A Fully Differential VCO Cell with Active Inductors for Gbps Serial Links

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Abstract

This paper investigates the distinct characteristics of cross-coupled fully differential ring VCO delay cells with resistor, current-source, and active inductor loads. In addition, it proposes a new cross-coupled fully differential ring VCO delay cell with self-biased active inductor load. The proposed VCO provides both rail-to-rail voltage swing and significantly increased oscillation frequency. Simulation results of 8-stage ring VCOs with various loads implemented in TSMC–0.18 μm 1.8 V 6-metal CMOS technology demonstrate that the delay of VCO cells with resistor load is much smaller than those with current-source load. The rise and fall times of the delay cell with self-biased active inductor are comparable to those with passive inductor load and are significantly smaller than those with resistor and current-source loads. VCOs with self-biased active inductor and rail-to-rail swing provide rail-to-rail output voltage swing and comparable oscillation frequency as compared with those with self-biased active inductor load only. The frequency tuning range of VCOs with the active inductor load is smaller as compared with those with resistor/current-source load. The sensitivity of VCOs with active inductor load to supply voltage variation is comparable to that with passive inductor load, and is smaller than that with resistor/current-source load.

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