BLOCK DIAGRAM OF A TYPICAL OPERATIONAL AMPLIFIER

An operational amplifier being a multistage amplifier,consists of some basic building blocks as shown in block diagram.

The block diagram of op-amp shown in fig.consists of a four stage direct-coupled amplifier in cascade.In summarized form,it can be explained as below.

The first stage is double-ended high-gain (60 dB) differential with a constant current source(in order to increase CMRR). In this stage high gain is desirable so that there would be a negligible effect on  the output of any shortcoming in the following stages. This is the reason why this stage is generally responsible for most of the gain of op-amp.Also this stage determines the input resistance of the op-amp.output of this stage is taken between collector of two emitter biased circuit so that output remains balanced and the dc voltage at output in quiescent condition maintains zero level.

The second stage,called the intermediate stage,is usually another differential amplifier,which is driven by the output of first stage, In most amplifier the intermediate stage is dual input,unbalanced (single-ended) output differential amplifier in order to increase the gain.The differential mode voltage gain of such an  amplifier is half of the gain of the dual-input,balanced output differential amplifier.In this amplifier output is  measured at the collector of only one of the two transistor w.r.t ground.In the quiescent condition some dc voltage exists at the output terminal and there is no other collector voltage at output to balance or nullify this output dc voltage.This is the reason why this type of amplifier is called unbalanced output type differential amplifier.This unbalanced dc voltage present at the output acts as an error voltage in the desired output signal.

The third stage,Known as level shifting   stage,is usually an emitter follower circuit in order to shift the dc level at the output of the intermediate stage downward to zero volt with respect to ground.It is noteworthy here that error signal is developed in the intermediate stage due to direct coupling and gets amplified in the succeeding stage.This increase in dc level tends to shift the operating point of the succeeding stages which also limits the output voltage swing or may distort the output signal.To overcome these problems, use of a level translator circuit becomes necessary to bring this dc level to zero volt.