Msc_thesis_XDeng.
Wideband Doherty Power Amplifier Design
for Base Station Application
Thesis submitted to the Office of Graduate Studies of
Delft University of Technology
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
in
Microelectronics
By
Xiaobo Deng
Student Number: 1530445
Supervisors:
Associate Prof. Dr. ing. Leo de Vreede, TU Delft
John Gajadharsing, NXP Semiconductors
Dr. Edmund Neo, NXP Semiconductors
August 2010
ii
Abstract
Doherty Power Amplifier (DPA) is employed to improve the efficiency when operated
with complex modulated signals. Due to its simplicity and high efficiency performance, it has
become the preferred choice of industry. However, practical implementations of DPA only
provide limited RF bandwidth, especially at high power level. The traditional narrow band
device matching network, required phase shift for proper load modulation and impedance
inverter seriously limit the bandwidth of DPA.
In this work, the frequency behavior of the ideal 2-way symmetrical DPA is analyzed
in detail, followed by the introduction of two new impedance inverters used to improve the
bandwidth of DPA. In order to fully exploit the wideband potential of the new impedance
inverters, the phase relation between the main and peak amplifier should be adjusted
according to the power level at every frequency, which can be stored in a lookup table. Based
on a previous wideband 20W DPA with mixed-signal input drive, in which the device output
capacitance is incorporated into the impedance inverter, a modified DPA using the idea of
compensated impedance inverter is designed and simulated. The prototype DPA design is
implemented with NXP LDMOS bare die device. Simulation results have shown more than
50% 6dB back off efficiency from 1.5GHz to 2.2GHz, compared with the original case whose
50% efficiency bandwidth is from 1.9GHz to 2.3GHz.
Since the prototype DPA is implemented at a low power level with bare die devices
and mixed-signal input drive, it cannot be used for the practical base station applications.
Traditional high power discrete DPA design method is introduced and the frequency behavior
is analyzed. It is found that for the high power DPA, the matching network and the offset line
are more important than the impedance inverter for the narrow bandwidth of DPA. A new
DPA structure was proposed for wideband operation. Simulation results show smaller gain
and power added efficiency spread in a 200MHz frequency band from 2.04GHz to 2.24GHz
than the traditional DPA.
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iv
Acknowledgments
Completion of this project and this thesis will be an unforgettable experience in my life. I
have not only gained a lot of knowledge about power amplifier design, but more importantly,
I have learnt how to do a project in a structured way with a great deal of patience, through
the opportunity to face and overcome the challenges. There is no doubt that this would not
have been possible without the support of a great number of people, of whom, I can mention
only a few here.
First of all, I would like to express my deep-felt gratitude to my supervisor, Dr. Leo de
Vreede, for giving me this opportunity to do such a project about Doherty power amplifier.
I wish to thank him for his encouragement, guidance and valuable discussion throughout my
master project.
I am also deeply grateful to Mr. John Gajadharsing, Senior Principle Engineer, NXP
Semiconductors, for sponsoring this project and the inspiring discussion during the project.
As my daily mentor at NXP, Dr. Edmund Neo has offered me great help from the beginning
to the end of this project. He has shown great patience during the discussion with me. My
heartfelt thanks to him.
I would like also to thank all the other staff in the base station group of NXP Semiconductors. Many thanks to Ji Zhao and Yilong Shen, not only for helping me get used
to the company life, but also for their valuable discussion. Special thanks to Igor Blednov,
Jordan Svechtarov, Komo Sulaksono and Lex Harm who sit in the same room with me for
teaching me a lot about power amplifier design. Besides, I will never forget the discussion
about life, money or even Kalashnikov with Igor Blednov. Jordan Svechtarov is a very good
teacher, who is always very kind for discussion. Through his support and encouragement, I
have proposed a patent during my project. Sjoerd van Nederveen and Ton Schouten have
helped me prepare the PCB, and Jawad Qureshi and Marco Pelk have helped me during the
measurement. I would like to thank them very much.
