Ngspice

ngspice Simulation Skill

Important — do not modify skill files during normal use. All new scripts, netlists, simulation outputs, and plots should go into the user's project working directory (outside .claude/). Do not modify the skill's assets/ folder or SKILL.md. Only edit skill-internal files when the user explicitly asks to improve or extend the skill itself.

Prerequisites & Installation

Dependencies: system-installed ngspice, Python 3 with numpy ≥ 1.20, matplotlib ≥ 3.3, scipy ≥ 1.7 (NumPy 1.x and 2.x both supported).

If ngspice is not found when a script starts, check_ngspice() will print a brief error and exit. Full platform-specific install instructions — including a portable Windows install that requires no PATH change, Chinese-network pip mirror fallbacks, and python-dateutil/six troubleshooting — are in:

references/installation.md

assets/
├── ngspice_common.py                   — shared utilities: path constants, runner, parsers, template renderer
├── models/
│   ├── ptm180.lib                      — PTM 180nm BSIM3v3
│   ├── ptm45hp.lib                     — PTM 45nm HP BSIM4
│   └── ptm22hp.lib                     — PTM 22nm HP BSIM4
├── netlist/
│   ├── tran_rc_charging.cir.tmpl       — Transient: RC charging voltage and current
│   ├── dc_nmos_iv.cir.tmpl             — DC: NMOS Id-Vds family curves
│   ├── ac_rc_filter.cir.tmpl           — AC: RC low-pass frequency response
│   ├── noise_rc_filter.cir.tmpl        — Noise: RC output noise spectral density
│   ├── tran_sample_hold_nmos.cir.tmpl  — Transient: 180nm NMOS sample-and-hold
│   ├── tran_sample_hold_ideal.cir.tmpl — Transient: ideal switch sample-and-hold
│   ├── tran_ktc_noise.cir.tmpl         — Transient: kT/C noise time-domain statistics
│   ├── dc_current_mirror.cir.tmpl      — DC: NMOS current mirror
│   ├── ac_cs_amp.cir.tmpl              — AC: common-source amplifier frequency response
│   └── dc_tgate_ron.cir.tmpl           — DC: transmission gate on-resistance
├── simulate_tran_rc_charging.py        — RC charging simulation engine
├── plot_tran_rc_charging.py            — RC charging plotting
├── run_tran_rc_charging.py             — RC charging entry point
├── simulate_dc_nmos_iv.py              — DC family-curve simulation engine
├── plot_dc_nmos_iv.py                  — DC family-curve plotting
├── run_dc_nmos_iv.py                   — DC family-curve entry point
├── simulate_ac_rc_filter.py            — AC + noise simulation engine
├── plot_ac_rc_filter.py                — AC + noise plotting
├── run_ac_rc_filter.py                 — AC + noise entry point
├── simulate_tran_sample_hold.py        — sample-and-hold simulation engine
├── plot_tran_sample_hold.py            — sample-and-hold plotting
├── run_tran_sample_hold.py             — sample-and-hold entry point
├── simulate_tran_ktc_noise.py          — kT/C noise simulation engine
├── plot_tran_ktc_noise.py              — kT/C noise plotting
├── run_tran_ktc_noise.py               — kT/C noise entry point
├── simulate_dc_current_mirror.py       — current mirror simulation engine
├── plot_dc_current_mirror.py           — current mirror plotting
├── run_dc_current_mirror.py            — current mirror entry point
├── simulate_ac_cs_amp.py               — common-source amplifier simulation engine
├── plot_ac_cs_amp.py                   — common-source amplifier plotting
├── run_ac_cs_amp.py                    — common-source amplifier entry point
├── simulate_dc_tgate_ron.py            — transmission gate Ron simulation engine
├── plot_dc_tgate_ron.py                — transmission gate Ron plotting
├── run_dc_tgate_ron.py                 — transmission gate Ron entry point
├── logs/                               — simulation logs (auto-created)
└── plots/                              — output figures (auto-created)

Naming convention: netlist templates are prefixed by simulation type (dc_, ac_, noise_, tran_); Python files follow the same naming (e.g. run_dc_nmos_iv.py).

Deployment and Running

1. Copy all files under assets/ to the project directory.
2. Run any entry-point script:

python run_tran_rc_charging.py    # RC charging (simplest — good for verifying ngspice install)
python run_dc_nmos_iv.py          # DC family curves
python run_ac_rc_filter.py        # AC + noise
python run_tran_sample_hold.py    # sample-and-hold
python run_tran_ktc_noise.py      # kT/C noise statistics
python run_dc_current_mirror.py   # current mirror
python run_ac_cs_amp.py           # common-source amplifier
python run_dc_tgate_ron.py        # transmission gate Ron

On Windows, set PYTHONUTF8=1 to avoid GBK encoding errors.

