User:Vsj 67gr

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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.

emitter follower

We are getting started with an explanation of the negative feedback. The first thing we ask each other is if there is such phenomenon in the real life. The first example we think of is the human speech. You have to listen to your voice in order to control it. So we have a current state X and we want to achieve other state Y.

Another example is when you get in a car and you want to achieve the goal to drive with 50km/h. You are in state X and you want to achieve state Y. But what do you need? The first thing you need is ENERGY. It's necessary but not enough. You have to have 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. When this difference is equal to zero, we have equilibrium.

The simplest regulator is the follower. Now we have to think about how we can realize a follower. An obvious follower is a wire. But is it a perfect follower? The answer is “no”. We prove it with the next example. The wire has resistance. When we have a resistor (1ohm), we have a wire (1ohm), and a voltage source (10V) and they are connected like it is shown on the figure.


So we use another (perfect) regulator – the emitter follower. We make the analogue of scheme? And a scheme with the transistor. The transistor is the voltage follower. The main idea is that the transistor observes the potential in point A and changes the potential in point B (by changing its resistance). And because this is a follower the voltage in point B should be equal to the voltage in point A. In fact it is almost equal, because we have the BASE-EMITTER VOLTAGE (which is approximately 0.6V).


Now we will test this scheme with Microlab.