Design of high-Q varactors for low-power wireless applications using a standard CMOS process

Abstract

The power consumption of an LC-tank oscillator is strongly affected by the varactor quality factor, whether the inductor is on-or off-chip. This paper proposes a new solution to realize high-Q, highly tunable on-chip varactors in a standard CMOS process, achieving a quality factor higher than 100 at 1 GHz, for a tuning ratio of 2. Other solutions are described and their respective advantages compared, while their characteristics are measured for a O S pm process.

A 56-GHz LC-Tank VCO With 17% Tuning Range in 65-nm Bulk CMOS for Wireless HDMI

“…They are placed in the same Nwell that is biased using a common voltage . The differential current path length is thus minimized [14]. Larger values are obtained using an array of basic cells with a waffle structure for distributing the common signals ( and the substrate biasing Subs), as shown in Fig.…”

“…A differential N poly/Nwell varactor operating in accumulation mode is used for continuous tuning. While previous works used two single-ended varactors connected in series [2]- [12], the differential varactor used in our VCO leads to an increased quality factor by reducing the differential current path length through the Nwell [14] and provides a smaller minimum capacitance due to the reduction of the routing interconnections. An additional bank of digitally controlled differential varactors provides coarse tuning for PVT calibration using three bits ( , and ).…”

MOS varactors with n- and p-type gates and their influence on an LC-VCO in digital CMOS

“…Quality factors can be quite high but the tuning ranges are unacceptably low. The approach of differential diode operation increases the quality factor but leaves the absolute capacitance tuning range ( ) still below 2 [33]. Downscaling of the supply voltage further aggravates the tuning problem of junction diodes [34].…”

“…More promising is the approach of tuning with the voltage-controlled gate capacitance of the MOS structure [21]. Quality factors are generally high and absolute capacitance tuning ranges well above 2 are possible [33]. Strong capacitance variation within a few hundreds of millivolts makes the MOS varactor devices useful at low supply voltages.…”

High-Frequency Oscillator Design for Integrated Transceivers

“…1g-ure 6.24(b) demonstrates that a MOS-varactor can be used in accumulation mode and inversion mode. Models describing CMOS and the quality factor for both modes can be found in [108][109][110]. In Section 9.3, the design of an LC oscillator with PMOS varactor will be described.…”