Hello, I'm not from Power electronics but I think the solution of this question might be Power electronics. Following is partial of the circuit. I want the value of Vreference to be the average value of the "Out" signal. The green signal is the output of (AM)envelope detector, this signal has 20kHz and 20mV peak-to-peak. How can I change the circuit between the green and blue circle, so that I can make the blue signal an average value of the green signal?

 

If your signal is an AC voltage, you need to firstly feed it into a precision rectifier circuit.
According to the graph you have provided, the green waveform does not go negative and hence does not require rectification.
To find the envelope, feed it into a low-pass filter (integrator).

 

If your signal is an AC voltage, you need to firstly feed it into a precision rectifier circuit.
According to the graph you have provided, the green waveform does not go negative and hence does not require rectification.
To find the envelope, feed it into a low-pass filter (integrator).

I forget to mention, the Vdet go through a precision rectifier then a LPF, then it outputs this green signal, so it is already the envelope, I now need to distinguish the modulation points. I use a comparator to compare the Vreference (the average value I try to obtain) and the green signal to demodulate it.

 

So you run the signal through another LP filter with a longer time-constant to get the long-term average.

It's not at all clear what you are trying to do.
What is the characteristic of the signal you are trying to "demodulate"?
What are modulation points?

 

So you run the signal through another LP filter with a longer time-constant to get the long-term average.

It's not at all clear what you are trying to do.
What is the characteristic of the signal you are trying to "demodulate"?
What are modulation points?

Thanks, I solved it, cos the majority of the schematic I did not upload. Sry for confusions.