Efficiency Enhancement of Linear GaN RF power Amplifiers Using

194 Pages·2008·7.28 MB·English

Asdesach Zena Markos

Efficiency Enhancement of Linear GaN RF power
Amplifiers Using the Doherty Technique
This work has been accepted by the faculty of electrical engineering / computer science of the University of
Kassel as a thesis for acquiring the academic degree of Doktor der Ingenieurwissenschaften (Dr.-Ing.).

Supervisor: Prof. Dr.-Ing. G. Kompa
Co-Supervisor: Prof. Dr.-Ing. A. Bangert

Defense day: 06th November 2008

Bibliographic information published by Deutsche Nationalbibliothek
The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie;
detailed bibliographic data is available in the Internet at http://dnb.d-nb.de.

Zugl.: Kassel, Univ., Diss. 2008
ISBN print: 978-3-89958-622-0
ISBN online: 978-3-89958-623-7
URN: urn:nbn:de:0002-6235

© 2009, kassel university press GmbH, Kassel
www.upress.uni-kassel.de

Printed by: Unidruckerei, University of Kassel
Printed in Germany
Dedicated to my parents
for their love, devotion and prayers.

Acknowledgments

I wish to express my sincere gratitude to Prof. Dr. -Ing. G. Kompa, for
giving me the opportunity to carry out this research work at the Department
of High Frequency Engineering (HFT), University of Kassel. His
intellectual supervision during the course of this work has been inspiring.
Special thanks goes to TARGET (Top Amplifier Research Group in a
European Team) for funding part of this research work. Particularly, I
would like to thank Prof. P. Colantonio, Prof. F. Giannini and Dr. R. Giofrè
of the Department of Electronic Engineering, University of Rome, “Tor
Vergata”, Italy. Their cooperation and supervision during my stay at their
department was tremendous.
I am indebted to Prof. Dr. -Ing. A. Bangert, for accepting to be a second
examiner and also to the members of the disputation committee Prof. Dr.
rer. nat. K. - J. Langenberg and PD Dr. -Ing. R. Marklein.
My gratefulness is directed to Mr. S. Dahmani, Mr. S. Embar, Mr. R. Ma,
Dr. -Ing. E. S. Mengistu, Mr. S. Monsi, Mrs. H. Nauditt, Dipl. -Ing. J.
Weide, Dipl. -Ing. B. Wittwer, Mr. A. Zamudio, and all other members of
the HFT who are not mentioned by name. Their kind help at the office and
in the laboratories is highly appreciated.
I am thankful to Mr. Beyene Aleme, Dr. Kassahun A. Belay, Dipl. -Ing.
Dawit Negash, Dr. Eva Rau, family members and friends for their
unreserved encouragement and support over the past several years.

Asdesach Zena Markos
Kassel, November 2008

Contents

Chapter 1: Introduction 1
1.1 Challenges in Power Amplifier Design..……….……………………………...2
1.2 Average Efficiency…………………….……………………………………....3
1.3 Thesis Organization………………….………………………………………...6
Chapter 2: AlGaN/GaN HEMT Modeling 8
2.1 GaN Material Properties……….………………………………………………8
2.1.1 AlGaN/GaN HEMT…………………………………………………….11
2.1.2 Performance Limiting Factors………………………………………….13
2.2 Device Modeling Approaches.....…………………………………………….14
2.3 Small-Signal Modeling………….…………………………………………....17
2.3.1 Extrinsic Parameter Extraction…………………………………………18
2.3.2 Intrinsic Parameter Extraction………………………………………….21
2.4 Large-Signal Modeling……………………………………………………….26
2.4.1 Gate Current and Charge Models……………………………………….28
2.4.2 Drain Current Model……………………………………………………29
2.4.3 Model Verification……………………………………………………...34
2.4.3.1 Pulsed I(V) Characteristics…………………………………….35
2.4.3.2 Output Power and Efficiency………………………………….35
2.4.3.3 Intermodulation Distortion Prediction…………………………36
Chapter 3: Linearity and Efficiency in Power Amplifiers 38
3.1 Class of Operation..…………………………………………………………..39
3.1.1 Conventional Amplifiers.………….……………..………………….…39
3.1.2 Harmonic Tuning Techniques.….………..…………………………….44
3.2 Nonlinearity and Memory Effect …………………………………………..49
3.2.1 Harmonics, AM/AM and AM/PM………………..………………….…49
3.2.2 Intermodulation Distortion....……………..…………………………….50
3.2.3 Adjacent Channel Power Ratio………..……………………………..…52
3.2.4 Memory-Effects...………………..……………………………………..53
3.3 Efficiency Enhancement Techniques…..…………………………………….55
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Chapter 4: Single-Stage GaN Power Amplifier Design 59
4.1 2W Class-AB GaN Power Amplifier Design……..…………………………...60
4.1.1 Matching Network Design………….…………………………………..61
4.1.1.1 Output Matching …….…………..……………………… ….61
4.1.1.2 Input Matching………....………………………………...……67
4.1.2 Bias Network Design…………………………………………………..69
4.1.3 Performance Evaluation..……..………………………………………..72
4.1.3.1 Single-Tone Power Sweep…………………………………….74
4.1.3.2 Two-Tone Test.….………...…..………...….………………...76
4.1.3.3 W-CDMA Characterization.……..……………………………79

