ngspice

ngspice Circuit Simulation Guide

SPICE Fundamentals

Netlist Structure

* Title line (first line, always a comment)
<components>
<directives>
.END

• .END must be last line. No statements after it.
• + at start of line continues previous statement.
• Comments: * (full line) or $ (inline). Note: ; is NOT the inline comment character in ngspice (that's LTspice).

Component Syntax

<ref> <node+> <node-> <value>
R1 in out 10k
C1 out 0 100n
V1 in 0 AC 1 PULSE(0 5 0 1n 1n 0.5m 1m)

Value Notation — CRITICAL

M means MILLI, not mega. Use MEG for 1e6. This is the #1 SPICE mistake. 1M = 0.001, not 1000000. Unrecognized suffix letters are silently ignored — no error, just wrong value.

Waveform Sources

PULSE(Vinitial Vpulse Tdelay Trise Tfall Ton Tperiod Ncycles)
SINE(Voffset Vamp Freq Td Theta Phi Ncycles)
EXP(V1 V2 Td1 Tau1 Td2 Tau2)
SFFM(Voff Vamp Fcar MDI Fsig)
PWL(t1 v1 t2 v2 ...)

Directives

.tran 5m                          $ transient, 5ms stop
.tran 0 5m 0 10u                  $ tstep, tstop, tstart, tmaxstep
.ac dec 200 10 100k               $ AC sweep, 200pts/decade, 10Hz-100kHz
.dc V1 0 5 0.01                   $ DC sweep V1, 0-5V, 10mV step
.op                               $ DC operating point
.noise V(out) V1 dec 200 10 100k  $ noise analysis
.tf V(out) V1                     $ DC transfer function
.include /path/to/model.lib       $ include library
.ic V(node)=1.5                   $ initial conditions (used with UIC)
.nodeset V(node)=1.5              $ hint for DC operating point solver

.ic forces node voltages at t=0 (use with .tran ... UIC). .nodeset is a solver hint only.

.MEAS Syntax

.meas TRAN vmax MAX V(out)
.meas TRAN vpp PP V(out)
.meas TRAN trise TRIG V(out) VAL=0.1 RISE=1 TARG V(out) VAL=0.9 RISE=1
.meas AC fc WHEN mag(V(out)/V(in))=0.707
.meas AC gain_1k FIND V(out) AT=1k
.meas TRAN avg_out AVG V(out) FROM=1m TO=5m
.meas TRAN energy INTEG V(out)*I(R1)
.meas TRAN slope DERIV V(out) AT=2m
.meas TRAN check param='tdiff < vout_diff ? 1 : 0'

Additional measurement types vs LTspice:

• MIN_AT, MAX_AT — return the time/freq of the min/max, not the value.
• DERIV — derivative at a point or when a condition is met.
• param='expression' — evaluate an expression using .param values and prior .meas results.
• par('expression') — inline algebraic expression on any output variable (uses B source syntax internally).
• SP analysis type for spectrum (fft) measurements (via meas command, not .meas line).

Gotchas:

• RISE/FALL/CROSS numbering starts at 1, not 0.
• If TRIG event never occurs, measurement silently fails.
• .meas is refused when batch mode (-b) is combined with -r rawfile — the invocation run_simulation uses. ngspice prints No .measure possible in batch mode (-b) with -r rawfile set! and computes nothing. Recovery: move the measurement into a .control ... run ... .endc block written as the DOT-LESS meas command — e.g. meas tran vmax MAX V(out) (a dotted .meas inside .control is not a valid command and silently does nothing). The result prints to the run's log.
• param and par are not available inside .control blocks — use let instead.

General Pitfalls

• Node "0" is ground. Using GND without .global GND or tying it to 0 creates a floating node — no error, wrong results.
• MOSFET requires 4 terminals: M1 d g s b — ngspice does NOT auto-connect bulk to source (LTspice does).
• Impedance ratios: Beyond ~1e16 cause numerical issues (64-bit doubles).
• Parameter sweep: ngspice has no native .step (that is LTspice syntax). The MCP runs parametric sweeps through configure_sweep + run_sweep, which generate and simulate one netlist per value. For a hand-written deck outside the MCP, use a .control block with an alter/loop. A .step line in a deck handed to run_simulation is rejected with a pointer to configure_sweep.

ngspice-Specific

Parameters and Expressions

.param Rval=10k
.param fc={1/(2*pi*Rval*Cval)}
.param combined='Rval + 10'
.func myfn(x) {x*2}

• Expressions in braces {expr} or single quotes 'expr' — both work.
• Expressions without delimiters work only when spaces are absent: .param c=a+123 OK, .param c = a + 123 FAILS silently (assigns only first token).
• Self-referential params fail silently: .param x = {x+3} does not work.
• Parameter names: must start with alpha; may contain ! # $ % [ ] _. Cannot use reserved words: time, temper, hertz, not, and, or, div, mod, sqr, sqrt, sin, cos, exp, ln, log, log10, arctan, abs, pwr, defined.
• String-valued params supported with limited concatenation.

