# 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:

Investigating the virtual ground point during Lab 2.

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.

An arrangement for investigating a virtual ground on a resistive wire.

Investigating the virtual ground point on a resistive wire.

## 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)

## Lab 4: How to make perfect circuits by series NFB

### Is the real diode a perfect component?

A "harmful" voltage VF across a diode.

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

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

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

The voltage drop VF enervates the input voltage.

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

### Making an "ideal" diode without VF

#### 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

Compensating the harmful forward voltage drop VF across a real diode by surplus voltage.

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?

Investigating the "ideal" diode by Microlab.

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!

### Enlarging the powerful "surplus" idea by overcoming...

#### ...multiple diode voltage drops...

Investigating multiple diodes (LEDs) connected in the feedback loop.

Compensating the harmful forward voltage drop VF across multiple diodes connected in the feedback loop.

Now the op-amp compensates doubled LED voltage drop (waveforms).

#### ...battery voltage...

The op-amp compensates even the voltage of a "positive" battery....

...and the voltage of a "negative" (opposite connected) battery.

The op-amp "lowers" its output voltage to compensate the harmful voltage drop across a "positive" battery (waveforms).

The op-amp "raises" its output voltage to compensate the harmful voltage drop across a "negative" battery (waveforms).

#### ...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:)

What are the two mystic plates and terminals named NFB?

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)

There are various components in the laboratory...

The op-amp overcomes the "harmful" voltage drop across a photoresistor

### Making the harmful voltage drop act as an input

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

Using the op-amp reaction as an output

### 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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