High-performance and area-efficient stacked transformers for RF CMOS integrated circuits

A solenoid layout technique is used to increase the magnetic coupling between the coils in multi-level spiral transformers. Using this technique, transformers with the following performance are measured: (a) magnetic coupling with coupling factor k>0.95 and insertion loss s/sub 21/ better than 0.8 dB at 5 GHz; (b) self-resonant frequency f/sub res/>15 GHz with peak Q>10. The effect of high and low resistive substrates is discussed with measured results. Using this structure, the size of the transformer can be reduced from 50 to 400%.