Design of Broadband Modified Class-J Doherty Power Amplifier With Specific Second Harmonic Terminations

This paper presents a symmetrical Doherty power amplifier (DPA) based on class-J mode for efficiency enhancement in a broad bandwidth. With a second harmonic suppression integrated into the output matching network of carrier power amplifier (PA) and peaking PA, respectively, the complex interaction of varying second harmonic impedances caused by dynamic load modulation can be eliminated and the second harmonic impedance remains the same reactance at both the output back-off (OBO) region and the saturated power. To meet the requirement of second harmonic termination, the DPA works in conventional class-J mode at the saturated power and a modified class-J continuum is proposed to expand the impedance design space at 8 dB OBO with good efficiency performance. A gallium nitride DPA is designed and fabricated to validate the method over a frequency band of 3.3-3.75 GHz. Under a 10%-duty-cycle pulse excitation, experimental results show the DPA delivers 48-48.8 dBm output power with a drain efficiency (DE) of 58%-71%. At 8 dB OBO, a measured DE of 44%-55% is achieved with a gain of 11.8-13.5 dB. When driven by a 2-carrier 40-MHz long-term evolution signal with a peak-to-average power ratio of 8 dB, the DPA exhibits an adjacent channel leakage ratio of -30 dBc at an average output power of 40.7 dBm at 3.45 GHz.