ScholarWorks@성균관대학교

초록

Previous studies have shown that broadband operation in Doherty power amplifiers (DPAs) can be achieved by employing simplified load networks (LNs) with minimal phase delay for both the carrier and peaking amplifiers, together with a broadband post-matching network (PMN). In this article, a three-element LN synthesis technique based on a pi-to-TF transformation is proposed to realize a broadband DPA with an extended output power back-off (OBO). Using the proposed transformation, the LNs for the carrier and peaking paths are synthesized into a minimal structure consisting of two series elements and one shunt element. This structural simplification facilitates broadband design when combined with a multi section PMN, since the load impedance at the combining junction remains invariant with respect to operating conditions. Closed form design equations are derived from the synthesis process, enabling the exact realization of the simplified structure. Among the feasible solutions obtained from the closed-form equations, the configuration providing the widest bandwidth was selected for implementation. To validate the proposed LN, a DPA was designed using 6- and 10-W gallium nitride high electron mobility transistors (GaN HEMTs), operating over the 2.7-3.8-GHz band and achieving a fractional bandwidth (FBW) of 34.3%. The fabricated DPA was evaluated with a 5G new radio (NR) signal of 100-MHz bandwidth and a peak-to-average power ratio (PAPR) of 7.8 dB, demonstrating a drain efficiency (DE) of 48.5%-56.8% at an average output power of 35.0 dBm. After applying digital predistortion (DPD), an adjacent channel leakage power ratio (ACLR) better than -49.6 dBc was achieved.

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