| Summary: | This paper presents a multi-mode asymmetric power amplifier (MMA-PA) with back-off efficiency enhanced. The back-off efficiency is improved without a lossy impedance inverter or signal separator which is used in conventional Doherty or outphasing amplifiers. The design considerations are provided in detail. In addition, the proposed MMA-PA operates in three different modes according to the input power level. The PA is fabricated in a 65-nm bulk CMOS technology. In the high-power (HP) mode, the MMA-PA exhibits a small-signal gain of 18.4 dB at 30 GHz and a 3-dB bandwidth of 4.5 GHz from 27.7 to 32.2 GHz. The output saturation power (<inline-formula> <tex-math notation="LaTeX">$\text{P}_{\text {sat}}$ </tex-math></inline-formula>) and 1-dB compression power are 16.7 and 15.1 dBm, respectively. While the peak power-added efficiency (PAE) is 24.7 %, the 6-dB back-off PAE keeps a high value of 14.1 %. In the medium-power (MP) mode, <inline-formula> <tex-math notation="LaTeX">$\text{P}_{\text {sat}}$ </tex-math></inline-formula> and peak PAE are 12.3 dBm and 21 %, respectively with a similar gain of 18.9 dB. In the low-power (LP) mode, the gain, <inline-formula> <tex-math notation="LaTeX">$\text{P}_{\text {sat}}$ </tex-math></inline-formula> and peak PAE are 11.5 dB, 7.5 dBm and 16.5 %, respectively. With a 100-MHz 64-QAM OFDM signal following the 3GPP 5G NR standard, the average output power in the HP mode is 6 dBm with EVM of −26 dB and ACLR < −34 dBc. In the MP and LP modes, the average output power are 3.3 and 1.1 dBm, respectively with EVM of −26.6 and −26.1 dB.
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