Multiple Turn Ratios of On-Chip Transformer With Four Intertwining Coils

This paper develops an on-chip transformer in which four coils are entwined tightly within a limited area. The proposed device has a total of six possible turn ratios. The flexibility of the turn ratio facilitates the convenient merging of two transformers with the RF design in a device area. To demonstrate the proposed device, seven devices are fabricated using 90-nm CMOS technology. Measurements reveal that one device has a wide range of turn ratios (n=1-5.8) and coupling coefficients (k=0.61-0.95) within an area of 0.01 mm2. A coil of one of the devices has an inductance of 28.4 nH and conducts with an inductance density of 2840 nH/mm2.