This thesis presents the design strategy, implementation and experimental results of a hybrid ClassAB amplifier. The applied design strategy consists in optimizing simultaneously the efficiency and linearity of a single-device class-AB power amplifier, while reaching given minimum output power and gain requirements. The simultaneous selection of optimum source and load terminations that provide the best trade-off among all of the requirements is described, and the synthesis of the matching networks is then presented. Based on the optimized design, a prototype is developed based on a 6 W packaged GaN device around 3.5 GHz, manufactured and measured. The amplifier achieves measured output power higher than 38 dBm with associated gain higher than 12 dB and saturated power-added efficiency in excess of 73% in a single-tone test at 3.25 GHz, while providing a 33% power-added efficiency and -30 dBc inter-modulation distortion in the 20 MHz two-tone test..