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2.3 Circuits and Subcircuits 59
In[12]:= Circuit[
Netlist[
{I1, {0, 1}, 1},
{Q1, {1 −> "B", 2 −> "C", 3 −> "E"},
Model −> NPNTransistor, Selector −> ACsimple},
{Q2, {3 −> "B", 2 −> "C", 0 −> "E"},
Model −> NPNTransistor, Selector −> ACsimple},
{Load, {0, 2}, Type −> Resistor, Value −> RL,
Pattern −> Impedance}
],
Model[
Name −> NPNTransistor,
Selector −> ACsimple,
Ports −> {"B", "C", "E"},
Definition −>
Netlist[
{RB, {"X", "E"}, RB},
{CC, {"B", "X", "C", "E"}, beta}
]
]
] // ExpandSubcircuits // DisplayForm
Out[12]//DisplayForm=
Netlist Flat, 6 Entries:
I1, 0, 1, 1
RB$Q1, X$Q1, 3, RB
CC$Q1, 1, X$Q1, 2, 3, beta
RB$Q2, X$Q2, 0, RB
CC$Q2, 3, X$Q2, 2, 0, beta
Load, 0, 2, Type Resistor, Value RL, Pattern Impedance
While ExpandSubcircuits (Section 3.4.1) has generated unique identifiers for the base resistor RB
and the current-controlled current source CC for each instance of the transistor model, it has not
made any changes to the associated symbolic element values RB and beta. Obviously, these have
been regarded as global symbols, resulting in identical base resistances and current gains of both
transistors. This is certainly not the result we wanted because the individual current gains of
the transistors in a Darlington stage may be quite different. Instead, ExpandSubcircuits should
instantiate the element values as well, and thus generate the symbols RB$Q1, RB$Q2, beta$Q1, and
beta$Q2 for the two resistors and the two current gains respectively.
The reason behind the unwanted behavior exhibited here is that ExpandSubcircuits has not been
explicitly instructed to treat RB and beta as what they really are, namely as parameters of the model.
We can solve this problem by adding the parameter declaration
Parameters −> {RB, beta}
to our Model definition, which instructs ExpandSubcircuits to replace all occurrences of these
symbols in the value fields of the subcircuit netlist by different identifiers for each subcircuit
instance.