All Prior Quiz Prep sheets, homework, lectures, and reading assignments. Don't forget that some problems similar to Exam #1 problems will be interspersed among the next several quizzes.
As always, you should be able to do all problems with either NPN or PNP transistors - know the difference!
1) Draw a typical Class AB output stage that is biased with a vBE multiplier circuit.
The output resistance of a typical Class AB output stage, assuming relatively small resistance in the source supplying the input signal, is simply the parallel combination of the emitter resistances of the two output transistors.
2) At what output voltage is the output resistance at its greatest?
3) For output voltages well away from zero, what is the output resistance in terms of the load resistance and the output voltage?
4) For output voltages near zero, what is the output resistance in terms of the quiescent current.
If a Class AB stage is biased using diodes (or diode-connected transistors) and the biasing diodes are not in close thermal proximity to the output transistors, thermal runaway can occur.
5) Describe what thermal runaway is and how it progresses.
6) What is the likely outcome of thermal runaway if left unchecked?
7) How does thermally coupling the biasing diodes to the output transistors protect the output transistors against thermal runaway?
A vBE multiplier is biased at a particular current but, as the output voltage changes, some of the bias current is bled off to provide base drive current for the output transistor. Therefore, it is important to know how the voltage across the multiplier changes in response to small changes in the current in it - this, by definition, is the small-signal output resistance of the multiplier.
8) Determine the output resistance of the the vBE multiplier shown in Fig 9.15 (p770).
9) For the circuit shown in Fig 9.16 (p771), if Vcc is 30V, the load resistance is 8 ohms, the beta of the output transistors is 10, and it is necessary to be able to deliver 1A of current to the load, what is the minimum value of Ibias if the base-to-base voltage across the output transistors is not to fall below 1.0V? Assume Q1 has a base-emitter voltage of 0.6V at a current of 1mA. What is the base-to-base voltage across the output transistors under quiescent conditions?