High Efficiency Microwave Amplifiers and
SiC Varactors Optimized for Dynamic Load
Modulation
CHRISTER ANDERSSON
Microwave Electronics Laboratory
Department of Microtechnology and Nanoscience – MC2
May 23, 2013
Thesis contributions


Theory and technology for energy efficient and high
capacity wireless systems
Power amplifier analysis
 Transistor technology and modeling
 Wideband design [A]

Transmitter efficiency enhancement
 Dynamic load modulation [B, C]
 Active load modulation [D]

Varactors for microwave power applications
 SiC varactors for DLM [E, F]
 Nonlinear characterization [G]
2
POWER AMPLIFIER ANALYSIS
Transistor technology
Simplified model:
Baredie 15-W GaN HEMT (Cree, Inc.)
Fano limit:

GaN HEMT
 High Ropt and high XCds/Ropt ratio
 Ideal choice for wideband high power amplifiers
4
Resistive harmonic loading [A]
ZL(f) = Ropt
Pout = class-B
η = 58%
Dimensions: 122 mm x 82 mm.
5
Measurements [A]



Decade bandwidth performance (0.4 – 4.1 GHz)
 Pout > 10 W
 η = 40 – 60%
DPD linearized to standard
 ACRL < –45 dBc
Envelope tracking candidate
6
Dynamic and active load modulation
TRANSMITTER EFFICIENCY
ENHANCEMENT
Dynamic load modulation (DLM) [B,C]


Load modulation
 Restore voltage swing and efficiency
Varactor-based DLM
 Reconfigure load network at signal rate
8
Class-J DLM theory [B]

DLM by load reactance modulation
 Ideal for varactor implementation

Theory enables analysis
 Technology requirements
 Power scaling [B] → [C]
 Frequency
reconfigurability
9
10-W demonstrator @ 2.14 GHz [B]
CuW-carrier dimensions: 35 mm x 20 mm.


3-mm GaN HEMT + 2x SiC varactors
Efficiency enhancement: 20% → 45% @ 8 dB OPBO
10
100-W demonstrator @ 2.14 GHz [C]
20V
30V
40V
Package internal dimensions: 40 mm x 10 mm.

Fully packaged
 24-mm GaN HEMT + 4x SiC varactors
 Record DLM output power (1 order of mag.)
 Efficiency enhancement: 10-15% units @ 6 dB

DPD by vector switched GMP model
 17-W WCDMA signal, η = 34%, ACLR < –46 dBc
11
Active load modulation [D]
β1

β2, φ
Mutual load modulation using transistors
 Both transistors must operate efficiently
 Co-design of MN1, MN2, and current control functions
• Successful examples: Doherty and Chireix
 Modulate current amplitudes and phase at signal rate
12
Dual-RF input topology [D]
β1


β2, φ
Complex design space – many parameters
Linear multi-harmonic calculations (MATLAB)
 Include transistor parasitics
 No assumption of short-circuited higher harmonics
 Optimize for wideband high average efficiency
• Output: circuit values + optimum current control(s)
13
Verification of calculations [D]

2 x 15-W GaN HEMT design
 Straightforward ADS implementation – plug in MATLAB circuit values
 Parasitics and higher harmonics catered for already

Good agreement with complete nonlinear PA simulation
WCDMA 6.7 dB PAPR
(MATLAB)
(ADS)
14
Measurements [D]
Dimensions: 166 mm x 81 mm.

Performance over 100% fractional bandwidth (1.0 – 3.1 GHz)
 Pmax = 44 ± 0.9 dBm
 PAE @ 6 dB OPBO > 45%

Record efficiency bandwidth for load modulated PA
15
Varactor-based DLM architecture.
Chalmers MC2 cleanroom.
14-finger SiC varactor (Cmin = 3 pF).
VARACTORS FOR MICROWAVE
POWER APPLICATIONS
Varactor effective tuning range

Increasing RF swing decreasing Teff
 Shape of varactor C(V) matters
 Nonlinear characterization [G]

