Inverting Summer Adds Currents - Electronic Circuits VI: Op-Amp Feedback and Exact Gain Networks Step by Step

An inverting summer adds exact input currents before feedback converts them to voltage. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Each input resistor contributes a current

The first input contributes 1 ampere, and the second contributes 1 ampere.

I_a + I_b = 1\ \text{A} + 1\ \text{A}

The feedback resistor converts total current to output

The total current of 2 amperes through 3 ohms gives -6 volts.

V_{\text{out}} = -\left(2\ \text{A}\right)\cdot 3\ \text{ohm} = -6\ \text{V}