SPICE Reference Guide #

Comprehensive reference for SPICE netlist syntax and commands supported in Breadpad. Breadpad uses ngspice, which is compatible with SPICE3 and includes many extensions.

Netlist Structure #

Basic Format #

* Circuit Title
* Comments start with asterisk

* Component Definitions
R1 nodeA nodeB 1k
C1 nodeB 0 100n
V1 nodeA 0 DC 5

* Analysis Commands
.TRAN 1u 10m
.PRINT TRAN V(nodeB)

.END

Netlist Rules #

First line: Circuit title (comment)
Node names: Alphanumeric, case-insensitive
Ground: Node 0 or GND
Comments: Start with * or ;
Line continuation: + at start of next line
Last line: .END command
Units: Use SI prefixes (k, M, m, u, n, p, f)

Component Syntax #

Passive Components #

Resistor

R<name> <node+> <node-> <value> [TC1=<val>] [TC2=<val>]

Examples:
R1 1 2 10k
R2 vcc out 4.7k TC1=0.001
RLOAD out 0 1Meg

Capacitor

C<name> <node+> <node-> <value> [IC=<val>]

Examples:
C1 in out 100n
C2 vcc 0 10u IC=0
CBYPASS 5 0 1u

Inductor

L<name> <node+> <node-> <value> [IC=<val>]

Examples:
L1 in out 1m
L2 vcc 0 100u IC=0
LCHOKE vcc vdd 10m

Semiconductor Devices #

Diode

D<name> <anode> <cathode> <model> [area=<val>]

Examples:
D1 vin vout DMOD
D2 0 input 1N4148

.MODEL DMOD D (IS=1e-14 N=1.8)
.MODEL 1N4148 D (IS=2.52n RS=.568 N=1.752 CJO=4p VJ=.4 BV=100 IBV=100u)

BJT (Bipolar Junction Transistor)

Q<name> <collector> <base> <emitter> [<substrate>] <model>

Examples:
Q1 col base emit NMOD
Q2 vcc in out 0 2N3904

.MODEL NMOD NPN (BF=100 IS=1e-14 VA=100)
.MODEL 2N3904 NPN (IS=6.734f BF=416.4 VA=74.03 BR=.7371 CJE=4.493p CJC=3.638p)

MOSFET

M<name> <drain> <gate> <source> <bulk> <model> [L=<val>] [W=<val>]

Examples:
M1 drain gate source bulk NMOS
M2 vdd in out 0 2N7000 L=1u W=10u

.MODEL NMOS NMOS (VTO=1 KP=20u LAMBDA=0.01)
.MODEL 2N7000 NMOS (VTO=2.0 KP=0.15 LAMBDA=0.001)

JFET

J<name> <drain> <gate> <source> <model>

Examples:
J1 drain gate source JMOD

.MODEL JMOD NJF (VTO=-2.0 BETA=1e-3)

Sources #

Voltage Source

V<name> <node+> <node-> [DC <val>] [AC <mag> <phase>] [<transient>]

DC Examples:
VCC vcc 0 DC 5
VBIAS base 0 DC 2.5

AC Examples:
VIN in 0 AC 1 0
VSIG in 0 AC 1.414 90

Transient Examples:
VPULSE in 0 PULSE(0 5 0 1n 1n 10u 20u)
VSIN in 0 SIN(0 1 1k 0 0)
VEXP in 0 EXP(0 5 0 1u 1m 0)

Current Source

I<name> <node+> <node-> [DC <val>] [AC <mag> <phase>] [<transient>]

Examples:
IBIAS 0 base DC 10u
IAC in 0 AC 1m
IPULSE 0 out PULSE(0 1m 0 1n 1n 100u 200u)

Subcircuits #

Definition

.SUBCKT <name> <pin1> <pin2> ... <pinN>
* Internal circuit
R1 pin1 internal 1k
C1 internal pin2 100n
.ENDS <name>

Example:
.SUBCKT OPAMP in+ in- out vcc vdd
* Op-amp internal circuit
.ENDS OPAMP

Usage

X<name> <node1> <node2> ... <nodeN> <subcircuit_name>

Example:
XAMP inp inm output vcc 0 OPAMP

Analysis Commands #

DC Operating Point #

.OP

Example circuit:
VCC vcc 0 DC 5
R1 vcc base 10k
R2 base 0 4.7k
.OP

DC Sweep #

.DC <source> <start> <stop> <step> [<source2> <start2> <stop2> <step2>]

Examples:
.DC VIN 0 5 0.1                    (Single sweep)
.DC VIN 0 5 0.1 TEMP -25 125 25    (Nested sweep)
.DC VCE 0 10 0.1 IB 0 100u 20u     (Family of curves)

Transient Analysis #

.TRAN <tstep> <tstop> [<tstart>] [<tmax>] [UIC]

