High-power UHF Doherty amplifier output combiner network optimization by 3-port sub-circuit X-parameters characterization

This letter presents the optimization of the output combining network for a high-power broadband Doherty power amplifier (DPA), based on the adoption of X-parameters. The technique relies on the large-signal vector network characterization of the prospective 3-port DPA subcircuit. The characterization stage was carried out implementing the devices termination by the multisection artificial transmission line technique, thus allowing to terminate the devices with more accurate impedance value across the frequency band. This was validated by the development of the output combiner of a DPA prototype based on a pair of silicon laterally diffused MOSFETs (Si-LDMOS). It was optimized in the band from 640 to 920 MHz, while the experimental characterization has been compared with a previous similar design. The DPA prototype exhibited a peak power of 54.4 dBm at centre frequency, with a peak drain efficiency (DE) up to 62% and 56% respectively at peak power and at 8-dB input power back-off. The data show a significant improvement with respect to the previous implementation of the same device technology and input network. This is due to the accuracy of the X-parameter model based simulations, which is estimated to be 5.6 E-3 in terms of normalized mean error.