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A Difference Amplifier

Sometimes it is desirable to obtain the difference between two signals. Of course, it is possible to apply one signal to an analog inverter and then sum the two together. However, perhaps there is an easier way to accomplish the same result.


A true difference amplifier.

The circuit to the right combines features of the normal inverting amplifier and the non-inverting amplifier. Note that there are two resistors Rf as well as two Rin resistors. For correct circuit operation, it is important that the two pairs be matched. With the circuit shown, the equation for the output voltage is:

Vout =  Rf  (V2 - V1)

Rin

This circuit operates cleanly and accurately, but does have a limitation. You cannot use it for summing multiple signals at the "-" input, because that input is no longer at a virtual ground. As a result, additional input signals to this point will interact with each other and produce distorted results.


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