Load-variation insensitivity, for impedance matching between power amplifiers (PAs) and transmitting antennas, contributes to challenging the design of millimeter-wave wireless systems. In this paper, a W-band two-way balanced PA based on a compact quadrature coupler with a broadside coupled stripline (BCSL) as the core is presented to enhance the load-variation insensitivity and stability. The proposed coupler is truly broadband with low amplitude and phase imbalance. The proposed W-band balanced PA achieves higher power-added efficiency (PAE) and unsaturated output power sat over wide frequency bandwidth. The W-band balanced PA is implemented in a 0.13-μm SiGe BiCMOS process and achieves a measured sat of 16.3 dBm and a peak PAE of 14.1% at 100 GHz (with 1.6-V power supply). The measured sat with 1-dB bandwidth is from 91 to 102 GHz. The measured results present the feasibility of the compact quadrature coupler. The total chip surface area (with pads) is 0.64 mm 2 , where the size of the proposed quadrature coupler area is only 0.04 mm2.