Doherty Power Amplifiers

From Fundamentals to Advanced Design Methods

Bumman Kim

• Publisher: Academic Press
• Year: 2018
• Print ISBN: 9780128098677
• eBook ISBN: 9780128098752
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Doherty Power Amplifiers: From Fundamentals to Advanced Design Methods is a great resource for both RF and microwave engineers and graduate students who want to understand and implement the technology into future base station and mobile handset systems. The book introduces the very basic operational principles of the Doherty Amplifier and its non-ideal behaviors. The different transconductance requirements for carrier and peaking amplifiers, reactive element effect, and knee voltage effect are described. In addition, several methods to correct imperfections are introduced, such as uneven input drive, gate bias adaptation, dual input drive and the offset line technique.Advanced design methods of Doherty Amplifiers are also explained, including multistage/multiway Doherty power amplifiers which can enhance the efficiency of the amplification of a highly-modulated signal. Other covered topics include signal tracking operation which increases the dynamic range, highly efficient saturated amplifiers, and broadband amplifiers, amongst other comprehensive, related topics.- Specifically written on the Doherty Power Amplifier by the world's leading expert, providing an in-depth presentation of principles and design techniques- Includes detailed analysis on correcting non-ideal behaviors of Doherty Power Amplifiers- Presents advanced Doherty Power Amplifier architectures

Chapter One

Introduction to Doherty Power Amplifier

Abstract

The most important property of the Doherty amplifier is the load modulation, which carries out the perfect combining of the asymmetrical powers from the two amplifiers. Thereby, only one amplifier (called carrier amplifier) operates at a low power level, and the two amplifiers (the second one is called peaking amplifier) generate powers at a higher power level. Also the carrier amplifier operates at the peak efficiency mode, while the peaking amplifier is operational due to the nice load modulation characteristic. Another important characteristic of the Doherty load modulation is that the overall gain of the amplifier is constant, providing a linear amplification. The Doherty circuit can modulate the real part of impedance only. To get the proper load modulation for a device with a complex impedance, an offset line is needed to provide the imaginary impedance transformation. The load modulation principle and the offset line operation are discussed. Several load modulation methods are also described.

Keywords

Load modulation; Offset line; Voltage-combined Doherty amplifier; Current-combined Doherty amplifier; Efficiency; Gain; Inverted Doherty amplifier; Transformer

1.1 Historical Survey

William H. Doherty was an American electrical engineer, best known for his invention of the Doherty amplifier. Doherty was born in Cambridge, Massachusetts, in 1907. He attended Harvard University, where he received his bachelor's degree in communication engineering (1927) and his Master's degree in engineering (1928). Doherty joined Bell Labs in 1929. At the Bell Labs., he worked on the development of high-power radio transmitters, which were used for transoceanic radio telephones and broadcastings.

Doherty invented this unique amplifier approach in 1936 using a vacuum tube amplifier. This new device greatly improved the efficiency of RF power amplifiers and was first used in a 50 kW transmitter that Western Electric Company designed for WHAS, a radio station in Louisville, Kentucky. Western Electric went on to incorporate Doherty amplifiers into, at least, 35 commercial radio stations worldwide by 1940 and many other stations, particularly in Europe and Middle East in the 1950s. Within the Western Electric, the device was operated as a linear amplifier with a driver that was modulated. In the 50 kW implementation, the driver was a complete 5 kW transmitter that could, if necessary, be operated independently of the Doherty amplifier, and the Doherty amplifier was used to raise the 5 kW level to the required 50 kW level.

As a successor to Western Electric Company Inc. for radio broadcast transmitters, the Continental Electronics Manufacturing Company at...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Acknowledgments
• Chapter One: Introduction to Doherty Power Amplifier
• Chapter Two: Realization of Proper Load Modulation Using a Real Transistor
• Chapter Three: Enhancement of Doherty Amplifier
• Chapter Four: Advanced Architecture of Doherty Amplifier
• Chapter Five: Linear Doherty Power Amplifier for Handset Application
• Index