptions are far more frequent than long time blackouts. According to [Schneider, p. 51-58] interruptions of 10ms are likely to occur every 200h in contrast to long time interruptions that occur around once every 10,000h. Voltage dips could lead to production halts that last much longer than the dip itself. According to [Terörde, p. 282] voltage dips of 100ms duration can lead to production halts of 24 hours.

Hazards
Drives of electric AC motors are very sensitive to voltage dips. The AC engine itself can perfectly cope with a transient in the supply. There is DC bus between de AC supply and the AC motor which contains a large capacitor to flatten the DC. When a voltage dip occurs at the supply side, the energy in this capacitor is consumed by the motor within a few milliseconds. The control loops of the engine drive draw their power from this DC bus. As soon as this DC bus goes under a predetermined voltage, the inverter shuts down in order to avoid possible damage. With an offline controller, the motor and production process remain uncontrolled, which can cause economical damage.

Solutions

 * One might enlarge the capacitor in the DC link. But this capacitor is already a major cost item. Enlarging it would make the drive even more expensive. Furthermore one might wonder if there exists any capacitor that can feed the hunger of multi-kW electromotors.
 * Ride-through scheme or kinetic buffering: The most interesting way of overcoming a voltage dip is to recover mechanical energy stored in the rotating masses of the motor and its load. In case of a voltage dip, the motor will be used in generator mode and will generate a minor amount of electrical power to maintain the DC bus and to keep the control logic alive. Actually this comes down to regenerative breaking. In essence there’s a dip detection mechanism that activates a preprogrammed ride-through scheme. This reverses the power flow in a matter of milliseconds. Of course the motor will slow down as this is regenerative breaking. The amount of kinetic energy in the motor is exhaustive, but should suffice for the duration of the voltage dip. When the power failure persists the motor will lose all its mechanical energy and won’t be able to restart. Eventually the control logic will shut down. But at least the process could be halted in a controlled way, without major damage. This concept is innovative and not yet common practice in the industries, but it could be in the future. Extensive explanations can be found in [Terörde p. 283].


text: User talk:41.224.39.105

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

title: User talk:41.224.39.105


text:
File:SendFootageButton.png

Licensing
/Images

title: File:SendFootageButton.png


text:
File:MontageConcepts.png

Licensing
/Images

title: File:MontageConcepts.png


text:
File:MontageSelectedFrames.png

Licensing
/Images

title: File:MontageSelectedFrames.png


text:
User:Srath

Sanjay Rath, Address: 15B Gangaram Hospital Road, New Delhi 110060, India

title: User:Srath


text: User talk:137.101.40.60

title: User talk:137.101.40.60


text:
File:RedoButton.png

Licensing
/Images

title: File:RedoButton.png


text:
User talk:Yasirniazkhan

Welcome message
Welcome, Yasirniazkhan!

Come introduce yourself at the new users page. If you have any questions, you can ask there or contact me personally. Webaware talk 12:42, 2 April 2008 (UTC)

title: User talk:Yasirniazkhan


text:
Chess Opening Theory/1. e4/1...e5/2. d4/2...exd4

Centre Game
Black goes in the direction of the Centre Game by capturing the e-pawn.

White's responses
The usual move is '''3. Qxd4'''.

3. c3 is the Danish Gambit.

3. Nf3, the Kieseritzky variation, often leads to several transpositions. Black can transpose to the Scotch with 3...Nc6 or 3...Bb4+, to the Philidor with 3...d6, to Petrov's Steinitz variation with 3...Nf6, or to the Lewis Gambit of the Bishop's Opening with 3...Bc5 (followed by 4. Bc4 Nf6). An independent line is 3...Bc5 4. Nxd4 Nf6.

Theory table
'''1. e4 e5 2. d4 exd4'''


text:
Talk:Indian Tipi/Tipi History

notes on sources

 * North American Indians of the Plains By Clark Wissler, 1912
 * http://epa.gov/greatlakes/native/2001/tepee.html

title: Talk:Indian Tipi/Tipi History


text:
Talk:Japanese/Lessons/Telling time

Untitled
I edited two mistakes. One, han was not written out in in the trascipt (hiragana) section. And two, "yo" was written in the transcription, but not the original dialouge. —The preceding unsigned comment was added by 61.7.11.141 (talk • contribs) 19:26, 30 September 2005  (UTC)

Incorrect information about use of 何（なに・なん）
In this lesson, there is an information about "nan" and "nani" work same as "what is" and "what's". It's not true. The use of "nan" or "nani" depends on the next sound. If the next sound is a consonnant, 何 is said "nan". if the sound is a vowel, 何 is said as "nani". This is a simple explanation, as there are exceptions and maybe other things. Someone with better knowledge than me should correct the article. --Luis Sergio Moura (talk) 14:36, 2 April 2008 (UTC)

X till Y
06:45 would be 七時十五分前, if I'm not mistaken. Adding how to state X till Y would be a nice addition here.

title: Talk:Japanese/Lessons/Telling time


text:
User:Jsigler

This I'm still a newbie to wikibooks, but I'm learning fast. I am using the wikijunior book [Famous Inventors] as a class project with my 3rd graders. I posted about our project at |Wikijunior Famous Inventors book Learning wikicode will be our biggest challenge, so we may need help. Thanks for your support from the wikibooks community.

title: User:Jsigler


text:
Circuit Idea/Group 65a

<<< contents - Group 64a - Group 66a - Group 67a - Group 68a - page stage >>>

<<< Group 64b - Group 65b - Group 66b - Group 67b - Group 68b >>>

65a Group Student Page

We are students from Faculty of Computer Systems, Technical University of Sofia. Our 65 group is divided into two sub-groups; we constitute the first 65a one. Here are our names:

Mehmed Yusuf, Nikolai Ninov, Vladimir Mladenov, Borislav Ivchev, Miglena Nesterova, Nikolai Dimitrov, Zdravko Zdravkov, Bozhkova, Maria Nikolaeva, TEN4O PETROV.

Lab 2: The genuine Ohm's experiment
Tuesday, March 18, 2008, 10.30 h

History. What did Ohm carry out in 1826? Do you know? Students: No... Please, let's someone write in the Google's window "Ohm's experiment" or "Ohm's law" to see what is written about the topic. Thank you ......... (insert your name here and make comments to obtain credits); please, make a folder in "Favorites" named "Ohm's law" and put there the most remarkable links.

Let's first consider his genuine experiment and then reproduce it here, in this laboratory, almost three centuries later. This is the first of all the ten exercises of this cycle; so, let's try to reproduce it in the most original form. So, what do we need to carry out it? Silence...

