Prototype for an Optimized Drive Signal Control System for a 2.5 GHz Doherty Power Amplifier

This paper presents an optimized drive signal control system for a 2.5 GHz Doherty power amplifier (PA). The proposed system enables independent control of the amplitudes and phases of the drive signals fed to the inputs of two parallel PAs that can be used in a Doherty PA architecture. This is achieved by incorporating a variable amplitude shifter (VAS) and a variable phase shifter (VPS) in each of the two parallel paths, which allows for driving uneven power levels with an arbitrary phase difference to the individual PAs. The specific VAS (Qorvo QPC6614) and VPS (Qorvo QPC2108) components that we used for the test system provide an amplitude attenuation range from 22 dB to 31.5 dB with a step size of 0.5 dB and a phase range from 0° to 360° at the intended operating frequency of 2.5 GHz, offering the benefit of characterizing the behavior of PAs under test for a large range of drive signals to optimize the output performance such as power added efficiency or adjacent channel leakage ratio. For demonstration, the proposed drive signal control system is integrated with two parallel GaN transistor-based PAs (Qorvo QPD0005) with a P
1dB
of 37.7 dBm, whose output signals are combined and then measured to provide system characterization. Each PA is preceded by a drive amplifier with a gain of 17.8 dB to boost the power fed into the PA. The proposed circuit also incorporates a 20 dB directional coupler and a 3 dB Wilkinson power splitter, which were designed, fabricated on an FR4 substrate, and experimentally verified. While Qorvo QPD0005 was used as a specific test-case in this demonstration, the proposed system can be used to characterize the behavior of a wide range of Doherty PAs.