Introduction

Feedback occurs where some part of the output of a system also appears back at its input and so modifies the input signal. It occurs in every system. If the feedback is undesired, then the input and output circuits of a system must be well separated and screened. Remember that if the circuit is meant to handle signal frequencies of a few megahertz, which are in the broadcast band, these will be readily radiated from the output circuit wiring and could easily be picked up by the input wiring. Also, if the input and output stages share a common power supply, then care must be taken to ‘decouple’ this as far as signal frequencies are concerned. Otherwise feedback could occur.

This chapter describes intentional feedback, which, when it is properly applied, can improve almost every performance feature of an amplifier. It can widen its frequency response, reduce the effects of component ageing, microphony and hum pickup, and stabilise the overall gain closely to some figure required by a designer. These are a few of its benefits which are more fully explained in the following sections. The subject is important because virtually every good quality or precision amplifier made today is likely to use feedback.

There is one troublesome effect of feedback on amplifier performance; it can create a tendency towards instability if the system is poorly designed. However, feedback which is specifically designed to make oscillators or switches is useful in its own right and it is described in chapter 6.