High Efficiency Microwave Amplifiers and SiC Varactors Optimized for Dynamic Load Modulation C...

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 |