The experimental design and characterisation of Doherty power amplifiers
Date
2006-12
Authors
Brand, Konrad Frederik
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
Abstract
Modern day digital modulation techniques in communication systems produce large peak-to-average
ratios. To maintain linearity, power amplifiers have to operate at backed-off levels. This results in low
efficiency with consequences such as high power consumption, short battery life and excessive heat
in power amplifiers. A Doherty amplifier is an efficiency enhancement technique which increases an
amplifier’s efficiency at backed-off levels.
This thesis presents a design procedure for a Classical Doherty amplifier. A method where Sparameter
measurements from a transistor are used to predict the transistor’s transmission phase
response for varying input power is presented. This method is found to be accurate by comparing it
to measurements done on a non-linear network analyser. The measured S-parameters are also
used to design the Doherty amplifier at its predicted peak output power.
Two Classical Doherty amplifiers are designed, manufactured and characterised. The
measurements are performed on a custom measurement setup using in-house developed Matlab
code to automate the measurements. The first Doherty amplifier used small-signal Siemens CFY30
GaAs FETs and the second Doherty amplifier used 10W Motorola MRF282 LDMOS transistors. The
performance of both amplifiers is compared to similar balanced amplifiers and shows improvements
in their efficiency.
The improvement in efficiency for the 10W Doherty power amplifier in relation to a balanced amplifier
is compared to results found in the literature and a good correspondence between the measured
and published results were obtained.
Description
Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006.
Keywords
Dissertations -- Electronic engineering, Theses -- Electronic engineering, Amplifiers, Radio frequency, Power amplifiers