It is not easy to live in a foreign country, especially when you are from a country with
totally different culture. I wish to acknowledge and give my appreciation to all my friends in
Delft and Nijmegen, without whom, I cannot imagine how boring my life will be in the Netherlands. In Delft, the big Chinese community helped me overcome my homesickness although
it is not good to improve my English. I will miss the parties we had after the exam or for the
Chinese festivals. Special thanks to the senior students, Xiaodong Guo, Guang Ge, Nan Li
and Bo Wu who are also from USTC, for giving me so many pieces of useful advice. I have
also been lucky to had many friends in Nijmegen and I will miss the enjoyable time with them.
Finally, I would like to acknowledge my parents, my sister and brother-in-law, my girlfriend Li Shen and my friends in China for their continuous support and love. Their encouragements have strengthened my confidence to face the difficulties during my master program.
I appreciated everything you have done for me.
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Contents
Abstract
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Acknowledgments
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1 Introduction
1.1 Background . . . . . . . . . . . . . .
1.2 Wireless Transceiver . . . . . . . . .
1.3 Figures of Merit of Power Amplifier
1.4 Design Challenges and Project Goals
1.5 Thesis Structure . . . . . . . . . . .
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1
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2 Theory of the Conventional and Doherty Power Amplifier
2.1 Conventional Power Amplifier . . . . . . . . . . . . . . . . . .
2.1.1 Device model . . . . . . . . . . . . . . . . . . . . . . .
2.1.2 Conventional power amplifier waveform analysis . . .
2.1.3 Ideal Class-B efficiency analysis . . . . . . . . . . . . .
2.2 Introduction to EER, LINC and DPA . . . . . . . . . . . . .
2.3 Doherty Power Amplifier . . . . . . . . . . . . . . . . . . . . .
2.3.1 Load modulation of DPA . . . . . . . . . . . . . . . .
2.3.2 Ideal DPA characteristics . . . . . . . . . . . . . . . .
2.3.3 Simulation results . . . . . . . . . . . . . . . . . . . .
2.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3 Frequency Behavior of the Ideal Doherty Power Amplifier
3.1 Frequency Behavior of an Ideal 2-way Symmetrical DPA . . . . . . . .
3.2 Two New Impedance Inverters . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Impedance transformation effects of three impedance inverters
3.2.2 New impedance inverter parameters tuning . . . . . . . . . . .
3.3 DPA with Different Impedance Inverters Bandwidth Comparison . . .
3.3.1 Main and peak amplifier impedance spread with frequency . .
3.3.2 Efficiency bandwidth comparison . . . . . . . . . . . . . . . . .
3.3.3 Average efficiency of DPAs with different impedance inverters .
3.3.4 Phase correction to improve output power . . . . . . . . . . . .
3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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4 A Wideband 20W LDMOS Doherty Power Amplifier
4.1 Previous 20W Doherty Power Amplifier Review . . . . .
4.1.1 π Network quasi-QWTL . . . . . . . . . . . . . .
4.1.2 Bias line . . . . . . . . . . . . . . . . . . . . . . .
4.1.3 Peak amplifier connection . . . . . . . . . . . . .
4.1.4 DPA performance . . . . . . . . . . . . . . . . .
4.2 Modified 20W Doherty Power Amplifier . . . . . . . . .
4.2.1 Compensated quasi-QWTL . . . . . . . . . . . .
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5 High Power Doherty Power Amplifier Bandwidth Discussion
5.1 High Power Doherty Amplifier Design . . . . . . . . . . . . . . . . . . . . .
5.1.1 Device characterization . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.2 Matching network . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.3 Offset line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.4 Doherty power amplifier performance . . . . . . . . . . . . . . . . .
