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Subtractor (difference amplifier)

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Objective

Subtract two voltages: Vout = (Rf/R1)·(V2 − V1).

Procedure

  1. Run and read Vout.
  2. Add the same offset to both inputs: Vout does not change.

Theory

With matched resistors R1 = R3 = 10 kΩ and Rf = R4 = 20 kΩ the output is 2·(V2 − V1) = 2·(2 V − 1 V) = 2 V, and a common-mode signal is rejected.

Questions to answer

  1. What limits the common-mode rejection in practice?

Components used

4 × Battery / DC source, Op-Amp LM741, 4 × Resistor 220 ohm 1/4W

About the model

Ideal wires; component models as listed in the catalogue (no parasitics, no temperature drift). Built without a breadboard so the schematic stays readable.

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