Op-Amp Basic Concepts

The circuit in Fig. (5.5) represents a very simple op-amp. It is virtually the same circuit as in the previous section, except there is an emitter follower stage and feedback added. In the circuit, the input signal is applied to Q1. Since the output of the diff-amp is taken from the collector of Q2, the base of Q1 represents the noninverting input. The output of Q2 is then input to Q4 which is forms a directly coupled emitter follower stage. The purpose of Q4 is to reduce the output resistance of the circuit. When feedback is applied, a fraction of the output voltage is then fed back to the inverting input (Q2). This provides negative feedback by in effect, reducing the total voltage between the bases of Q1 and Q2.

  

Figure 5.5: Simple Op-Amp-Like Circuit