5.2 Main Amplifier Frequency Behavior Analysis . . . . . . . . . . . . . . . . .
5.2.1 Single TL matched Class-AB with ideal load modulated impedance .
5.2.2 Wideband matched Class-AB with ideal load modulated impedance
5.3 The Effect of the Peak Amplifier Offset Line . . . . . . . . . . . . . . . . . .
5.4 Proposed Bandwidth Improved DPA topology . . . . . . . . . . . . . . . . .
5.4.1 DPA output matched to different values . . . . . . . . . . . . . . . .
5.4.2 Proposed wideband DPA topology . . . . . . . . . . . . . . . . . . .
5.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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6 Conclusions and Recommendations
6.1 Conclusions . . . . . . . . . . . . .
6.2 Recommendations . . . . . . . . .
6.2.1 RF power technologies . . .
6.2.2 Integrated DPA . . . . . . .
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4.3
4.4
4.2.2 Modified compensated quasi-QWTL
4.2.3 Directly adding compensation line to
4.2.4 New output network . . . . . . . . .
4.2.5 Modified 20W DPA performance . .
Layout and Measurement Results . . . . . .
Conclusion . . . . . . . . . . . . . . . . . .
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amplifier
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A DPA bandwidth varies with power
81
B Coupled Line Impedance Transformer
B.1 Coupled Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B.2 Coupled Line with Feedback Transmission Line . . . . . . . . . . . . . . . . .
B.3 Alternative Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Bibliography
89
List of Figures
1.1
1.2
Block diagrams of a generic transmitter and receiver . . . . . . . . . . . . . .
Generic power amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2
2.1
2.2
2.3
Ideal device transfer characteristic . . . . . . . . . . . .
Reduced conduction angle current waveform . . . . . . .
Optimum load impedance (a), RF power and efficiency
conduction angle . . . . . . . . . . . . . . . . . . . . . .
Envelop PDFs of differently modulated RF signals . . .
EER system configuration . . . . . . . . . . . . . . . . .
LINC system configuration . . . . . . . . . . . . . . . .
DPA system configuration . . . . . . . . . . . . . . . . .
Simplified DPA configuration . . . . . . . . . . . . . . .
Ideal 2-way symmetrical DPA characteristics . . . . . .
5
6
2.4
2.5
2.6
2.7
2.8
2.9
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
3.13
3.14
3.15
3.16
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(b) as
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a function of
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Efficiency Vs. frequency of DPA at different input drive amplitudes . . . . .
Real and imaginary part of Zm Vs. normalized frequency . . . . . . . . . . .
Coupled line impedance inverter . . . . . . . . . . . . . . . . . . . . . . . . .
Compensated impedance inverter . . . . . . . . . . . . . . . . . . . . . . . . .
Impedance transformation effects comparison schematic of 3 different
impedance inverters(Up to down: QWTL, Coupled line, Compensated
impedance inverters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Impedance transformation of 3 different impedance inverters with different
effective load impedance(RL =0.5Ω, 1Ω) . . . . . . . . . . . . . . . . . . . . .
Impedance transformation effects comparison schematic of 3 different
impedance inverters(Up to down: QWTL, Coupled line, Compensated
impedance inverters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Impedance transformation effects of 3 different impedance inverters at back off
(6dB back off, when Im =0.5 and Ip =0) . . . . . . . . . . . . . . . . . . . . . .
Impedance transformation effects of 3 different impedance inverters between
back off and full power (3.1dB back off, when Im =0.7 and Ip =0.4) . . . . . .
Flattest impedance transformation of 3 different impedance inverters between
back off and full power (3.1dB back off, when Im =0.7 and Ip =0.4) . . . . . .
Flattest impedance transformation of 3 different impedance inverters between
back off and full power (0.46dB back off, when Im =0.9 and Ip =0.8) . . . . . .
Flattest impedance transformation of 3 different impedance inverters at full
power (Im =1 and Ip =1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Performance of compensated impedance inverters with different Zsh compared
with QWTL impedance inverter . . . . . . . . . . . . . . . . . . . . . . . . .
DPA with LC compensated impedance inverters . . . . . . . . . . . . . . . .
Impedance transformation of LC compensated impedance inverter compared
with QWTL impedance inverter(Red: QWTL when RL =0.5; Black: QWTL
when RL =1; Blue: LC compensated inverter when RL =0.5; Magenta: LC
compensated inverter when RL =1) . . . . . . . . . . . . . . . . . . . . . . . .
