A continuous analysis of the oscillation amplitude in MOS LC-VCOs

In this paper, a closed-form expression for oscillation amplitude in source-tailed MOS LC-VCOs versus the oscillator bias current and LC-tank resistive losses is proposed. The developed analysis is based on the utilization of analog saturation functions to provide a continuous equation among the oscillator's current-limited and voltage-limited regimes, which takes also into account the oscillation start-up condition. Quantitative predictions from the derived equation are in good agreement with Spectre simulations results from a 0.25-µm technology LC-VCO circuit. This expression can be used in an equation-based algorithm serving to control the amplitude and to optimize the phase noise versus frequency setting in switched-resonator based LC-VCOs.

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