4.2 3W Class-F GaN Power Amplifier Design ...…..……………………………80
...
4.2.1 Output Matching Network……….....…...…….…………… ……...…82
4.2.2 Input Matching Network………..………….……………………………83
4.2.3 Waveform Simulations ....…....….……………………………………...85
4.2.4 Performance Evaluation .………….…………………………………….86
4.2.4.1 Output Power and Efficiency……………………………….…87
4.2.4.2 Linearity Characterization……………………………………..89
Chapter 5: Doherty Amplifier Design 92
5.1 Doherty Operation………..…………………………………………………...92
5.1.1 RF Characteristics…...……………………………………………….…100
5.1.2 Implementation Technique…………………...………………………...104
5.1.2.1 Uneven Doherty Technique………………………………….106
5.1.2.2 Practical Considerations……………………………………..110
5.1.3 Linearity of Doherty Amplifier…………………...……………………112
5.2 4W GaN Doherty Amplifier Design…….…………………………………..115
5.2.1 Main Amplifier……….…………...……….……..….………………….117
5.2.2 Peaking Amplifier……….…………….…..……………………...……120
5.2.3 Power Divider and Combiner……….….….…………………………...122
5.2.3.1 Uneven Power Divider………………………………………122
5.2.3.2 Combiner…………………………………………………….124
5.2.4 Load Modulation…………………...…………………………………..125
5.2.5 Performance Evaluation…………...…………………………………...126
5.2.5.1 Output Power and Efficiency ……………………………...128
5.2.5.2 Linearity Characterization...…………………………………131
5.3 7W GaN Doherty Power Amplifier Design…….…………………………..133
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5.3.1 Design and Simulation……….…………………………………………134
5.3.2 Performance Evaluation……...………………………………………...138
5.3.2.1 Output Power and Efficiency… ………………………...139
5.3.2.2 Linearity Characterization …………………………………142
Chapter 6: Conclusion and Further Work 145
Appendices 150
A. Measurement Setups……………………………………………………....150
A.1 Time Domain Measurement Setup.....………………………………150
A.2 Frequency Domain Measurement Setup..…………...………………151
B. Switch Mode Amplifier....……………………..………………………….153
B.1 Class-D Amplifier..…………………………………………………153
B.2 Class-E Amplifier..…………………………………………………154
C. Lumped Element Characterization .………………………………………156
D. Schematics Circuits of Designed Power Amplifiers…..………….……….158
References 163
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List of Figures

Figure 1.1 Power loss in a 10W amplifier as function of its efficiency……….....…….4
Figure 1.2 Efficiency of an ideal class-B amplifier as a function of output
power back-off……………………...……………………………….…..….4
Figure 1.3 Illustration of average efficiency in a single-stage amplifier
driven with variable envelope signal with 8.5 dB PAR …………….…..….5
Figure 2.1 Basic AlGaN/GaN HEMT structure………………….………………..….11
Figure 2.2 AlGaN/GaN HEMT on Si…..……………………………………………..12
Figure 2.3 Large-signal table based model derivation process ………………………16
Figure 2.4 Photo of the investigated 2-mm (10 x 200 µm)
AlGaN/GaN on Si HEMT. ………………..….…………………..…....….17
Figure 2.5 A 22-element distributed model for a GaN HEMT.………………..….….17
Figure 2.6 Pinch-off S-parameter fitting …………….... …………………….......…..21
Figure 2.7 Extracted C , G C and C ………………...…..…………………....…22
gs m, gd ds
Figure 2.8 Extracted G R, τ, G G and R ………………………………..…....24
ds, i gdf, gsf gd
Figure 2.9 Extracted C as a function of the intrinsic voltages…………………..…..25
gs
Figure 2.10 Comparison of measured and simulated S-parameters.…………………..25
Figure 2.11 Large-signal model topology for AlGaN/GaN HEMT..…..….………..…26
Figure 2.12 Extracted gate current sources...…………………………….……….…...29
Figure 2.13 Extracted charge sources………………….. …………………….….…...29

Figure 2.14 Pulsed I(V) measurements to characterize surface trapping………….…..30
Figure 2.15 Pulsed I(V) measurements to characterize buffer trapping………….. …..31
Figure 2.16 Pulsed I(V) measurements with a 10Ω serious stabilization resistor ….…32
Figure 2.17 Pulsed I(V) measurements to characterize self heating effect ……….…..32
Figure 2.18 Extracted bias-dependent fitting parameters to model the drain current…33
Figure 2.19 Comparison of measured (lines) and simulated (symbols) S-parameters...34
Figure 2.20 Pulsed I(V) simulations (lines) and measurements (symbols) …….….….35
Figure 2.21 Single-tone power sweep simulations and measurements (class-AB )..... .36
Figure 2.22 Single-tone power sweep simulations and measurements (class-C)….......36
Figure 2.23 Two-tone measurement and simulation (f = 2.14 GHz, ∆f = 200 kHz).....37
0
Figure 3.1 Load-lines of various class of operations...……………..……...…..…...…39
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