Efficiency Enhancement of Doherty Amplifier Through Mitigation of the Knee Voltage Effect

This paper presents an approach to maximize the efficiency of a Doherty power amplifier (PA) with the knee voltage effect. Since the carrier PA with , which is the usual matching impedance for a carrier PA at a low power region, does not reach to the saturated operation at the 6-dB back-off power level, the maximum efficiency could not be achieved. However, the carrier amplifier can be driven into the saturation using the load impedance larger than and can deliver the maximum efficiency even under the knee voltage effect. The optimized design for the maximum efficiency at the back-off level is derived. The optimized amplifier is analyzed and simulated in terms of its load modulation behavior, efficiency, and output power, then compared with the conventional Doherty PA. The enhanced performance is demonstrated by the Doherty PA built using CREE's GaN HEMT CGH40045 devices at 2.655 GHz. For worldwide interoperability for microwave access applications with a 7.8-dB peak-to-average power ratio, the proposed PA delivers an efficiency of 49.3% at an output power of 42 dBm with an acceptable linearity of 23.1 dBc. The linearity is improved to 43 dBc by employing a digital feedback predistortion technique, satisfying the system linearity specification.

[1]  Jangheon Kim,et al.  Analysis of a Fully Matched Saturated Doherty Amplifier With Excellent Efficiency , 2008, IEEE Transactions on Microwave Theory and Techniques.

[2]  W.H. Doherty,et al.  A New High Efficiency Power Amplifier for Modulated Waves , 1936, Proceedings of the Institute of Radio Engineers.

[3]  Bumman Kim,et al.  Optimum operation of asymmetrical-cells-based linear Doherty power Amplifiers-uneven power drive and power matching , 2005, IEEE Transactions on Microwave Theory and Techniques.

[4]  Jaehyok Yi,et al.  Adaptive Digital Feedback Predistortion Technique for Linearizing Power Amplifiers , 2007, IEEE Transactions on Microwave Theory and Techniques.

[5]  Robert H. Caverly,et al.  HF, VHF, and UHF systems and technology , 2002 .

[6]  L.E. Larson,et al.  Design of wide-bandwidth envelope-tracking power amplifiers for OFDM applications , 2005, IEEE Transactions on Microwave Theory and Techniques.

[7]  Jaehyok Yi,et al.  Optimum design for linearity and efficiency of a microwave doherty amplifier using a new load matching technique , 2001 .

[8]  Frederick Raab,et al.  Efficiency of Doherty RF Power-Amplifier Systems , 1987, IEEE Transactions on Broadcasting.

[9]  P. Asbeck,et al.  High-Efficiency Envelope-Tracking W-CDMA Base-Station Amplifier Using GaN HFETs , 2006, IEEE Transactions on Microwave Theory and Techniques.

[10]  Jinsung Choi,et al.  Power Amplifiers and Transmitters for Next Generation Mobile Handsets , 2009 .

[11]  J. Kim,et al.  Highly Efficient Three-Way Saturated Doherty Amplifier With Digital Feedback Predistortion , 2008, IEEE Microwave and Wireless Components Letters.

[12]  J. Moon,et al.  High-Efficiency Hybrid EER Transmitter Using Optimized Power Amplifier , 2008, IEEE Transactions on Microwave Theory and Techniques.

[13]  K. Horiguchi,et al.  Efficiency Enhancement of 250W Doherty Power Amplifiers Using Virtual Open Stub Techniques for UHF-band OFDM Applications , 2006, 2006 IEEE MTT-S International Microwave Symposium Digest.

[14]  R. Sweeney.  Practical Magic , 2008, IEEE Microwave Magazine.

[15]  P. Asbeck,et al.  An extended Doherty amplifier with high efficiency over a wide power range , 2001, 2001 IEEE MTT-S International Microwave Sympsoium Digest (Cat. No.01CH37157).

[16]  Y. Suzuki,et al.  Highly efficient feed-forward amplifier using a Class-F Doherty amplifier , 2003, IEEE MTT-S International Microwave Symposium Digest, 2003.

[17]  Feipeng Wang,et al.  An Improved Power-Added Efficiency 19-dBm Hybrid Envelope Elimination and Restoration Power Amplifier for 802.11g WLAN Applications , 2006, IEEE Transactions on Microwave Theory and Techniques.

[18]  J. Laskar,et al.  Analysis and design of a high-efficiency multistage Doherty power amplifier for wireless communications , 2005, IEEE Transactions on Microwave Theory and Techniques.

[19]  S. C. Cripps,et al.  RF Power Amplifiers for Wireless Communications , 1999 .

[20]  A. Inoue,et al.  Efficiency enhancement of Doherty amplifier with combination of class-F and inverse class-F schemes for S-band base station application , 2004, 2004 IEEE MTT-S International Microwave Symposium Digest (IEEE Cat. No.04CH37535).

[21]  Bumjae Shin,et al.  A fully matched N-way Doherty amplifier with optimized linearity , 2003 .

[22]  Leonard R. Kahn,et al.  Single-Sideband Transmission by Envelope Elimination and Restoration , 1952, Proceedings of the IRE.

[23]  C. Weitzel,et al.  RF power amplifiers for wireless communications , 2002, 24th Annual Technical Digest Gallium Arsenide Integrated Circuit (GaAs IC) Symposiu.

[24]  Bumman Kim,et al.  A highly efficient Doherty power amplifier employing optimized carrier cell , 2009, 2009 European Microwave Conference (EuMC).

[25]  F. Raab,et al.  Power amplifiers and transmitters for RF and microwave , 2002 .

[26]  S. Sano,et al.  A 80W 2-stage GaN HEMT Doherty Amplifier with 50dBc ACLR, 42% Efficiency 32dB Gain with DPD for W-CDMA Base station , 2007, 2007 IEEE/MTT-S International Microwave Symposium.