QWTL transformation effect . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3.17 Compensated impedance inverter transformation effect . . . . . . . . . . . . .
3.18 Impedance seen by the main amplifier of DPAs with different impedance inverters at different frequencies(from f0 to 1.4f0 with a step of 0.1f0 ) . . . . .
3.19 Impedance seen by the peak amplifier of DPAs with different impedance inverters at different frequencies(from f0 to 1.4f0 with a step of 0.1f0 ) . . . . .
3.20 Eff and Pout Vs. Freq of DPA with 3 different impedance inverters when drive
voltage=0.5, 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.21 Eff Vs. Pout @ 1.2f0 (Red line: QWTL, Magenta line: Coupled line, Blue line:
Compensated) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.22 WCDMA signal PDF and average efficiency of DPA with different impedance
inverters at different frequencies calculated with this PDF . . . . . . . . . . .
3.23 Impedance seen by the main and peak amplifier of DPAs with different
impedance inverters and optimized phase at different frequencies(from f0 to
1.4f0 with a step of 0.1f0 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.24 Efficiency of DPA with two new impedance inverters before(blue line) and
after(red line) phase optimization at 1.2f0 . . . . . . . . . . . . . . . . . . . .
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
4.11
4.12
4.13
4.14
4.15
4.16
4.17
4.18
4.19
Simplified block diagram of the 20W NXP Gen6 wideband DPA . . . . . . .
Simplified output network of 10W bare die LDMOS device output . . . . . .
π-network quasi QWTL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Original QWTL and quasi-QWTL . . . . . . . . . . . . . . . . . . . . . . . .
Drain efficiency versus output power at different frequencies for the original
wideband 20W DPA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Drain efficiency of the original 20W DPA at 6dB back off and full power at
different frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Compensated quasi-QWTL schematic . . . . . . . . . . . . . . . . . . . . . .
Compensated quasi-QWTL . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Impedance transformation of original QWTL and quasi-QWTL(Blue line: original QWTL; Red line with circles: quasi-QWTL; Magenta line: compensated
quasi-QWTL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LDMOS with 28V supply voltage loss mechanism . . . . . . . . . . . . . . . .
Shunt short circuited QWTL’s impedance . . . . . . . . . . . . . . . . . . . .
Modified shunt short circuited TL’s impedance . . . . . . . . . . . . . . . . .
Impedance transformation of original QWTL and quasi-QWTL(Blue line: original QWTL; Red line with circles: quasi-QWTL; Magenta line: modified compensated quasi-QWTL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The schematic of peak device with compensation line . . . . . . . . . . . . . .
Drain efficiency versus output power at different frequencies(Red line: original
DPA; Blue line: DPA with compensation line) . . . . . . . . . . . . . . . . . .
Original output network with compensation line(upper part) and new output
network(lower part) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
New output network microstrip line structure . . . . . . . . . . . . . . . . . .
New input matching network microstrip line structure . . . . . . . . . . . . .
Modified DPA efficiency versus output power compared with original DPA(Red
line: modified DPA; Blue line: original DPA) . . . . . . . . . . . . . . . . . .
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4.20 Drain efficiency versus output power at 6dB back off (a) and full power (b)
comparison between original and modified DPA; Peak output power of two
DPAs (c) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.21 Main amplifier loading impedances at back off and full power . . . . . . . . .
4.22 Layout of the modified DPA . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10
5.11
5.12
5.13
5.14
5.15
5.16
5.17
5.18
5.19
5.20
5.21
5.22
5.23
5.24
5.25
5.26
5.27
6.1
6.2
Conventional symmetrical 2-way distributed DPA schematic . . . . . . . . . .
DC characteristic of LDMOS BLF7G22LS-130 . . . . . . . . . . . . . . . . .
Load pull simulation result of LDMOS BLF7G22LS-130 . . . . . . . . . . . .
The network for which equation (5.1) applies . . . . . . . . . . . . . . . . . .
