A differential (or difference) amplifier is a two-input circuit that amplifies only the difference between its two inputs. An operational amplifier or op-amp (Figure 1) is an example of a difference amplifier. The formula that describes the behavior of the circuit is:
Vout = A * (Vin+ - Vin-) where A is the gain of the amplifier
Figure 1. Operational Amplifier Symbol
Differential amplifiers are useful in electrically noisy environments where a low amplitude electrical signal can be easily corrupted by the effect of unwanted external noise. In this scenario, a single-ended amplifier would be unsuitable since it would also amplify the unwanted noise signal as well as the desired input signal. A differential amplifier works on the principle that unwanted electrical noise couples equally onto both input terminals of the amplifier and will therefore be rejected allowing only the wanted signal to be amplified.
They are used at the front end of analog systems being used to detect a low amplitude electrical signal e.g. an ECG monitor to detect heart rate where the electrical signal may be of the order of only a few millivolts. The purpose of the differential amplifier is to increase the amplitude of the heart signal to a level where it can be converted into a digital form. The gain of the circuit can be adjusted by appropriate selection of external resistors connected between the output and input terminals.