## 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.

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