Impedance-Balancing Rule for Op-Amps

Abstract

This note demonstrates a rule that aids circuit design by allowing the component values around multi-input summing amplifiers or comparators to be easily calculated. It was first published 27 years ago in 1991 and again in 1993, in two relatively obscure journals. Now, in 2018, I have scanned and re-formatted the original text, because I still think that it describes a useful tool for the electronic circuit designer. The rule states … The gain from any input of a summing amplifier is the feedback impedance divided by that input impedance, provided that the sum of the admittances connected to the inverting input of the amplifier is equal to the sum of the admittances connected to the non-inverting input. Given this condition, amplifier bias currents will not contribute an error.