Op-amp differential amplifier is given in figure bellow.
Simulate op-amp differential amplifier.
Input in op-amp is already realized as differential amplifier with discrete components. Voltage gain from inverting input to the output is the same as voltage gain from non-inverting input to the output, but with different sign. Overall voltage difference gain is almost infinite for op-amp without negative feedback. It means that input range is zero if there is no negative feedback. For that purpose negative feedback with resistors R1 and R2 is set. By using superposition theorem, we have
By combining upper two equations and having following condition satisfied:
i.e. upper circuit operates as an analog op-amp differential amplifier, with much wider range for input signals V1 and V2 comparing to differential amplifier with discrete components. If additional condition
is fulfilled, both analog inputs V1 and V2 has the same input impedance. CMRR factor of op-amp differential amplifier is highly dependent of how well resistors R1, R2, R3 and R4 are matched. Better version of op-amp differential amplifier is with three op-amps, scheme known as op-amp instrumentation amplifier.