Research And Design Of S-band Broadband Doherty Power Amplifier

The Doherty Power Amplifier(DPA)is widely used in various wireless transmitters because it can maintain a high efficiency within a certain power back-off region,and achieves a good balance between efficiency and linearity.However,the working bandwidth of the conventional DPA is narrow,which limits its applications in broadband communication systems.This work is based on the above application backgrounds,mainly completes the designs of two broadband DPAs.At the same time,due to the needs of the test,a driver-stage power amplifier(PA)is also designed in this work.In this work,firstly,the basic indicators of radio frequency power amplifier(RFPA)are introducted,and the basic theories of the DPA are analyzed.Then the designs of the driver-stage PA and the symmetric broadband DPA are completed.The working frequency bands of the two PAs are 3.4GHz-3.6GHz.Besides,in order to expand the working bandwidth of the two PAs,broadband matching networks in the form of low pass filter are applied to their input and output matching networks.Measured results show that the designed symmetric broadband DPA achieves a PAE of 26%-39% within 6dB power back-off(PBO)region,the peaking output power and drain efficiency are 41 dBm and 51% respectively.After summarizing the the design experiences of the driver-stage PA and symmetric broadband DPA,the asymmetric broadband DPA is improved from the following aspects: firstly,an improved power combination network is adopted which can decrease theimpedance transformation ratio of the 1/4 wavelength transmission lines;secondly,the gate bias networks are moved forward before the input matching networks,which aims to reduce the influence on input matching networks;thirdly,considering the impedance migration effect of the GaN transistors,an extra mathching bandwidth is preset outside the low frequency band during the input matching network design of auxiliary PA;finally,in order to extend the power back-off region,an asymmetric configuration is adopted.In the working frequency band of 3.3GHz-3.6GHz,measured results of the asymmetric broadband DPA show that the peaking output power is approximately 42 dBm,the PAE is 36%-45% within the 9dB power back-off region,the peaking PAE is between 58%-62%,and the gain is 10dB-13.5dB when the output power is below 40 dBm.Besides,the output spectrum and Adjacent Channel Power Ratio(ACPR)of the asymmetric broadband DPA are measured by using broadband modulated signals.The methods used in this workare beneficial to improve the bandwidth,power back-off region and efficiency of DPAs,and can provide corresponding theoretical reference and design experience for the subsequent ultra-wideband and high efficiency DPAs.