Low Insertion-loss Stacked Transformers Using Tapered Spirals for High Performance RFICs

Spiral inductors with gradually varying width and space (taper) across the turns are known to have higher quality factor (Q). In this paper, for the first time, stacked transformers are shown to significantly benefit from tapered primary and secondary spirals. It is also revealed that tapered stacked transformers are more effective with increased primary/secondary spiral thickness. Prototype stacked transformers are fabricated using a $0.35 {\mu}{{\mathrm {m}}}$ BiCMOS technology with dual thick metal option. Measurements show Q improvements more than 21% (9.9 - 12.04) for primary, 20% (9.81 - 11.78) for secondary thereby resulting in 15% (1.07 - 0.91) reduction in insertion loss of the transformer.