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-um CMOS and the performance was verified experimentally.
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