A novel MEMS-based co-designed power amplifier is presented. To introduce the analysis and evaluate the impact of realistic on-chip losses, two design approaches are discussed and compared. Then the study of the novel circuit, based on the integration of high-Q BAW resonators with a Class E PA, is described. The integration method is explained, demonstrating how a careful co-design can allow to reach optimum performances. To confirm the theory a design example in a standard 0.18 mum CMOS technology is provided, showing an output power of 14.7 dBm and a drain efficiency up to 56% at 2.44 GHz.