## 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)  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. 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 Frequency (log scale) Bandwidth _________ (no feedback) Bandwidth (with 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.