What's the function of this circuit?

[saturn]

Hello,
Can anyone please tell what's the function of this circuit? How it works out?

Thanks.

 

[Kral]

The first stage is an AC coupled differential amplifier. It's a little unusual in that the gain from the inverting input is different from the gain from the non inverting input. The usual configuration would be to have R20 = R12, and to have an additional capacitor (= C9) in parallel with R20 to keep everything balanced for maximum common mode rejection. The AD847 spec sheet calls for input resistors (R18, R19) to protect the input transistor bases from excessive current in the presence of transient inputs. These resistors, along with C22, C23 provide a low pass filter corner frequency of 1/(2PiR18C22). C9, along with R12 provide a low pass corner frequency of 1/(2PiR12C9).
.
The 2nd stage is a half wave precision rectifier.
.
I have no idea what C13 is for, since pin 1 is one of the null inputs.
.
The schamatic is chopped off after C20, but R22, R25, C20 seem to provide scaling an filtering to convert the half wave output to DC. Anything beyon dC20 is chopped off in my view of the circuit.
Regards,
Kral

 

[leomecma]

I think C13 do a dominant pole, because buffer configurations work very nearly of 180º nyquist fase, and consequently can be non-estable. Mainly when the output have a RC network, that brings the fase to more nearly of 180º.