Large-signal analysis of MOS varactors in CMOS -Gm LC VCOs

MOS varactors are used extensively as. tunable elements in the tank circuits of RF voltage-controlled -oscillators (VCOs) based on submicrometer CMOS technologies. MOS varactor topologies include conventional D = S = B connected, inversion-mode (I-MOS), and accumulation-mode (A-MOS) structures. When incorporated into the VCO tank circuit, the large-signal swing of the VCO output oscillation modulates the varactor capacitance in time, resulting in a VCO tuning curve that deviates from the dc tuning curve of the particular varactor struc-w ture. This paper presents a detailed analysis of this large-signal effect. Simulated results are compared to measurements for an example 2.5-GHz complementary -G(m) LC VCO using I-MOS varactors implemented in 0;35-mum CMOS technology.