Large-signal analysis of MOS varactors in CMOS -G/m/ LC VCOs

• Published in: IEEE journal of solid-state circuits Vol. 38; no. 8; pp. 1325 - 1332
• Main Authors: Bunch, R L, Raman, S
• Format: Journal Article
• Language: English
• Published: 01-08-2003
• ISSN: 0018-9200
• DOI: 10.1109/JSSC.2003.814416

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 structure. 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.