High Efficiency Microwave Amplifiers and SiC Varactors Optimized for Dynamic Load

Modulation

CHRISTER ANDERSSON

Microwave Electronics LaboratoryDepartment of Microtechnology and Nanoscience – MC2

May 23, 2013

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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]

POWER AMPLIFIER ANALYSIS

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Transistor technology

GaN HEMT High Ropt and high XCds/Ropt ratio Ideal choice for wideband high power amplifiers

Fano limit:

Baredie 15-W GaN HEMT (Cree, Inc.)

Simplified model:

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Resistive harmonic loading [A]

ZL(f) = Ropt

Pout = class-Bη = 58%

Dimensions: 122 mm x 82 mm.

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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

TRANSMITTER EFFICIENCY ENHANCEMENT

Dynamic and active load modulation

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Dynamic load modulation (DLM) [B,C]

Load modulation Restore voltage swing and efficiency

Varactor-based DLM Reconfigure load network at signal rate

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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

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10-W demonstrator @ 2.14 GHz [B]

3-mm GaN HEMT + 2x SiC varactors Efficiency enhancement: 20% → 45% @ 8 dB OPBO

CuW-carrier dimensions: 35 mm x 20 mm.

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100-W demonstrator @ 2.14 GHz [C]

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

Package internal dimensions: 40 mm x 10 mm.

40V30V20V

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Active load modulation [D]

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

β1 β2, φ

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Dual-RF input topology [D]

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)

β1 β2, φ

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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)

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Measurements [D]

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

Dimensions: 166 mm x 81 mm.

VARACTORS FOR MICROWAVE POWER APPLICATIONS

14-finger SiC varactor (Cmin = 3 pF).

Chalmers MC2 cleanroom.

Varactor-based DLM architecture.

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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

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Schottky diode SiC varactors [E,F]

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]

Engineer doping profile Higher doping

• Lower loss• Higher electric fields

Wide bandgap SiC High critical electric field

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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

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Acknowledgment

• ”Microwave Wide Bandgap Technology project”• ”Advanced III-Nitrides-based electronics for future microwave communication and sensing systems”• ”ACC” and ”EMIT” within the GigaHertz Centre

This work has been performed as part of several projects:

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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]

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Model simplifications [A-D]

Linear transistor (constant gm) Load line in saturated region

(no compression) Class-B bias

Sinusoidal drive → half-wave rectified current

Bare-die parasitics mainlyshunt-capacitive Effective ”Cds” found by load-pull

15-W GaN HEMT (Cree, Inc.)

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Power amplifiers (PA)

Final stage amplifier before antenna High power level → efficiency most critical

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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)

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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]

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Comparison [A]

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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

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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

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Doherty-outphasing continuum [D]

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

(class-B efficiency)

WCDMA 6.7 dB PAPR

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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

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Nonlinear characterization [G]

Active multi-harmonic source/load-pull system Study of an abrupt SiC varactor

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Power dependent detuning and loss [G]

Capacitance and loss increase with RF swing Dependent on varactor and circuit topology

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Effect of 2nd harmonic loading [G]

Q–factor drop due to resonance Relevance in tunable circuit design Varactors inherently nonlinear devices

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Nonlinear varactor characterization [G]

Active multi-harmonic source/load-pull system Study of an abrupt SiC varactor

Capacitance and lossincrease versus RF swing Harmonic loading dependent

| RF | | RF |