Hello, everyone
I'm looking for a fixed output amplitude amplifier. For example, if a have a input voltage of 1Vp I must get 1Vp, if the input now is 0.2Vp I still get 1Vp, if the input now is 5 Vp, the output remains 1Vp and so on.
I have been trying with some FET AGC amplifiers, they claim they keep the amplitude constant. However, they don't, or maybe I'm testing them wrong.

How could I solve this? Hope You would help me.

 

How constant the output is kept depends (at least) on the gain in the feedback loop.
Show your schematic and we may be able to suggest improvements.

 

Hey! This is the design I'm trying...

 

So if you put some gain between R11 and Q1 gate this will tighten up the change in output level.

 

A number of comments and questions:
What power supply voltages are you using?
If you are using a single-voltage power supply you have to bias the op-amp inputs to the middle of the power supply voltage.
You should always have some resistance in series with a variable resistance (such as RV3) that connects to a op-amp virtual ground.
I would put a fixed resistor from the inverting input to ground and ac couple into the non-inverting input.
The 2n3819 has a high gate threshold -- several volts.
Rectifying and filtering a 1V peak signal with 1N4148 silicon diode will only give a maximum of about 0.3 volts at the gate of the FET.
C2 and C3 must not be polarized caps.

 

What is the absolute range of input voltage?

What characteristic of the input are you trying to detect? The amplitude seems to be of no concern.

Is it just any positive polarity?

What is the repetition rate? How often does the input change?