Examples:
.TRAN 1n 1u           (1ns steps, 1μs total)
.TRAN 1u 10m 0 10n    (1μs steps, 10ms total, max 10ns internal step)
.TRAN 1n 1u UIC       (Use initial conditions)

AC Small-Signal Analysis #

.AC <sweep_type> <points> <fstart> <fstop>

Types: DEC (decade), OCT (octave), LIN (linear)

Examples:
.AC DEC 10 1 100k     (10 points per decade, 1Hz to 100kHz)
.AC LIN 1000 1k 10k   (1000 linear points, 1kHz to 10kHz)
.AC OCT 8 1 1Meg      (8 points per octave)

Temperature Sweep #

.DC TEMP <start> <stop> <step>

Example:
.DC TEMP -40 125 25   (Temperature from -40°C to 125°C in 25° steps)

Monte Carlo Analysis (ngspice) #

.CONTROL
let run = 0
repeat 100
  alter R1 = 1k*(1 + gauss(0, 0.05/3, 6))
  run
  let run = run + 1
end
.ENDC

Example with tolerance:
R1 1 2 {1k*(1+gauss(0,0.05/3,6))}    (1kΩ ±5% tolerance)

Model Statements #

.MODEL Syntax #

.MODEL <name> <type> (<parameter>=<value> ...)

Types:
  D     - Diode
  NPN   - NPN BJT
  PNP   - PNP BJT
  NMOS  - N-channel MOSFET
  PMOS  - P-channel MOSFET
  NJF   - N-channel JFET
  PJF   - P-channel JFET

Common Diode Parameters #

.MODEL <name> D (
+  IS=<saturation current>          default: 1e-14
+  N=<emission coefficient>         default: 1
+  RS=<series resistance>           default: 0
+  CJO=<zero-bias capacitance>      default: 0
+  VJ=<junction potential>          default: 1
+  M=<grading coefficient>          default: 0.5
+  BV=<breakdown voltage>           default: inf
+  IBV=<breakdown current>          default: 1e-3
)

Common BJT Parameters #

.MODEL <name> NPN (
+  IS=<saturation current>          default: 1e-16
+  BF=<forward beta>                default: 100
+  BR=<reverse beta>                default: 1
+  VA=<Early voltage>               default: inf
+  VJE=<B-E junction potential>     default: 0.75
+  VJC=<B-C junction potential>     default: 0.75
+  CJE=<B-E capacitance>            default: 0
+  CJC=<B-C capacitance>            default: 0
+  TF=<forward transit time>        default: 0
+  TR=<reverse transit time>        default: 0
)

Common MOSFET Parameters #

.MODEL <name> NMOS (
+  VTO=<threshold voltage>          default: 0
+  KP=<transconductance>            default: 2e-5
+  LAMBDA=<channel modulation>      default: 0
+  PHI=<surface potential>          default: 0.6
+  GAMMA=<body effect>              default: 0
+  CBD=<B-D capacitance>            default: 0
+  CBS=<B-S capacitance>            default: 0
+  CGSO=<G-S overlap cap>           default: 0
+  CGDO=<G-D overlap cap>           default: 0
)

Output Commands #

.PRINT #

.PRINT <analysis> <output1> <output2> ...

Examples:
.PRINT DC V(out) I(VCC)
.PRINT TRAN V(in) V(out) V(base)
.PRINT AC VM(out) VP(out) VDB(out)

.PLOT #

.PLOT <analysis> <output1> <output2> ...

Example:
.PLOT TRAN V(in) V(out)

.PROBE #

.PROBE [<output1> <output2> ...]

Example:
.PROBE            (All voltages/currents)
.PROBE V(out)     (Specific outputs)

Control Commands #

.OPTIONS #

.OPTIONS <option1> <option2> ...

Common options:
.OPTIONS ABSTOL=1p      (Absolute current tolerance)
.OPTIONS RELTOL=0.001   (Relative tolerance)
.OPTIONS VNTOL=1u       (Voltage tolerance)
.OPTIONS ITL1=100       (DC iteration limit)
.OPTIONS ITL4=10        (Transient iteration limit)
.OPTIONS TRTOL=7        (Transient tolerance)
.OPTIONS METHOD=TRAP    (Integration: TRAP or GEAR)
.OPTIONS TEMP=27        (Default temperature in °C)

.PARAM (Parameters) #

.PARAM <name>=<value> ...

