A Four-Way Series Doherty Digital Polar Transmitter at mm-Wave Frequencies

This article presents an efficient digital polar transmitter (DPTX) at mm-wave frequencies that exploit a novel N -way series Doherty combiner (SDC) to enhance its drain and system efficiency at deep power back-off (PBO). The proposed N -way SDC is scalable and can be implemented elegantly using N transformers and N-1 shunt capacitors. As a proof of concept, a four-way Doherty DPTX is realized with the proposed SDC in which four identical but independent digital phase modulators deliver a phase-modulated constant envelope signal to their corresponding digital power amplifiers to perform the required amplitude modulation. Fabricated in a 40nm CMOS process, the proposed DPTX occupies a core area of 1.1 \mathrm {mm^{2}} and exhibits 18.7dBm saturated output power and