The differential amplification circuit built in the input circuit of an operational amplifier is designed to amplify only the difference between two input signals, so the structure is completely symmetric, as shown in the following figure.
Assuming that the two input voltages, V1 and V2, are equal, and their respective resistors and transistors in the circuit are exactly equivalent in terms of characteristics, collector currents I1 and I2 are equal, and output voltages Vo1 and Vo2 are the same values, and the differential circuit is totally balanced.
Vo1 = Vo2 ≈ V+ - Io × Rc/2
Next, if V1 increases and IB1 (the input current of Q1) increases, I1 correspondingly increases, so that output voltage Vo1 decreases.
However, since the sum of I1 and I2 is always equal to constant current source Io, I2 necessarily decreases and Vo2 increases, so that Vo1 ≠ Vo2 and the balance is lost. In fact, differential output voltage Vo1 - Vo2 occurs, and this voltage is proportional to differential input voltage V1 - V2. Moreover, if V2 increases until it becomes the same voltage as V1, the circuit again becomes balanced, with no differential output voltage.
In this manner, differential amplification circuits have extremely interesting characteristics in terms of responding only to the differential voltage (differential component) between two input pins.