Building the laboratory setup. As far as I know, Ohm has discovered the local voltages along a copper wire that was supplied by a strong voltage source. So, we need a wire, a power supply, a voltmeter and an ammeter. But what kind of wire we need? Bare or insulated? Copper, silver, iron, or other? Student: Silver will work excellent...  We haven't a silver wire, let's try with a copper one:) Oh, it has welded!?! It is so well that the power supply has a current limitiing circuit (2.5 A)! That is why Ohm made a strong voltage source by applying the just invented by Seebek thermoelectric effect! Now, it is clear we need a resistive wire (nitinol, NiChrome, etc.); so, let's unwind a piece of wire (about 50 cm) from a heater. ...... (insert your name here), please, meausure its resistance. The resistance is 10 Ω, so we have 0.2Ω/cm.

Experiment 1. Now, fix the two ends of the wire in porcelain insulated terminals (holders) and apply a voltage (for instance, 10 V) first to the left end of the wire. What can we investigate now in this arrangement? What do we measure with the voltmeter? The usual viewpoint is to think of a resistor as a point, as a something that has not dimensions, as a two-terminal element that has only a property of resistance. But here we have the unique chance to peep inside the "resistor"! What will you "see" along the wire? What will the voltmeter show when we slide it from right to left? What are the local voltages along a resistor, if there is no current? Student 1: Zero... Student 2: 10 V... What is the voltage at the right? Zero... And what is it at the middle? Zero...

Well, let's try. Just touch the wire by the voltmeter probe and move it along the wire to measure all the voltage drops (for now, regarding to the ground)! If you are tired, stick a crocodile clips on the probe, "bite" the wire and move this "slider" along it! The result: everywhere local voltages are 10 V!

Experiment 2. Now, ground the right end of the wire. Oh! The wire is hot! Have I instructed you about all the dangers in the laboratory:)? Include this one: do not touch a hot wire! See the ammeter - it shows 1 A; so, the power is P = V.I = V2/R = I2.R = 10 W. By the way, are these calculations right (has the resistance a constant value)? We see, it is too hard for this thin wire to dissipate the power to the ehviroment and it is heating.

Now, move again the "crocodile slider" along the wire and measure the local voltage drops. As far as I know, Ohm did exactly the same. He moved the probe from one position to other, measured the corresponding potentials, made the difference between them and calculated the ratio (V2 - V1)/(L2 - L1) = (V2 - V1)/(r2 - r1) = dV/dR = I. Thus he has established that this ratio (it was the current I) is constant along the wire; so, Ohm has concluded that V/R = I.

We can see that moving the "crocodile slider" the voltage drops decrease gradually from 10 to 0 volts. What is the input and what is the output quantity here? Eureka! Using the "slider" movement as an input and measuring the voltage drop as an output we have "invented" the legendary (but misnamed) potentiometer, movement-to-voltage converter, sensor... Is it a linear converter? Why? A hint: the resistance is distributed linearly along the wire.

Experiment 3. But why do not we change the role of the quantites? With the same success we can stop the "crocodile slider" at the middle of the wire and then to vary the input voltage as an input and, as before, to measure the voltage drop (regarding to the ground) as an output. Eureka (again)! Now we have "invented" the legendary voltage divider (voltage-to-voltage converter)! What is the relation between the input and output voltage (the so-called "transfer ratio"? We can see directly that VOUT = VIN.L2/(L2 + L1).

Experiment 4. We are ready to do more sophisticated experiments... Are there more interesting points (different from the ubiquitous ground) to fix the negative voltage probe? Of course, we may touch any wire point! Well, let's begin with the middle point: "bite" it with the black (negative) negative "aligator" and slide the red (positive) "aligator" along the wire. Only, first change the unipolar voltmeter with a bipolar one. Why? What might we expect? The result is more than surprising: the output voltage varies from -5 V to +5 V! Eureka! We have obtained a bipolar movement-to-voltage sensor!

Experiment 5. What happens, if we move simultaneously both the voltmeter probes in the same direction? Students: The voltage will not change... Right! They name this "common mode input signal".

Experiment 6. And what happens, if we move simultaneously both the voltmeter probes in opposite direction? Students: The voltage will change... Right! It will even change more rapidly than above. They name this "differential input signal".

Experiment 7. Now stop both the "sliders" and wiggle the input voltage. This is a quite odd voltage divider with flying output. What is the relation between the input and output voltage (the transfer ratio? We can see directly that VOUT = VIN.(L2 - L1)/L.

We can measure the voltage drops along the resistive wire by using more unusual indicators, e.g. electric bulbs. What do you think, is there any problem to supply a bulb in this exotic way? If yes, when and why?

Applications. Where may we see this phenomenon in life? The answer is simple: everywhere where a current flow through bad conductors. In all these cases, a gradually decreasing voltage drop appears across the "conductor". A "car example": if you move the probe lamp along the heater of the rear window, you will see the voltage drop across the resistive wire.

Lab 3: Building a simple common-emitter transistor amplifier
Tuesday, April 01, 2008, 10.30 h

Lab 4: Op-amp circuits with series negative feedback
Tuesday, April 15, 2008, 10.30 h

What is op-amp?

Inputs. Grounded voltage source... What is single-ended input? Floating voltage source... What is differential input? Why do we need a differential input? Input resistance... Do input currents flow? How does a non-inverting input behave? How does an inverting input behave?

Output. How the op-amp output is made? What is the idea of a complementary, push-pull emitter follower? Output resistance...

Supplying. The idea of bipolar (split) supply (+12 V and -12 V here)... Why do we need a bipolar supply? Where currents flow?

Gain. Typical value is A > 200000. It seems too high. What is the maximum input voltage to keep the op-amp in the active region (assume 10 V maximum output voltage)? Can we use a bare op-amp as an amplifier? Or we can use it as another useful device having analog inputs and descrete output (write its name here)? We may conclude: If we use the op-amp as an amplifier, the voltage difference between its inputs is zero. But if we use it as a comparator, the input voltage might be whatever.

Transfer characteristic. Microlab can help us to obtain this curve on the screen. There is no time in this graphical presentation; there is only input and output quantities. We might discern three regions on this curve: negative saturation (large), active region (narrow) and positive saturation (large).

Generalization. What does the op-amp actually do? A possible answer is: an op-amp converts the steady power supply into varying voltage source.

