Figure 1.  Basic Circuit Diagram for a Simple Operational Amplifier (Op-Amp)

Figure 1 shows the basic circuit diagram of a very simple operational amplifier.

An op amp basically has 4 circuit stages: 1) an input stage; 2) an intermediate stage; 3) a level-shifting stage; and 4) an output stage.  The input stage of an op-amp is usually a pair of matched transistors configured as a dual-input differential amplifier.  The output of this input stage is taken from across the outputs (collectors in the above example) of the paired transistors. This balanced output is fed into another dual-input differential amplifier that serves as the intermediate stage.  The output of this intermediate stage is taken from just one of the transistors, i.e., it is single-ended and therefore not balanced.

The dc level at the output of the intermediate stage is high with respect to ground, so a level-shifting circuit such as an emitter follower is used to shift it down closer to ground.  The output stage of an op amp usually consists of a push/pull pair of complementary transistors which increases the swing of the output voltage and enhances the load current capacity of the op amp.  The gains of the input and intermediate stages of an op amp are high, while those of the emitter follower and output stage are generally close to 1. 

See Also:  Operational Amplifiers

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