Please have in front of you 
and refer to it
3L1 Understand the source and use of an AGC voltage.
The automatic gain control or AGC is a voltage controlled part of the receiver with the voltage coming from the link between the second IF and the detector stage. The level of the voltage is directly related to the strength of the signal.
Now looking at or the previous superhet circuits in
this section you will see the AGC bias line.
As a strong signal alters the bias on the base of the transistor by reducing the voltage and thereby reduces the gain of the transistor. This voltage is used to control the gain of the receiver and so enable the receiver to handle a wide range of signal strengths with little or no change in output audio level.
This same voltage can be used to drive a Signal strength meter or "S" meter.
For further understanding...
Signal Detection:
The AGC system continuously monitors the input signal's strength (volume or amplitude). When the signal strength increases or decreases beyond a set threshold, AGC kicks in to adjust the gain.
Adjusting Gain:
If the incoming signal is too weak, the AGC boosts the gain to amplify it. If the signal is too strong and could cause distortion, AGC reduces the gain to prevent clipping or overloading.
Feedback Loop:
AGC systems often use a feedback loop that continually adjusts the gain in response to the varying signal levels, ensuring the output is stable and at the target level.
Time Constants:
AGC can have different time constants, which control how quickly it responds to signal changes. For example, it might respond more slowly to avoid reacting to quick, transient changes in volume, but faster in cases of more significant volume shifts.
Recall that the speed of the AGC response can be adjusted on both attack and decay
If your Receiver has AGC and the input amplitude increase the term "Attack time" or "Fast Attack" refers to that time which the AGC takes to respond and lower the output amplitude. The fast attack is required to stop unwanted large increases in signal from reaching the speaker or head phones where hearing could be damaged.
Similarly the length of time the AGC responds to a decrease in input amplitude is called "decay time" and is a measure to the time to boost the input amplitude.
Thus according to how the amplitude of the input signal is varying will depend as to how you want the settings e.g. a slow fading can require longer response time than quick fading.
In summary, AGC helps to automatically adjust the gain in real-time to maintain consistent audio levels, improving user experience and preventing distortion.
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