A fully-matched two-stage power amplifier MMIC has been successfully developed operating with the frequency range from 27 GHz to 29 GHz. Platy-transmission lines with low characteristic impedance have been utilized for matching at an intermediate stage and an output one in order to accomplish a compact chip area. Four HEMTs have been combined at an output stage in the developed MMIC. A difference in load impedance at each input port is suppressed by two short stubs in the final output circuit. The stubs are also effective for coincidence of through-phases between four input ports and an output port of the final output circuit. The amplifier has a linear gain of 8 dB and an output power of greater than 30 dBm at 29 GHz. The ultra compact chip area of 1.6 /spl times/ 1.8 mm/sup 2/ has been achieved using the platy-TRL matching method (PTMM). The developed MMIC using the PTMM will be able to be applied to a higher output power amplifier as a basic cell that has an output power of 1 watt.