Large-signal analysis of MOS varactors in CMOS -G/sub m/ 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 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/sub m/ LC VCO using I-MOS varactors implemented in 0.35-μm CMOS technology.

I. Introduction

The explosive growth in wireless communications has led to an increased demand for wireless products that are low-cost, low-power, and compact in size. Recently, there has been considerable interest in the use of CMOS technology to implement RF components such as low-noise amplifiers (LNAs), mixers, and voltage-controlled oscillators (VCOs) [1], [2]. The benefits include leveraging high-volume silicon CMOS fabrication capacity for RF products and the potential for achieving high-levels of RF/analog/digital integration, rapidly approaching single-chip system implementations [3].