Operational Amplifier Fundamentals Unit 5 – The Inverting Summer
UNIT 5 – THE INVERTING SUMMER
UNIT OBJECTIVE
At the completion of this unit, you will be able to describe the operation of an inverting summing
amplifier using a three-input op amp summing configuration.
UNIT FUNDAMENTALS
An inverting summing amplifier is a multiple input inverting op amp circuit. The circuit output
voltage is proportional to the algebraic sum of the input voltages.
The polarity of the circuit output voltage is opposite that of the summed input voltages.
The junction formed by the input resistors, feedback resistor, and the op amp inverting terminal
is the circuit summing point.
Direct summation of the input voltages and feed-back voltage occurs at the circuit summing
junction. The circuit summing junction is at virtual ground (the op amp is zero based) because
the noninverting terminal is referenced to circuit common.
Circuit output voltage is proportional to the sum of each input voltage multiplied by the ratio of
RF to the respective input resistor (Rx).
VO = -{[(RF/R1) x V1] + [(RF/R2) x V2] + [(RF/R3) x V3]}