A basic differential amplifier can be used as a subtractor as shown in the above figure. If all resistors are equal in value, then the output voltage can be derived by using superposition principle.
To find the output V01 due to V1 alone, make V2 = 0.
Then the circuit of figure as shown in the above becomes a non-inverting amplifier having input voltage V1/2 at the non-inverting input terminal and the output becomes
Similarly the output V02 due to V2 alone (with V1 grounded) can be written simply for an inverting amplifier as
Thus the output voltage Vo due to both the inputs can be written as
Vo= V 01+ V 02= V 1@V2