Building an op-amp follower

Disturbing the negative feedback follower

Converting the NFB follower into an amplifier

Resources
Operational amplifier Differential and Single-Ended Inputs Labs: group 66a and group 67a. Op-amp amplifiers with negative feedback

Building an op-amp inverter

Converting the op-amp inverter into an amplifier

<<< contents - Group 64a - Group 66a - Group 67a - Group 68a - page stage >>>

<<< Group 64b - Group 65b - Group 66b - Group 67b - Group 68b >>>

title: Circuit Idea/Group 65a


text:
Circuit Idea/Group 66a

<<< contents - Group 64a - Group 65a - Group 67a - Group 68a - page stage >>>

<<< Group 64b - Group 65b - Group 66b - Group 67b - Group 68b >>>

66a 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 first - 66a. Here are our names:

Dafar Shaban, Dilyana Dilova, Irina Hadjieva, Miroslava Hristova, Victor Glavev, Alexandra Georgieva, Danail Dekov, Liliya Bancheva, Elina Lazarova, Mihaela Borisova, Tzvetan Tzvetkov, Hristiana Stancheva, Silviya Nakova, Nataliya Genova, Vasil Tzanov, Ivan Tzvetkov.

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

Using the heritage of group 65a
Welcome to the laboratory. The picture on the whiteboard represents the laboratory setup that your colleagues from group 65a have used for reproducing the genuine Ohm's experiment. You can use the results of their laboratory experiment to introduce the next great idea named voltage diagram. It will help us to visualize the voltages that your colleagues have measured along the resistive film.

Introducing the idea of pressure diagram (an opened water circuit)

By the way, we can derive this idea from the well-known hydraulic analogy (plumbing) that we can see everywhere around us. For example, imagine a large vessel filled of water that supplies a long thin pipe; let's first the pipe to be tapped. The question is: "What is the pressure inside the pipe?" And more precisely speaking, "What are the local pressures along the pipe?"

We can get to know, if we drill small holes at equal intervals along the pipe (if we want to be more precise, we might stick vertically thin glass pipes acting as local manometers). The result is expectable for us: all the water levels (accordingly, all the local pressures along the pipe) are equal. This picture shows the pressure distribution along the pipe; we can name it "pressure diagram".

The pressure diagram of a closed water circuit

Now open the pipe; the water will begin flowing. What are the local pressures along the pipe now? At the left end the pressure is maximum; at the right end it is minimum. Our intuition suggests that the local pressures will decrease gradually from left to the right.

Really, the levels of the water bars (accordingly, the local pressures along the pipe) decrease gradually from left to the right. The envelope of the pressure diagram is a triangle.

Conveying the idea of pressure diagram to voltage one
Let's now apply this powerful idea to draw the analogous electrical voltage diagrams. The idea is obvious: if we think of voltage as a kind of pressure, we may present the local voltages by local voltage bars in exactly the same way as we have presented the local pressures along a pipe by a local water bars! As above, the lengths of the voltage bars are proportional to the magnitudes of the local voltages regarding to ground (we might set the zero voltage level at the height of the resistor and then to draw the positive voltage bars above and the negative voltage bars below the resistor's level). The set of these voltage bars forms the whole voltage diagram. We can use the envelope of the voltage diagram instead the very set of voltage bars, in order to simplify the image.

Making a computer draw a "living" voltage diagram on the screen
Basic idea. But it is too hard and boring for us, human beings, to draw all the voltage diagrams when the circuit attributes (voltages and resistances) vary. Because these diagrams have to be not static, dead pictures; they have to be "living" diagrams (animations) that change accordingly to the circuit state. So, wee need a computer that "watches" closely what we do with the circuit under test and builds the according "living" voltage diagram on the screen. Then, let's build such a computer-based system and leave it to do this donkey work:). Then we will only look at the picture on the screen and think about the circuit phenomena behind it!

Components. First at all, we need some computer. It may be humble enough, if only it has some graphical possibilities (for example, the computer that is used in this laboratory to build Microlab system in 1986 is a version of the famous Apple II). Then we have only to connect to the PC buses a few analog-to-digital converters (ADC's) acting as analog inputs and a few analog-to-digital converters (ADC's) acting as analog inputs and our computer-based system Microlab is ready!

Power supply. Note that the analog-digital periphery "sucks" energy directly from the poor computer:); so, it acts not only as a computer but it supplies the periphery too. Then can we supply the very resistive object as well? Can we use the very DAC's to supply the resistor under test? This will allow the computer to control the obect! Only, the DAC's can give maximum 10 mA by their op-amp outputs. So, we can't supply the low-resistive (10 Ω) wire of group 65a!

Resistor. Do you remember the linear 4.7 kΩ varying resistor from the previous laboratory exercise? As far as I can remember, we carried out a "reverse engineering" then taking it to pieces:) What do we do now? Suggest a solution!  A student: Yes, I do... Obviously the colleague suggests to supply such a resistor by the DAC's 10 mA outputs without any problem (calculate the maximum current). Thus we can use it instead the low-resistive wire. Then we supplied only the one of the resistor's terminals. Now, let's complicate the arrangement and make it more confusing:) by supplying both the ends (we suspect it might be very interesting). Well, connect AO1 (DAC1) to the right and AO2 (DAC2) to the left end of the resistor.

Voltmeters. What do you think, wheather to connect the two old-fashioned but attractive bipolar voltmeters V1 and V2 to observe the input voltages? Or to rely only on the abstract digit VOM's and on the screen digit meausements? I see, you like the good old meters maybe because they are something "live", moving, geometrical, spatial, real... People trust such genuine things... But is there any problem as these "antiques" have 20 kΩ internal resistance)?

Ground. Now, let's say also some words about ground. But what is ground? We can find a possible answer in the Wikipedia article about virtual ground. Shortly, ground is a reference point, regarding to which we measure voltages. The PC power supply that we use is the so-called "split supply". It consists of two 12 V supplies connected in series (- + >>> - +). The middle point serves as ground in this arrangement. The DAC's grounds are connected internally to this ground. We have also to connect the black test ends of our voltmeters to this ground.

Software. More than twenty years ago, I prepared a program that can visualize the voltage diagram on the screen as a "living" animation. For this purpose, the program make computer "interest" in the local voltages in three key points - the left end point, the slider intermideate point and the right end point. Let's then satisfy its curiosity:) by connecting, for a start, the DAC1's output to the ADC1's input and the DAC2's output to the ADC2's input.

Lab 3: Biasing the simple common-emitter transistor amplifier
Tuesday, April 01, 2008, 13.45 h

Making biasing batteries

Lab 4: Op-amp circuits with series negative feedback
Tuesday, April 15, 2008, 13.45 h

What is op-amp?

