Question about AGC stability.

Hi every one:
When designing an AGC circuit, what should be pay attention to make sure the designed AGC is in absolute stable state?

Firstly, you should know the frequency characteristics of the chosen level detector.
Then, what is the relationship between control voltage and gain for your VGA ?
And what is the frequency characteristic of the controller (in most cases a LPF).
At the end you should try to analyze the loop gain of the whole circuit - linearized around the normal working point

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