All paths resolve automatically via Path(__file__).resolve().parent — no configuration needed.

Example 1: Transient — RC Charging

Circuit: R + C to ground, 1V DC step input, initial Vcap = 0.

Configurations:

Simulation: .tran to 10τ (slow config), UIC + IC=0.

Output: plots/tran_rc_charging.png (top panel: voltage; bottom panel: current).

Sanity checks:

• V(out) = Vin × (1 − e^(−t/τ)); reaches 63.2% at t = τ
• I = Vin/R × e^(−t/τ); initial current = 1V/1kΩ = 1mA (or 100µA for 10kΩ)
• Capacitor essentially fully charged (>99%) after 5τ

Files: tran_rc_charging.cir.tmpl → simulate_tran_rc_charging.py → plot_tran_rc_charging.py → run_tran_rc_charging.py

Example 2: DC — NMOS Id-Vds Family Curves

Circuit: PTM 180nm NMOS, W=10µm, L=0.18µm.

Simulation: .dc Vds 0 1.8 0.01, Vgs = {0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8} V.

Output: plots/nmos_dc_iv.png.

Sanity checks:

• Vgs = 0.4V (near Vth ≈ 0.4V): Id ≈ 59 µA (near threshold / weak inversion)
• Vgs = 0.6V, Vds = 0.9V (saturation): Id ≈ 561 µA, Id/W ≈ 56 µA/µm
• Vgs = 1.0V, Vds = 0.9V (saturation): Id ≈ 2.45 mA, Id/W ≈ 245 µA/µm
• Saturation region clearly visible (curves flatten for Vds > Vgs − Vth)

Files: dc_nmos_iv.cir.tmpl → simulate_dc_nmos_iv.py → plot_dc_nmos_iv.py → run_dc_nmos_iv.py

Example 3: AC — RC Low-Pass Filter Bandwidth

Circuit: R + C to ground, AC source amplitude 1V.

Configurations:

Simulation: .ac dec 100 1Meg 10G.

Output: plots/rc_ac_bw.png (top panel: gain).

Sanity checks:

• −3 dB point matches theoretical fc: 159.2 MHz (1kΩ), 15.92 MHz (10kΩ)
• Roll-off above fc: −20 dB/decade (first-order filter)
• Low-frequency gain = 0 dB

Example 4: Noise — RC Filter Output Noise

Circuit: same as AC example (R + C to ground).

Simulation: .noise v(out) Vin dec 100 1Meg 10G.

Output: plots/rc_ac_bw.png (bottom panel: noise spectral density).

Sanity checks:

• White noise floor = √(4kTR): 4.07 nV/√Hz (R=1kΩ), 12.87 nV/√Hz (R=10kΩ)
• Integrated noise = √(kT/C): 64.3 µVrms (C=1pF) — classic kT/C noise
• Noise bandwidth = π/2 × fc (first-order filter)
• Noise rolls off at −20 dB/decade above fc

AC and noise share a single entry script run_ac_rc_filter.py (same circuit, natural teaching flow).

Example 5: Transient — Sample-and-Hold Switch Comparison

Circuit: NMOS switch (180nm, W=4µm) + Csamp=1pF, 10MHz sinusoidal input, 100MHz clock.

Simulation: .tran 0.1n 250n.

Output: plots/sample_hold_compare.png.

Model comparison:

Sanity checks:

• Hold voltage remains flat during switch OFF (1pF capacitor charge conservation)
• 180nm NMOS Ron ≈ 400Ω → acquisition time constant τ = Ron × C = 400 × 1p = 0.4 ns
• Clock feedthrough (ΔV = Cov/Csamp × Vclk) visible as small step at clock edges
• Ideal switch shows no clock feedthrough or charge injection

Files: tran_sample_hold_nmos/ideal.cir.tmpl → simulate_tran_sample_hold.py → plot_tran_sample_hold.py → run_tran_sample_hold.py

Example 6: Transient — kT/C Noise Time-Domain Statistics

Circuit: noisy R (400Ω, equivalent switch Ron) + Csamp; DC input 0.9V. A trnoise voltage source injects thermal noise into the resistor.

Principle: Resistor thermal noise density 4kTR, bandwidth-limited by RC, gives capacitor noise variance = kT/C. Sample v(out) every 5τ over a long transient to obtain 10000 independent samples.

Configurations:

Output: plots/tran_ktc_noise_hist.png (noise trace + histogram + fitted Gaussian + statistics summary).

