Weak Inversion MOS Varactors for Tunable Integrators

Abstract

A tunable integrator is the basic building block of a tunable filter. Traditionally, tunability can be accomplished using a MOSFET-C structure with MOS devices replacing resistors; or, using switches and banks of resistors and capacitors for discrete tuning; or, using transconductance-C techniques; or, using a varactor-R structure with varactors replacing capacitors. A MOSFET-C structure typically requires the MOS devices to be in strong inversion, which might not be feasible given the ultra-low supply voltage requirement. Using switches in the signal path would require voltage boosting to turn on the switches, which raises reliability concerns. The design of highly linear tunable transconductors at very low supply voltages is very challenging. We have thus investigated the use of varactor-R techniques.