| As the development of3G&4G（LTE-Advanced）wireless communication system, thespectrum resource becomes fewer and fewer. Multi carriers, broadband and digital non-constantenvelope modulation technology have been used to meet the requirement of high speed datatransfer rate. The new modulation has improved the transfer rate, while increasing the signal peak-to-average power ratio (PAPR). The broadband and high PAPR signal makes a big challenge to thedesign of the power amplifier. Thus, we have to consider the efficiency requirements and itslinearity at the same time. The problem of efficiency and linearity needs to be solved in the4Gcommunication system.Comparing with other technologies to improve the efficiency of power amplifier modules,Doherty power amplifier structure is relatively simple, easy to implement, low cost, and highefficiency for high PAPR signals. Doherty structure combined with digital pre-distortiontechnology (DPD) can improved the linearity and efficiency of power amplifier significantly.Hence, it can be applied to the4G communication system.In this paper, the main work is divided into four parts:In the first part, we introduce the performance index and classification of power amplifiers.Then we make the introduction of several key technologies for improving power amplifierefficiency. Comparing with other technologies, the Doherty power amplifier has advantages interms of circuit complexity, cost, and effeciency.In the second part, we make a detailed analysis of the two-way traditional symmetricalDoherty amplifier’s basic principle and its entire work process. After actual test，we find that themain amplifier and the peak amplifier cannot reach saturation power at the same time，so the peakamplifier cannot make the complete active load-pull to the main amplifier. For this issue, we usethe two-way asymmetrical Doherty power amplifier and three-way Doherty power amplifier. Thenthe principle of the two kinds of Doherty structure is analyzed.In the third part, we design and test the symmetrical Doherty power amplifier. When weemploy the actual communication signal (a5-carriers LTE signal with100MHz bandwidth) tomake test, the output power is40dBm. The gain flatness is±0.5dB. At this time, the drain efficiency of the Doherty PA can reach40%, and the ACPR is-34.93dBc and-35.43dBcrespectively. It can satisfy the requirements of the project.In the fourth part, we summarize and analyze the symmetrical Doherty power amplifierdesigned in the third part. On this basis, we decide to use the asymmetrical Doherty poweramplifier to improve its insufficient. In the design of the asymmetrical Doherty power amplifier,we analyze and optimize the output combined structure, and make PCB board to test itsperformance. When using a3-carrier WCDMA signal (15MHz bandwidth) to test the board, theoutput power can reach50.8dBm. At8.5dB back-off point, the PAE of the ADPA can reach38%.It is improved about3%, comparing with the symmetrical Doherty power amplifier. |
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