This paper presents the analysis and design of a high-efficiency broadband Class-EM power amplifier (PA). The inherent broadband characteristic of the Class-EM PA that turns out due to the resistive-reactive impedance terminations at the fundamental and second harmonic frequency is first investigated and then extended using double reactance compensation technique by incorporating a parallel resonator LPCP into the main circuit’s load network. Designed using GaN HEMTs, the PAs deliver 23.7% (without LPCP) and 60.4% (with LPCP) fractional bandwidth, within which the PAE is larger than 60% and the output power is relatively constant.