An S-band GaN Microwave Monolithic Integrated Circuit Power Amplifier(MMIC PA) is designed and manufactured by exploiting 0.25 μm AlGaN/GaN High Electron Mobility Transistor(HEMT) technologies on SiC substrate. The proposed power amplifier can be applied to the small-cell base. To reduce the size and loss, quarter-wave impedance transformer can be fully integrated using lumped passive elements, and the branch-line couplers are represented by lumped elements in the input matching network. The fabricated PA exhibits, at 3-3.2 GHz in pulse-wave conditions, an output power of 10 W, with a power-added efficiency of 38% at 6 dB of output power back-off. The chip size is 4.0 mm×2.4 mm.