Electronic – How to mirror resistor? (use same variable resistance for multiple op-amp gains)


I want to create parallel op-amp circuits/channels and use the value from a single potentiometer to adjust the gains of each uniformly by making each op-amp see a 'copy' of the resistance to set the feedback gain – the amps in question are TL072's and I'm looking to make at least 4 seperate channels.

Is this possible – do any circuits come to mind?

Best Answer

If you are working at audio frequencies I would recommend using a monolithic Voltage Controlled Amplifier (VCA), with the analog control voltage coming from a single potentiometer, and perhaps a buffer driver for low impedance to drive the higher impedance gain control inputs of your 4 channels.

A single channel VCA is the SSM2018 from Analog Devices. The 4-channel SSM2154 is obsoleted. I am sure there are similar devices from other vendors.

If you are working with anything higher than audio frequencies then you should look at the vendors' selections of Variable Gain Amplifier (VGA) and Programmable Gain Amplifier (PGA) devices. However these are likely to be set by a 6-bit to 8-bit digital control and not via an analog control voltage that would easily be obtained from a potentiometer.

A simple approach is to use a JFET shunt VCA topology. This picture is courtesy of Elliott Sound Products (https://sound-au.com/articles/vca-techniques.html)


National Semiconductor AN32 from 1970 has a JFET as an attenuator at the input to an op-amp. In this configuration you would use a fixed gain to gain-up your output to a maximum and attenuate it down at the final stage using the control voltage. The image below would be the final stage, after your gain stage.

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