This article describes the effects of active Q enhancement on the performance of monolithic LC oscillator. Phase noise being the most important parameter for oscillators used in communication circuits, a lot of research efforts have been put in the direction of improving phase noise of fully on-chip LC oscillators over the past few years. Poor quality factor (Q) of on-chip passive components, specially that of spiral inductors limit the phase noise performance of LC oscillators. Use of active Q enhancement techniques has been proposed to improve phase noise but has not been proved by rigorous theory or supported by conclusive measurement results and thus require further investigation. In this article, it has been shown that active Q enhancement using transformer coupling, in fact, degrades the phase noise. The conclusion was reached based on theoretical analysis supported by simulation results. © 2007 Springer Science+Business Media, LLC.