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

Suffix	Meaning	Value
f	femto	1e-15
p	pico	1e-12
n	nano	1e-9
u	micro	1e-6
m	milli	1e-3
k	kilo	1e3
MEG	mega	1e6
G	giga	1e9
T	tera	1e12

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 does NOT work in batch mode (-b) when combined with -r rawfile — data is streamed to disk and not available for analysis. Use interactive mode or a .control block instead.
• 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.

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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):

Op	Prec	Description
!	1	unary NOT
**, ^	2	power
*	3	multiply
/, %, \	3	divide, modulo, integer divide
+, -	4	add, subtract
==, !=/<>	5	equality
<=, >=, <, >	5	comparison
&&	6	boolean AND
||	7	boolean OR
c ? x : y	8	ternary

^ 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 <filename> <section> — requires a section name. Omitting the section silently loads nothing (no error!). For unconditional inclusion, use .include instead.
• .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 currents are saved (can create huge files).
• Adding even ONE .save line drops all defaults — only listed signals are saved.
• To keep defaults plus extras: .save all @m2[vdsat]
• .save @r1[i] for resistor current (not available via I() syntax).

.control / .endc Blocks

ngspice has a built-in scripting language for post-simulation analysis:

.control
run                               $ execute the simulation
plot V(out)                       $ plot results
print V(out)                      $ print to stdout
let vmax = maximum(V(out))        $ create vector
set filename = "results.csv"      $ create string variable
write $filename V(out) I(Vin)     $ save to rawfile
wrdata output.txt V(out)          $ save as CSV-like text
.endc

Variables vs vectors — a critical distinction:

• set creates string/shell variables: set myvar = "hello" — access with $myvar
• let creates numeric vectors: let x = 2*pi — access with $&x to convert to number
• Mixing them up causes silent failures.
• $&param dereferences a circuit .param into a control variable.

Control structures:

$ While loop
while condition
  ...
end

$ Repeat
repeat 10
  ...
end

$ Foreach
foreach val 1 2 3 4 5
  echo value is $val
end

$ If/else
if condition
  ...
else
  ...
end

$ Also: dowhile, break [n], continue [n], label, goto

foreach values are space-separated (no commas). Can use variable expansion: foreach var $myvariable.

Key commands: run, plot, print, let, set, write, wrdata, alter, altermod, echo, meas, linearize, fft, define, source

Monte Carlo

ngspice has no .mc directive. Monte Carlo is done via .control block loops:

.control
define unif(nom, var) (nom + nom*var * sunif(0))
define gauss(nom, var, sig) (nom + nom*var/sig * sgauss(0))

let mc_runs = 100
let run = 1
dowhile run <= mc_runs
  alter c1 = unif(1e-09, 0.1)
  alter r1 = gauss(10k, 0.05, 3)
  tran 1u 1m
  $ ... store/process results ...
  let run = run + 1
end
.endc

Random functions:

• sunif(0) — single uniform random number in [-1, 1]
• sgauss(0) — single Gaussian random number (mean=0, stddev=1)
• .options seed=<value> or seed=random — set random seed

Statistical functions (.param context):

• gauss(nom, rvar, sigma) — Gaussian, relative variation
• agauss(nom, avar, sigma) — Gaussian, absolute variation
• unif(nom, rvar) — uniform, relative
• aunif(nom, avar) — uniform, absolute
• limit(nom, avar) — binary: nom+avar or nom-avar

Using .param statistical functions requires multiple ngspice runs (each run re-evaluates). Using .control loops with alter varies within a single run.

.options Flags

General:

Flag	Effect
SEED=val|random	Random number seed
TEMP=x	Operating temperature (default 27C)
TNOM=x	Nominal temperature (default 27C)
SAVECURRENTS	Auto-save all device terminal currents
KLU	Use KLU matrix solver (faster for large MOS circuits)
INTERP	Interpolate output to fixed TSTEP grid

Convergence:

Flag	Default	Effect
RELTOL	0.001	Relative tolerance
ABSTOL	1e-12	Absolute current tolerance
VNTOL	1e-6	Absolute voltage tolerance
GMIN	1e-12	Minimum junction conductance
ITL1	100	DC iteration limit
ITL4	10	Transient iteration limit
METHOD	trap	Integration method: trap or gear
MAXORD	2	Max integration order for Gear (2-6)
TRTOL	7	Transient error tolerance factor
XMU	0.5	Trapezoidal damping (reduce slightly to suppress ringing)

Matrix conditioning:

.options rshunt=1e12              $ resistor from every node to ground
.options rseries=1e-4             $ series resistor on every inductor
.options cshunt=1e-13             $ capacitor from every node to ground

Use rshunt for "no DC path to ground" errors. Use rseries when inductors parallel voltage sources cause OP failure. Use cshunt for oscillation/noise issues.

AUTOSTOP — stops transient analysis after all .meas conditions are satisfied.

XSPICE

Mixed-signal simulation with code models (requires XSPICE-enabled build):

A1 [in] [out] lut1
.model lut1 d_lut(rise_delay=1n fall_delay=2n input_load=0.5p
+ table_values="0110")

Digital device types: d_and, d_or, d_nand, d_nor, d_xor, d_inverter, d_buffer, d_flop, d_latch, d_lut, etc.

Digital nodes use [name] bracket syntax for buses.

Key Differences from LTspice

• Inline comments: $ not ;
• Conditional in B source: ternary ?: not IF()
• MOSFET bulk terminal: required (4 pins), not auto-connected
• GND node: must explicitly tie to 0 or use .global
• No startup keyword in .tran
• No A-devices (mixed-signal primitives) — use XSPICE instead
• No Unicode mu issue — ngspice preserves u as-is
• .func definitions cannot be recursive — causes hang, not error
• Different .raw file format (all doubles vs LTspice mixed precision)
• .backanno needed for current probing in post-processing