Abstract: In this paper, we study the large-signal modulation effect of accumulation-mode MOS (A-MOS) varactor incorporated into the VCO tank circuit. A gain model is presented for A-MOS varactor in CMOS LC VCOs. Simulated results show good agreement with those obtained by spectreRF for an example 2.4 GHz LC VCO, which has used A-MOS varactors implemented in Chartered 0.35 μm CMOS 2P4M RF technology.