Design And Implementation Of Broadband Doherty RF Power Amplifier With High Back-off Efficiency

With the rapid development of modern mobile communication system,wireless signals' modulation has evolving form QPSK,8-PSK to higher order like 16-QAM,64-QAM,256-QAM because of the requirements of high throughout.The signal with high order modulation exists a high peak to average ratio(PAR),which means the linearity and high return efficiency of the transmitter are demanded.To maintain the efficiency of RF power amplifier(PA)working in the state of back-off mode,a circuit topology named Doherty is widely applied in RF power amplifier design areas.The traditional Doherty architecture has its limit in only has a good narrowband working characteristics.Yet,due to the employed spectrum's fragmentation distribution and the forward compatibility design of the system,the working band range from 800 MHz to 3.6 GHz and has a large spread.Many designs in power amplifier have consider back-off efficiency and bandwidth as the focus of research because the communication systems,demands in these aspects.Based on a modified harmonic impedance matching technique and the design of auxiliary power amplifier output matching circuit design,this paper presents a method of design broadband RF power amplifier with high back-off efficiency and finally give an implementation of Doherty PA of this method.The main content of this paper is as follows:The harmonic impedance matching method based on modified simplified real frequency technique(SRFT).By the analyzation of PA's Load-line model at the back-off mode,the optimal fundamental and harmonic impedance is found in broadband operation mode.Based on modified SRFT,the implementation of broadband matching network for output of PA at fundamental and harmonic is well validated.A single branch broadband PA is designed to proof the effective of this matching algorithm.The "two point" matching design method based on analysis of active load modulation on GaN PA transistor model.Using this analysis,the optimal impedance matching goals of auxiliary PA in saturation mode and back-off mode is found.And the matching network is implemented according to the analysis of PA model.Finally,a broadband Doherty power amplifier with high back-off efficiency is given.The proposed PA has a 40%bandwidth(1.8-2.7 GHz).The output power range from 42 dBm to 43 dBm,a 55-65%saturation drain efficiency and a 52-55%,6 dB back-off drain efficiency is measured.Which gives a good validation to those methods.