Subject: Priniciples of Communication Engineering

Difficulty : Medium

Marks : 05

Signals from various radio stations reaching at the receiver input are not of same strength. Signals from the strong stations are strong and those from weak stations are weak. If the receiver gain is constant then the receiver output will fluctuate proportional to the strength of the input signal. This is not desirable so the automatic gain control (AGC) is used to adjust the receiver gain automatically so as to keep the receiver output constant irrespective of the strength of input signal.

Various types of AGC are:

• Simple AGC
• Dlayed AGC
• Ideal AGC.

In simple AGC is a system which will change the overall gain of the receiver automatically, this is done in order to keep the receiver output constant even when the signal strength at the input of the receiver is changing. In AGC system a dc voltage (AGC bias) is derived from the detector. This AGC bias is thus proportional to the strength of received signal.

AGC bias is applied to a selected number of RF and IF amplifiers and mixer stage. The transconductance and hence the gain of the devices connected in these stages is dependent on the applied AGC bias.

Simple AGC characteristics are clearly an improvement over no AGC situation. But its disadvantage is that reduction in the gain of the receiver will take place even for work signals. In delayed AGC gain is not reduced for weak signals. And it is reduced only for strong signals characteristics of delayed AGC is close to the ideal AGC characteristics delayed AGC characteristics are adjustable.