High-Q Characteristics of Variable Width Inductors With Reverse Excitation

This brief proposes a technique to increase the performance of spiral inductors through appropriate selection of excitation port, especially in the case of variable width spirals. In this brief, forward excitation refers to applying ac voltage at the outer or upper terminal of the inductor with the inner or lower terminal grounded, while in reverse excitation, the inner or lower terminal is connected to the signal. Performance improvement with reverse excitation is explained using the voltage profile of the spiral. While tapered layout increased the peak-Q from 15.9 to 22.8, reverse excitation increased it further to 26.7. Moreover, reverse excitation also resulted in an increased peak-Q frequency from 5 to 8 GHz and self resonant frequency from 12 to 14.6 GHz in these tapered inductors. On the other hand, a similar approach is shown to be detrimental in the case of series stacked inductors, while symmetric inductors remain unaffected.