Summing and Difference Amplifiers
Inverting summer
VOUT = -RF(V1/R1 + V2/R2 + ...)
Each source contributes current to the summing node. Equal resistors give equal weighting; different resistors create weighted sums.
Audio mixing
An analog mixer can give each source its own resistor so one source does not directly drive another.
Difference amplifier
A four-resistor network can amplify V2-V1. Excellent common-mode rejection requires closely matched resistor ratios.
A high-CMRR op-amp cannot rescue a difference amplifier whose resistor ratios are badly mismatched.
Instrumentation amplifier
Instrumentation amplifiers provide high input impedance and excellent differential accuracy, commonly using three amplifier stages or an integrated matched-resistor structure.
Transimpedance amplifier
VOUT = -IINRF
This current-to-voltage circuit is fundamental for photodiodes and many sensor interfaces.