| With the development of modern wireless communication systems, wireless communication is moving towards the direction of large capacity and high data transfer rates. To meet the high data rate requirements, the modulation signal becomes more complex, however, the complex modulation signal leads to the signal bandwidth expansion and peak to average power ratio(PAPR: peak-to-average power ration) increases, which needs a new type of power amplifier’s structure and design techniques for the communications signals. Doherty is the most popular technology in base station, because of its simple structure and high efficiency in back-off area. Under this background, this paper expands research on bandwidth and efficiency of Doherty power amplifier. The main contents are summarized as follows:Firstly, based on the theory of the traditional Doherty, this thesis analyzes several factors which limits its bandwidth, such as:the limitation of broadband matching, the impact restrictions of quarter impedance conversion line for Doherty amplifier, the saturated current of main and peaking amplifier have influence on the load modulation of the Doherty amplifier, the compensation line of the peaking amplifier has impact on the main amplifier.Secondly, this thesis proposes an improved load modulation structure of Doherty power amplifier,based on a general expression of the load of the main and peaking amplifier deriving directly from the load modulation network,The proposed new structure is then compared to the traditional Doherty power amplifier. We implemented a Doherty amplifier with Cree’s 10 WGaN HEMT transistor as carrier amplifier and peaking amplifier, respectively. At 3.5GHz, saturated output power of the DPA is more than 43 dBm, saturated drain efficiency is greater than 75%; Under two-tone signal spacing by 5MHz interval testing, drain efficiency of 6d B back-off is greater than 50%, and IM3 is less than-30dBcFinally, this thesis extends the the bandwidth of Doherty power amplifier with improved load modulation structure based on the broadband matching theory. In the first step, the S parameters and the reflection coefficient theory are used to analyze the characteristics of the output network of the main power amplifier, and the reflection coefficient of the input port of the output matching network is obtained. In order to verify the validity of the above analysis, this thesis designs and implements an asymmetric wideband Doherty power amplifier with Cree’s 10 WGaN HEMT transistor and Cree’s 25 WGaN HEMT transistor as carrier amplifier and peaking amplifier, respectively, which works at 3.2—3.7GHz. The saturated output power of the DPA is more than 45 dBm, and the average drain efficiency of 7.5dB back-off greater than 47%, at 9dB back-off, the average drain efficiency is more than 40%. The simulation results are consistent with the theoretical analysis. |
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