Electrical & Electronic Engineering, Imperial College London
Doi
Abstract
Millimeter-wave communications have experienced phenomenal growth in recent
years when limited frequency spectrum is occupied by the ever-developing communication
services. The power amplifier, as the key component in the transmitter/receiver module
of communication systems, affects performance of the whole system directly and receives
much attention.
For minimized distortion and optimum system performance, the non-constant en-
velope modulation schemes used in communication systems have challenging requirements
on linearity. As linearity is related to communication quality directly, several linearization
techniques, such as predistortion and feedforward, are applied to power amplifier design.
Predistortion method has the advantages over other techniques in relatively simple struc-
ture and reasonable linearity improvement. But current predistortion circuits have quite
limited performance improvement and relatively large insertion loss, which indicate the
need for further research. In most of millimeter-wave amplifier design, great effort has
been spent on output power or gain, while linearity is often ignored. As almost all the
predistortion circuits operate at the RF frequencies, the linearized millimeter-wave com-
munication circuit is still relatively immature and very challenging.
This project is dedicated to solve the linearity problem faced by millimeter-wave
power amplifier in communication systems, which lacks of e®ective techniques in this field.
Linearity improvement with the predistortion method will be the key issue in this project
and some original ideas for predistortion circuit design will be applied to millimeter-wave
amplifiers.
In this thesis, several predistortion circuits with novel structure were proposed,
which provide a new approach for linearity improvement for millimeter-wave power am-
plifier. A millimeter-wave power ampli¯er for LMDS applications built on GaAs pHEMT
technology was developed to a high engineering standard, which works as the test bench
for linearization. Actual operation and parasitic elements at tens of gigahertz have been
taken into consideration during the design.
Firstly, two novel predistorter structures based on the amplifier were proposed, one
is based on an amplifier with a fixed bias circuit and the other is based on an amplifier with
a nonlinear signal dependant bias circuit. These novel structures can improve the linearity
while improving other metrics simultaneously, which can effectively solve the problem of
insertion loss faced by the conventional structures. Besides this, an original predistortion
circuit design methodology derived from frequency to signal amplitude transformation was
proposed. Based on this methodology, several transfer functions were proposed and related
predistortion circuits were built to linearize the power amplifier. As this methodology is
quite different from the traditional approach, it can improve the linearity signifficantly
while other metrics are affected slightly and has a broad prospect for application