Engineer C(V) to be less abrupt
17
Schottky diode SiC varactors [E,F]


Engineer doping profile
 Higher doping
• Lower loss
• Higher electric fields
Wide bandgap SiC
 High critical electric field

SiC varactor performance [E,F]





Moderate small-signal tuning range
High breakdown voltage
High Q-factor
Highest tuning range when |RF| > 5 V
Used in [B,C,d,g,h]
18
Conclusions

Energy efficient wideband power amplifiers





Simplified modeling (XCds/Ropt)
Resistive harmonic loading [A]
Varactor-based dynamic load modulation [B,C]
Active load modulation [D]
Varactors for microwave power applications
 Nonlinear characterization [G]
 Novel SiC varactor [E,F]
• Dynamic load modulation one of many applications

Theory and technology for energy efficient high capacity
wireless systems
19
Acknowledgment
This work has been performed as part of several projects:
• ”Microwave Wide Bandgap Technology project”
• ”Advanced III-Nitrides-based electronics for future microwave communication and sensing systems”
• ”ACC” and ”EMIT” within the GigaHertz Centre
20
21
22
Power amplifiers (PA)

Final stage amplifier before antenna
 High power level → efficiency (η) critical

PA internals





FET
Input matching network
Load matching network
Nonlinear circuit
Propose simplifications to allow linear analysis
 These are used in [A-D]
23
Model simplifications [A-D]
15-W GaN HEMT (Cree, Inc.)

Linear transistor (constant gm)
 Load line in saturated region
(no compression)

Class-B bias
 Sinusoidal drive →
half-wave rectified current

Bare-die parasitics mainly
shunt-capacitive
 Effective ”Cds” found by load-pull
24
Power amplifiers (PA)

Final stage amplifier before antenna
 High power level → efficiency most critical
25
Typical PA

Transistor
 Microwave frequency FET

Input network
 Gate bias, stability, source impedances (current wave shaping)

Load network
 Drain supply, load impedances (voltage wave shaping)
26
Transistor equivalent circuit

Complete model is complicated





Nonlinear voltage-controlled current source
Nonlinear capactiances
Feedback
Package parasitics
Propose simplifications to allow linear analysis
 These are used in [A-D]
27
Comparison [A]
28
PA efficiency and modern signals


PA efficiency drops in output power back-off (OPBO)
Modern signals
 High probability to operate in OPBO
 Average efficiency is low

Need an architecture to restore the efficiency in OPBO
29
Dynamic load modulation (DLM)


PA efficiency drops in output power back-off (OPBO)
Load modulation


Restore voltage swing and efficiency
Varactor-based DLM
 Reconfigure load network at signal rate
 Linearization: RF input + baseband varactor voltage
30
Doherty-outphasing continuum [D]
(class-B efficiency)
WCDMA 6.7 dB PAPR

Dual-RF input PA – optimum current control versus power & frequency
 Classic Doherty impedances & short-circuited higher harmonics
 Classic Doherty transmission line lengths not best choice
• Adding 90° includes outphasing operation and gives higher efficiencies
31
Reality check [D]

Realistic circuit
 Cannot assume short-circuited higher harmonics
 Must consider transistor parasitics


Complicated design space (not suitable for ADS)
Linear multi-harmonic calculations (MATLAB)
 Assume simplified transistor model
 Optimize circuit values
• Relatively cheap calculation
• Brute-force evaluation of 14M circuits vs. drive and frequency
32
Nonlinear characterization [G]


Active multi-harmonic source/load-pull system
Study of an abrupt SiC varactor
33
Power dependent detuning and loss [G]

Capacitance and loss increase with RF swing
 Dependent on varactor and circuit topology
34
Effect of 2nd harmonic loading [G]

Q–factor drop due to resonance
 Relevance in tunable circuit design
 Varactors inherently nonlinear devices
35
Nonlinear varactor characterization [G]

Active multi-harmonic source/load-pull system
 Study of an abrupt SiC varactor

Capacitance and loss
increase versus RF swing
 Harmonic loading dependent
| RF |
| RF |
36