A Transformer-Based Dual-Mode Ka-Band CMOS Power Amplifier With Enhanced Back-Off Efficiency

This paper presents the differential-to-single-ended load-modulated (DISEL)-power amplifier, a transformer-based dual-mode Ka-band CMOS power amplifier designed to enhance power-added efficiency (PAE) under output back-off conditions while maintaining high output power. Implemented in a 65-nm RFCMOS process, the DISEL-power amplifier employs a reconfigurable architecture that switches between differential and single-ended modes according to output power levels without requiring complex impedance matching networks. In high-power mode, the power amplifier operates as a fully differential three-stacked FET structure to deliver high output power, while in low-power mode, one differential branch is selectively deactivated to increase the load impedance seen by the active path, thereby improving PAE under output back-off conditions while maintaining linearity. Measurement results with a 28.0 GHz continuous wave input demonstrate a saturated output power exceeding 23.1 dBm and a peak PAE of 28.5% in high-power mode. Furthermore, at a 10 dB output back-off point relative to the high-power mode, the low-power mode achieves a 2.3 percentage point improvement in PAE. These results verify that the proposed DISEL-power amplifier structure can maintain high output power while improving PAE under output back-off conditions.