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In this paper a two-stage 2-GHz GaN HEMT amplifier with 15-W output power, 28-dB power gain, and 70% power-added efficiency (PAE) is presented. The power stage is designed to operate under class F conditions. The driver stage operates under class F-1 conditions and feeds the power stage with both fundamental and second harmonic components. The inter stage matching is designed to target a quasi-half sine voltage shape at the intrinsic gate port of the power stage. The goal is to reduce aperture angle of the power stage and get PAE improvements over a wide frequency bandwidth. In addition to the amplifier design description, this paper reports original time-domain waveform measurements at internal nodes of the designed two-stage power amplifier using calibrated high-impedance probes and large signal network analyzer. Furthermore, waveform measurements recorded at different frequencies show that aperture angle remains reduced over large frequency bandwidth. In this study, a PAE greater than 60% is reached over 20% frequency bandwidth.