Block Diagram Representation of Op-amp

Block Diagram Representation of Op-amp

As mentioned earlier, now a days op-amps are available in an integrated circuit form. Commercial integrated circuit op-amps usually consists of four cascaded blocks. The block diagram of IC op-amp is shown in the Fig. 2.3.1.

1. Input Stage

The input stage requires high input impedance to avoid loading on the sources. It requires two input terminals. It also requires low output impedance. All such requirements are achieved by using the dual input, balanced output differential amplifier as the input stage. The function of a differential amplifier is to amplify the difference between the two input signals. The differential amplifier has high input impedance. This stage provides most of the voltage gain of the amplifier.

2. Intermediate Stage

The output of the input stage drives the next stage which is an intermediate stage. This is another differential amplifier with dual input, imbalanced i.e. single ended output. The overall gain requirement of the op-amp is very high. The input stage alone cannot provide such a high gain. The main function of the intermediate stage is to provide an additional voltage gain required. Practically, the intermediate stage is not a single amplifier but the chain of cascaded amplifiers called multistage amplifiers.

3. Level Shifting Stage

All the stages are directly coupled to each other. As the op-amp amplifies d.c. signals also, the coupling capacitors are not used to cascade the stages. Hence the d.c. quiescent 

voltage level of previous stage gets applied as the input to the next stage. Hence stage by stage d.c. level increases well above ground potential. Such a high d.c. voltage level may drive the transistors into saturation. This further may cause distortion in the output due to clipping. This may limit the maximum a.c. output voltage swing without any distortion. Hence before the output stage, it is necessary to bring such a high d.c. voltage level to zero volts with respect to ground.

The level shifter stage brings the d.c. level down to ground potential, when no signal is applied at the input terminals. Then the signal is given to the last stage which is the output stage.

The buffer is usually an emitter follower whose input impedance is very high. This prevents loading of the high gain stage.

4. Output Stage

The basic requirements of an output stage are low output impedance, large a.c. output voltage swing and high current sourcing and sinking capability.

The push-pull complementary amplifier meets all these requirements and hence used as an output stage. This stage increases the output voltage swing and keeps the voltage swing symmetrical with respect to ground. The stage raises the current supplying capability of the op-amp.

In short, the overall block diagram can be shown as in the Fig. 2.3.2.

Review Questions

1. Draw the schematic block diagram of the basic op-amp and briefly explain each stage.

Dec.-06, Marks 8

2. Explain the functions of all the building blocks of an op-amp.