Multistage Amplifiers

Abstract

The three variants of the single-stage transistor amplifier, namely the common-emitter, the common-base and the common-collector have been analysed and discussed in chapters 4 and 5. However, a single stage by itself may not provide enough gain or the input and output resistances may be unsuitable for a particular application. In these situations, the required performance can be obtained by coupling two or more single stages together to form a multistage amplifier. Stage configurations incorporated in the complete amplifier should be selected on their merits for the particular job concerned. Table 4.1 (page 94) which lists the small-signal properties of the configurations, is a useful guide. For example, the design of an amplifier with low input and output resistances could involve a common-base input stage and a common-collector output stage. Often a common-collector stage is used as the first or last stage of a multistage amplifier to achieve either a high input resistance or a low output resistance. A common-emitter stage followed by a common-base stage has a good high-frequency response. This is because the common-base stage provides a very low load resistance for the common-emitter stage and has itself an inherently good high-frequency performance.