SPICE the circuit simulator - some notes

December 2019, Ottawa, Canada

I don't use SPICE that often, and every single time I forget the commands and the syntax. So, here are a couple notes and examples - mainly for my own reference.


Quick intro

First, under GNU/Linux you will be most likely using "ngspice".
To start, draw a circuit on a piece of paper, name the components with a label and name each node, as shown in the examples below. Note that the reference (ground) is always labeled "0".
A node is where two components, seen as lumped elements with a number of pins, connect to each other. It's where you would physically measure voltages with your tester. If you have a wire connecting two components, the whole wire will be only one single node (we assume it's equipotential).
The input file for SPICE is called a "netlist", and it includes the components and if you wish also the operations to be performed by SPICE (or you can type them manually after, in the SPICE interface). When inserting a component in the spice netlist, you name a component and write where it attaches to, and its value. The first character of the name tells SPICE which component it is, then following characters are just helpful for you to understand what you have typed. Successive numbers are the pins where you attach it, then the value.

Rload 3 4 22K

You can also name nodes like "node33" etc etc if it helps.

Example 1: voltage divider - transient simulation

Consider the simplest example, the voltage divider shown below.

Write the following to a file and save it as you wish (but use no spaces in the name).

* This is a comment
* Start with the title
Voltage divider

* Then put the components
V1  1  0  10V
R1  1  2  5
R2  2  0  5


Call it using "ngspice filename". This will open the SPICE interface. Let us run a transient simulation for example (even if this particular case is simple, since it is steady state...). Type into the SPICE interface:

tran 0.1 1

This will simulate what happens with a step of 0.1 s, for a total of 1 second. Then, you can plot the voltages on the nodes. Type:

plot v(1) v(2)

This should produce the following image:

Example 2: voltage divider with DC sweep

So, another option is running the previous circuit with a DC analysis. This will short all inductances and open all capacitors. SPICE has the "DC" command, which performs a DC sweep. Note that the analysis performed by SPICE in this mode is non-linear (cool!)
Load the same circuit as before, then type into SPICE:

dc V1 0 12 1

This will overwrite the values of the voltage source V1, with values ranging from 0V to 12V, with step 1V. Then, plot the voltages at the nodes using:

plot v(1) v(2)

And rightfully, you will see something like this next plot:

Example 3: simple class A amplifier - transient simulation

Consider for example the class A common-emitter amplifier circuit in the following figure (actual values are not the best, it's just an example!):

We run a transient simulation with the "tran" command, specifying the timestep and the final time. Then we plot the voltage at nodes 7 (the output of our ampli) and at node 2 (the input). Here is the code (write it in a file "ampli.net" and, in GNU/Linux, call it with "ngspice file.net").

** Asterisk denotes a comment.
** DC Voltage source:
** AC Voltage source (amplitude is Vpp/2):
** Vn  "PLUS"  "MINUS" SINE(offset  amplitude  frequency  delay  theta_damping_factor)
** Resistor: (use "100K" for 100 kOhm)
** Transistor:
** Qn  collector_NODE  base_NODE  emitter_NODE   NAME
*** Start by the title of this file:
Simple ampli

*** Model for the transistor (find it online) 
.MODEL 2n2222a  NPN (IS=2.20f NF=1.00 BF=240 VAF=114
+ IKF=0.293 ISE=2.73p NE=2.00 BR=4.00 NR=1.00
+ VAR=24.0 IKR=0.600 RE=0.194 RB=0.777 RC=77.7m
+ XTB=1.5 CJE=24.9p VJE=1.10 MJE=0.500 CJC=12.4p VJC=0.300
+ MJC=0.300 TF=371p TR=64.0n EG=1.12 )

*** Write the connections here
V1 node4 0 12
V2 node1 0 SINE(0 0.001 10000 0 0)
Rs node2 node1 50
C1 node3 node2 47n
R1 node4 node3 33K
R2 node3 0     3.3K
Rc node4 node5 14K
Re node6 0     1.2K
Ce node6 0 47n
C2 node7 node5 47n
Rl node7 0 100K
Q1 node5 node3 node6 2n2222a

*** Here some commands to be executed
tran 0.000001 0.001
plot node5 node7


Example 4: RC filter - AC small signal analysis

SPICE can do a lot more cool stuff. For example it can do small signal AC analysis. We can simulate the transfer function of a circuit, such as the simple low-pass filter shown below.

The netlist for this circuit is:

Low-pass filter

VS  1  0  AC  1  SIN(0  1  2KHZ)
R1  1  2  1K
C1  2  0  0.032UF

ac   dec 5 10 10MEG
plot mag(v(2)/v(1))


SPICE will basically overwrite the voltage source, applying a frequency from 10 Hz to 10MHz, placing 5 points per decade. Note that the AC small signal analysis in SPICE starts with a DC analysis to find the DC bias. This is done automatically, without requesting it explicitly.
To plot the magnitude of the transfer function, you can compute the ratio of the output and input voltages, then compute the module (magnitude):

plot mag(v(2)/v(1))

CAVEAT: capacitors in series and DC analysis

Something important here: in some versions of SPICE, such as "ngspice", if you try to put two capacitors in series and launch a DC or a transitory analysis, the computation may crash. Indeed, the problem of finding the voltage between two capacitors in series, is ill posed for a purely DC calculation. Most SPICE flavors automatically put a large resistor in parallel to the capacitor. As far as I know, ngspice does not do it, so you should do it yourself. If you have capacitors in series, put a 100G resistor in parallel to each of them, and you'll be fine.

Plotting transistor characteristic curves

So, don't you hate when a datasheet does not show the characteristic curves of a transistor? Actually (assuming your SPICE transistor model is accurate enough), you could plot them using spice! I'm sure there is a good way to do it directly withing spice, anyway click here for an Octave script that does it.
The result is something like this:

The way this is done is by setting up a circuit like below. There are two generators. The V1 generator is fixed to a value and the DC value near the base is swept from 0 to some volts, saving the resulting (1) base current, (2) collector current and (3) collector-emitter voltage. This is repeated for various values of V1 from some 0 to 12 V. Here is the circuit:


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