In this paper, we present a new topology to implement a passive 180° phase shifter using on-chip auto-transformer in an SOS process. The measured results indicate a phase variation of less than 2° over the frequency range of 1.8-2.4GHz. An insertion loss of 2.3dB was measured at 2.1GHz. The phase shifter has a small footprint of 0.3 × 0.7mm², which is almost three times less than a traditional high-pass/low-pass design. Post-layout simulations indicate similar performance in terms of bandwidth, phase accuracy and insertion loss for both circuits. Stacked switches are used to increase the power handling to +20dBm.