Design and Characterization of a 6 W GaN HEMT Microwave Power Amplifier with Digital ...

175 Pages·2011·18.58 MB·English

by  Dragan Mitrevski

Design and Characterization of a 6 W
GaN HEMT Microwave Power Amplifier
with Digital Predistortion Linearization
Dragan Mitrevski
Master of Science in Electronics
Submission date: March 2011
Supervisor: Morten Olavsbråten, IET
Norwegian University of Science and Technology
Department of Electronics and Telecommunications

Problem Description
The activity around the subject of power amplifiers have increased considerably in the last decade,
both in companies developing radio equipment for wireless services, and in a number of
universities and research centers around the world. A power amplifier is of great economic and
technical importance, because it accounts for a significant portion of the costs in both development
and production of radio equipment, and is a very critical component in today's wireless world. In
portable devices, it is the power amplifier that drains the battery the most, and for both hand-held
devices and base station equipment (infrastructure) the power amplifier must satisfy several
stringent requirements. Technically good power amplifier solutions will thus offer great
competitive advantages.
The main topic of this thesis is to characterize a 6 W GaN HEMT transistor from Cree Inc. through
load pull simulations and measurements, and implement digital predistortion for linearization.
Thus, the tasks will consist of:
- Be familiarized with simulation tools to perform nonlinear simulations.
- Be familiarized with class F/inverse class F power amplifier design techniques.
- Be familiarized with measurement equipment to perform load pull measurements.
- Implement and evaluate different nonlinear black-box methods for characterizing the nonlinear
behavior (AM/AM, AM/PM, etc) of the power amplifier in MATLAB.
- Implement routines for digital predistortion in MATLAB.
- Measure the linearity performance of different power amplifiers (AM/AM, AM/PM, ACPR, etc) and
compare the performance with and without linearization applied.
Assignment given: 06. October 2010
Supervisor: Morten Olavsbråten, IET

Dragan Mitrevski
Design and Characterization
of a 6 W GaN HEMT
Microwave Power Amplifier with
Digital Predistortion Linearization
Thesis for the degree of Master of Science
Trondheim, March 2011
Norwegian University of Science and Technology
Faculty of Information Technology, Mathematics
and Electrical Engineering
Department of Electronics and Telecommunications
NTNU
Norwegian University of Science and Technology
Master’s thesis
for the degree of master of science in Electrical Engineering
Faculty of Information Technology, Mathematics and Electrical Engineering
Department of Electronics and Telecommunications
© Dragan Mitrevski
Master’s thesis at NTNU, 2011
Abstract
In this thesis, characterization of a 6W GaN HEMT power amplifier for optimal
operating conditions through load pull simulations and measurements is investi-
gated. The purpose is to find source and load impedances to achieve for instance
maximum efficiency and maximum output power, and investigate whether the
simulated results can be replicated in a measurement setup.
Simulations show that when matching for maximum output power, a peak
output power of 13W is achieved, while in 1dB compression, an output power
of 11.40W together with a power added efficiency of nearly 67% is obtained, a re-
markable for a 6W device. Matching for maximum efficiency yields a peak power
added efficiency of 78% and a peak drain efficiency of 90%, however, in 1dB com-
pression these levels are 71% and 74%, respectively. Load pull measurements are
performed with a prototype design, however, due to problems with the software
controlling the measurement equipment, the tuning process had to be done man-
ually, which effectively limited the possibility of replicating the simulated results.
With manual tuning in coarse steps, two sets for semi–optimal operating con-
ditions are found. For the first set of impedances, a peak output power greater
than 11W, with a peak power added efficiency of 68% is obtained, while for the
second set of impedances, an output power of 7.40W with a peak power added
efficiency of 70% is achieved. The measured performance indicate that the simu-
lated results can be replicated once the measurement setup is working optimally.
In addition, the topics of black–box characterization and digital predistortion lin-
earizationofmicrowavepoweramplifiersareinvestigated. Microwavepowerampli-
fiers obtain maximum operational performance when operated close to the satura-
tion point. However, due to the nonlinear behavior of the device when operated in
this region, the output signal will be distorted and cause interference with neigh-
boring channels. Thus, in order to maintain the high operational performance
and avoid adjacent channel interference, it is of great importance to characterize
the nonlinear behavior of the device, and compensate for the introduced distor-
tion terms. Digital predistortion linearization is considered to be among the most
cost–effective linearization schemes as of today, providing flexibility and good per-
formance. The design of a digital predistortion unit amounts to three main steps:
deriving a behavioral model of the microwave power amplifier, estimating the
model parameters for the inverse characteristic, and the implementation of the
preinverse filter. For behavioral modeling, well–known nonlinear models, such as
theVolterraseriesareinvestigated,andusedwithanindirectlearningarchitecture
to estimate the parameters of the inverse system.
Simulations of the different linearization algorithms show that in an ideal envi-
ronment,theadjacentchannelpowerratiocanbereducedwithupto25dB,almost
independent of the model used for the predistorter design. However, when used in
a real measurement setup, a maximum reduction in adjacent channel power ratio
of only 15dB is obtained. The relatively large difference in performance is most
likely caused by a combination of long term memory effects due to semiconductor
trapping phenomena, noise, and modeling errors.

Preface
This thesis is submitted in partial fulfillment of the requirements for the degree of master
of science (MSc) at the Department of Electronics and Telecommunications, Norwegian
University of Science and Technology (NTNU). The work was carried out in the pe-
riod October 2010 to March 2011, under the supervision of Associate Professor Morten
Olavsbråten, who is with the Department of Electronics and Telecommunications at
NTNU.
Acknowledgment
First and foremost I would like to thank my supervisor Morten Olavsbråten, both for
introducing me, and giving me the opportunity to work with the topic of RF power am-
plifiers. His vast, and seemingly never–ending insight within the topic, combined with
his enthusiasm, have been a great source for both motivation and inspiration during
this work. I would also like to thank head engineer Terje Mathiesen for all his help
with the production of the PCBs, together with Tore Landsem and Tore Berg from the
engineering workshop for cutting heat sinks, and other mechanical structures that I’ve
asked for. Finally, special thanks to my friend and fellow student Hans Herman Hansen
for valuable input on my thesis - and to the rest of my friends I’ve made during my time
at NTNU for five unforgettable years.
Trondheim, Norway, March 2011
Dragan Mitrevski
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