Examples:
.PARAM VCC=5 IB=10u
.PARAM RC={VCC/10m}    (Calculated parameter)

Usage in components:
R1 vcc out {RC}
V1 vcc 0 DC {VCC}

.FUNC (Functions) #

.FUNC <name>(<args>) {<expression>}

Examples:
.FUNC parallel(r1,r2) {r1*r2/(r1+r2)}
.FUNC Q(c,v) {c*v}

Usage:
R1 a b {parallel(1k,2k)}

Advanced Features #

Initial Conditions #

Capacitor IC

C1 in out 100n IC=2.5

Inductor IC

L1 in out 1m IC=10m

Global IC

.IC V(nodeA)=5 V(nodeB)=2.5

UIC (Use Initial Conditions)

.TRAN 1u 10m UIC

Temperature #

Component Temperature Coefficient

R1 1 2 1k TC1=0.001 TC2=1e-6

Simulation Temperature

.TEMP 27        (Set to 27°C)
.TEMP -40 25 125 (Simulate at multiple temperatures)

Expressions #

Arithmetic Operators

+ - * / ^      Addition, subtraction, multiplication, division, power

Functions

abs(x)         Absolute value
sqrt(x)        Square root
exp(x)         Exponential
ln(x), log(x)  Natural and base-10 logarithm
sin(x)         Sine (radians)
cos(x)         Cosine
tan(x)         Tangent

Statistical Functions

gauss(nom, rvar, sigma)    Gaussian distribution
unif(nom, rvar)            Uniform distribution

Waveform Sources #

PULSE #

PULSE(v1 v2 td tr tf pw per)

Parameters:
  v1  - Initial value
  v2  - Pulsed value
  td  - Delay time
  tr  - Rise time
  tf  - Fall time
  pw  - Pulse width
  per - Period

Example:
VIN in 0 PULSE(0 5 0 1n 1n 10u 20u)

SIN #

SIN(vo va freq td theta)

Parameters:
  vo    - Offset
  va    - Amplitude
  freq  - Frequency
  td    - Delay
  theta - Damping factor

Example:
VIN in 0 SIN(0 1 1k 0 0)

EXP #

EXP(v1 v2 td1 tau1 td2 tau2)

Parameters:
  v1   - Initial value
  v2   - Final value
  td1  - Rise delay
  tau1 - Rise time constant
  td2  - Fall delay
  tau2 - Fall time constant

Example:
VIN in 0 EXP(0 5 0 1u 1m 1u)

PWL (Piecewise Linear) #

PWL(t1 v1 t2 v2 t3 v3 ...)

Example:
VIN in 0 PWL(0 0 1u 0 1.1u 5 10u 5 10.1u 0 20u 0)

Measurement Output #

Voltage Measurements #

V(node)           - Voltage at node (relative to ground)
V(node1, node2)   - Voltage between nodes (node1 - node2)
VM(node)          - Magnitude (AC analysis)
VP(node)          - Phase in degrees (AC analysis)
VDB(node)         - Magnitude in dB (AC analysis)
VR(node)          - Real part (AC analysis)
VI(node)          - Imaginary part (AC analysis)

Current Measurements #

I(Vsource)        - Current through voltage source
I(element)        - Current through element
IM(Vsource)       - Magnitude (AC analysis)
IP(Vsource)       - Phase (AC analysis)
IDB(Vsource)      - Magnitude in dB (AC analysis)

SPICE Shell Commands (ngspice) #

Access via SPICE Shell in Breadpad:

run              - Execute the simulation
stop at <time>   - Set breakpoint
resume           - Continue simulation
print <output>   - Display value
plot <output>    - Graph output
write <file>     - Save results
setplot          - Select plot
display          - Show available plots
quit             - Exit shell

Best Practices #

Netlist Organization #

* Power Supply Design Example
* Author: Your Name
* Date: 2025-01-10

***** Power Sources *****
VIN vin 0 DC 12
VREF ref 0 DC 2.5

***** Input Stage *****
R1 vin div 10k
R2 div 0 10k
C1 div 0 100n

***** Output Stage *****
RLOAD out 0 1k
COUT out 0 10u

***** Analysis *****
.TRAN 1u 10m
.PROBE
.END

Naming Conventions #

Descriptive names: RLOAD not R27
Grouping: VCC, VDD, VEE for supplies
Signals: IN, OUT, SIGNAL_NAME
Nodes: base, collector, drain

Convergence Aids #

* Add small resistances
RGMIN sensitive_node 0 1G

* Add series resistance to voltage sources
VSRC vsrc 0 DC 5
RSRC vsrc actual_vcc 0.001

* Parallel resistance for inductors
L1 a b 1m
RP1 a b 1G

* Series resistance for capacitors
C1 in mid 100n
RS1 mid out 0.001

Common Units & Prefixes #

Prefix	Symbol	Value	Example
Tera	T	10¹²	1TΩ
Giga	G	10⁹	1GHz
Mega	M, Meg	10⁶	1MΩ, 1Meg
Kilo	k, K	10³	1kΩ
Milli	m	10⁻³	1mA
Micro	u	10⁻⁶	1uF
Nano	n	10⁻⁹	1nF
Pico	p	10⁻¹²	1pF
Femto	f	10⁻¹⁵	1fF

Note: Use Meg for mega-ohms to avoid confusion with milli (m).