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 <−40dBc LO feedthrough. It demonstrates a drain efficiency of 33%/36%/22% at 0/4.5/11.5dB PBO at a 29.5GHz carrier frequency. While transmitting a 300MHz 64-QAM OFDM signal with a peak-to-average power ratio of 10.7dB, the DPTX achieves 18%/8% average drain/system efficiency, −27.6dB error vector magnitude, and −27.5dBc adjacent channel leakage ratio. To the best of our knowledge, this work is the first reported mm-wave Doherty transmitter that includes the entire chain all the way from the binary data stream up to the modulated mm-wave output signal.