Systematic Design of a Microwave GaN Doherty Power Amplifier by Kenneth Previn Samuel A ...

112 Pages·2017·5.26 MB·English

Systematic Design of a Microwave GaN Doherty Power Amplifier
by
Kenneth Previn Samuel
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
Graduate Department of Electrical and Computer Engineering
University of Toronto
(cid:13)c Copyright 2017 by Kenneth Previn Samuel
Abstract
Systematic Design of a Microwave GaN Doherty Power Amplifier
Kenneth Previn Samuel
Master of Applied Science
Graduate Department of Electrical and Computer Engineering
University of Toronto
2017
Doherty PAs are the standard for efficient transmission of amplitude modulated signals. However, its
designprocessvariestremendouslyacrossusecase,frequencyrange,powerrange,anddevicetechnology.
Due to this, the design procedure of a Doherty PA is often subjected to tuning and optimization. A
step-by-stepsystematicapproachindesigningaDohertyPAisintroducedinthisthesisasaconsequence
ofanovelmethodtomatchdevicestotheiroptimumterminations. Thedesignmethodologyintroduced
in this thesis can be applied to any use case irrespective of the operating conditions. To display the
functionalityofthisprocedure,amicrowaveGaNDohertyPAisdesignedfromsimulationsusingAgilent’s
Advanced Design System (ADS). It is then fabricated and measured for performance. Good agreement
between measured and simulated data is demonstrated.
ii
This work of my hands I dedicate to my beloved parents
Previn Inbaraj Samuel & Jessie Carolyne Samuel,
whose continued support, sacrifice, and unending love has led me to great heights.
To my grandmother
Victoria Devasundaram.
And to the memory of my late grandparents
Inbaraj Samuel & Premila Inbaraj Samuel,
& Raphael Devasundaram.
Unto God,
under whose steady and mighty hand I have humbled myself and overcome all things.
I waited patiently for the lord,
and he inclined unto me and heard my cry.
He drew me up from the pit of destruction,
out of the miry bog,
and set my feet upon a rock,
making my steps secure.
He put a new song in my mouth,
a song of praise to our God.
Many will see and fear the lord
and put their trust in him.
Psalm 40:1-3
iii
Acknowledgements
It would be foolish to believe that one’s accomplishments belong to themselves alone. Without friends,
family, mentors, or companions, I would be likened to wilderness that does not bring forth any fruit,
subject only to desolation and decay. However, I’ve been blessed enough to have all four in my
possession, and their waters have turned this wilderness into a garden that bears good fruit. If it
weren’t for the likes of these people, this feat of mine would not have come to pass.
First and foremost,
Prof.George V. Eleftheriades,
for believing in me and giving me this opportunity.
My Parents.
My sister Kathleen.
Rev.David Kalison, whose support, spiritual guidance, and efforts I will not forget.
My grandmother Victoria & my late grandparents.
My extended family who have been a constant source of strength, love, and guidance.
Especially my two uncles Melvin Samuel & Joshua Alexander, whom I’ve adored.
Elders in my family that have counseled me, especially Mercy Joy,
whose sacrifice has enabled generations hence.
My confidant Naif, for his counsel, advice and brotherly love.
My friends Sorooban, Nishant, Kajani, Swakhar, Vijith, Raj, Pradeep, Shalini, Sean, Vinny, Presanna
and Jordan. For your companionship through the years.
Ben Joyner, you inspired the fire in me.
Eman Assem, for your comforting words that have enabled me to see beyond limitations.
Trevor R. Cameron, whose constant companionship and friendship during my time here has enabled
me to breakthrough critical technical challenges associated with this work.
All my friends in the research group, whom I have shared great moments with in the past few years.
May there be many of them.
Dr.Khoman Phang and Prabhu Mohan for your support in my search for admission into this program.
Venrick Azcueta for photographing the work presented in this thesis.
The teachers that have inspired within me the curiosity and have nourished it.
