Difference Amplifier Subtracts First - Electronic Circuits VI: Op-Amp Feedback and Exact Gain Networks Step by Step

A balanced difference amplifier subtracts two inputs and applies an exact resistor ratio. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

The resistor ratios are matched

Both resistor ratios are 2, so the difference network is balanced.

\frac{R_f}{R_{\text{in}}} = \frac{R_{\text{bottom}}}{R_{\text{top}}} = 2

Subtract first, then multiply

The inputs differ by 3 volts, and ratio 2 gives 6 volts.

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