Tuesday, 10 March 2026

Operational Amplifiers – Complete Theory

Page 7 – Summing Amplifier (Adder)

The Summing Amplifier is an op-amp circuit used to add multiple input voltages.

It is also called an Adder Circuit.

Circuit Configuration

Multiple input signals connected through resistors
All inputs applied to the inverting terminal
Non-inverting terminal grounded
Feedback resistor connected from output to input

Virtual Ground Concept

Since the non-inverting terminal is grounded:

V− ≈ 0

This node behaves like a virtual ground.

Input Currents

Each input produces a current:

I1 = V1 / R1

I2 = V2 / R2

I3 = V3 / R3

Total current entering the node:

I = I1 + I2 + I3

Output Voltage

The output voltage is:

Vout = −Rf ( V1/R1 + V2/R2 + V3/R3 )

Special Case (Equal Resistors)

If

R1 = R2 = R3 = Rf

Then

Vout = − (V1 + V2 + V3)

Applications

Audio mixers
Signal processing
Digital to analog converters
Analog computation

GATE Important Points

Summing amplifier adds multiple inputs
Uses virtual ground concept
Output is inverted
Weighted sum possible

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