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Contents
1 Introduction 1
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Matching for Load Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Known Bandwidth Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Proposed Design Overview & Goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4.1 Goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 Power Amplifier Theory 6
2.1 Power Amplifier. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.1 Ideal Non-Linear Device Current-Voltage Model . . . . . . . . . . . . . . . . . . . 6
2.1.2 Classes of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.3 Efficiency and Conduction Angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.4 Back-Off Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2 Load-pull . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3 Load Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.4 Doherty Power Amplifier. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3 Design Considerations of a Doherty Amplifier 20
3.1 Choosing a Power Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.2 Realizing a Doherty Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.2.1 Characteristic Impedance of the Transformer . . . . . . . . . . . . . . . . . . . . . 24
3.2.2 Auxiliary Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.3 Harmonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.4 Parasitics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.5 Load-pull for Doherty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.6 Output Impedance of the Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4 Design of a GaN Doherty Amplifier 39
4.1 Design Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.2 10W CREE GaN RF Power Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.3 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.4 Load-pull . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.5 Double Impedance Matching Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.6 Transformer & Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
v
4.7 Simulation of the Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.8 Bandwidth Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
5 Fabrication & Measurements 59
5.1 Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.1.1 Broadband Wilkinson Power Divider & Input Delay . . . . . . . . . . . . . . . . . 59
5.1.2 Source Matching Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.1.3 Output Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.2 Layout & List of Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.3 Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.4 Measurement Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6 Bandwidth of an Inverter 76
6.1 The Quarter-Wave Transmission Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
6.2 Group Delay Contributions of Poles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
6.3 π, T, and n-Stage Ladder Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6.4 Wheatstone Bridge or Lattice Phase Equalizer . . . . . . . . . . . . . . . . . . . . . . . . 90
6.4.1 The ABCD Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
6.5 Lattice Network as Impedance Inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
6.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
7 Conclusion 95
7.1 Recent Research Efforts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
7.2 Key Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
7.3 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
7.4 Closing Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Bibliography 100
vi
List of Figures
1.1 Typical matching solutions for devices used in PAs . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Ideal Non-linear Device Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Classification of amplifiers based on biasing. Drain current output driven by an input
signal v shown, for varying V bias points . . . . . . . . . . . . . . . . . . . . . . . . . . 8
in gs
2.3 Maximized drain current and voltage swing due to optimum load . . . . . . . . . . . . . . 9
2.4 Drain Efficiency and Output Power vs. Conduction Angle for optimum loads . . . . . . . 10
2.5 Efficiency restored in back-off operation when load resistance is increased . . . . . . . . . 11
2.6 load-pull Contour for pP Delivered Power . . . . . . . . . . . . . . . . . . . . . . . . . 12
max
2.7 Power delivered reduced due to sub optimum load impedance . . . . . . . . . . . . . . . . 13
2.8 Load modulation of common load R by a second source . . . . . . . . . . . . . . . . . . . 14
2.9 Load modulation with input impedance shifted by a transmission line . . . . . . . . . . . 15
2.10 Load modulation given by Eq.2.18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.11 Loadmodulationrequiredforoptimumefficiencyatbothback-offandpeakpoweroperation 17
2.12 Doherty Amplifier Voltage, Current, Power and Efficiency relations . . . . . . . . . . . . . 18
3.1 Performance limits of power devices by technology . . . . . . . . . . . . . . . . . . . . . . 21
3.2 Traditional Doherty Amplifier topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.3 Load inversion of Z . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
m(cid:48)
3.4 Impedance modulation given in Example3.2.1 . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.5 Overshoot caused by choosing z >z . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
o P
3.6 Load modulation bandwidth is reduced as a result of being pushed farther out on the
Smith chart when z <z .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
o P
3.7 Current characteristics of the main and auxiliary amplifiers . . . . . . . . . . . . . . . . . 27
3.8 Current harmonic components plotted against conduction angle α. Harmonics are nor-
malized to I =1. Note the disappearance of the 3rd harmonic when α=π. . . . . . . 29
max
3.9 A resonant parallel LC filter used to short out harmonic components in an ideal device.
The filter presents a short at every other frequency other than the fundamental. Z is
m
the modulated load impedance seen at the fundamental frequency by the device. L is
∞
an RF choke and C is a DC blocking capacitor. . . . . . . . . . . . . . . . . . . . . . . . 30
∞
3.10 The resonant LC tank is replaced with a short circuited shunted stub of length λ/4. This
stub shorts out the termination impedance of the device at the 2nd harmonic frequency.
