Slide 19/3/2002 S. Xie, V. Paidi, R. Coffie, S. Keller, S. Heikman, A. Chini, U. Mishra, S. Long, M. Rodwell Department of Electrical and Computer Engineering, University of California, Santa Barbara Phone: (805) Fax: (805) Topical Workshop on Power Amplifiers September 2002, UCSD High Linearity Class B Power Amplifiers in GaN HEMT Technology

Slide 29/3/2002 Outline UCSB Introduction How does Class B PA work? Why single-ended Class B? Highly linear single-ended Class B PA design and simulation Measurement results Conclusions S. Xie, V. Paidi

Slide 39/3/2002 Two identical devices working in 50% duty cycle with 180° phase shift. Half sinusoidal drain current on each device, but full sinusoidal drain voltage. Even harmonics are suppressed by symmetry => wide bandwidth (limited by the power combiner). Class B: Ideal PAE 78.6%; feasible PAE 40-50% (typical GaN HEMT at X- band); Class A: Ideal PAE 50%, feasible PAE 20-30%. V in V out = V DS1 – V DS V in -V in V DS 2 V DS 1 UCSB How does push-pull Class B PA work? V. Paidi, S. Xie

Slide 49/3/2002 To obtain high efficiency (78%), a half-sinusoidal current is needed at each drain. This requires an even-harmonic short. This can be achieved at HF/VHF frequencies with transformers or bandpass filters. However, 1.Most wideband microwave baluns can not provide effective short for even-mode. Efficiency is then poor. 2.They occupy a lot of expensive die area on MMIC. UCSB Why not push-pull for RF MMIC V. Paidi, S. Xie

Slide 59/3/2002 I D1 V in -V in V in + -I D2 = I D Zero Z at 2fo IDID V in -V in I D2 I D1 Push-pull Class B Single-ended Class B with Bandpass filter Even harmonics suppressed by symmetry Conclusion: From linearity point of view, push-pull and single-ended Class B with bandpass filter B are equivalent – same transfer function. Even harmonics suppressed by filter UCSB Single-ended Class B = push-pull Bandwidth restriction < 2:1 V. Paidi, S. Xie

Slide 69/3/2002 Ideal Class B Bias too low: Class CBias too high: Class AB UCSB Why is biasing critical for Class B? I D1 V in + I D2 = I D VpVp I D1 V in + I D2 = I D VpVp I D1 V in + I D2 = I D VpVp V. Paidi, S. Xie

Slide 79/3/2002 Class C, nonlinear Class B, linear Class AB, nonlinear Class B is linear given that the current transfer function is linear UCSB Voltage Transfer Function as a Function of Bias Voltage V. Paidi, S. Xie

Slide 89/3/2002 The distortion will be minimum when the amplifier is biased at Class B by using GaN HEMT on SiC. Highly linear characteristics of GaN HEMT on SiC Bias point UCSB A source of IM3 distortion: Transconductance distortion V. Paidi, S. Xie The third order term in the Taylor expansion is very small when biased at pinch off.

Slide 99/3/2002 UCSB Lossy input matching network to increase the bandwidth Cds is absorbed into the  - section output lowpass filter Single-ended Class B Power Amplifier S. Xie, V. Paidi Lossy input matching  - section lowpass filter

Slide 109/3/2002 Simulation of class B Saturated PAE ~48% Class B bias: Saturated output power ~ 37 dBm, Saturated PAE ~ 48% UCSB Simulation performance of Class B S. Xie, V. Paidi Waveforms of drain voltage and current Saturated output power ~37 dBm

Slide 119/3/2002 Best IM3 suppression is achieved at Class B and Class A UCSB Simulation of Intermodulation Suppression and S. Xie, V. Paidi Class B bias: C/IMD3~44dBc PAE ~ 48% Class A bias C/IMD3~42dBc PAE ~ 35% Class AB bias Class C bias

Slide 129/3/2002 F t ~55 GHz RF Performance UCSB Device Performance of GaN HEMTs Performance of 12 fingers (1.2mm) device: L g ~ 0.25um I dss ~ 1A/mm f t ~ 55 GHz (~ 50 GHz for dual gate) V br ~ 40V (~ 55V for dual gate) DC I_V curve ~1.2 A V. Paidi, S. Xie

Slide 139/3/2002 Chip photograph of Class B power amplifier (Approximately 6mmX1.5mm) Air bridges Source Drain Gate 1 Gate 2 UCSB V. Paidi, S. Xie

Slide 149/3/2002 Measurement setup UCSB Single tone from 4 GHz to 12 GHz; Two-tone measurement at f1 = 8 GHz, f2 = GHz; Bias sweep: Class A (Vgs = -3.1V), Class B (Vgs = -5.1V, Class C (Vgs = V) and AB (Vgs = -4.5 V). Measurements: S. Xie, V. Paidi

Slide 159/3/2002 Class B power amplifier measurement result Gain vs. frequency Class AB Class B 3 dB bandwidth: 7GHz - 10GHz S. Xie, V. Paidi UCSB

Slide 169/3/2002 Class B = - 5.1V Single tone f 0 = 8GHz: Two tone f 1 =8GHz, f 2 =8.001GHz : f 1,f 2 2f 1 -f 2, 2f 2 -f 1 PAE (maximum) ~ 34% Saturated output power 36 dBm Good IM3 performance: 40dBc at Pin = 15 dBm, and > 35 dBc for Pin < 17.5 dBm S. Xie, V. Paidi UCSB

Slide 179/3/2002 Class A = - 3.1V f 1,f 2 2f 1 -f 2, 2f 2 -f 1 Single tone f 0 = 8GHz: Two tone f 1 =8GHz, and f 2 =8.001GHz : Saturated output power 36 dBm Good IM3 performance at low power level but becomes bad rapidly at high power levels Saturated output power each tone ~ 33dBm S. Xie, V. Paidi UCSB PAE (maximum) ~ 34%

Slide 189/3/2002 Summary of IM3 suppressions Class B Class A Class C Class AB Psat Low output power levels (Pout 36 dBc, Class A > 45 dBc). Higher output power levels, Class A behaves almost the same as Class B. Class AB and C exhibit more distortion compared to Class A and B. S. Xie, V. Paidi UCSB

Slide 199/3/2002 Class B vs. Class A Class B Class A IM3 suppression and PAE of two-tonePAE of single tone Class B Class A While maintains the same IM3 suppression as Class A, Class B can get more than 10% of PAE. S. Xie, V. Paidi UCSB

Slide 209/3/2002 For a less than octave bandwidth, a push-pull Class B amplifier can be replaced by a single-ended Class B amplifier with bandpass or lowpass filter. A single-ended Class B MMIC power amplifier in GaN HEMT is designed and 36dBm of saturated power and 35dBc of IM3 suppression are obtained. Class B is better than Class A because it can get good IM3 performance comparable to that of Class A, while providing PAE ~10% higher than that of Class A. UCSB Conclusions S. Xie, V. Paidi

Slide 219/3/2002 This work was supported by the ONR under grant (N ) Special thanks to Dr. Walter Curtice, who provide us the C_FET3 model for simulation; Thanks to L.-Y. (Vicky) Chen and Likun Shen, who helped us for the measurement. UCSB Acknowledgements S. Xie, V. Paidi