  This paper presents the first multiband mm-wave linear Doherty PA in silicon
for broadband 5G applications. We introduce a new transformer-based on-chip
Doherty power combiner, which can reduce the impedance transformation ratio in
power back-off (PBO) and thus improve the bandwidth and power-combining
efficiency. We also devise a "driver-PA co-design" method, which creates
power-dependent uneven feeding in the Doherty PA and enhances the Doherty
operation without any hardware overhead or bandwidth compromise. For the proof
of concept, we implement a 28/37/39-GHz PA fully integrated in a standard
130-nm SiGe BiCMOS process, which occupies 1.8mm2. The PA achieves a 52% -3-dB
small-signal S21 bandwidth and a 40% -1-dB large-signal saturated output power
(Psat) bandwidth. At 28/37/39GHz, the PA achieves +16.8/+17.1/+17-dBm Psat,
+15.2/+15.5/+15.4-dBm P1dB, and superior 1.72/1.92/1.62 times efficiency
enhancement over class-B operation at 5.9/6/6.7-dB PBO. Moreover, the PA
demonstrates multi-Gb/s data rates with excellent efficiency and linearity for
64QAM in all the three 5G bands. This PA advances the state of the art for
Doherty, wideband, and 5G silicon PAs in mm-wave bands. It supports drop-in
upgrade for current PAs in existing mm-wave systems and opens doors to compact
system solutions for future multiband 5G massive MIMO and phased-array
platforms.