Inputs. Grounded voltage source... What is single-ended input? Floating voltage source... What is differential input? Why do we need a differential input? Input resistance... Do input currents flow? How does a non-inverting input behave? How does an inverting input behave? Can we use an op-amp with a differential input as an op-amp with single-ended (inverting or non-inverting) input? What do we do in these cases?

Output. How the op-amp output is made? What is the idea of a complementary, push-pull emitter follower? Output resistance...

Supplying. The idea of bipolar (split) supply (+12 V and -12 V here)... Why do we need a bipolar supply? Where currents flow? We can imagine the output part of the op-amp consists of two parts - a positive and a negative. Each of them works when its output voltage has the according polarity. By the way, is it possible to supply the op-amp with only one (e.g., positive) voltage and, at the same time, to obtain the opposite (negative) output voltage? And can the op-amp output voltage exceed the supply voltage? Maybe, it would be possible, if there were capacitive or inductive differentiating circuits inside the op-amp... But (for now) an op-amp contains only resistive and active components:(

Gain. Typical value is A > 200000. It seems too high. Can we manage to utilize all this gain? What is the maximum input voltage to keep the op-amp in the active region (assume 10 V maximum output voltage)? Can we use a bare op-amp as an amplifier? Or we can use it as another useful device having analog inputs and descrete output (write its name here)? We may conclude: If we use the op-amp as an amplifier, the voltage difference between its inputs is zero. But if we use it as a comparator, the input voltage might be whatever.

Transfer characteristic. Microlab can help us to obtain this curve on the screen. There is no time in this graphical presentation; there is only input and output quantities. We might discern three regions on this curve: negative saturation (large), active region (narrow) and positive saturation (large).

Generalization. What does the op-amp actually do? A possible answer is: an op-amp converts the steady power supply into varying voltage source; it acts as a voltage-controlled voltage source.

Analogies

Driving: keeping up a constant speed, direction, temperature, volume... More negative feedback analogies: studying, teaching, keeping up a room temperature (body weight, mental equilibrium...) In all these cases we do all that is possible to realize our goals, to move toward the goal...

Structure (block diagram)
So, the simplest negative feedback follower consists of only three components: power (energy) source E, regulating element R and a subtractor (-).

Operation
This humble device operates according to a simple algorithm: if X > Y, then increase Y; if X < Y, decrease Y; if X = Y, do nothing. As a result, always X = Y. So, this is an "active copying" principle and this device is an "active follower". This "algorithm" is so simple that humble devices (tubes, transistors, op-amps...) having no any intellect can perform it.

Advantages
Why do we make followers in this odd way? What are the advantages of this "negative feedback" approach?

Passive electric follower

Now, we have only to convey this powerful idea from real life to electronics. Well, let's begin.

VIN will represent X in our electric follower; accordingly, VOUT will represent Y. But let's first persuade ourselves what is the use of building such an active follower instead a "passive" one.

What is the simplest voltage follower? Of course, a piece of wire can act as a humle voltage follower. It works perfectly, if there is no load (there is no current flowing). Only, is it still a follower, if we connect a big load at the end of the wire (especially, if it is a thin and long wire)? And imagine what happens, if the load and wire resistances vary... For example, the wire length can vary...

Another nasty problem of the passive voltage follower is that the load consumes energy from the input voltage source.; so, it droops when loaded.

Op-amp negative feedback follower

No, the bare wire is not a perfect follower in all the cases; so, it is worth the trouble to build an active negative feedback follower...

First, we connect a power supply - a voltage source V representing the power source E from the block diagram. We assume a bipolar input voltage; so, we get a bipolar (split) power supply. Then, we connect an op-amp OA acting as the regulating element R. Finally, we need a voltage subtractor to make the op-amp compare its output voltage VOUT with the input voltage VIN. What is the simplest voltage subtractor? Of course, this is the bare loop (a piece of wire) according to Kirchhoff's voltage law (KVL). In other words, we have to connect the two voltage sources in series and in opposite directions (+ -, - +) traversing the loop, in order to subtract their voltages. Then, we have to cut the loop and to feed the op-amp input by the voltage difference.

Operation

We are human beings, not computers... It seems strange but, in order to understand what the abstract op-amp really does in this circuit, we have to think of the op-amp as a "slow-thinking" device, not as a fast, non-inertial, instant device... From this human viewpoint, the op-amp compares continuously its output voltage VOUT with the input voltage VIN and changes VOUT in the right direction so that to keep almost zero difference VOUT - VIN = 0. It does exactly the same what we, human beings, were doing in all the analogies above.

Where currents flow in this circuit? Draw them with whole loops in the two cases - at a positive and a negative input voltage. Remember: a current always arrives at the point from where it has gone. Another tip: in the two cases (at positive and negative input voltage) the load current will flow through different power supplies (accordingly, through the positive and negative voltage sources). Imagine also the complementary output emitter follower inside the op-amp: the upper n-p-n transistor will "blows" the output current when the op-amp "wants" to produce a positive voltage and the lower p-n-p transistor will "sucks" the output current when the op-amp "wants" to produce a negative voltage.

Disturbing the negative feedback follower
Let's now "provoke" our op-amp by various disturbancies to see how it will react:) Firstly, we may introduce and vary a line resistance. Then, we may vary the load resistance. Finally, we might vary both the resistances. If we are curious enough, we can vary the voltages of the power supply as well (the one, the other and both).

Output resistance. Can we say that we have realized one eternal dream of electricians - to build a varying perfect voltage source:)? Can we think of an op-amp follower as an (almost) ideal voltage source?

Conclusion: in electronics, we make ideal devices by appllying negative feedback.

Converting the NFB follower into an amplifier

Resources
Operational amplifier

Differential and Single-Ended Inputs

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.

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.

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) 08:22, 8 May 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) forvard 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 fordward voltage drop on a real diode with an 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!

...forward LED voltage drop...

...battery voltage

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

<<< top - Group 64a - Group 65a - Group 67a - Group 68a - page stage >>>

<<< Group 64b - Group 65b - Group 66b - Group 67b - Group 68b >>>

title: Circuit Idea/Group 66a


text:
Ido/Lesson III5

Lektado
Reading ========= Lektar - to read =======

title: Ido/Lesson III5


text:
User talk:Luis Sergio Moura

Welcome!
Welcome, Luis Sergio Moura!

