Research On Doherty Power Amplifier With High Efficiency And High Back-off Based On Asymmetrical Structure

With the rapid development of wireless communication technology,the future mobile communication systems have increasingly higher requirements for system efficiency and bandwidth.System performance will largely depend on the available RF hardware.Therefore,the RF power amplifier needs to meet the performance of high back-off,high efficiency,larger output power and wider bandwidth,which greatly increases the difficulty and complexity of the design.Doherty power amplifier（DPA）with dynamic load modulation is one of the most commonly used techniques to improve the efficiency of power amplifier.However,the power back-off range of traditional DPA is small,which cannot meet the requirements of future mobile communication systems.Therefore,this thesis takes high power back-off DPA as the research object,adopts the reactance compensation method and the 3-port output network design method to design different output combined network structures to effectively improve the back-off range and efficiency to meet the design requirements of future wireless communications.The main contents are as follows:1、The overall goal of this thesis is to improve the back-off range and efficiency of RF amplifiers to adapt to the development of wireless communication systems.Therefore,firstly,the main technical specifications of the power amplifier are summarized.Then,the existing high efficiency and high back-off power amplifier technologies,such as envelope elimination and restoration,harmonic injection network,generalized combining network and so on,are briefly introduced,which provides a theoretical basis for further research on power amplifiers.2、In order to expand the power back-off range of high efficiency DPA,a design method of asymmetric DPA based on reactance output impedance is studied.Firstly,the working principle of asymmetric DPA is introduced.Secondly,for a specific output power back-off range,an analysis method is proposed to determine the required reactance output impedance.Finally,the two-point matching network is used to design a suitable peaking output matching network to realize the reactance output impedance,so as to increase the efficiency of the carrier amplifier when the power is back-off.Compared with the conventional design method,the output power back-off range can be increased by about 2 d B by using the proposed design.To verify the feasibility of this method,10 and 30 W Ga N HEMT transistors are used as carrier and peaking amplifiers respectively,and the 3.4-3.6 GHz asymmetric DPA with a power back-off range of 12d B is designed.The measured results show that over the entire frequency range,the efficiency is47%-49%at 12d B power back-off and 63-66%at saturated power.For a 40 MHz LTE signal with a carrier frequency of 3.5 GHz,the adjacent channel leakage rate after linearization is-50d Bc,and the average efficiency is higher than 50%.3、In order to simplify the design of DPA output matching network and improve the efficiency at the same time,an optimal design method of DPA based on 3-port output combined network is proposed.Firstly,the principle based on 3-port network S-parameter matrix is analyzed and deduced,and verified by traditional DPA and high back-off DPA.Secondly,based on the DPA designed in Chapter 3,determine the design parameters required for the 3-port output combination network design,selectθ₃₁ as a free variable,and determine the bestθ₃₁ value based on performance comparison.Then,the theoretically derived S parameter value is used as the optimization target value to optimize the topological structure of different output matching networks.Finally,through the comparison of efficiency,back-off range,bandwidth and other performance indicators,the optimal scheme is selected to complete the design of the DPA.In order to compare and verify the effectiveness of the research method,an asymmetric DPA with high back-off efficiency and high saturation efficiency in the 3.4-3.6GHz band is designed using the same carrier and peaking transistors as in Chapter 3.The measured results report that in the entire frequency range,the efficiency at 12d B power back-off is higher than 50%,and the efficiency at saturation power reaches 67.8%-71%.Compared with the previous design,the method used can increase the back-off and saturation efficiency by about 3%and 5%,respectively.The results show that after using the proposed design method,the efficiency of the power amplifier at back-off and saturated power is effectively improved,which can better meet the high efficiency requirements of the future mobile communication system.