RF Power Amplifier, 2nd Edition

Chapter 4Class D RF Power Amplifiers

4.1 Introduction

Class D radio-frequency (RF) resonant power amplifiers [1–20], also called Class D dc–ac resonant power inverters, were invented in 1959 by Baxandall [1] and have been widely used in various applications [2–19] to convert dc energy into ac energy. Examples of applications of resonant amplifiers are radio transmitters, dc–dc resonant converters, solid-state electronic ballasts for fluorescent lamps, LED drivers, induction heating appliances, high-frequency electric heating applied in induction welding, surface hardening, soldering and annealing, induction sealing for tamper-proof packaging, fiber-optics production, and dielectric heating for plastic welding. In Class D amplifiers, transistors are operated as switches. Class D amplifiers can be classified into two groups:

• Class D voltage-switching (or voltage-source) amplifiers.
• Class D current-switching (or current-source) amplifiers.

amplifiers

Class D voltage-switching amplifiers are fed by a dc voltage source. They employ (1) a series-resonant circuit or (2) a resonant circuit that is derived from the series-resonant circuit. If the loaded quality factor is sufficiently high, the current through the resonant circuit is sinusoidal and the current through the switches is a half-sine wave. The voltage waveforms across the switches are square waves.

In contrast, Class D current-switching amplifiers are fed by a dc current source in the form of an RF choke (RFC) and a dc ...