In this work, a high efficiency power amplifier based on Class-E topology is presented. When considering the reduction in overall size and cost and the trade-off between efficiency and output power, a PA with finite drain inductance manifests its superiority over an RFchock topology. An on-chip transformer with multiple functionalities is also designed, its primary coil works as the finite drain inductance and it achieves power recombination and impedance matching simultaneously. The circuit is fabricated in 65nm CMOS technology. According to post-simulation, the PA can provide 37.28 mW output power while consuming 96.8mW DC power.