Gateways Into Electronics

8

DYNAMIC RESPONSE OF SEMICONDUCTOR CIRCUITS

8-1 INTRODUCTION

In this chapter we will look into the small-signal frequency response of the circuits presented in Chapter 7. One of our principal objectives will be to show that the simplicity of the fragments discussed there is largely conserved and that calculations in terms of such fragments remain quite straightforward. For completeness, we will consider the dynamic response for large signals in a few representative cases.

As we have done so far, we will stick to the basics and avoid special methods. In the exercises, however, we will introduce the y-parameter model for the common-emitter (or common-source) configuration because it is so similar to the small-signal models developed in Chapter 6 that it is no burden on memory, yet it has the advantage of making calculations much easier.

The small-signal models of bipolar and field-effect transistors are formally identical; we will therefore continue to concentrate on bipolar circuits since our calculations can be applied to field-effect versions by making the current gain infinite.

8-2 PREUMINARY EXERCISE: THE SHORT-CIRCUIT FORWARD CURRENT GAIN

The short-circuit forward current gain h_fe(s) of a bipolar transistor is the small-signal current gain from the base to the collector when the emitter is grounded and the collector is held at a fixed voltage, as shown in Fig. 8-la. It is thus one of the parameters of the obsolete h-parameter model of bipolar transistors; we will maintain ...