AMPICBench

A collection of analog circuits for benchmarking sizing, simulation, and layout tools.

AMPICBench is maintained by the AMPIC group. It provides ready-to-simulate ngspice netlists of analog building blocks in the open-source SkyWater sky130A process, together with per-corner operating-region data, so that automated analog design flows (including LLM-based ones) can be tested against the same reference circuits.

Contents

AMPICBench/
├── Bias_Circuits/
│   ├── sim_Bias_gen/                          Simple current-reference bias generator
│   └── sim_Cascode_bias/                      Wide-swing cascode bias network
├── Opamps/
│   ├── sim_Diffamp1/                          Differential amplifier (open-loop AC)
│   ├── sim_Telescopic_Amp/                    Telescopic cascode amplifier (open-loop AC)
│   ├── sim_FC_Amp/                            Folded-cascode amplifier (open-loop AC)
│   ├── sim_Opamp1_stb/                        Opamp1 with loop-gain / stability analysis
│   ├── sim_Opamp_2stg_Diff_ClassA_STB/        Two-stage: diff-pair first stage + class-A second stage
│   ├── sim_Opamp_2stg_FC_ClassA_STB/          Two-stage: folded-cascode first stage + class-A second stage
│   └── sim_Opamp_2stg_Telescopic_ClassA_STB/  Two-stage: telescopic first stage + class-A second stage
└── Sky130_char/
    ├── nmos_char/                             sky130 LVT NMOS operating-point / DC sweep characterization
    └── pmos_char/                             sky130 LVT PMOS operating-point / DC sweep characterization

Each circuit folder contains:

File	Purpose
sim_<name>.cir	Top-level ngspice netlist: parameters, subcircuits, testbench, and .control block
sim_<name>_cells.spice	The same subcircuits with all parameters evaluated to plain numbers (W/L in um)
corners.json	Per-corner (tt/ff/ss/fs/sf at -40, 27, 125 C) worst saturation margin in mV and the list of devices in triode
.spiceinit	ngspice settings needed for the sky130 models

The Sky130_char folders contain only the .cir and .spiceinit files.

Analyses included

• Bias circuits and device characterization: DC sweep (dc VV1 0 1.8 0.036) plus operating-point dump.
• Open-loop amplifiers (Diffamp1, Telescopic, FC): operating point and AC sweep, 10 Hz to 1 GHz.
• Opamps with _stb: operating point and loop-gain AC sweep, 10 Hz to 10 GHz, using Tian double-injection (the ngspice equivalent of Spectre stb). The control block measures unity-gain frequency, phase margin, DC gain, and gain margin.

Every simulation writes op_nodes.txt and op_devices.txt (id, vgs, vds, vth, vdsat, gm, gds, cgs, cgd).

Simulation results and schematics

RESULTS.md collects the loop-gain (stb) and unity-gain step (tran) plots, the open-loop AC plots of the single-stage amplifiers, and a schematic of every circuit. Each circuit folder also has a results/ folder with the plots (PNG), the data (CSV) and the extracted metrics (results.json). Schematics are in schematics/ (PNG, PDF and SVG).

Circuit	DC gain (dB)	UGF (MHz)	Phase margin (deg)
Opamp1_stb	60.6	79.0	69.2
Opamp_2stg_Diff_ClassA_STB	60.6	126.0	31.5
Opamp_2stg_FC_ClassA_STB	73.9	71.3	32.0
Opamp_2stg_Telescopic_ClassA_STB	81.9	68.7	49.3

Results use ngspice 42 with the SkyWater models from skywater-pdk-libs-sky130_fd_pr (tt, 27 C). Other PDK builds or simulators can give slightly different numbers.

Tools

File	Purpose
tools/run_sims.py	Runs stb and unity-gain step simulations and writes plots, CSV and metrics (python3 tools/run_sims.py . --in-place)
tools/draw_schematics.py	Draws the transistor-level schematics from the netlists and an operating-point run (python3 tools/draw_schematics.py . schematics results_summary.json)
tools/analog_sheet.py, tools/ams_symbols.py	Drawing helpers (SchemDraw) and the AMS symbol loader
tools/ams_symbols/	The AMS symbol library: one SVG per symbol plus the Visio stencil AMS.vss

The drawing tools need pip install schemdraw matplotlib and the Liberation Sans / Serif fonts.

Requirements

• ngspice
• The SkyWater sky130A PDK, for example installed via open_pdks

Running a simulation

The netlists reference the PDK model library with an absolute path:

.lib "/opt/pdks/sky130A/libs.tech/ngspice/sky130.lib.spice" tt

Change this line to point at your own PDK install (or symlink your PDK to /opt/pdks). Then run from inside a circuit folder so that .spiceinit is picked up:

cd Opamps/sim_FC_Amp
ngspice -b sim_FC_Amp.cir

To change the process corner, edit the corner name at the end of the .lib line (tt, ff, ss, fs, sf) and the .temp value.

Provenance

The netlists were converted from Cadence ADE Spectre testbenches (sky130 kit) to ngspice. Device area and perimeter parameters (as, ad, ps, pd) reproduce the Cadence pcell geometry via the cds_* functions in each netlist. Design changes relative to the original Cadence designs are recorded as DESIGN CHANGE comments at the top of the affected .cir files.

Citing

If you use AMPICBench in your work, please cite it. GitHub shows a "Cite this repository" button generated from CITATION.cff. BibTeX:

@misc{saxena2026opera,
      title={OPERA: OPamp Design by an Expert-Taught Reasoning Agent, from Specifications to Layout}, 
      author={Vishal Saxena and Soumyajit Mandal},
      year={2026},
      eprint={2610.39395},
      archivePrefix={arXiv},
      primaryClass={cs.AR},
      url={https://arxiv.org/abs/2610.39395}, 
}

A reference to the accompanying paper will be added here once it is available.

License

Copyright (c) 2026 Vishal Saxena, AMPIC Group.

AMPICBench is released under the Creative Commons Attribution-NonCommercial 4.0 International license (CC BY-NC 4.0); see LICENSE. You may share and adapt the material for non-commercial purposes with attribution. For commercial use, please contact the AMPIC group.

The sky130 PDK and models are not part of this repository and remain under their own license (Apache-2.0).