Three separate expression parsers exist in ngspice — this is a known source of confusion:

1. Front-end (.param, brace expressions) — evaluated at netlist expansion time
2. B source / behavioral — evaluated during simulation (no braces)
3. .control block — operates on its own vectors/variables

These have slightly different function sets and precedence rules. Braces {...} are "compile-time"; bare expressions in B sources are "run-time".

Operator precedence (.param expressions):

^ behavior depends on compatibility mode:

• Default (hs compat): x^y = pow(fabs(x), y) for x>0; rounds y for x<0; 0 for x=0
• LTspice compat (lt): x^y = pow(x, y) if y is close to integer; else 0 for x<0

Built-in functions (.param):

• Trig: sin, cos, tan, asin, acos, atan, arctan
• Hyperbolic: sinh, cosh, tanh, asinh, acosh, atanh
• Exp/log: exp, ln, log (base e), log10
• Power: sqrt, pow(x,y), pwr(x,y) (= pow(fabs(x),y))
• Rounding: nint (nearest, half to even), int (toward 0), floor, ceil
• Selection: min, max, sgn
• Conditional: ternary_fcn(x,y,z) (= x ? y : z)
• Statistical: gauss(nom,rvar,sigma), agauss(nom,avar,sigma), unif(nom,rvar), aunif(nom,avar), limit(nom,avar)
• Special: var(name) (interpreter variable), vec(name) (vector value)

Behavioral Sources (B sources)

B1 out 0 V=<expression>
B2 out 0 I=<expression> [tc1=x] [tc2=x] [temp=x]

Conditional: uses ternary cond ? true : false — NOT IF() (that's LTspice). Put a space before ? so the parser doesn't confuse it with other tokens. Nested ternaries need explicit parentheses.

Available functions (B source context): cos, sin, tan, acos, asin, atan, cosh, sinh, acosh, asinh, atanh, exp, ln, log, log10, abs, sqrt, u (unit step), u2 (ramp 0-1), uramp, floor, ceil, min, max, pow, **, pwr, ^, i(device)

Special variables: time (transient), temper (circuit temp in C), hertz (AC frequency). time is zero during AC; hertz is zero during transient.

Piecewise linear in B source:

Bdio 1 0 I = pwl(v(A), 0,0, 33,10m, 100,33m, 200,50m)
Blimit b 0 V = pwl(v(1), -4,0, -2,2, 2,4, 4,5, 6,5)

x values must be monotonically increasing — non-monotonic stops execution. Can use time or expressions as the independent variable.

Gotchas:

• exp() is internally capped at argument=14 — beyond that it becomes linear (for convergence).
• log/ln/sqrt of negative values use fabs() automatically — no error, may give unexpected results.
• Division by zero or log(0) causes an error.
• B source par('expression') can be used in .plot/.print output lines.
• Non-linear R/L/C can be synthesized from B sources using the subcircuit template pattern (see ngspice manual 5.1).

Subcircuits

.subckt myfilter in out rval=100k cval=100nF
R1 in p1 {2*rval}
C1 p1 0 {cval}
.ends myfilter

X1 input output myfilter rval=1k cval=1n

Key differences from LTspice:

• Parameters on .subckt line do NOT need params: keyword — just name=value after nodes.
• .lib behavior is compatibility-mode dependent, and no single .lib form works in every mode — so for unconditional whole-file inclusion use .include <file>, which resolves in every ngspice mode (verified on ngspice-42). If you use .lib:
  • ngspice-native modes (hsa, plain default): .lib <file> <section> loads the named .lib section … .endl block; a bare .lib <file> with no section does NOT load the file's models.
  • This server's default kiltpsa (a PSPICE-family mode) inverts this: a bare .lib <file> loads an unsectioned file, but a sectioned .lib <file> <section> (the PDK corner-select idiom) is mis-split by the lt/ps tokens into two plain includes that drop the section, surfacing as a missing include (could not find include file). Set [simulator] ngbehavior = "hsa" in ltspice-mcp.toml (or LTSPICE_MCP_NGBEHAVIOR=hsa) and restart the server to parse the section; run_simulation emits this hint when a failed run matches the pattern.
• .param inside subcircuits is local scope (masks globals). Nesting up to 10 levels.
• Subcircuit and model names are global — must be unique across the entire netlist.

.save Directive

.save V(out) I(Vin)               $ save only these signals
.save @m1[id] @m1[gm]             $ save internal device parameters
.save all @m2[vdsat]               $ save defaults PLUS extras

• Without .save, all node voltages and source cur

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