At the fundamental frequency, the stub presents an open in parallel with the load Z ,
m
leaving it untouched. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
vii
3.11 A λ/12 open circuited stub presents a short at the 3rd harmonic f , which is 3 times the
3
fundamental frequency f . At f , the impedance seen looking into the stub is j0.58Z ,
1 1 o
≈
where Z is the characteristic impedance of the stub. . . . . . . . . . . . . . . . . . . . . . 31
o
3.12 Circuit model for a GaN HEMT proposed in [1], complete with parasitic elements. Bond
wire inductances, pad capacitances, junction to junction resistances and capacitances are
shown. Face A is the access point of device’s drain terminal. . . . . . . . . . . . . . . . . 32
(cid:48)
3.13 Depiction of how load-pull contours shift when parasitics are accounted for. Drain and
sourceparasiticsareshown,andtheSmithchartdepictsimpedanceshiftgoingfromfaceA
to A. Concentric contours on the Smith chart represent levels of degrading performance,
(cid:48)
with optimum performance in the centre. The blue impedance is the termination the
device should see, where as the red is its conjugate which will be used in designing the
matching network. The red curves see an increase in transmission length going from
A A due to the parasitics, so they shift in a clockwise fashion on the Smith chart.
(cid:48)
→
Whereas, the blue curves being the conjugate of the red, shift counterclockwise. . . . . . . 33
3.14 A load-pull characterization depicting power delivered contours, blue, and PAE contours,
red. Thefigurehighlightshowtheoptimumimpedancemaydifferforthetwometrics. In
such a case, adesign compromise between thetwopoints is made; shown by theyellowline. 34
3.15 Example load-pull contours shown for both peak and back-off conditions. Z and Z
P BO
contours shift with parasitics. Z is a certain VSWR away that is greater than that of
BO
Z , given that Z is close to Z . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
P P o
3.16 Theroleofthematchingnetworkisshown. Itmatchesthecomplexoptimumimpedances
to a real value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.17 The common load point is a junction of three branches, where the main, the auxiliary
and the common load are all shunted together. Z is the impedance of the common
L
load presented by the output matching network of the amplifier that matches it to the
standard 50Ω. Z is the modulated impedance and Z is its inverted version, both these
m(cid:48) m
impedancesarepurelyreal. Z istheimpedancethatistohitbothoptimumimpedances
dev
Z and Z of the main device, it is a complex valued impedance transformed from the
BO P
purely real impedance Z by the device matching network. Z is the impedance seen
m out
looking into a matched auxiliary amplifier. . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.18 OutputimpedanceZ oftheauxiliaryamplifierwhenturnedoff,spansaroundtheouter
out
rim of the Smith chart due to the passive lossless components of the matching network.
At the fundamental frequency f , it must be an open circuit. . . . . . . . . . . . . . . . . 38
1
4.1 K–∆ analysis of the CREE CGH40010 device under V = 28V and I = 200mA. S-
d q
parameter simulation from 500MHz to 6GHz shows K and ∆ plotted over frequency.
