| Due to the rapid development of Artificial Intelligence(AI)and Internet of Things(IoT)areas,as well as the very tensive spectrum resources,higher transmission rate of the modulated signals is just around the corner for the wireless communication systems.The current fourth generation wireless communication system has been unable to meet the rising needs of the contemporary society.Therefore,the fifth generation wireless communication system using multi-input multi-output(MIMO)antenna array technology is adopted for data transmission in millimeter-wave band and has gradually attracted worldwide attentions from scientists in this field.Specially,as one of the core components of modern wireless communication system,the RF power amplifier shows significant impact on the transmission quality and distance of modulated signal to a certain extent.Among the existing choices,Doherty power amplifier has become one of the most popular choices for engineers to deal with the challenges since it is able to amplify peak-to-average ratio modulated signals even if in the power back-off region.According to the background of this research,investigations on the bandwidth and working efficiency of symmetric and asymmetric Doherty power amplifiers are carried out through the broadband design of power amplifier matching circuit and the optimization of the power synthesis network and compensation line tuning.The main contributions and results of this work are as follows:Firstly,the basic indicators and categories of RF power amplifiers are introduced in detail.Then,the principle of Doherty structure is discussed using active load traction to achieve high efficiency amplification.The voltage and current characteristics of Doherty power amplifier are analyzed by formulation under different conditions.By introducing the change of its output impedance and the function of compensation line,the relationship between these parameters and the performance indices such as the efficiency of the power amplifier and the power of the back-off is analyzed.Secondly,combining with theoretical analysis and software simulation,the design process of symmetrical Doherty power amplifier is introduced in detail by utilizing the characteristics of Doherty power amplifier which can effectively amplify the peak-to-average ratio signal in power regress area.The experimental results show that the amplifier can achieve 400 MHz bandwidth and maintain over 30% drain efficiency at 6 dB power back.When the input signal frequency is 3.5 GHz,the saturated output power of the amplifier is 40 dBm and the drain efficiency is up to 48%.Finally,in order to deal with the increasing peak-to-average ratio of modulated signals in communication systems,based on the symmetric broadband Doherty power amplifier,the asymmetric Doherty power amplifier is designed by using an asymmetric microstrip power divider combined with an appropriate gate bias.At the frequency of 3.5 GHz,the saturated output power of the amplifier is 42 dBm and the drain efficiency is 24.6% when the power regress area is extended to 9 dB.The nonlinearity of the amplifier is investigated by using an equal amplitude and phase dual-tone signal working at 3.5 GHz central frequency with a frequency space of 5 MHz.The IMD3 reaches-39.4 dBc at 6 dB power return point.In this paper,it is proven that Doherty power amplifier illustrate the advantage in dealing with the peak-to-average ratio signal via numerical simulations and systematical measurements.Most importantly,it future demonstrates that the proposed symmetric and asymmetric Doherty power amplifiers are able to work with high-efficiency in a broad frequency band of interest,which provides instructive significance for the future wireless communications. |
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