A novel proportional series combining transformer using for power amplifier (PA) is presented. Compared with balanced series combining structures, the high efficiency range is extended by sophisticated power adjusting and impedance tuning. As a proof-of-concept, a 2.5 V dual-channel PA with the proposed transformer was implemented in standard 0.18-µm CMOS process. The fabricated PA achieved a saturated output power (P sat ) of 28 dBm and a maximum linear output power (P 1-dB ) of 26.8 dBm with the power added efficiency (PAE) of 33.5% and 31%, respectively. With power mode control, the high efficiency range exceeded 8.2 dB. The PA satisfied EVM requirements of LTE 2.3 GHz 20 MHz/64-QAM signal and WLAN 802.11g signal, respectively. Keywords: power amplifier, transformer, power combiner, proportional, power mode control Classification: Integrated circuits

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