Tricky Circuit

An inverting summing amplifier

Mix two signals with resistors

A summing amplifier adds input signals. Op amp means Operational Amplifier, a powered circuit that amplifies the difference between two inputs. Negative feedback returns output to the subtracting input. A virtual ground stays near 0 V through feedback, without a wire to ground.

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EXPERIMENT BENCH

Change the circuit settings, predict the readings, then check a challenge. These are learning models, with the assumptions explained in each lesson.

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How it works

Ideal amplifier with ±12 V output limits; two equal 10 kΩ input resistors. The gain knob sets feedback resistance. No noise, bandwidth, load or resistor error is modeled.

V1 and V2 are input voltages. Each contributes current through its own resistor. Their currents add in the feedback resistor Rf. Gain magnitude g = Rf/10 kΩ; output Vout reverses the sum’s sign.

Difficulty

  1. Build · Observe

    Power on at V1 = 1 V, V2 = 1 V and g = 1. Obtain −2 V.

    Need a hint?

    V1=1 V; V2=1 V; g=1 → −2 V

  2. Explore · Explore

    Set V1 = 1 V, V2 = 2 V and g = 2. Obtain −6 V.

    Need a hint?

    V1=1 V; V2=2 V; g=2 → −6 V

  3. Solve it · Solve

    Keep V1 = 1 V, V2 = 2 V and set g = 3. Obtain −9 V.

    Need a hint?

    V1=1 V; V2=2 V; g=3 → −9 V

Extra reading and circuit recipes

An inverting summing amplifier

An audio mixer combines several channels without wiring their sources directly together. Here two steady voltages let you inspect the same addition rule without a sound waveform.

Vout = −g × (V1 + V2); g = Rf/Rin

Equal input resistors give both sources equal weight. Changing the common feedback resistor scales both contributions together.

A useful connection

At equal resistor ratios, the output is the inverted sum, not the average.

Beyond the model

The amplifier supply provides output energy. Adding voltages does not create free power.

A useful trap to avoid

Virtual ground is not a spare ground terminal. Its near-zero voltage depends on working linear feedback.

Both inputs are 1 V. Gain changes from 1 to 2. The output becomes…

  1. −2 V
  2. −4 V
  3. +4 V
Read more

The sum is 2 V; inversion and gain two give −4 V.

Two equal voices

Use 1 V at each input with gain 1.

The inverted sum is −2 V.

Make the mix larger

Use inputs 1 V and 2 V with gain 2.

The inverted sum grows to −6 V.

Try another change

Keep gain 1. Compare inputs 0 V + 2 V with 1 V + 1 V: the same sum gives the same −2 V output.

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