Talk:Circuit Idea/Building an Emitter Follower
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Here is the building "scenario" that my students (group 67a) and I were using during Lab 3 to consider the famous transistor circuit of emitter follower. Pay special attention to this work as it is entirely made by the very students. Circuit-fantasist (talk) 07:27, 28 April 2008 (UTC)
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[edit] Lab 3: Building an emitter follower
Tuesday, April 01, 2008, 16.45 h
authors: Angel,Virginia & Vladimir
[edit] Building a negative feedback follower
[edit] 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 50km/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?
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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)
