A single-stage MMIC PA with a center frequency of 75 GHz was designed, which was fabricated based on the InP DHBT in a 0.8 μm process. The ft/fmax of the device was 171/250 GHz. This circuit used a two-layer common-base stack structure, in which the lower CB layer was directly grounded at the base. The emitter at the input port was supplied with a negative voltage of -0.96 V, and the bias voltage Vc2 was 4 V. In order to increase the output power, the devices in the upper and lower layers were in parallel with four fingers. Furthermore, the same devices were used to design a stack structure with a common emitter below. The CB stack was compared with international stack structure circuits manufactured in advanced InP processes , while the CB stack structure had better performances than the CE stack in terms of gain and peak PAE.