Design of a broadband Doherty power amplifier with a modified load modulation network

Abstract

This article proposes a modified load modulation network (LMN) to adjust the structure of the traditional Doherty power amplifier (DPA) by changing one λ/4 microstrip line at the output of the main power amplifier into two in the traditional DPA and connecting a terminal open-circuit λ/2 line at the junction point which can reduce the frequency dispersion effect to achieve expanding bandwidth while maintaining saturation efficiency and back-off efficiency. A 2.6-3.6 GHz DPA is designed and fabricated for verification. The measured results show that the saturated output power is 42.5-44 dBm and the saturated efficiency is between 63% and 69% in target band with a gain from 8.5 to 9.5 dB. At 6 dB output power back-off, the drain efficiency is 44%-50%. And the adjacent channel power ratio (ACPR) is better than À32 dBc. K E Y W O R D Sback-off efficiency, broadband, Doherty power amplifier (DPA), load modulation network | INTRODUCTIONWith the rapid development of wireless communication technology, spectrum resources are becoming increasingly scarce. Communication operators usually adopt more complex modulation methods to increase the amount of data carried per unit bandwidth. However, this will also increase the peak to average ratio (PAPR) of the signal, which puts forward higher requirements for the output power back-off (OBO) efficiency of the RF power amplifier (PA) which is an important part of the communication system. Doherty power amplifier (DPA) is widely used in wireless communication systems because of its simple structure and high back-off efficiency, but its narrowband effect limits its development. [1][2][3][4] One of the main reasons is the influence of load modulation network (LMN). According to the transmission line theory, when the operating frequency deviates from the center frequency, there is a frequency dispersion effect which cause the input impedance of the LMN deviates from the designed value.