Symmetric High Turns Ratio Stacked Transformers in 0.35 μm SiGe Technology

A simple, area efficient and symmetric stacked transformer with high turns ratio is proposed in this paper. The primary winding of the transformer is a single spiral realized on the top metal. The secondary winding is conventional symmetric inductor realized on the bottom metal. The width of the primary spiral is same as the composite width of the all the secondary spiral turns resulting in high turns ratio. A simple method to vary the turns ratio is also discussed. Prototype transformer fabricated in a 0.35 μm BiCMOS technology measured turns ratio of 4, while achieving a very low insertion loss of 0.88 dB at 3 GHz.