Block diagram of OP amp
Input Stage
The input stage is a Dual input balanced output differential amplifier. The two amplifiers are applied at inverting or non inverting terminals. This stage provides most of voltage gain of the op-amp and decides input resistance value R1.
Intermediate Stage
This is usually another differential amplifier. It is driven by output of the input stage. This stage is dual input unbalance output differential amp. This stage provides additional voltage gain to the input signals.
Level shifting stage
This is third stage in the block diagram of op-amp. Due to direct coupling between first two stage the input of level shifting stage is an amplifying system with non-zero DC level. Level shifting stage is used to bring this DC level to a zero volt with respect to ground.
Output Stage
This is normally complementary output stage. It increases magnitude of voltage and rises the current supplying capacity of the op-amp. It also provides low output resistance. The output stage is a push pull of two transistors.