Come introduce yourself at the new users page. If you have any questions, you can ask there or contact me personally.--AdRiley (talk) 15:38, 2 April 2008 (UTC)

Japanese Wikibook discussion
Hi Luis Sergio Moura,

I just added a comment on Talk:Japanese which I wanted to draw your attention to as you've been involved in this book. --Swift (talk) 14:40, 10 June 2008 (UTC)

Your account will be renamed
Hello,

The developer team at Wikimedia is making some changes to how accounts work, as part of our on-going efforts to provide new and better tools for our users like cross-wiki notifications. These changes will mean you have the same account name everywhere. This will let us give you new features that will help you edit and discuss better, and allow more flexible user permissions for tools. One of the side-effects of this is that user accounts will now have to be unique across all 900 Wikimedia wikis. See the announcement for more information.

Unfortunately, your account clashes with another account also called Luis Sergio Moura. To make sure that both of you can use all Wikimedia projects in future, we have reserved the name Luis Sergio Moura~enwikibooks that only you will have. If you like it, you don't have to do anything. If you do not like it, you can pick out a different name.

Your account will still work as before, and you will be credited for all your edits made so far, but you will have to use the new account name when you log in.

Sorry for the inconvenience.

Yours, Keegan Peterzell Community Liaison, Wikimedia Foundation 23:26, 17 March 2015 (UTC)

title: User talk:Luis Sergio Moura


text:
Circuit Idea/Group 67a

<<< contents - Group 64a - Group 65a - Group 66a - Group 68a - page stage >>>

<<< Group 64b - Group 65b - Group 66b - Group 67b - Group 68b >>>

67a Group Student Page

We are students from Faculty of Computer Systems, Technical University of Sofia. Our 67 group is divided into two sub-groups; we constitute the first 67a one. Here are our names:

Virginia, Angel, Vladimir...

(add your name and place a photo here )

Lab 2: The genuine Ohm's experiment
Tuesday, March 18, 2008, 16.45 h

The theme of the practical lesson is: Walking along the resistive film The first thing we did on this practical lesson was to reproduce Ohm's experiment, because Ohm's law is the base of the electronics we study. The idea of our practical and theoretical lessons in our course of electronics is to invent everything, every single element (even the resistor). So we started imagining how Ohm did his famous experiment and found the fundamental relationship between the current, voltage and resistance. He did his experiment in the beginning of the 19th century. This means only thing he could use as a power supply was the forerunner of the electrical battery, created by Alessandro Volta. Unfortunately this experiment failed, because Ohm used a wire with very low resistance so the power source was loaded. For reproducing Ohm's experiment we used a resistive wire. In the lab we used a power supply (maximum current 2.5A, maximum voltage 10V), voltmeter and ammeter. So we started the experiments.

Experiment N1:  The scheme we were examining is shown on fig.4 in the main module. We move the voltmeter in this way: one of its ends is connected with the ground and the other is walking on the resistive film by a crocodile clip as a slider. Apparently the measured voltage is 10V no matter where the crocodile clip is along the resistive film.

Experiment N2: The scheme we were examining is shown on fig.5 in the main module. Now we have a change in the measurement of the voltmeter. We can see change is linear.

So now is appropriate to make the hydraulic analog: the tapped pipe and the opened pipe. The illustrations are shown at the main module.

We slide the crocodile clip along the resistive film. This is equal to using a potentiometer. The potentiometer is practically a movement-to-voltage converter. According to the fact that the resistance is distributed linearly along the wire, we can conclude that this converter is linear. So there was a new idea: why don't we stop moving the crocodile slider, leaving it in a certain point on the resistive wire and start changing the input voltage?

Experiment N3: We have 2 voltage sources. One of them (the left one) varies and the other is constant. We see the results on the screen of the computer. You can see the illustration attached it the subtopic in the main module: V1 varies: a left-controlled voltage-to-voltage converter.

$$\ Vout = \frac{r2}{r1+r2}.Vin1 $$

Experiment N4: Now the right one varies and the left one is constant. We see the results on the screen of the computer. You can see the illustration attached it the subtopic in the main module: V2 varies: a right-controlled voltage-to-voltage converter.

$$\ Vout = \frac{r1}{r1+r2}.Vin2 $$

Experiment N5: Now both of the voltage sources vary. We see the results on the screen of the computer. You can see the illustration attached in the subtopic in the main module: Both V1 and V2 vary: a resistive summer.

$$\ Vout = \ \frac{r2}{r1+r2}.Vin1\ + \frac{r1}{r1+r2}.Vin2 $$

As we can see this is a summer with coefficients:

$$\ a = \frac{r2}{r1+r2} $$

$$\ b = \frac{r1}{r1+r2} $$ Since now we have seen the results only when the input voltages are positive. Now we change the polarity.

'''We can see that there is one point we can see on the screen on the computer with zero-potential. This is the famous virtual ground. This point moves on the horizontal axis. So there was a new question – can we make this moving point a fixed one? The answer is yes. There is one element that controls its output in according with the input voltage. This is the operational amplifier. This element keeps the virtual ground at point A. See the illustration: a passive resistive subtractor and of course the negative feedback game.'''

Lab 3: Building an emitter follower
Tuesday, April 01, 2008, 16.45 h

authors: Angel,Virginia & Vladimir

Looking for analogies of negative feedback phenomenon
Speaking. We are getting started with an explanation of the term negative feedback. The first thing we ask each other is if there is a similar phenomenon in real life. Thus we have found we might consider human speech as an example: you have to listen to your voice in order to control its volume. In this way you are able to communicate with those around you appropriately. The noisier the surrounding environment is, the louder your voice becomes and vice versa. This basic idea explains roughly the essence of negative feedback.

Driving. Another example of the idea of negative feedback might also be the following situation. Imagine you are getting in your car and striving to drive with 50 km/h. Let us now suppose that we have a current state X and we want to achieve another goal state Y. In order to complete this task we should monitor the process of its development and control it in a desirable way by adjusting all necessary parametres. You are in state X and you would like to achieve state Y. But what do you need?

Generalizing the analogies into a block diagram
The first thing you need is ENERGY. It's necessary but not enough. You need also a visualization of your speed in order to control it. In other words you need a REGULATOR.

But what is the regulator doing? It compares the difference between the purpose state and the current state; then, it does what can do to eliminate it. When this difference is equal to zero, we have an equilibrium – this shows that the goal state has been achieved.

Building an electrical follower
The most simple regulator is the so called follower. Now we have to think about how we can realize a follower.

Is the humble wire a follower?
An obvious example of electric follower is... a simple wire (?!?). This remedy is simple: if you want the potential of point B to follow the potential of point A, just connect them to each other by piece of wire. But is that wire a perfect follower? The answer is “no”. We will provide reasons why with the next example.

