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Switchmode RF Power Amplifiers

# Chapter 1: Power-Amplifier Design Principles

This introductory chapter presents the basic principles for understanding the power-amplifier's design procedures in principle. Based on the spectral-domain analysis, the concept of a conduction angle is introduced, by which the basic classes A, AB, B, and C of the power-amplifier operation are analyzed and illustrated in a simple and clear form. The frequency-domain analysis is less ambiguous because a relatively complex circuit often can be reduced to one or more sets of immittances at each harmonic component. The different nonlinear models for MOSFET, MESFET, HEMT, and bipolar junction devices including HBTs, which are very prospective for modern microwave monolithic-integrated circuits of power amplifiers, are given. The effects of the input-device parameters on the conduction angle at high frequencies is explained. The design and concept of push-pull amplifiers using balanced transistors are presented. The possibility of the maximum power gain for a stable power amplifier is discussed and analytically derived. Finally, the parasitic-parametric effect due to the nonlinear collector capacitance and measures for its cancellation in practical power amplifier are discussed.

## 1.1 Spectral-Domain Analysis

The best way to understand the electrical behavior of a power amplifier and the fastest way to calculate its basic electrical characteristics like output power, power gain, efficiency, stability, or harmonic suppression is to use a spectral-domain analysis. Generally, such an analysis is based on the determination of the output response of the nonlinear active device when applying the multi-harmonic signal to its input port, which analytically can be written in the form of

where i(t)