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High Efficiency Microwave Amplifiers and SiC Varactors Optimized for Dynamic Load Modulation C HRISTER A NDERSSON 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 R opt

and

high X Cds /R opt ratio Ideal choice for wideband high power amplifiers 4

Resistive harmonic loading [A]

Z L (f) = R opt P out = class-B η = 58%

Dimensions: 122 mm x 82 mm.

Measurements [A]    Decade bandwidth performance (0.4 – 4.1 GHz)   Pout > 10 W

= 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,

= 34%, ACLR < –46 dBc 11

β 1

Active load modulation [D]

β 2 , φ

 Mutual load modulation

using transistors

  

Both

transistors must operate efficiently Co-design of MN 1 , MN 2 , and current control functions • Successful examples: Doherty and Chireix Modulate current amplitudes and phase at signal rate 12

β 1

Dual-RF input topology [D]

β 2 , 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)

Measurements [D] Dimensions: 166 mm x 81 mm.

  Performance over 100% fractional bandwidth (1.0 – 3.1 GHz)  P max = 44 ± 0.9 dBm  PAE @ 6 dB OPBO > 45%

Record efficiency bandwidth for load modulated PA

Varactor-based DLM architecture.

Chalmers MC2 cleanroom.

14-finger SiC varactor (C min = 3 pF).

VARACTORS FOR MICROWAVE POWER APPLICATIONS

Varactor effective tuning range   Increasing RF swing decreasing

T eff

 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 (X Cds /R opt )    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

Power amplifiers (PA)  Final stage amplifier before antenna  High power level → efficiency (

) 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 g m )  Load line in saturated region (no compression) Class-B bias  Sinusoidal drive → half-wave rectified current Bare-die parasitics mainly shunt-capacitive  Effective ”C ds ” 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

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

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