Automatic Gain Control AGC

Thread Starter


Joined May 12, 2021
I have to implement a circuit which is supposed to amplify an input amplitude to constant amplitude 8Vpp output even if the input amplitude is varied (0.5-5V). The input have a 6V offset and the supply voltage is 12V. I do the simulation with the program LT-Spice. I need to build an AGC for a project (LED light organ).

-Block diagram:

-Here I have an approach for the circuit (

-I also built this circuit. It's not an AGC. It works for low input (input 1V) but not for high input (e.g. 5V):

-I prefer the first circuit (, but it works poorly.

What do you think about an optocoupler?

Thank you in advance for your help. I'm a newbie and I would be very grateful for every suggestion :D


Joined Jan 29, 2010
hi Max,
Welcome to AAC.
It would help to improve the responses and suggesting of a solution, if you placed the documentation here on AAC.



Joined Jul 10, 2017
The opto-coupler will work with a very high dynamic range. Be aware that they mention a "resistive" opto-coupler. That can be made using a LED or incandescent light source closely coupled to a cadmium sulphide photo-detector.


Joined Mar 14, 2008
Since you changed Q1 from an NPN in my circuit to a PNP (why?) and turned the JFET upside down, then you're on your own to determine why your circuit doesn't work. :rolleyes:

Note that the OP07 op amp output can go no closer than within about 1V of the supply rails or 10Vpp for a 12V supply.
If you want to go to the supply rails you need a rail-rail type op amp.
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