PRINCIPLES OF FEEDBACK

For a simple amplifier the voltage gain is defined as the ratio of output voltage to input voltage. This is written as AV = VO/VI as shown in Fig. 2(a). Addition of a feedback of magni­tude 3, as shown in Fig. 2(b), will result in a modified value for the voltage gain given by the equation: AV = AV/(1 — 3AV). The term /SAV, called the feedback factor, can be either positive or negative. A study of the variation of AV with posi­tive as well as negative values of 3 is shown in Fig. 2(c, d). It is observed that the value of AV becomes infinite with only 10% of positive feedback. However, this should not be viewed as advantageous, because positive feedback greatly increases distortion in the output. Mathematically it is true that the gain approaches infinity; however, in reality, the circuit be­gins to oscillate. Positive feedback does not find many applica­tions.

voltage Vj

voltage VO

(a)

PRINCIPLES OF FEEDBACK

PRINCIPLES OF FEEDBACK

Figure 2. (a) Block diagram of an amplifier with AV = VO/Vi. (b) Block diagram of an amplifier with feedback. The dashed line en­closes the entire amplifier including the feedback; its gain is AV = VO/Vi.

PRINCIPLES OF FEEDBACK

Midfrequency level

flat

Vfl,

(b)

Figure 1. Frequency response curve of an audio amplifier.

Consider the case when PO = JPj. Then the gain in decibels is 101og10 = -3.0103

Therefore, for audio engineers the point of interest lies where the gain falls by 3 dB. The frequencies at which this occurs are called half-power frequencies. Let the flat portion of the

AV

Av/V2 = AV-3 dB a’v

AV -3 dB = A’v/V2

With negative feedback

PRINCIPLES OF FEEDBACK

Frequency (log scale)

Bandwidth _________

(no feedback)

Bandwidth (with feedback)

PRINCIPLES OF FEEDBACK

At

A

(S/N )f,

nofeedback

However, in 1932, Harry Nyquist of Bell Telephone Labo­ratories extended this theory at length and published his fa­mous paper on regenerative theory. His principles laid the foundation for the development of feedback oscillators. There­fore, positive feedback is also called regenerative feedback. While designing frequency-selective feedback circuits for audio amplifiers, one may use positive feedback either as a bass or as a treble boost.

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