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Using DSP to Improve the Performance of a Doherty Amplifier

Using DSP to Improve the Performance of a Doherty Amplifier. Yu Zhao, Masaya Iwamoto, Larry Larson and Peter Asbeck. High Speed Device&Circuit Group. Introduction. To reduce DC power consumption in wireless communication Increase overall efficiency of RF PA Use Doherty structure

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Using DSP to Improve the Performance of a Doherty Amplifier

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  1. Using DSP to Improve the Performance of a Doherty Amplifier Yu Zhao, Masaya Iwamoto, Larry Larson and Peter Asbeck High Speed Device&Circuit Group

  2. Introduction • To reduce DC power consumption in wireless communication • Increase overall efficiency of RF PA • Use Doherty structure • To maintain performance of transmitter • Achieve adequate linearity ( for CDMA and OFDM) • Use DSP to control Doherty Amplifier

  3. Agenda • Extended Doherty amplifier • architecture • measured results without DSP • Simulated application of DSP to Doherty amplifier • control strategy • behavioral model • simulation result • Conclusions

  4. Reff l/4 Zc=gRL Pin Pout Main PA RL Auxiliary PA l/4 Zo 10dB output back-off 1 Extended Doherty (g=4) 0.9 Classical Doherty (g=2) 0.8 0.7 0.6 Class B Efficiency 0.5 0.4 Class A 0.3 0.2 0.1 0 0 0.2 0.4 0.6 0.8 1 P (normalized) out Advantages of Doherty Architecture • Low power range • only Main-PA operates • Reff = g2RL • High power range • Main-PA goes into saturation, Aux-PA turns on • Reff = gRL • Extended Doherty uses g =4 • Achieves wider high efficiency range

  5. 60 Drain efficiency 50 40 PAE 30 20 Gain 10 0 -10 -5 0 5 10 15 20 25 30 P (dBm) out GCS InGaP/GaAs HBT Experimental Results --- Gain,Efficiency&ACPR • Circuit shows good efficiency over wide power range Gain ACPR1 ACPR2 • ACPR is still a concern

  6. Pout=16dBm 46 12.5 10.0 PAE=+14.2% ACP1=+47.2dB ACP2=+62.6dB Load:69.00+15.00j PAE=+13.2% Gain=+7.0dB ACP1=+47.8dB ACP2=+63.4dB UCSD_P16_V4.0_F950.bin center=50.00+0.00j Experimental Results --- Load-Pull Pout=25dBm 38 40 42 44 46 48 50 ACPR1 PAE 52 54 35.0 56 30.0 58 25.0 20.0 17.5 15.0 12.5 PAE=+33.0% ACP1=+46.9dB ACP2=+55.6dB Load:69.00+15.00j PAE=+35.1% Gain=+8.1dB ACP1=+51.4dB ACP2=+55.1dB UCSD_P25_V4.0_F950.bin center=50.00+0.00j ACPR1=-38 to -58 dBc (3:1 VSWR) PAE=12 to 35% (3:1 VSWR) ACPR1=-46 to -48 dBc (3:1 VSWR) PAE=10 to 15% (3:1 VSWR) Measurements taken at Conexant Systems Inc.

  7. Φ2-Φ1 I Q DSP Input match Input match Pre_amp Vgg2 I Q Design Issues where DSP Can Help Main l/4 Zc=gRL A1, Φ1 l/4 Auxiliary A2, Φ2 • Relative input of Main & Aux amplifier- A2/A1, Φ2-Φ1 • Maintain magnitude&phase balance • Bias point of Auxiliary amplifier – Vgg2 • Maintain magnitude&phase balance and good efficiency • Eliminate additional phase shifter

  8. Main PA l/4 RF_signal1 Auxiliary PA RF_signal2 Simulation Results --- Phase Difference Φ1, Φ2 Sweep Φ2 - Φ1 A2/A1 = 1.5 constant Bias = - 4.0 V constant A1, Φ1 A2, Φ2 10 1 Normalized Gain Normalized Phase 5 0.5 0 0 Gain Distortion(dB) Phase Distortion(degree) -0.5 -5 -10 -1 -1.5 -15 -2 -20 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Pout (dBm) Pout (dBm)

