This article proposes a broadband differential GaN power amplifier (PA) with coupled-line-based matching networks for Ku-band satellite communication. The developed matching technique can achieve flat wideband impedance matching with a compact area. Also, this work employs a combination of series and parallel power combining topology to improve the impedance conversion ratio of the output matching network, thereby reducing the insertion loss. As proof of the above concept, our two-stage PA is realized in a 0.25-μm GaN pHEMT process. The PA scores 18.2-to-21.2 dB gain, 38.6-to-39.9 dBm saturated output power (Psat), and 35%-to-42% peak power added efficiency (PAE) from 10.5 to 15.5 GHz. The PA shows a flat RF performance across 10.7-to-14.9 GHz with <1 dB gain ripple and <0.7 dB Psat ripple. Meanwhile, the total chip size is 2.7 × 2.4 mm2, corresponding to a power density up to 1.47 W/mm2.