Found in a 1980s telecommunications documentation, the circuit presented acts as a product detector. The modulator or mixer in the square may be that of the previous circuit.
The circuit presented emulates an OR gate with the LM3900, operational transconductance amplifier....
This circuit was obtained from a 1962 magazine. It can still be mounted to be powered from 6 V...
The figure shows how to implement a conventional current monitor with differential amplifier on an H...