Subthreshold Variable-Gain Amplifier Core for Hearing-Aid-Class Audio Compression

Submission to the IEEE SSCS Open-Source Ecosystem "Code-a-Chip" Travel Grant Awards, ISSCC 2027.

A reproducible, gm/ID-driven automated design flow for a subthreshold variable-gain amplifier (VGA) core — the gain element of an ultra-low-power hearing-aid audio compressor — implemented entirely with open-source tools on the SkyWater SKY130 PDK.

Scope. This delivers the variable-gain core only. The envelope detector and closed gain-control loop are not implemented, so no compression ratio, threshold, or attack/release characteristic is claimed. Full detail in notebook.ipynb, §10.2. This line does not get softened or deleted as the project fills in — see the plan doc.

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Team

Name	Affiliation	Role	IEEE / SSCS Member	Email
Ayush <Surname>	VIT Vellore, India	Lead / travel-grant recipient	<Yes/No>	<email>
<Team member 2>	VIT Vellore, India	Co-designer	<Yes/No>	<email>
<Faculty advisor>	VIT Vellore, India	Advisor	<Yes/No>	<email>

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Repository structure

vga-codeachip/
├── LICENSE                  # Apache 2.0
├── README.md                # this file
├── notebook.ipynb           # the submission notebook — run this
├── cache/                   # committed: LUTs, optimiser results, environment capture
│   ├── lut_sky130.parquet
│   ├── pareto_results.json
│   └── environment.json
├── netlists/                # xschem exports + testbench templates
├── scripts/                 # importable Python modules (kept out of notebook cells)
│   ├── char.py              # characterisation sweep + parsing
│   ├── lut.py                # LUT interpolators
│   ├── sizing.py             # inverse sizing
│   ├── optimize.py           # pymoo NSGA-II problem
│   └── measure.py            # FoM extraction from raw SPICE output
├── xschem/                  # .sch / .sym schematic sources
├── layout/                  # .mag, .gds, DRC/LVS reports
└── images/                  # every figure used in the notebook/README, committed

cache/ and images/ are committed on purpose — they're what let a reviewer reproduce every result without re-running expensive simulations. Raw ngspice output (.raw, .log) is never committed; see .gitignore.

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Reproducing the results

git clone <this-repo-url>
cd vga-codeachip
# Tools already on $PATH if using IIC-OSIC-TOOLS; otherwise see notebook §2.2 for micromamba install
jupyter notebook notebook.ipynb
# Run top to bottom with default flags (REBUILD_LUT=False) — reproduces
# every result in Sections 5-9 from the cached artefacts in cache/.

Target: a clean-room clone should reproduce all figures/tables in under ten minutes. This is verified twice before submission (see project plan, Phase 7).

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Target specification

Full target table with rationale/citations lives in notebook.ipynb Table 2 — this is the condensed version for anyone browsing the repo first.

Parameter	Target	Unit
Supply voltage ($V_{DD}$)	<1.8>	V
Total quiescent current	<x>	µA
Mid-band gain range	<x> – <y>	dB
$-3$ dB bandwidth	≥ <10>	kHz
Phase margin	≥ 60	°
Integrated input noise	≤ <x>	µV$_{rms}$
THD @ <x> mV$_{pp}$	≤ 1	%
NEF	≤ <x>	–

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Block diagram

<Insert images/flow.png — the whole flow: characterisation → inverse sizing → Pareto sweep → verification → layout → post-layout re-verification.>

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Results summary

Full tables (spec compliance, LUT-vs-SPICE validation, PVT corners, pre/post-layout deltas, prior-art comparison) are in notebook.ipynb §9. Headline numbers only, below — fill in once Phase 5/6 complete.

Metric	Target	Achieved (post-layout, tt/27°C)	Worst corner
$A_v$ (dB)
$I_{tot}$ (µA)
$V_{n,rms}$ (µV)
NEF
DRC	—	<clean / N errors>
LVS	—	<clean / not converged>

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Reuse and attribution

Reused artefact	Source	How it is used here
gm/ID (C/ID) LUT-driven sizing flow	A. Adair, Code-a-Chip ISSCC'25	Device characterisation and inverse sizing methodology
LUT + pymoo optimisation structure	A. M. Elhelly, Code-a-Chip ISSCC'26	Multi-objective optimiser wrapper pattern

This project is a port of a decades-old, clinically-validated circuit function (subthreshold log-companding hearing-aid AGC, in the lineage of Sarpeshkar-style analog cochlear/hearing-aid work) onto a fully open PDK — it is not presented as a novel circuit. Prior art is cited directly in notebook.ipynb §12.

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Tool versions

Pinned versions are captured automatically into cache/environment.json during Phase 0 setup and reproduced in notebook.ipynb Table 1. Do not upgrade any tool mid-project — see the risk register in the plan doc.

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License

Licensed under the Apache License, Version 2.0 — see LICENSE. SPDX-License-Identifier: Apache-2.0

Acknowledgements

<Advisor, lab, any mentor — one or two lines.>