RF Power Amplifiers
9.1 TRANSISTOR CONFIGURATIONS
In Chapter 8, class A amplifiers were treated. Some discussion was given to its application as a power amplifier. While class A amplifiers are used in power applications where linearity is of primary concern, they do so at the cost of efficiency. This chapter describes power amplifiers that provide higher efficiency than the class A amplifier. Before describing these in detail, it should be recalled that a single-transistor amplifier can be installed in one of four different ways: common emitter, common base, common collector (or emitter follower), and common emitter with emitter degeneracy. Although there are always exceptions, the common emitter circuit is used in amplifiers where high-voltage gain is required. The common base amplifier is used when low input impedance and high output impedance is desired. This is accompanied with a current gain ≈ 1. The emitter follower is used when high input impedance and low output impedance is desired. This is accompanied with a voltage gain ≈ 1. The common emitter with emitter degeneracy configuration is used when improved repeatability is needed with respect to differences in the transistor short-circuit current gain (β). The emitter resistance provides negative feedback and thus causes some loss in voltage gain. These configurations (minus bias circuits) are illustrated in Fig. 9.1. These properties are described in detail in most electronics texts.