This paper presents a W-band three-stage amplifier using 0.13-μm SiGe BiCMOS process, which is implemented with transformers for inter-stage matching and single-to-differential transformation. However, for balun operation, conventional transformer exhibits imbalance characteristic resulting from the complex parasitic effects arising in the millimeter-wave frequency range. An anti-pair capacitor structure is devised and applied to improve amplitude and phase balance and further benefit the RF performance. A two-way transformer-based parallel combiner embedded with several co-optimizing elements is designed to implement power combining and optimum load impedance matching. The amplifier achieves a peak gain of 28.2 dB at 93 GHz, with a 3-dB bandwidth from 85 to 96 GHz. The measured saturated output power (Psat) and maximum power added efficiency (PAE) at 94 GHz are 14.1 dBm and 9.2%, respectively. The power amplifier with only 0.6-mm2 area exhibits excellent gain and maintains competitive output power and PAE in W-band.