| Power amplifier is a very important component in Wireless communication system. With the development of the third generation mobile communication technology, the linearization of power amplifier has become a very important issue and also is a hot research field. This research is aimed at the feedforward linearization of the power amplifier.In this paper first the nonlinear properties of RF power amplifier and the conversion distortion of AM-AM and AM-PM is analyzed, also the effects of nonlinear distortion to radio transmitters is analyzed. Second the usual linearization method is discussed according to the domestic and overseas latest research, including: feedback, pre-distortion and feedforward linearization technique. Finally the decision factors of the feedforward system gains are discussed, and the effect that the insertion loss of the time delay elements has on the overall efficiency is theoretically analyzed.The main work of this paper is to design a three-stage power amplifier with the center frequency of 880MHz, 10MHz bandwidth, the output power of 40W for CDMA system. The output stage of this amplifier consists of two single-stage amplifiers connected in parallel. We discuss the matching circuit of the output-stage in detailed, then make S parameter simulation and Harmonic Balance simulation using ADS , the result is P1dB=43dBm with very low ports reflection loss. This power amplifier is linearized using feedforward linearization technique. Because of the high requirements of amplitude and phase balance, vector modulator is used to adjust the amplitude and phase balance, and the least power detection method is used to tune the I/Q coefficient of the vector modulator. Considering the adaptive instrument, the RF power amplifier which fulfils the practical requirement is finally designed and simulated. Two-tone Harmonic Balance simulation shows that IMD3 has a reduction of 25dB, the total IMD3 cancellation of the power amplifier achieves anticipated -65dBc. |
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