RF amplifier

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. In this chapter a description of 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 3 1. The common emitter with emitter degeneracy is used when improved stability is needed with respect to differences in the transistor short circuit current gain with some degradation in the voltage gain. These are illustrated in Fig. 9.1 in which the bias supplies are not shown. These properties are described in detail in most electronics texts.

The transistor itself can be in one of four different states: saturation, forward active, cutoff, and reverse active. It is in the forward active region, when for the bipolar transistor, the base�Cemitter junction is forward biased and the base�C collector junction is reverse biased. These states are illustrated in Fig. 9.2 for a npn transistor. An actual bipolar transistor requires a base�Cemitter voltage greater than 0.6 to 0.7 volts for it to go into the active state.