This work presents a compact on-chip outphasing power amplifier with a parallel-combining transformer (PCT). A series-combining transformer (SCT) and PCT are analyzed as power-combining transformers for outphasing operations. Compared to the SCT, which is typically used for on-chip outphasing combiners, the PCT is much smaller. The outphasing operations of the transformer combiners and class-D switching PAs are also analyzed. A tuning inductor method is proposed to improve the efficiency of class-D power amplifiers (PAs) with power-combining transformers in the out-of-phase mode. The proposed PA was implemented with a standard 0.18 µm CMOS process. The measured maximum drain efficiency is 37.3% with an output power of 22.4 dBm at 1.7 GHz. A measured adjacent channel leakage ratio (ACLR) of less than −30 dBc is obtained for a long-term evolution (LTE) signal with a bandwidth of 10 MHz.
A Fully Integrated Compact Outphasing CMOS Power Amplifier Using a Parallel-Combining Transformer with a Tuning Inductor Method