MK120 explanation part of receiver

Kits Mini Kits Range
1

Hello,

can anybody explain the part of the Opamp IC1B in the schematic?
http://www.velleman.be/downloads/files/schema/receiver.jpg
The rest of the schematic is clear to me.
It seems to be some kind of negative feedback, but can anyone explain how it works?

I’m doing some work for school.

Thanks.

2

IC1B is an active rectifier. It allows you to rectify signals which are smaller than one diode drop. The feedback is a precaution, so that negative signals are heavily feedbacked, to avoid slewing (slow down) of the amplifier.
A textbook on electronics will explain this circuit in detail.