Summing and Difference Amplifiers

Add or subtract several signals with resistor networks around an op-amp.

Intermediate

Inverting summer:

$V_{out} = -R_f\left(\frac{V_1}{R_1} + \frac{V_2}{R_2} + \dots\right)$

With equal resistors this simply adds the inputs (inverted) — the basis of audio mixers.

Difference amplifier (four resistors, matched ratio $R_2/R_1$ ):

$V_{out} = \frac{R_2}{R_1}(V_2 - V_1)$

It rejects voltages common to both inputs, which is essential for measuring the small voltage across a current-sense resistor.

Sub-topics (1)

A three-op-amp circuit with very high input impedance and common-mode rejection, gain set by one resistor.

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