A 39 GHz Doherty-Like Power Amplifier With 22dBm Output Power and 21% Power-Added Efficiency at 6dB Power Back-Off

The design of a Doherty-like power amplifier for millimetre-wave (mm-wave) applications is presented in this work. The designed power amplifier employs a novel symmetrical load-modulated balanced amplifier (S-LMBA) architecture. This design is advantageous in minimizing the undesired impedance interaction often encountered in the classic LMBA approach. Such interactions are typically due to the use of a non- $50~\Omega $ load at the isolation port of the output quadrature coupler. Moreover, magnitude and phase control networks are carefully designed to generate the specific magnitude and phase information for the designed S-LMBA. To demonstrate the proposed ideas, the S-LMBA is fabricated in a 45-nm CMOS SOI technology. At 39 GHz, a 22.1 dBm saturated output power ( $\text{P}_{\mathrm {sat}}$ ) with a maximum power-added efficiency (PAE) of 25.7% is achieved. In addition, 1.68 times drain efficiency enhancement is obtained over an ideal Class-B operation, when the designed S-LMBA is operated at 6 dB power back-off. An average output power of 13.1 dBm with a PAE of 14.4% at an error vector magnitude (EVM $_{\mathrm {rms}}$ ) above -22.5 dB and adjacent channel power ratio (ACPR) of -23 dBc is also achieved, when a 200 MHz single carrier 64-quadrature-amplitude-modulation (QAM) signal is used. Including all testing pads, the footprint of the designed S-LMBA is only 1.56 mm2.