Report

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