Circuit Idea/Group 64b
Vasil Tsanov, Martin Bochvarski, Selver Ismail, Georgi Drumev, Ivailo Kirov, Josif Hristov, Stoycho Barov, Maya Zhecheva, Vasil Tashev.
Lab 1: Investigating passive resistive circuits by Microlab system
Thursday, March 27, 2008, 13.45 h
Before the exercise our group is very excited to reproduce Ohm's experiments from the past.
Lab 3: Investigating transistor circuits with parallel negative feedback
(Building a transistor "zener" diode)
Thursday, April 10, 2008, 13.45 h
Lab 4: How to make perfect circuits by series NFB
Is the real diode a perfect component?
Diodes are two-terminal semiconductor elements, characterized by allowing the current to flow in the forward direction when the voltage is positive but not in the reverse one when the voltage is negative. So diodes represent the action of a valve and so were they called in the past. However, the diode needs a little (approximately 0.5-0.7V) voltage-push in order the current to start flowing freely. The result is small but noticeable voltage drop - VF. What is this voltage drop - useful or harmful? Sometimes it is useful; other times it is harmful...
"Useful" examples. When we make voltage stabilizers, we need this voltage drop. In these cases we do all that is possible to create and increase this useful voltage drop appearing across various diode component: diodes, LEDs, zeners, multiple diodes connected in series...
"Harmful" examples. In other cases, when we use a diode as a switching element, we do all that is possible to remove and to remove this harmful voltage drop... Then we need an ideal diode without forward voltage drop VF...
Making the simplest series diode rectifier
A rectifier is an electrical element that converts alternating current to direct current. Rectifiers form the basis for electronic power supplies and battery-charging circuits. Furthermore, they are used in signal processing to demodulate radio signals and in the precision conversion of AC to DC voltage in electronic voltmeters. However, keeping in mind that the diode allows a current to flow in one (positive) direction but not in the other (negative), we can easily use the term "rectifier" to name a diode.
So let's make the diode perform as a rectifier and thus participating in a half-wave rectifier circuit (the circuit diagram is shown on the left). We have a sinusoidal source (for the experiment it is 1V) and a resistive load. Maya.zhecheva
When the source voltage is positive, the diode is in the so-called forward-bias region. If the diode was ideal the source voltage will appear across the load. However, the diode we have is not ideal; so it performs a voltage drop equal to the push-up voltage of the diode (approximately 0.5 - 0.7V). So during the positive wave the output voltage is less than the source voltage (for our example it will be VL = VS - VF = 1 - 0.7 = 0.3V. The result is shown in the diagram on the right.
During the negative wave of the sinusoidal voltage the diode is in its reverse-bias region and no current flows through the load. Thus only the positive half-cycles result on the load, which makes the diode suitable as a rectifier. Maya.zhecheva
Making an "ideal" diode without VF
Deriving the basic idea from our human routine
Implementing the powerful idea into an electrical circuit
Making an almost ideal "op-amp" series diode rectifier
What can act as a varying voltage source in our electronic circuit? What can "help" the imperfect diode by adding so much voltage to the input voltage as it loses across the diode? At last Lab 3 we were using transistors for such a purpose; let's now, for the sake of change, use an op-amp...
Now the op-amp has to "insert" the "helping" voltage VF into the circuit; so, how to connect it?
Eureka! We have invented an (almost) ideal diode without (any) forward voltage VF! Let's investigate it by Microlab system.
Firstly, let's remind what an operational amplifier (op-amp) is: it's a device having a differential input (it may be current, voltage, mechanical motion?!?Circuit-fantasist (talk) 18:09, 7 May 2008 (UTC), etc.) and the output that varies according to the input but with a larger amplitude. Almost always they are used with negative feedback (the output signal is returned to the input in opposite direction to the source signal). However, they may be also used with a positive feedback (the output signal that is returned is summed with the input original signal) but much rarely. For ideal OA in a negative-feedback circuit, the network returns a fraction of the output to the inverting input and thus makes the differential input voltage toward zero.
So let's now go back to our experiment. What we are trying to do is to compensate the harmful forward voltage drop on a real diode with a operational amplifier. But isn't the result of this try an ideal operational amplifier?!
We have a source voltage Vin and at the point named "1" we will have that voltage. However, if we want to compensate the voltage drop across the diode, we should use an op-amp that will have on its output a voltage equal to the source voltage plus an additional amount that is equal to this voltage drop. This will be the voltage we will have at point "2". If this increased voltage is applied before the diode, the voltage after the diode will be the same as our initial source voltage and that is what we have at point "3". Which is our purpose!
Enlarging the powerful "surplus" idea by overcoming...
...multiple diode voltage drops...
...resistor voltage drop
With the same success we can overcome the "harmful" voltage drop across a resistor. What a circuit can we "invent" in this way? Write its name here.... (reifilpma gnitrevni-non:)
Application. Now look at the old fashioned laboratory power supply that has helped us to carry out the famous Ohm's experiment. In addition to the positive and negative terminals it has also two mystic terminals named NFB. What are they? Why and when do we need them? Describe here and draw a picture about a possible application of this arrangement. By the way, when I was student and then young engineer, I have managed to burn out a few costly microchips when I was trying to supply them:((( What do you think, how have I managed to do that "heroism"? Circuit-fantasist (talk) 08:25, 27 April 2008 (UTC)