  9. A2/A1 I Q DSP Input match Input match Pre_amp Vgg2 I Q Design Issues where DSP Can Help • Relative input of Main & Aux amplifier- A2/A1, Φ2-Φ1 • Maintain magnitude&phase balance • Bias point of Auxiliary amplifier – Vgg2 • Maintain magnitude&phase balance and good efficiency • Eliminate additional phase shifter Main l/4 Zc=gRL A1, Φ1 l/4 Auxiliary A2, Φ2

  10. Main PA l/4 RF_signal1 Auxiliary PA RF_signal2 Simulation Results --- Power Split Ratio A1, A2 Set A1=1,Sweep A2 Φ2 - Φ1 = 90° constant Bias = - 4.0 V constant A1, Φ1 A2, Φ2 1 10 Normalized Gain 8 0.5 Normalized Phase 6 0 4 Gain Distortion(dB) 2 -0.5 Phase Distortion(degree) 0 -1 -2 -4 -1.5 -6 -2 -8 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Pout (dBm) Pout (dBm)

  11. I Q DSP Input match Input match Pre_amp I Q Design Issues where DSP Can Help • Relative input of Main & Aux amplifier- A2/A1, Φ2 - Φ1 • Maintain magnitude&phase balance • Bias point of Auxiliary amplifier – Vgg2 • Maintain magnitude&phase balance and good efficiency • Eliminate additional phase shifter Main l/4 Zc=gRL A1, Φ1 Vgg2 l/4 Auxiliary A2, Φ2

  12. 60 50 40 Drain Efficiency (% ) 30 20 10 0 0 5 10 15 20 25 30 Pout (dBm) 1 10 Normalized Gain Normalized Phase 8 0.5 6 0 4 Gain Distortion(dB) Phase Distortion(degree) 2 -0.5 0 -1 -2 -1.5 -4 -6 -2 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Pout (dBm) Pout (dBm) Simulation Results --- DifferentBias change bias of Aux-PA A2/A1 = 1.5 constant Φ2 - Φ1 = 90° constant

  13. DSP ControlStrategy • Given input signal &desired Pout, choose optimum Vgg2, A2/A1 and Φ2(t)-Φ1(t) to maintain relatively constant gain, phase with maximum efficiency DSP Block Bias of Aux PA Power level Look-up-table a1(t)/a2(t) θ2(t)-θ1(t) X1(t) --- to Main PA Base band X(t) Signal splitter X2(t) --- to Aux PA

  14. DSP Control Algorithm ---Based on simulation data When Pin < 10 dBm Choose: A1/A2=1.5 Φ2-Φ1 = 100° Vgg2 = - 4.4 V When 10 < Pin < 15 dBm Change Vgg2 (-3.8 to -3.92 V) Drain Efficiency (% ) Vgg2 Pout (dBm) Vgg2 Vgg2 Normalized Phase Gain Distortion(dB) Gain Phase Distortion(degree) Pin (dBm) Pin (dBm)

  15. DSP Control Algorithm ---Based on simulation data When 15 < Pin < 18 dBm Choose: A1/A2=1.5 Φ2 - Φ1 = 90° Vgg2 = - 3.80 V Drain Efficiency (% ) Vgg2 Pout (dBm) Vgg2 Vgg2 Normalized Phase Gain Distortion(dB) Gain Phase Distortion(degree) Pin (dBm) Pin (dBm)

  16. DSP Control Algorithm ---Based on simulation data When Pin > 18 dBm Choose: A1/A2=1.5 Φ2 - Φ1 = 85° Vgg2 = - 3.92 V Drain Efficiency (% ) Vgg2 Pout (dBm) Vgg2 Vgg2 Normalized Phase Gain Distortion(dB) Phase Distortion(degree) Gain Pin (dBm) Pin (dBm)

  17. Simulated Amplifier with DSP Control PA’s behavioral model with DSP control (Blue) and the “best” without DSP control (Red) (choose A2/A1 = 1.5,Φ2 - Φ1 = 90°,Vgg2= -3.44V) Normalized Phase Gain Gain Distortion(dB) Phase Distortion(degree) Pin (dBm) Pin (dBm)

  18. Simulation Results --- ACPR&Efficiency Efficiency(%) ACPR (dBc) Use matlab to compute spectrum of CDMA signal Pout (dBm) Blue,with DSP Red,without DSP

  19. Conclusions • Extended Doherty amplifier can achieve high efficiency over wide output power range • PA Simulation based on behavior model shows CDMA ACPR specification can be met with DSP optimization • We believe DSP can make circuit design easier and improve performance significantly

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