Sanity checks:

• 1 pF: measured σ ≈ 64 µV (±10% statistical variation)
• 100 fF: measured σ ≈ 203 µV (±10% statistical variation)
• Histogram is Gaussian; fitted curve matches theoretical √(kT/C)
• Measured/theoretical ratio in [0.85, 1.15]

Files: tran_ktc_noise.cir.tmpl → simulate_tran_ktc_noise.py → plot_tran_ktc_noise.py → run_tran_ktc_noise.py

Example 7: DC — NMOS Current Mirror Output Characteristics

Circuit: PTM 180nm NMOS 1:1 current mirror; M1 diode-connected (reference), M2 output.

Parameters: W=10µm, L=0.5µm (longer channel for better matching), Iref=100µA.

Simulation: .dc Vout 0 1.8 0.01.

Output: plots/dc_current_mirror.png (Iout vs Vout + mirror ratio).

Sanity checks:

• Vout > Vov (≈0.2V), M2 in saturation: Iout ≈ 100 µA
• Channel-length modulation causes Iout to increase slightly with Vout (slope ≈ 1/ro)
• Output resistance ro ≈ VA/Id ≈ 50–100 kΩ
• Vout < Vov: M2 enters triode, Iout < Iref

Files: dc_current_mirror.cir.tmpl → simulate_dc_current_mirror.py → plot_dc_current_mirror.py → run_dc_current_mirror.py

Example 8: AC — Common-Source NMOS Amplifier Frequency Response

Circuit: PTM 180nm NMOS common-source amplifier with resistive load and load capacitance.

Parameters: W=10µm, L=0.18µm, Vgs=0.6V, Rd=2kΩ, CL=1pF, VDD=1.8V.

Simulation: .ac dec 100 1k 100G.

Output: plots/ac_cs_amp_bode.png (gain Bode plot + phase).

Sanity checks:

• DC gain |Av| = gm × Rd ≈ 12.9 dB
• −3 dB frequency ≈ 132 MHz (theoretical 1/(2πRdCL) ≈ 80 MHz; higher in practice due to MOSFET parasitic capacitance)
• Low-frequency phase ≈ 180° (inverting amplifier)
• High-frequency roll-off at −20 dB/decade (single pole)

Files: ac_cs_amp.cir.tmpl → simulate_ac_cs_amp.py → plot_ac_cs_amp.py → run_ac_cs_amp.py

Example 9: DC — Transmission Gate On-Resistance

Circuit: NMOS + PMOS complementary transmission gate, W/L=100, fully on (clk=VDD, clkb=0).

Method: Apply 10mV across the gate, sweep Vpass; Ron = 10mV / I.

Technology node comparison:

Output: plots/dc_tgate_ron.png (Ron vs Vpass, log scale, 3 curves).

Sanity checks:

• 180nm: min Ron ≈ 38 Ω; lowest resistance in the mid-Vpass range
• 45nm HP: min Ron ≈ 52 Ω
• 22nm HP: min Ron ≈ 103 Ω (lower VDD limits gate drive)
• Ron rises at Vpass near 0 and VDD (NMOS / PMOS turning off respectively)
• Complementary structure ensures conduction across the full Vpass range

Files: dc_tgate_ron.cir.tmpl → simulate_dc_tgate_ron.py → plot_dc_tgate_ron.py → run_dc_tgate_ron.py

Shared Module: ngspice_common.py

Netlist Template Conventions

• Template filenames are prefixed by simulation type: dc_, ac_, noise_, tran_
• Use Python str.format() placeholders: {R}, {model_path}
• Never put {...} inside SPICE comment lines — they are parsed as format placeholders and raise KeyError
• Convert paths with spath() to forward slashes (Windows compatibility)
• Temporary netlists are written to tempfile.NamedTemporaryFile(suffix='.cir') before execution

Plotting Conventions

• Never call plt.show(); always use fig.savefig(path, dpi=150, bbox_inches='tight')
• Save figures to plots/
• Axis labels in ASCII + LaTeX (e.g. $V_{DS}$, $I_D$)
• Do not use Chinese characters in matplotlib labels

Reference Documents

See references/conventions.md for full details:

• §1 Project structure and path conventions
• §2 ngspice execution modes and command-line arguments
• §3 Netlist template placeholder list
• §4 Output parsing formats
• §5 Physical sanity-check values quick reference
• §6 Common errors and fixes

See references/installation.md for installation details:

• §1 Windows (portable 7z, official installer, choco, winget)
• §2 Verifying the installation
• §3 Python dependencies, six/dateutil, pip mirror fallbacks
• §4 PATH troubleshooting table