| Modern communication systems have developed rapidly over the past decade.However,the faster data rate makes the peak-to-average power ratio(PAPR)of digitally modulated signals in the system a great challenge to the design of power amplifiers.Although the Doherty power amplifier has been proved to be a good choice for modern high-performance transmitters,especially in the design of base-station amplifiers,which accounts for a considerable proportion,5G communication systems impose stricter requirements on the bandwidth,efficiency,and linearity of power amplifiers.Therefore,based on expanding the Doherty bandwidth,this paper will take the enhancement of the efficiency and back-off range as the main research direction.The main contents of this paper are as follows:Firstly,the research background of Doherty power amplifier is summarized,and the research achievements and progress of broadband,post-matching and three-way Doherty are summarized respectively.Secondly,based on the bandwidth limitation of traditional Doherty,a post-matching structure which can improve the working bandwidth of Doherty amplifiers is analyzed.Its working principle is deduced,and its new requirements for the carrier branch output matching network are pointed out.Based on the analysis of a low-order impedance inverter suitable for the post-matching structure,a high-pass low-order impedance converter suitable for improving the efficiency of the traditional Doherty power amplifier is proposed,and a Doherty power amplifier working at 3.45 GHz is designed.The simulation results show that its saturation output power is greater than 45.2dBm,the drain efficiency at saturation point is 78.9%,and the drain efficiency is greater than 68.3% within the 6dB back-off range.Then,a design combining three-way Doherty and post-matching technology is adopted to expand the bandwidth and the back-off range,and also maintain high efficiency when the signal with high PAPR modulation is amplified.The feasibility and design method of the low-order impedance inverter used in the design of the three-way post-matching Doherty power amplifier are derived.The technical shortcomings of converting the low-order impedance inverter designed based on lumped parameter element to distributed parameter element are pointed out,and the improvement method is put forward when it is used in the three-way post-matching Doherty power amplifier.In order to verify the effectiveness of the scheme,a three-way post-matching Doherty amplifier working at 1.8~2.7GHz is designed.The simulation results show that its saturation output power is 46.3~47.5dBm,the gain at saturation point is 8.5~10.5dB,and the drain efficiency within 9.5dB back-off range is greater than 55%.Finally,the paper is sorted and summarized,and some suggestions for improving shortcomings are put forward. |
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