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68b Group Student Page
Lab 3: Investigating a transistor current source with negative feedback.

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 1: Investigating passive resistive circuits by Microlab

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...

Inside the Chinese multimeter (the picture is made by Lora_Kirilova)
After that we used the multimeter for measuring, but one of our colleagues had to retain hold of the jacks (the picture is made by Lora_Kirilova)
The idea of finding the virtual ground (the picture is made by Lora_Kirilova)
Exercise 2: Ohm's law (the picture is made by maniac_353W)
The higher the voltage is (its absolute value), the brighter the probe bulb is (the picture is made by Lora_Kirilova)
When we applied too high voltage to the wire it became red hot (the picture is made by maniac_353W)
Then we tried to cut a pen... (the picture is made by Lora_Kirilova)
Supplying a small DC motor by a voltage drop across a part of the resistive wire (the picture is made by Lora_Kirilova).
...and the result was - cut again. Also we had a sheet of paper catch fire, but fortunately, we put the fire out quickly! (the picture is made by Lora_Kirilova)
Depending on the polarity of the voltage drop and the distance between the two crocodile clips the DC motor spins around its axis in different directions and with different speed - getting closer to the 'ends' (+10V/-10V) it speeds up! (the picture is made by Lora_Kirilova)

Lab 3: "Inventing" a negative feedback BJT current source

Thursday, April 10, 2008, 16.45 h

Transistor as a current-stable resistor

A BJT acts as a current-stable resistor (by Lora Kirilova).
Showing how a BJT acts as a current-stable resistor (by Lora Kirilova).

Comparison with a voltage-stable resistor

The pointers represent two IV curves (by Lora Kirilova).

Making transistor behave as a voltage-stable resistor

Building the simplest negative feedback follower

The simplest negative feedback follower consists of power source, a regulating element and a comparator (by Lora Kirilova).

Building an emitter follower on the whiteboard

The transistor of the emitter follower acts as a regulating element (by Lora Kirilova).

Mounting the emitter follower on the PCB

Mounting the circuit... (by Lora Kirilova)
... on the PCB (by Lora Kirilova).

Investigating the emitter follower

Investigating the emitter follower on the PCB (by Lora Kirilova).

Making transistor behave again as a current-stable resistor

Investigating the "invented" transistor current source (by Lora Kirilova).

Lab 4: How to make perfect components by parallel NFB

Is the real diode a perfect component?

A "harmful" voltage VF across a diode.

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

The forward voltage drop VF across a real diode disturbs the input source.

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?

Making an "ideal" diode without VF

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

A wonder: VF has disappeared!
Removing the harmful forward voltage drop VF across a real diode by "antivoltage".
The imperfect real diode has become an almost ideal one.

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)

Making an almost ideal "op-amp" diode

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...

An op-amp can "help" the imperfect diode by adding a voltage VF in series with the input voltage source.

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!

Now the op-amp does this boring "donkey" work; we might relax... (by Lora Kirilova).
Students (Lora, Tanya, Jordan...) investigate a diode connected in the feedback loop.
Students (Antonia, Maria, Stilian & Hristo) investigate the same phenomenon.
Investigating a diode connected in the feedback loop (by Lora Kirilova).
How the op-amp compensates the harmful forward voltage drop VF across a real diode (waveforms).

Enlarging the powerful "helping" idea by destroying...

...multiple diode voltage drops...

Investigating two diodes connected in the feedback loop.
Now the op-amp compensates doubled voltage drop VF (waveforms).

...zener diode voltage drop...

Investigating a zener diode connected in the feedback loop (by Lora Kirilova).
The op-amp can compensate even the voltage drop across a zener diode.

...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

Investigating a photoresistor connected in the feedback loop (by Lora Kirilova).

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

Generalizing the powerful "helping" idea

A recipe: imperfect component + varying battery = perfect element.

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

Investigating the DAC by Microlab.
Investigating Vout given varying Vin and a constant digital code (a waveform).
After we included an additional resistor (dozens of kOhms), we observed a saturation.

The pictures are made by Jordan Slavov.


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