Differential amplifier

Recall the two rules:

1. No Current Flows into the Input Terminals, i.e. $I_1=I_f$
2. The Differential Input Voltage is Zero i.e. $V_a = V_b $

When resistors, $R_1 = R_2$ and $R_3 = R_4$

              

When $R_1 = R_2 = R_3 = R_4$, it is called Unity Gain Differential Amplifier and the voltage gain of the amplifier will be exactly one or unity.