RF Power Amplifier, 2nd Edition

Chapter 8Class F RF Power Amplifiers

8.1 Introduction

Class F radio-frequency (RF) power amplifiers [1–28] utilize multiple-harmonic resonators in the output network to shape the drain-to-source voltage such that the transistor loss is reduced and the efficiency is increased. These circuits are also called polyharmonic or multiresonant power amplifiers. The drain current flows when the drain-to-source voltage is flat and low, and the drain-to-source voltage is high when the drain current is zero. Therefore, the product of the drain current and the drain-to-source voltage is low, reducing the power dissipation in the transistor. This method of improving the efficiency is the oldest technique and was invented by Tyler in 1919 [1]. Class F power amplifiers with lumped-element resonant circuits tuned to the third harmonic or to the third and fifth harmonics have been widely used in high-power amplitude-modulated (AM) broadcast radio transmitters in the low-frequency (LF) range (30–300 kHz), the medium-frequency (MF) range (0.3–3 MHz), and the high-frequency (HF) range (3–30 MHz). Class F power amplifiers with a quarter-wavelength transmission line control all the odd harmonics and are used in very high-frequency (VHF) (30–300 MHz) frequency-modulated (FM) broadcast radio transmitters [1]. They are also used in ultrahigh frequency (UHF, 300 MHz–3 GHz) FM broadcast radio transmitters [3]. Dielectric resonators can be used in place of lumped-element resonant circuits. Output power ...