3.2 Differential Amplifier

Differential amplifiers are designed to take two input signals and subtract them to get only the difference between the input signals. Differential amplifiers are related to, but distinct from the Instrumentation Amplifier. Figure 3-2 shows a differential amplifier implementation.

One of the most important differences compared to the instrumentation amplifier is that the differential amplifier adds a bias voltage to the output. This is beneficial in that the output may remain valid if the inputs are flipped. However, the gain of each input is not matched, which may lead to unexpected outputs if both inputs change at the same time.

For the most predictability, either fix one of the inputs at a known constant, or measure one of the inputs before solving for the other. The output formula is shown below:

V_{O U T} = (1 + \frac{R_{2}}{R_{1}}) * V_{(I N +)} - \frac{R_{2}}{R_{1}} * V_{(I N -)}