Quarter wavelength transformer matching bandwidth comparison for two
impedance levels(2.2Ω and 22Ω) . . . . . . . . . . . . . . . . . . . . . . . . .
Basics of an inshin output match . . . . . . . . . . . . . . . . . . . . . . . . .
Single TL and multi-TLs matching effect at output and input of the device .
Equivalent-load networks at a low-power operation for the DPAs without and
with offset line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Main amplifier with offset line power sweep at different load conditions . . .
Peak amplifier off-state impedance rotation by the offset line . . . . . . . . .
Ideal power splitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DPA performance from 2.11GHz to 2.17GHz . . . . . . . . . . . . . . . . . .
DPA performance compared with Class-AB at 2.14GHz . . . . . . . . . . . .
DPA performance from 1.94GHz to 2.34GHz . . . . . . . . . . . . . . . . . .
Simplified schematic of Class-AB amplifier with offset line, impedance inverter
and 25Ω load impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Performance of the Class-AB amplifier with offset line and 100Ω load impedance
Comparison between wideband and single TL matched Class-AB . . . . . . .
Performance of the wideband Class-AB amplifier with offset line and 100Ω load
impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Performance of the wideband output matched DPA from 2.04GHz to 2.24GHz
with a step of 0.05GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
S parameter looking into the peak amplifier from 2.04GHz to 2.24GHz . . . .
Power added efficiency of the DPA with different peak offset line length from
2.04GHz to 2.24GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power added efficiency of the DPA matched to different values from 2.04GHz
to 2.24GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DPA output matched to 25Ω without traditional QWTL impedance inverter .
The performance of the DPA in Figure 5.23 compared with the conventional
DPA from 2.04GHz to 2.24GHz (Red lines: 2.04GHz; Blue lines: 2.14GHz;
Magenta lines: 2.24GHz) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0.2dB asymmetry rat-race splitter and its performance . . . . . . . . . . . . .
The performance of the DPA in Figure 5.23 with rat-race power splitter . . .
The performance of the wideband input matched DPA in Figure 5.23 with rat
race power splitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
From discrete DPA to integrated DPA . . . . . . . . . . . . . . . . . . . . . .
Integrated DPA building blocks . . . . . . . . . . . . . . . . . . . . . . . . . .
51
52
53
55
56
57
58
59
60
61
61
62
63
64
64
64
65
65
66
67
67
68
69
69
70
71
72
73
74
74
79
79
xii
A.1 Simplified DPA configuration . . . . . . . . . . . . . . . . . . . . . . . . . . .
81
B.1 Coupled line. The port names are in agreement with directional coupler terminology for a backward-wave coupler, while the numeration is the same as that
used in filter design with coupled line sections. . . . . . . . . . . . . . . . . .
B.2 Coupled line with feedback transmission line. . . . . . . . . . . . . . . . . . .
B.3 Another configuration of coupled line with feedback transmission line. . . . .
83
84
87
List of Tables
2.1
Bias point and conduction angle of different classes . . . . . . . . . . . . . . .
3.1
3.2
3.3
3.4
3.5
3.6
A set of impedance inverter parameters . . . . . . . . . . . . . . . . . . . . .
3 impedance inverters’ transformation bandwidth at back off and full power level
Bandwidth of DPA with different impedance inverters . . . . . . . . . . . . .
Output power of DPA with different impedance inverters when Vdrive =1 . . .
Average efficiency of DPA with different impedance inverters . . . . . . . . .
Optimum phase of the peak amplifier with different impedance inverters for
maximum output power when Vdrive =1 . . . . . . . . . . . . . . . . . . . . . .
20
21
32
34
34
5.1
5.2
Load-pull simulation results summary . . . . . . . . . . . . . . . . . . . . . .
Load-pull measurement results summary . . . . . . . . . . . . . . . . . . . . .
57
58
6.1
Material properties of Si and GaN . . . . . . . . . . . . . . . . . . . . . . . .
78
xiii
6
34
xiv
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