| Radio frequency power amplifier is the core component in the communication system,and to improve its energy and spectral efficiency are the two main requirements for the new 5G wireless communication systems.Doherty technique is utilized as the solutions to enhance energy efficiency,with active load-pull modulation to improve the efficiency in the power back-off region,which is rated as one of the most promising techniques.Linearization is the key technology to improve the efficiency of spectrum utilization.Of all the linearization techniques,DPD is one of the most cost-effective options and it creates an expanding nonlinearity that is complementary to the nonlinear characteristic of the power amplifier and cancels the distortion.The difficulty of the amplifier design lies in the precise control of the microstrip line impedance characteristics,and the conversion from transistor input and output impedance to 50 ohm.In this paper,1.85 GHz MRF6404 RF transistor is used to design the matching network by two-element method,resonance method and microstrip method,and each method is analyzed.Finally,microstrip line components are used to design the overall circuit and circuit boards are fabricated and measured.The main work and research results of this thesis are as follows:(1)To solve the impedance difference between the input and output impedance of the transistor in the RF power amplifier and the standard 50 ohm impedance of the RF system,the input impedance matching circuits are designed by using two-element method,resonance method and microstrip method based on the impedance characteristic of MRF6404 transistor.Three matching circuits are built in the software platform,verified and fabricated.The experimental results show that there are some differences between the test results of the three methods and the simulation results due to the existence of the high frequency parasitic effects of the lumped elements.The matching circuit designed by resonance method can obtain the wide bandwidth by controlling the quality factor,and the impedance matching circuit designed by microstrip method has the advantage of lower mismatch loss than the circuit of two-element method and resonance method.(2)To solve the center frequency shift of the amplifier matching circuit and thewhole layout,the simulation circuits are built to study the influence of the bias capacitance and the matching capacitance position deviation and the microstrip discontinuity on the center frequency of the microstrip matching circuit.The results show that the center frequency is shifted from 1.85 GHz to 1.87 GHz when the position deviation of the bias capacitance is 2 mm,and another two position deviations of the matching capacitance will cause the center frequency to shift from1.85 GHz to 1.89 GHz and 1.75 GHz,respectively.The microstrip discontinuity will shift the center frequency from 1.85 GHz to 1.83 GHz.These center frequency shifts can also cause mismatches to increase.(3)The input and output matching circuits are redesigned based on the research results of the deviation from the center frequency.The test results show that the center frequency of the original input impedance matching circuit is 2.27 GHz,resulting in a deviation of 23%.The improved circuit reduces the deviation to 3.7%.The center frequency of the original output impedance matching circuit is 1.97 GHz.The improved circuit reduces the deviation to 3.6% and the reflection coefficient is optimized from-7.5 dB to-42.6 dB;the center frequency deviation of the overall layout is reduced from 16.8% to 3.2%. |
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