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Instrumentation amplifier (three op-amps)

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Objective

Amplify a small differential voltage with a three-op-amp instrumentation amplifier and reject the common-mode level.

Procedure

  1. Run and read Vout.
  2. Change Rg to 2 kΩ: the gain falls to 11.

Theory

Gain = (1 + 2R/Rg)·(R3/R2) = (1 + 2·10 k/1 k)·1 = 21. A 100 mV difference on a 2 V common-mode level gives 2.1 V while a pure common-mode input gives ≈ 0 V.

Questions to answer

  1. Why is the input resistance of an in-amp so high?

Components used

8 × Battery / DC source, 3 × Op-Amp LM741, 7 × 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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