The wire as you know has its own resistance. Let us now connect a resistor (1 ohm), our wire (1 ohm) and a voltage source(10V) like it is shown in the second picture. After measuring the output voltage we see that it has been reduced with the voltage drop caused by the wire’s internal resistance. The reason for this is clear – the current flowing in the circuit.

Building an emitter follower on the whiteboard
So we decide to make use of another (this time more “perfect”) regulator – an emitter follower. Having already come to the idea of constant regulating and adjusting a desired value according to the changes in its environment, we proceed with a real scheme with a transistor. The transistor here acts as the regulating element of the negative feedback follower above. The essence is that it “observes” the potential in point A and changes the potential in point B (by changing its own resistance) so that to keep an almost zero difference between them. But let's give a description of our practical laboratory exercise.

Putting the powerful idea into practice
(How our laboratory exercise passed over)

Preparation. In order to reveal to us the "secrets" of ubiquitous transistor, our teacher Cyril Mechkov provided us with the opportunity to build our own test circuit and observe its behaviour. As curious students who usually enjoy taking things apart, we were desperate to "invent" our own emitter follower and get a preconception of the so called negative feedback. We already knew some of the basic applications of the transistor having already studied digital circuitry but most of us lacked any practical experience. Having realized that, firstly prof. Mechkov briefly outlined the idea of the laboratory session and then gave us a soldering iron, a test board and all the necessary equipment to carry out the investigation of our device. The old Apple II based PC proved ideal for the experiment - it not only served as a power supply:) but together with the MicroLab system was able to "watch" our scheme and draw real-time voltage diagrams. Armed with this powerful "weaponry" we felt like real inventors!

Investigation. Some of the girls were terrified when Miroslav (a colleague) switched on the scheme, but fortunately nothing exploded;) Everything was well under control. After connecting our newly constructed emitter follower to the prototyping board we started the investigation by gradually varying its power supply (ranging from -10 to 10 Volts). As you can see from the pictures in the region of the positive values the transistor's behaviour coincides with theory. The voltage drop of the emitter-base junction always "strives" to remain the same - exactly 0,7 Volts. In accordance with Kirchoff's laws the potential of the output emitter was just like we expected - the input voltage minus the voltage drop. By analogy, the same linear dependency was observed in the negative input range. The lab results in that case are reproduced in the picture below.

Problem. However, quite unexpectedly we suggested double-checking the scheme by connecting an old-fashioned analogue votmeter in parallel to the digital VOM. Miroslav then attached one of the crocodile clips to the emitter and the other to the ground of the board (notice that the emitter - collector power supply is bipolar). At first, the new result on the screen seemed quite strange - in the negative region there was a section with a constant potential. Something had made the emitter base junction reverse biased. Professor Mechkov smiled being aware of the "mischevous intruder" in the voltage diagram but wanted us to find out the cause. We didn't hesitate for long and changed the old-fashioned voltmeter with a Chinese 3$ one.

Voila! The key to the "mystery" was finally discovered - the reason for switching off the transistor was the voltmeter's internal resistance. In the case of the old electromagnetic device it was about 20 kOhms. This internal resistance had become the main reason for the advent of a current flow through one of the power supplies, our load and the voltmeter. This newly appeared current kept the potential at the output unchanging when the base supply was low enough (see the picture). We realized the n-p-n transistor can only "source" current; it can't sink current.

Strengthening the problem. Having discovered this effect, we decided to replace the voltmeter with a lower resistor (100 Ohms) in order to simulate a voltage breakdown through the reverse biased base-emitter junction. In that particular case the transistor acted like a zener diode stabilizing the output voltage in certain ranges. A reproduced PC based simulation is shown on the last picture.

Conclusions
'''The final conclusion from the laboratory exercise was that a simple emitter follower might not be so simple and predictable in certain situations. We gathered some really valuable experience by observing the changes in our transistor's behaviour - at first it acted as an emitter follower, then it became a zener diode finally it turned into a voltage controlled switch. The small transistor was like a skilful magician but one playing tricks with current. Electronics is really fascinating, isn't it!'''

 * Angel, Virginia and Vladimir, thank you for your participation! Your material is excellent; it is so fascinating, captivating and funny! It was a great pleasure for me to join your team and to do my best to refine your work. Thank you again! Circuit-fantasist (talk) 17:57, 14 April 2008 (UTC)

Lab 4: Op-amp circuits with series negative feedback
(Converting the imperfect emitter follower into a perfect op-amp follower)

Authors: Angel with the help of Virginia

Do you remember our scheme from the previous exercise? We used a single transistor as an emitter follower illustrating in this way the concept of the negative feedback. We also noticed that our device was a bit more perfect compared to the most primitive voltage follower - a single piece of wire. However, when we put our obedient transistor to tests, we discovered that it felt a constant "hunger" - drawing power from the input and thus causing a voltage drop of 0,7 Volts. For the purpose of the laboratory experiment that was acceptable, but what would you say about implementing this scheme in real, extremely precise devices? Of course, it depends on the application, but one thing is certain - our emitter follower is not "ideal" simply because it disturbs the input.

If we look around ourselves in reality everything abides by these laws of nature. For example, if we have to complete a specific task, we need a proportional amount of energy. This energy is not given "for free", it has been accumulated as a result of previous activities which in their own turn have come to fruition thanks to the efforts of others and so on. The same regularity is valid for electronics. Perhaps, the basic philosophical principle of each human activity and schemotechnics too, in our context, is converting and processing some kind of input energy (usually represented by an electrical quantity) into a desired output. In this way we are able to represent, process and store information in electrical circuits in various ways. However, we should always be aware of the "losses" in our circuitry because in order to function properly each element requires sufficient resources.

After these introductory reflections, you might ask the following question. What is an ideal scheme and could it actually exist? Some people would suppose that such schemes are virtually non-existent because they cannot be practically constructed. Others would conclude that the term ideal is a bit imaginary - ideal things are in fact our own ideas of how things should function in the best possible conditions. Still others may qualify ideal elements as fairly theoretical - as an abstraction of mathematics and science. In fact all these three different groups of people would be right to a certain extent. Absolutely ideal elements may never come to life, but don't you think that our aspiration for developing more and more efficient utilities matters even more? Isn't that desire the main driving force that pushes forward all aspects of human creativity? These were some of the questions we asked ourselves at the fourth exercise in specialized computer electonics. Such kind of discussion was necessary because we had to examine the next step in the evolution of schemotechnics - his majesty - the operational amplifier.

