This paper introduces a broadband S-band Outphasing power amplifier. The low-pass matching network is applied for both the post-matching structure of Doherty power amplifier(PA) and the phase compensation of combiner to achieve high drain efficiency at saturation and power back-off in a wide frequency range. The design was simulated using Cree's GaN HEMT CGH40010F transistor. The results of the proposed Outphasing PA show that the drain efficiency is higher than 63% within a 46.5dBm saturated output power and the 6dB power back-off drain efficiency is 45%-50% in 3.0-3.8 GHz