All op-amps consist of a differential amplifier at their input stage. If two different voltage signals are applied to the two input terminals of the op-amp, the resulting output signal is proportional to the “difference” between the two signals. Thus the differential amplifiers amplify the difference between two voltages measured with respect to a common reference. The differential amplifier stage of an op-amp is as shown below.

The two transistors Q1 and Q2 have identical characteristics. The two input signals Vi1 and Vi2 are applied to the base terminals of Q1 and Q2 respectively. Note that the differential amplifier has two output terminals V_O1 and V_O2 .

Ideally, the output voltage is zero when both the inputs are equal. When Vi1 is greater than Vi2, the output terminal V_O1 will be positive and V_O2   will be negative. When V_i2 is greater than V_i1, the output terminal V_O2  will be positive with respect to V_O1.

The output V_O is given by,

A differential amplifier can be configured in four different ways:

·         Dual input balanced output differential amplifier.

·         Dual input unbalanced output differential amplifier.

·         Single input balanced output differential amplifier.

·         Single input unbalanced output differential amplifier.