The device is potentially unstable for frequencies in the shaded region because K dips
below 1 in that region. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.2 A stabilization parallel RC network is added in series with the gate with R = 15Ω and
C = 10pF. The resistor adds loss to the transmission path, thereby decreasing the gain
and making the device unconditionally stable. The capacitor improves high frequency
operation by providing a bypass path. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
viii
4.3 The stabilization network causes the device to become unconditionally stable for most of
the usable frequency range, where K is now > 1. K and ∆ are plotted before and after
stabilization. With solid lines showing values after stabilization and dashed lines before
stabilization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.4 Load-pull test bench used in ADS for the CREE CGH40010 . . . . . . . . . . . . . . . . . 43
4.5 Harmonic balance one-tone load-pull of the CREE CGH40010 device model. Power is
sweptandthemaximumpowerdelivered,maximumgainandgaincompressionisrecorded
by sweeping the load across the Smith chart. Z is chosen to be 20+j20Ω as a suitable
P
compromise between the three metrics. At about 4dB away from Z , Z is chosen
P BO
optimally to be 15+j45Ω. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.6 2nd harmonicload-pullcontoursshowingPAE.Thefundamentalloadisfixedandthe2nd
harmonic load is swept to plot these contours. (a) shows that when Z is presented and
BO
the amplifier is operating at back-off condition, the 2nd harmonic termination must be
close to j24Ω for improved efficiency. (b) shows that when Z is presented at the peak
P
operating condition, the termination required is much more relaxed. . . . . . . . . . . . . 46
4.7 The optimum impedances obtained from Fig.4.5, Z and Z are matched to a real
BO P
valued impedance. Matching network is first terminated with the conjugate of the opti-
mum impedances, Z and Z . The first component in the matching network is a series
B∗O P∗
inductive element with reactance j0.26. This component transforms Z and Z such
B∗O P∗
that they have approximately the same susceptance. Next, a shunt inductor with suscep-
tance j0.68 moves both impedances to the real line. The final transformed real valued
−
impedances ZB∗(cid:48)O and ZP∗(cid:48) are 107−j4Ω and 28+j0.3Ω respectively.. . . . . . . . . . . . 47
4.8 Reflection Coefficient of the matching network of the two load points . . . . . . . . . . . . 48
4.9 Complete ADS test bench of the Doherty PA . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.10 Simulated performance of the designed Doherty PA showing PAE over frequency and
output power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.11 Simulationof(a)gainversusoutputpowerand(b)gaincompressionversusoutputpower
of the designed Doherty PA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.12 Circuit used to simulate the impedance seen by the main amplifier . . . . . . . . . . . . . 54
4.13 A load-pull contour analysis of BW 1GHz to 1.4GHz . . . . . . . . . . . . . . . . . . . . 56
4.14 A load-pull contour analysis of BW 1.5GHz to 1.7GHz . . . . . . . . . . . . . . . . . . . 57
4.15 A load-pull contour analysis of BW 1.8GHz to 2.0GHz . . . . . . . . . . . . . . . . . . . 58
5.1 Layout of the multistage broadband Wilkinson power divider and input delay line . . . . 60
5.2 Layout of source matching network and stability components . . . . . . . . . . . . . . . . 61
5.3 Reflection coefficient of the source matching network . . . . . . . . . . . . . . . . . . . . . 61
5.4 Layout and schematic of the output network of the Doherty PA . . . . . . . . . . . . . . . 62
5.5 Tuning of the matching network accounting for pad delay and component dimensions. . . 64
5.6 Layout and schematic of the fabricated Doherty PA . . . . . . . . . . . . . . . . . . . . . 65
5.7 Close-up of the lumped components used . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.8 The fabricated Doherty PA photographed- showing top view . . . . . . . . . . . . . . . . 67
5.9 Photograph of the Doherty PA mounted onto a heat-sink . . . . . . . . . . . . . . . . . . 68
5.10 The test bench setup for measuring the performance of the Doherty PA . . . . . . . . . . 69
5.11 Measured DE plotted against output power delivered . . . . . . . . . . . . . . . . . . . . . 72
ix
5.12 Comparison of back-off DE of measurement, co-simulation, and full-simulation . . . . . . 72
5.13 Measured harmonic distortion of the Doherty PA . . . . . . . . . . . . . . . . . . . . . . . 73
5.14 Co-simulated OIP3 of the Doherty PA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6.1 Geometry of group delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
6.2 S poles of low pass and high pass Π networks . . . . . . . . . . . . . . . . . . . . . . . . 80
21
6.3 Phase analysis of the low pass Π network . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
6.4 Coefficients of n-stage Π network on Pascal’s triangle . . . . . . . . . . . . . . . . . . . . . 84
6.5 Poles of an n-stage Π network, shown up to 4 stages . . . . . . . . . . . . . . . . . . . . . 86
6.6 Phaseandphasedepartureversusnumberofstages,showingthen-stageΠnetworkmodels
a transmission line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.7 Regiononthecomplexplanewhereconjugatepolescontributeaphasevaluethatisgreater
than the normalized group delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.8 Schematic of the lattice phase equalizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
6.9 Poles and zeroes of a lattice phase equalizer or Wheatstone bridge network . . . . . . . . 91
6.10 ABCD analysis of a lattice network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
x