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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 C HRISTER A NDERSSON Microwave Electronics Laboratory Department of Microtechnology and Nanoscience – MC2 May 23, 2013

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2 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] 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]

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POWER AMPLIFIER ANALYSIS

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4 Transistor technology GaN HEMT High R opt and high X Cds /R opt ratio Ideal choice for wideband high power amplifiers GaN HEMT High R opt and high X Cds /R opt ratio Ideal choice for wideband high power amplifiers Fano limit: Baredie 15-W GaN HEMT (Cree, Inc.) Simplified model:

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5 Resistive harmonic loading [A] Z L (f) = R opt P out = class-B η = 58% Dimensions: 122 mm x 82 mm.

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6 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 Decade bandwidth performance (0.4 – 4.1 GHz) Pout > 10 W η = 40 – 60% DPD linearized to standard ACRL < –45 dBc Envelope tracking candidate

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TRANSMITTER EFFICIENCY ENHANCEMENT Dynamic and active load modulation

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8 Dynamic load modulation (DLM) [B,C] Load modulation Restore voltage swing and efficiency Varactor-based DLM Reconfigure load network at signal rate Load modulation Restore voltage swing and efficiency Varactor-based DLM Reconfigure load network at signal rate

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9 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 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 10-W 2.14 GHz [B] 3-mm GaN HEMT + 2x SiC varactors Efficiency enhancement: 20% → 8 dB OPBO 3-mm GaN HEMT + 2x SiC varactors Efficiency enhancement: 20% → 8 dB OPBO CuW-carrier dimensions: 35 mm x 20 mm.

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W 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% 6 dB DPD by vector switched GMP model 17-W WCDMA signal, η = 34%, ACLR < –46 dBc Fully packaged 24-mm GaN HEMT + 4x SiC varactors Record DLM output power (1 order of mag.) Efficiency enhancement: 10-15% 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. 40V 30V 20V

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12 Active load modulation [D] 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 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 β1β1 β 2, φ

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13 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) 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) β1β1 β 2, φ

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14 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 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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15 Measurements [D] Performance over 100% fractional bandwidth (1.0 – 3.1 GHz) P max = 44 ± 0.9 dBm 6 dB OPBO > 45% Record efficiency bandwidth for load modulated PA Performance over 100% fractional bandwidth (1.0 – 3.1 GHz) P max = 44 ± 0.9 dBm 6 dB OPBO > 45% Record efficiency bandwidth for load modulated PA Dimensions: 166 mm x 81 mm.

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VARACTORS FOR MICROWAVE POWER APPLICATIONS 14-finger SiC varactor (C min = 3 pF). Chalmers MC2 cleanroom. Varactor-based DLM architecture.

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17 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 Increasing RF swing decreasing T eff Shape of varactor C(V) matters Nonlinear characterization [G] Engineer C(V) to be less abrupt

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18 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] 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 Engineer doping profile Higher doping Lower loss Higher electric fields Wide bandgap SiC High critical electric field

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19 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 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

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20 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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23 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] 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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24 Model simplifications [A-D] 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 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 15-W GaN HEMT (Cree, Inc.)

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25 Power amplifiers (PA) Final stage amplifier before antenna High power level → efficiency most critical Final stage amplifier before antenna High power level → efficiency most critical

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26 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) 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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27 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] 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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28 Comparison [A]

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29 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 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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30 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 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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31 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 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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32 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 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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33 Nonlinear characterization [G] Active multi-harmonic source/load-pull system Study of an abrupt SiC varactor Active multi-harmonic source/load-pull system Study of an abrupt SiC varactor

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34 Power dependent detuning and loss [G] Capacitance and loss increase with RF swing Dependent on varactor and circuit topology Capacitance and loss increase with RF swing Dependent on varactor and circuit topology

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35 Effect of 2nd harmonic loading [G] Q–factor drop due to resonance Relevance in tunable circuit design Varactors inherently nonlinear devices Q–factor drop due to resonance Relevance in tunable circuit design Varactors inherently nonlinear devices

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36 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 Active multi-harmonic source/load-pull system Study of an abrupt SiC varactor Capacitance and loss increase versus RF swing Harmonic loading dependent | RF |

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