Radio Frequency Integrated Circuit Design - Webs

354 Radio Frequency Integrated Circuit Design
Figure 10.6 Power amplifier circuit with tuned load.
any of these classes. Waveforms for the base voltage vB , collector voltage vC ,
and collector current iC are shown in Figure 10.7 for class A operation and in
Figure 10.8 for class B and C operation. Class A amplifiers can be designed to
have more gain than class B or class C amplifiers. However, as will be seen
later, the achievable output power is nearly the same for a class A, class AB, or
class B amplifier. For a class C amplifier, where the transistor conducts for a
short part of the period, the output power is reduced.
The maximum sinusoidal collector voltage is shown from approximately
0V to 2VCC . The assumption that the collector voltage can swing down close
to 0V is justified in that it simplifies the analysis and typically results in only
a negligible error. While the collector voltage is assumed to be sinusoidal because
of the filtering action of the tuned circuit, the collector current may be sinusoidal,
Figure 10.7 Waveforms for class A power amplifier.