IEEE Circuits and Systems Magazine - Q2 2023 - 29

Feature
Continuous-Time
Analog Filters for
Audio Edge Intelligence:
Review on Circuit Designs
Kwantae Kim, Member, IEEE, and Shih-Chii Liu, Fellow, IEEE
Abstract
Edge audio devices can reduce data bandwidth requirements by
pre-processing input speech on the device before transmission
to the cloud. As edge devices are required to ensure always-on
operation, their stringent power constraints pose several design
challenges and force IC designers to look for solutions that use
low standby power. One promising bio-inspired approach is to
combine the continuous-time analog filter channels with a small
memory footprint deep neural network that is trained on edge
tasks such as keyword spotting, thereby allowing all blocks to be
embedded in an IC. This paper reviews the historical background
Digital Object Identifier 10.1109/MCAS.2023.3267893
Date of current version: 20 July 2023
SECOND QUARTER 2023
1531-636X/23©2023IEEE
IEEE CIRCUITS AND SYSTEMS MAGAZINE
29
of the continuous-time analog filter circuits that have been used
as feature extractors for current edge audio devices. Starting from
the interpretation of a basic biquad filter as a two-integrator-loop
topology, we introduce the progression in the design of secondorder
low-pass and band-pass filters ranging from OTA-based to
source-follower-based architectures. We also derive and analyze
the small-signal transfer function and discuss their usage in edge
audio applications.
Index Terms-Auditory, silicon cochlea, two-integrator-loop,
band-pass filter (BPF), continuous-time (CT) filter, flipped voltage
follower (FVF), second-order filter, source-follower (SF), super
source-follower (SSF).
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IEEE Circuits and Systems Magazine - Q2 2023

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