All Prior Quiz Prep sheets, homework, lectures, and reading assignments.
As always, you should be able to do all problems with either NPN or PNP transistors - know the difference!
It is very evident that people still do not understand the basic concepts behind the small signal transistor models and the corresponding parameters. This lack of fundamental understanding is readily apparent in the type of faulty logic that is being seen on the homework and on the quizzes. The fact that this material has been fair game on every quiz and has appeared on half of them has not encouraged people to actually learn these concepts. Therefore, in yet another effort to encourage this before the exam, the following questions WILL be on Quiz #5:
1) What is the basic equation that relates the collector current to the base-emitter voltage in a BJT?
2) For EACH of the following small signal parameters of a BJT: transconductance, emitter resistance, input resistance, and output resistance:
- What is the definition of this parameter?
- Derive the relationship between this parameter and the transistor's DC operating point parameters
3) What is ( ro * gm )?
4) SHOW that the transconductance in parallel with the input resistance is identical to the emitter resistance.
The only things that you should "memorize" are the answer to #1 (which is the same level of expectation as asking you to "memorize" Ohm's Law for a resistor) and the answers to part (a) in #2 (which is merely asking you to know four fundamental definitions which is the same level of expectation as asking you to "memorize" that acceleration is defined as dv/dt and that velocity is defined as dx/dt).
In order to answer (2b) for the output resistance, you can either start with the result from #1 modified to take into account the Early Voltage or you can start with a graphical representation using the sloping iC vs. vCE characteristic as long as you define ro in terms of that graph.
Practice working these four questions until you can comfortably do them in well under ten minutes.
It is also apparent that there is a great deal of confusion between a differential pair and a current mirror - these are in no way the same thing nor do they in any way operate on the same basic principals. A differential pair operates on the basis of splitting a fixed current and steering that current along two parallel paths with the fraction of the total current going down each path dependent on an applied differential voltage. A current mirror works by using a diode-connected transistor to develop a base-emitter voltage corresponding to the reference current passing through it and then applying that voltage to the base-emitter junction of another transistor in order to generate a nearly identical current in that transistor. The confusion that I'm seeing leads me to believe that people are trying to memorize equations without making the effort understand what these circuits are, how they operate, or what the purposes of them are. This won't get you very far at all. If you understand these basic concepts, then you can determine the needed equations from those fundamentals. If all you do is memorize equations, then all you will do is regularly apply the wrong equation to problems - and I am seeing a lot of that happening not only on the quizzes but on the homework. Given that you aren't having to use anything that's memorized when working the homework, this tells me that the problem is in not understanding how these circuits work and therefore not being able to use the equations that are sitting in front of you when you have the book open.
Additional review topics:
1) Explain why the addition of a cascode stage improves the output gain of an actively loaded differential pair.
2) Why does the use of a current mirror as an active load result in an input offset voltage?
3) In a practical circuit using a passively loaded BJT differential pair, what phenomenon places a limit on the achievable gain?
4) The small-signal differential transconductance of a BJT differential pair with an active load is approximately equal to the transconductance of the input transistors. Why? In other words, walk through the reasoning outlined on p523.
5) Since the differential voltage gain of an actively loaded BJT differential pair is given by Ad = gmRo it is recommended by a colleague that you can get a very large gain simply by using a very large valued resistor from the output to ground. What is the flaw in this reasoning?