Professor Mechkov challenged us by further developing the topic from the previous session. His main aim was to prepare us for the introduction to this both mysterious and powerful electronic component. Having already understood the role of the negative feedback, we felt it was the right time to study the principles of the op amp schemes and their practical applications. At first, we were a bit baffled because we had already encountered the op amp in other disciplines in the form of complete circuits. The problem was that nobody had explained to us the philosophy behind these circuits. Just remembering several facts or block schemes by heart is not quite useful for future engineers like us. Professor Mechkov knew that pretty well, so in order to make us more familiar with the topic he fostered our imagination giving us a simple but yet effective description of the behavior and advantages of the operational amplifier. In short an op amp may be compared to a strong and obedient weight-lifter. Imagine you have to move some quite heavy furniture at home but unfortunately you lack the power. On top of that you have to carry out the task very carefully so as not to scratch the wooden floor. Suddenly you remember that you can call one of your friends for help - he is a professional well trained weight-lifter. In five minutes he comes and thanks to your directions finishes the job in time. You are very happy and treat him a drink for his efforts. Of course this is a very basic and broad illustration but in fact it quite well represents "the abilities" of the op amp. So, with this figurative idea in mind we may proceed with a formal description of the operational amplifier. It refers to a division of high-gain DC coupled amplifiers with two inputs and a single-ended output. Some of its most significant characteristics are its extremely high gain - typically 200 000 or even a million, its very high input impedance and low output impedance, and last but not least the need for a split power supply (+/-12 Volts for example). We won't go into further details about the internal structure because this is not our goal. We will just mention that a typical op amp is built up of several resistors and transistors connected in a special way.

So, for the sake of practice we should know that an op amp is able to work with two types of input signals – either single-ended or differential ones. Similarly to the transistor an op amp actually doesn't amplify anything - what it is doing is precisely controlling its output according to the changes of the inputs boosting the output from its own power supply. The op amp needs external energy in order to do this trick. Remember that the principle we outlined at the beginning of the text always remains valid! What deserves mentioning here is that the term amplification is used not literally but has appeared perhaps as a result of the overall effect of the functioning of the op amp. The idea is again simple but genius! We use a relatively weak signal or electrical magnitude so as to command a more powerful circuit. This time, however, in contrast to the transistor, we don't draw energy directly from the input, but use an external bipolar power supply. In this way, the op amp is able to respond to the variations of the inputs and adjust qiute precisely its output.

Comparator

Let us now look back at the exercise. Having the above mentioned ideas in mind professor Mechkov described the first and maybe most logical application of the op amp - its usage as a comparator. The scheme we discussed is shown in the next picture.

We observe that the potential of the non-inverting input is lower than that on the inverting. In this particular case we say that the input is differential. Judging by the simulation, we realized that if there was even a tiny difference between V1 and V2 the output was either driven to +VCC or -VCC depending on the magnitude of the inputs. Practically, the output voltage is determined as a multiplication of the voltage difference and the so called open-loop gain. This dependency is illustrated in the following diagram.

As a whole, perhaps the main disadvantage of the scheme is that its operating range is extremely narrow due to the op amp's high sensitivity. At some point the output remains the same - this means that the op amp has reached its upper or lower saturation threshold.

Voltage Follower

Next, thinking on how we can "disturb" the op amp so as to build a voltage follower, we came to the idea of the negative feedback. What we did was to take part of the output signal back to the input but with negative sign. Again the same well known principle but this time realized in an almost perfect way (or let's say the most ideal known nowadays :)). By connecting the output to the negative input and applying some potential to the positive one, we managed to "invent" an immaculate voltage follower. The interesting thing here was that the op amp always strive to keep the difference between the input potentials zero - this is the so called virtual ground. Of course this happens only if you have applied some kind of feedback. We also found out that the inputs drew almost no current - as a result of the op amp's extremely high input impedance.

The scheme and the simulation are the following. Notice that this kind of voltage follower has a gain equal to one.

Non-inverting Amplifier

After these tasks we decided to increase the gain of the negative feedback schemes. For that reason, we put a resistor in the circuit of the feedback trying to cheat our humble op amp in such a way so that it would pull up the potential of its output twice or even more. This case illustrated the so called compensatory mechanism and the respective device is called a non-inverting amplifier. Vout/Vin = 1 + R1/R2

Positive Feedback

Last but not of least importance was the simulation of the infamous Schmitt trigger. Until now we were discussing schemes with negative feedback, so we decided to see what would happen if we had a positive feedback. Look at the next diagram.

Let us mark the highest voltage the operational amplifier can produce ±Vmax. The negative input is connected to the ground and bearing in mind that the op amp keeps the potential difference between its inputs equal to zero, we can conclude the potential in point A is zero or Va=0V.

I1 = ±(Vin – Va) /R1 = ±Vin/R1

(depends on the polarity of the input voltage)

I2 =± (Vout – Va)/R2= ±Vout/R2

(depends on the polarity of the output voltage)

Let us now assume that this scheme is currently in its higher state +Vmax.

Therefore I2=Vmax/R2. As we already mentioned the op amp's most simple role is to function as a comparator, so if we take the input voltage with positive polarity - therefore I1 = Vin/R1. Again knowing that the op amp inputs draw almost no current we can say the I1=-I2. Then Vmax/R2=- I1 = Vin/R1 <=> Vin= -Vmax.R1/R2 - so Vin should drop below −(R1/R2)Vmax to get the output switched. Once the comparator output has switched to −VS, the threshold becomes +(R1/R2)VS to switch back to the higher state. After these preliminary notes we can observe from the simulation that this is a circuit with hysteresis.

What is the good thing about that hysteresis and why is the op amp called a trigger? The device is called a trigger because it keeps its output unchanged until the input changes are sufficient to trigger a change. That's why we say that op amps with positive feedback have "memory".

Now let's pay attention on the hysteresis: its main advantage is that it has two limiting thresholds. If we have a device with one threshold and a noisy input signal, the device may change its output in very unwanted ways - of course these changes might be very fast. In contrast, if we have a Schmitt trigger the scheme will be much more reliable and resistant to any input deviations because of its "improved" characteristics.

Final Words

After all, operational amplifiers are not so dreadful as their name suggests, are they! In fact these little "smarties" are so versatile and powerful that there are widely used in many areas of practical electronics. Thanks to professor Mechkov we have managed to find our way through the "jungle" of op amps and represent our "discoveries" on this page. We hope that our brief description has helped you gain a better understanding of the topic. If you have any ideas or suggestions we would be happy if you - our readers, take part in the discussion!

