Input Stage of an op-amp
The input stage is a dual input,balanced output differential amplifier.It has 2 inputs Vin1 and Vin2 which are applied at the bases B1 and B2 of transistors Q1 and Q2.The output V0 ois measured between the two collectors C1 and C2 which are at the same dc potential.Because of the equal dc potential at the two collectors with respect to ground,the output is referred to as balanced output.
Intermediate Stage
The next stage is dual input,unbalanced output difference amplifier.Here two input signals are used however the output is measured at only one of the two collectors with reference to ground.The output is referred to as an unbalanced output because the collector at which the output voltage is measured is at some finite dc potential with reference to ground.In other words,there is some dc voltage at the output terminal without any input signal applied.
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