Fully Tunable Analog Biquadratic Filter for Low-Power Auditory Signal Processing in CMOS Technologies

Abstract

A novel Gm-C structure of a second-order continuous-time filter is proposed that allows for the independent control of the filter’s natural frequency (ω0) and quality factor (Q). The structure consists of two capacitors and four transconductors. Two transconductors together with the capacitors form a lossless second-order circuit with tunable ω0. The other two transconductors form a variable gain amplifier (VGA) which realizes an adjustable loss and thereby adjustable Q. The proposed solution can be used to implement low-voltage and low-power tunable front-end filter banks for fully integrated CMOS cochlear implants and edge intelligence accelerators. An example filter bank powered by 0.5 V and consuming 40 nW of power per single filter is designed and simulated using a 180 nm CMOS process. Circuitries for the adaptive control of transistor bias at a reduced supply voltage are proposed. The ω0 and Q control circuitries are also proposed: a delay-locked loop (DLL)-based system for fine ω0 tuning and a binary-weighted current mirror for Q adjustment. The proposed solution allows for the independent regulation of ω0 and Q within the ranges of 0.25–8 kHz and 1–14, respectively, with a relative tolerance of up to 5% across a filter bank.