 * Angel and Virginia, your essay was so exciting that, believe me, it moved me to cry! If only a few students of every group were as you, I will be no end happy! I have an idea: can you visit the rest nine labs in future to pass your experience on your colleagues:)))? Will you assist me during Lab 5?:) By the way, I would like to make some remarks on your work but I dare not to touch it:) Best regards! Circuit-fantasist (talk) 17:23, 21 April 2008 (UTC)

Resources
Operational amplifiers is an excellent material written by famous Tony Kuphaldt. Operational amplifier Differential and Single-Ended Inputs Labs: group 65a and group 66a. Op-amp amplifiers with negative feedback

Lab 5A: Building the op-amp inverting integrator (student's version:)
During our laboratory course we have already carried out experiments with passive "mischevous" elements(resistors for example) and active ones (transistors and op amps) being able to command their output according to the variations of the input signals. Having understood the role of each of these groups we may be able to "harness" their potential to a maximum extent only if we examine their behaviour in more complex time-dependent schemes. Why time-dependent? - you might ask? The answer is simply because the representation of time as a variable is what we need in combination with the input and output values in order to simulate or examine all kinds of real-life phenomena. Thus, "playing" around with our electric building blocks we will be able to construct an endless number of various useful circuits. During the fifth exercise we developed the above-mentioned conception of time-sensitive schemes by constructing an inverting op-amp integrator. The "main characters" in that session were the op-amp from the previuos lab and a new "actor" - a capacitor - acting as a current driven voltage integrator. The role of a typical integrator as its name suggests is to function as an accumulator of a particular value. In this way depending on the physical essence of accumulation, we may be able to measure time intervals or even more precisey work with time in general. A single passive voltage integrator as the one shown below in professor Mechkov's description is imperfect mainly because of the non-linear voltage drop across the capacitor which disturbs the input source. In fact, this is the typical "disadvantage" of all kinds of passive schemes - their behaviour influences the input. However, according to the application this effect may be either useful or unwanted. What is more important here is grasping the idea and trying to perfect its practical realization. Following the principles from the previous two laboratory exercises, we came to the idea of connecting a single capacitor in the series of the negative feedback. This time the op-amp was "deluded" in an unconevtional way - the change in the negative input was exponential in itself but thanks to the op-amp's extremely high gain - the output changed in a linear way(not infinitely of course). Observing this effect, we concluded that it would be quite easy for us to construct a universal measuring device, allowing us to explore and even improve the behaviour of all kinds of electronic components - diodes, resistors, etc. All we need is a suitable power supply, an op-amp and a little imagination. Pure mathematical formulae or boring readily-built schemes cannot do the trick ;)

Current-to-voltage C integrator

What is the most elementary electrical integrator? Of course, this is the humble capacitor. So, if we drive a capacitor C by a constant current source I, it acts as an ideal current-to-voltage integrator with a current input IIN and a voltage output VOUT = VC. Note that the output voltage changes linearly through the time.

Building an op-amp inverting integrator - step 1 Keeping a constant current by using an ideal current source

Voltage-to-voltage RC integrator

Only, we need usually an integrator with voltage input and voltage output (voltage-to-voltage integrator). For this purpose, we can build a compound voltage integrator just connecting a voltage-to-current converter (a resistor) before the integrator:

V-to-I converter + I-to-V integrator = V-to-V integrator

Only, a problem arises here - the voltage drop VC across the capacitor C "enervates" the input voltage thus decreasing the input current. As a result, the output voltage changes exponentially through the time.

Building an op-amp inverting integrator - step 2 Keeping a constant current by depreciating the load

Active RC integrator

Remember what we do in real life when an obstacle stands in our way - we remove it by an equivalent useful "antidisturbance". Following this recipe, we may remove the "harmful" voltage VC by an "antivoltage" -VC. That means to connect an additional varying voltage source and to make its voltge equal to -VC. As a result, the "harmful" voltage VC disappears and the upper point becomes a virtual ground! The compound current source VIN-R is "fooled": it doesn't "understand" that there is a capacitor connected; it "thinks" that its output is shorted.

Building an op-amp inverting integrator - step 1 Keeping a constant current by adding an additional voltage

But where to take an output from? We have three possibilities.

First, we might use the old output; but we have already destroyed this voltage!

Second, we may use the "original" voltage as an output. It is possible but bad solution to connect the load across the capacitor for two reasons: the load has to have a differential output; the load will shunt the capacitor thus affecting the current.

Finally, we can use the "copy" voltage as an output! What a great idea! First, the load will be connected to the common ground; second, it will consume energy from the supplementary source instead from the input one!

Building an op-amp inverting integrator - go to step 3

Op-amp inverting RC integrator
Finally, we have only to replace the varying voltage source with a real one. Now the op-amp doses the voltage of the power supply thus producing a compensating voltage -VC. In other words, the combination of an op-amp and a steady battery acts as a varying voltage source.

The op-amp "observes" the potential of the virtual ground (the difference between the two voltages) and changes instantly its output voltage so that this point to stay always at zero volts. Doing that, the op-amp compensates the "harmful" voltage drop across the capacitor by copying and adding it to the voltage of the input source; doing that, the op-amp "helps" the input source.

It's time to make conclusions. What have we actually done here? How does it operate? What is the final result?

The answer is amazing: we have made a "botomless" capacitor having infinite capacitance. Looking from the side of the input source this "ideal" capacitor acts just... as a piece of wire...?!? What do you think about this speculation? Is it always right?

We are already true magicians as we can transform any imperfect component into an almost ideal one! In this lab, we have transmuted the "imperfect" capacitor into a perfect infinite one having no any voltage drop across it although a current flows continuously through it! But with the same success we can make an ideal diode, zero resistance, etc... For this purpose, we just incorporate a varying battery to the imperfect component that compensate the losses inside the component. Remember: all the op-amp inverting circuits exploit this clever trick. Circuit-fantasist (talk) 16:50, 6 May 2008 (UTC)

How to make perfect components by parallel NFB Building an op-amp inverting integrator - go to step 4

Resources
How do we build an op-amp RC integrator? shows a reliable four-step building "scenario". Building an op-amp inverting integrator is an interactive multimedia flash minitutorial that builds the circuit in four steps.

<<< contents - Group 64a - Group 65a - Group 66a - Group 68a - page stage >>>

<<< Group 64b - Group 65b - Group 66b - Group 67b - Group 68b >>>

title: Circuit Idea/Group 67a


text: User talk:67.131.76.100

title: User talk:67.131.76.100