Contents Introduction
Analog power amplifiers (PAs) have been dominant at mm-wave frequencies. Recently, an outphasing transmitter (TX) [1] and a Doherty PA [2] were introduced in the Ka-band, obtaining high efficiency (>30%) and high output power (>20dBm). Outphasing and Doherty have been dependable techniques to improve the back-off efficiency, but require complex passive networks. Inverse class-D architectures lead to high voltage stress on the output-stage devices. Both architectures require high-speed DACs, which decreases the system efficiency (SE). To overcome this, power DACs [3],[4] were proposed. In [3], multi-stacked FET current cells work as mm-wave DACs, using Gilbert-cell upconverters; however, due to the wide-bandwidth specification, the SE got degraded to 10.3%. Additionally, at mm-wave, distributed effects contribute significantly to load-impedance variation, resulting in reduced output power. The synthesized impedance variation technique [4] offers means to compensate the distributed effects in the combination network and increases SE to 22%. Voltage-mode switched-capacitor RFDACs (SCPAs, [5]) have proven nearly optimal for wireless TXs in CMOS because they occupy small chip area, are energy efficient and benefit from scaling. Every transistor in an SCPA is switched (i.e., devices do not function as linear current sources); hence, the power consumption scales . The SCPA is a segmented class-D amplifier (Fig. 1a), where the switches and capacitors are segmented in parallel paths that are digitally (en/dis)abled to provide linear amplitude control. In this work, we present for the first time an SCPA operating in the mm-wave regime. Operation at high frequencies is enabled by edge-combining, in-slice LO generation and careful design of distributed effects in the capacitor array.
