A Methodology for Long Time Constant Log-Domain Filters in CMOS

A simple methodology for implementation of low-order, current-mode, log-domain filters in CMOS technology is presented. The key transistors in the circuit are operated in weak inversion and in contrast with previous approaches may pass into the triode regime. The concept is particularly suited to implementation in silicon-on-insulator technology, because dielectric isolation of the transistors eliminates leakage currents, and because influence of the body effect on circuit function is limited. Very long time constants, on the order of 1s or more, are obtainable. A simple elaboration of the basic unit circuit allows the time constant to be controlled by a bias current.