This paper presents the problem of the parasitic effect of the transistor for the practical design of high power and high-efficiency power amplifiers (PAs). To solve the problem, we propose a new method: transferring the impedances at the intrinsic plane into those at the package plane with the help of the package model. In this case, the convenience of the design of the output matching network is improved a lot. Moreover, the design methodology of PAs with compact microstrip resonant cell (CMRC) as well as the topology of the transmission-lines (TLs) are also proposed. The CMRC can provide the required open-circuit for the third harmonic. On this basis, the harmonic impedance conditions can be easily realized by the tuning TLs. The insertion loss of the proposed CMRC at the fundamental is low and the physical dimension is relatively small. To verify the feasibility of the proposed circuit, using a 10W GaN HEMT CGH40010F transistor, a switch-mode class E/F PA operating at 2.2 GHz is designed as a prototype. Simulation results show the power-added efficiency of 78.4%, output power of 40.1 dBm, and power gain of 12.1 dB.