Tunable coupled inductor Q-enhancement for parallel resonant LC tanks

A tunable transformer-based Q-enhancement method for monolithic inductors used in parallel resonant LC tanks is presented. The Q of the inductor that forms the transformer primary is enhanced by cancelling the voltage drop across the series loss resistance of the primary with a voltage of opposite polarity that is induced by a current in the secondary. The circuit allows DC voltage control of Q-enhancement. Techniques to address stability issues associated with the positive feedback are described. The circuit was fabricated in 0.18 (mu)m CMOS and the performance was verified experimentally.