Subcircuits

When you look at different functions of an electrical circuit, they are easier to understand by hiding information in a block, or a subcircuit.

The main syntax for a subcircuit is "subckt myckt ... ends ckt".

A basic example

To understand the syntax for a subcircuit take a look at voltage divider.

Enter the following lines in a file vdiv.ckt:

* voltage divider
subckt vdiv in out
vi in 0 1.0
r1 1 out 10K
r2 out 0 10K
ends vdiv

In Gnucap this subcircuit can be loaded with:

gnucap> get vdiv.ckt

Now you can check that the netlist is loaded:

gnucap> list
.subckt vdiv ( in out )
vi ( in 0 )  DC  1.
r1 ( 1 out )  10.K
r2 ( out 0 )  10.K
.ends vdiv

Now enter a circuit that uses the subcircuit:

gnucap> b
>vsrc 1 0 5
>x1 1 0 vdiv

Then calculate the DC operating points:

gnucap> probe op V(nodes)
gnucap> op
#           V(1)       V(2)      
 27.        1.         0.5

to be removed

As you can see, the voltage on node 2 is half of voltage on node 1. This calculation took 27 internal steps (Todo: check the meaning of # 27.)

And sweep the input voltage for example:

• run DC sweep

  width out=80

You can plot the results with:

plot dc v(1)(0,1) v(2)(0,1)
dc vi 0.0 1.0 .1

If you would do the process by reading the content of a file where everything is written inside, you would append a dot before each command(except for build) and pass the name of the file with the -b option

#gnucap -b test.ckt

the content of the file would be:

    * voltage divider
    vi 1 0 1.0
    r1 1 2 10K
    r2 2 0 10K
    .probe op V(nodes)
    .op
    .width out=80
    .plot dc v(1)(0,1) v(2)(0,1)
    .dc vi 0.0 1.0 .1
    .end

Note that a there's a end command. It simple tells the program that it have done his job.