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Summing amplifier

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Objective

Add three input voltages with an inverting summer: Vout = −Rf·(V1/R1 + V2/R2 + V3/R3).

Procedure

  1. Run and read Vout.
  2. Change one input and check the change in Vout equals −(Rf/R)·ΔV.

Theory

All inputs feed the virtual-ground node, so the currents add: Vout = −10 k·(1 V/10 k + 2 V/10 k + 0.5 V/5 k) = −4 V.

Questions to answer

  1. How would you give input 2 twice the weight?

Components used

5 × 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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