Mumbai University > Electronics Engineering > Sem4 > Fundamentals of Communication Engineering

Marks: 5M

Year: Dec2014, May 2014

• Automatic gain control (AGC) is a mechanism wherein the overall gain of the radio receiver is automatically varied according to the changing strength of the received signal. This is done to maintain the output at a constant level.
• If the gain is not varied as per the input signal, consider a stronger input signal, then the signal might probably be distorted with some of the amplifiers reaching saturation level.
• AGC is applied to the RF, IF and mixer stages, which also helps in improving the dynamic range of the receiver antenna to 60-100 dB by adjusting the gain of the various stages in the radio receiver.
• The AGC derives dc bias voltage from the part of the detected signal to apply to the RF, IF and mixer stages to control their gains. The transconductance and hence the gain of the devices used in these stages of the receiver depends on the applied bias voltage or current.
• When the overall signal level increases, the value of the applied AGC bias increase leading to the decrease in the gain of the controlled stages.
• When there is no signal or signal with low value, there is minimum AGC bias which results in amplifier generating maximum gain.
• AGC facilitates tuning to varying signal strength stations providing a constant output.
• AGC smoothens the amplitude variations of the input signal and the gain control does not have to be recalibrated every time the receiver is tuned from station to station.
• An AGC which is not designed correctly can lead to considerable distortion to a smooth signal.

• There are two types of AGC circuits:

  i. Simple AGC: the gain control mechanism is active for high as well as low value of carrier voltage.

  ii. Delayed AGC: AGC bias is not applied to the amplifiers until signal strength crosses a predetermined level, after which AGC bias is applied.