Chapter 9. RF Power Amplifiers

  1. W. Alan Davis PhD1 and
  2. Krishna K. Agarwal PhD2

Published Online: 9 OCT 2001

DOI: 10.1002/0471200689.ch9

Radio Frequency Circuit Design

Radio Frequency Circuit Design

How to Cite

Davis, W. A. and Agarwal, K. K. (2001) RF Power Amplifiers, in Radio Frequency Circuit Design, John Wiley & Sons, Inc., New York, USA. doi: 10.1002/0471200689.ch9

Author Information

  1. 1

    University of Texas at Arlington

  2. 2

    Raytheon Systems Company

Publication History

  1. Published Online: 9 OCT 2001

Book Series:

  1. Wiley Series in Microwave and Optical Engineering

Book Series Editors:

  1. Kai Chang

Series Editor Information

  1. Texas A&M University

ISBN Information

Print ISBN: 9780471350521

Online ISBN: 9780471200680

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Keywords:

  • RF power amplifiers;
  • transistor configuration;
  • class B amplifier;
  • dead band;
  • efficiency;
  • class C amplifier;
  • input bias voltage;
  • class D power amplifier;
  • class F power amplifier;
  • feed-forward amplifiers;
  • problems

Summary

Earlier in Chapter 7, 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. In this chapter a description of power amplifiers that provide higher efficiency than the class A amplifier is given. 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. 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–emitter junction is forward biased and the base–collector junction is reverse biased. The voltage swing of a class A amplifier will remain in the forward active region throughout its entire cycle.

The class B amplifier is biased so that the transistor is on only during half of the incoming cycle. The other half of the cycle is amplified by another transistor so that at the output the full wave is reconstituted.

The class C amplifier is useful for providing a high-power continuous wave or frequency modulation.

The class D amplifier looks like a class B amplifier except for the input side bias. In class D operation the transistors act as near ideal switches that are on half of the time and off half of the time. The input ideally is excited by a square wave.

A class F amplifier is characterized by a load network that has resonances at one or more harmonic frequencies as well as at the carrier frequency. The class F amplifier was one of the early methods used to increase amplifier efficiency and has attracted some renewed interest recently. These amplifiers, as well as feed-forward amplifiers are discussed.