In this paper a wideband, high-power amplifier that achieves an output power of 20 W with a bandwidth greater than one octave in the L and S bands is presented. Two ~10 W Class AB PAs are implemented with Gallium Nitride-high electron mobility transistor devices and a low loaded-Q matching network to achieve wideband performance. High power added efficiency (PAE) is achieved by combining outphased saturated power amplifiers using a wideband isolated combiner. We introduce new analysis detailing proper selection of the isolation resistance in isolated outphasing combiners. The impedance transforming Wilkinson combiner is designed to interface the system with a 50-$\Omega$ load. The L-S band (1.2~2.5GHz) amplifier was simulated, fabricated, and characterized. The fabricated HPA provides a peak output power of 43 dBm, gain of >15 dB with a peak PAE of >40% across the band. A 10 MHz, 64 QAM, long-term evolution signal is measured and achieves ACLR <; -30 dBc, EVM = 4.8%-rms at an average output power and efficiency of 39.3 dBm and 33.1%, respectively.