Homework Assignment #6

DATE ASSIGNED: 04 Mar 03

DATE DUE: 13 Mar 03

Problems #9.(1, 3, 4, 5, 7, 11, 14)

Notes, Hints and Suggestions:

The diode (Q₃) in Fig 9.2 is a diode connected transistor (notice the device reference is Q₃ and not D₃). This is a shorthand convention for indicating a diode connected BJT and the assumption, given that you understand how bias circuits work from Chapter 6, is that you recognize this portion of the circuit as a basic current mirror in which R sets up a reference current in Q₃ that is then mirrored to a matched transistor Q₂.

In this chapter we are generally interested in the power dissipation of various items - the load, the supply, the amplifier as a whole or specific components within it. As with many of the equations in Chapter Six - most of the equations in Chapter Nine are applicable only to the specific problem for which they were derived.

For instance, the Equation 9.10 on page 758 is not a general result for any Class A amplifier - it is only applicable to the particular Class A amplifier shown in Fig 9.2 and even then the results are specific to output signals that are sinusoids. Hence the conclusion that the maximum power efficiency is 25% is likewise limited to that particular Class A amplifier topology (which is a very common one) when dealing with sinusoidal signals (a very common situation). As you will see in the homework problems, the power efficiency of a Class A amplifier can approach 100% under the right conditions - conditions under which you can deliver significant power to a load while dissipating very little power in the amplifier.

Because of this, it is even more important in this chapter than in Chapter Six that you NOT try to simply memorize a bunch of equations - because even if you could successfully memorize every equation in the chapter, it only takes a slight tweak to either the circuit or the signals and none of the "result" equations would apply at all. It is very important that you understand the basic concepts and, for this chapter, these are concepts with which you should already be very comfortable - such things as instantaneous power being the power across a device multiplied by the current through that device and such as average power being the integral of the instantaneous power over a certain interval of time divided by the length of the time interval.