Gateways Into Electronics

7

LOW-FREQUENCY CIRCUIT FRAGMENTS

7-1 INTRODUCTION

Transistor circuits are multifarious in appearance, but most of them can be reduced to combinations of easily recognizable substructures or fragments with remarkably simple properties. These fragments, in turn, can be understood in terms of three basic one-transistor configurations, characterized by which terminal of a transistor is held at constant voltage.

Our purpose in this chapter is to become thoroughly acquainted with the low-frequency properties of the basic configurations and construct the fragments most commonly found in integrated circuits. With these elements in hand, we will see that the low-frequency behavior of circuits with dozens of transistors can often be predicted with just a few calculations. Much of our effort will go into calculating the small-signal input and output impedances of the basic configurations and of the fragments we construct with them; this will allow us to see how fragments affect one another.

In examples and exercises we will present a variety of the elegant circuits that become feasible if it can be assumed-this is one of the great advantages of integrated circuits-that certain critical transistors have identical characteristics and, implicitly, the same temperature.

We will concentrate on bipolar transistors because unique circuits can be constructed by exploiting the exponential dependence of the collector current on the base-emitter voltage in these devices. Another reason for this choice ...