A New Compact Load Network for Doherty Amplifiers Using an Imperfect Quarter-Wave Line

In this paper, we propose a new compact load network for high-power Doherty amplifiers using an imperfect quarter-wave line. A compact $\pi$-network quarter-wave transmission line, which consisted of two shunt capacitors and a series microstrip line, was used to replace a bulky microstrip transmission line. A simple $L$–$C$ matching network was then deployed after the quarter-wave line. To take the inherent internal parasitic components in the packaged high-power transistors into account, the shunt capacitors of the $\pi$ -network quarter-wave transmission line were readjusted, which made an imperfect quarter-wave line. For verification, both the conventional and compact Doherty amplifiers were designed and implemented for the 859-MHz band using high-voltage laterally diffused metal–oxide–semiconductor field-effect transistors. The measured linearities and efficiencies of both amplifiers were compared to each other with respect to the reference performance of the class-AB amplifier for two-tone and down-link wideband code-division multiple-access signal excitations. The compact Doherty amplifier with a significantly reduced size for the load network by 75.7% performed even better than the conventional Doherty amplifier.