8.1 INTRODUCTION

The term feedback is the basic backbone for all amplifier systems. An electronics engineer named Harold Black working with the Western Electric Company back in 1928 had been searching for various means of gain stabilization in amplifiers used in telephone repeaters. It was then that he intuitively conceived and invented the feedback amplifier. The process or technique by which a fraction of the output voltage or current is combined back/fed back to the input, to produce a desired output of a system is called feedback. But the question is – why does an amplifier circuit need feedback for gain stabilization!

As discussed in Chapters 4 and 5, in a BJT, parameters like DC common-emitter current gain β, base-emitter voltage VBE and collector current IC may vary with temperature. In the manufacturer's data sheet, a range of values for these parameters are specified for a given type of transistor group because of material property tolerance. As a result of this, voltage and current gain, quiescent point and input/output impedances usually tend to differ from one circuit to another and also become temperature dependant. By applying the feedback mechanism, transistor circuit characteristics are made independent of the aforesaid parameters of transistors. Although feedback is being formally introduced to the reader in the present chapter, the concept of feedback was utilized in Chapter 5 in connection with biasing of transistor. For example, in the emitter feedback bias and voltage-divider bias circuits, an emitter resistor (RE) was introduced in common-emitter circuits to stabilize the Q-point. In these circuits, when the collector current increases, the drop across the emitter resistor (RE) increases which decreases the base-emitter voltage and that, in turn, further opposes any change in the collector current. This opposition is actually referred to as negative feedback. Thus, the readers encountered the technique of feedback even before they were introduced to it formally.

The feedback technique is generally known by its two types: positive or regenerative feedback, used in designing oscillators, and negative or degenerative feedback, used in designing amplifiers.