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198 3. Reference Manual
Any symbol which appears in the value field of a model netlist entry or in a set of behavioral
equations can be designated as a parameter. The difference between parameters and other symbols
is that you can assign instance-specific values only to parameters. Other symbols are regarded
as global quantities which cannot be bound to instance-specific values in a model reference. In
addition, if a model parameter p is left unbound in a model reference, it will be replaced by a
uniquely instantiated symbol p$instance upon subcircuit expansion whereas symbols representing
global quantities are not changed. The following line shows a valid example for the Parameters
declaration of a BJT model:
Parameters −> {IS, N, BF, BR}
In some cases it is necessary to override the value of a global circuit parameter locally for a particular
model instance. For example, you may have set the environmental temperature TEMP in a Circuit
object with the GlobalParameters statement
Circuit[
Netlist[ ],
GlobalParameters[TEMP −> 300.15]
]
but you want to specify a different operating temperature for an individual device, such as a
temperature sensor or the output transistor of a power amplifier. In order to override the global
value of TEMP for this device, the local temperature value must be specified explicitly in the
corresponding model reference:
{QOUT, {VCC −> C, 5 −> B, out −> E},
Model −> BJT, Selector −> DC,
IS −> 1.*^−16, N −> 1., BF −> 100., BR −> 1., TEMP −> 340.}
However, TEMP is a global quantity and not an instance parameter of the model BJT/DC. Therefore,
the local setting TEMP −> 340. in the model reference QOUT has no effect unless the Parameters
specification in the model definition is modified as follows:
Parameters −> {IS, N, BF, BR, Global[TEMP]}
Now the setting Global[TEMP] allows you to change the value of TEMP individually for any instance
of BJT/DC while the setting from the GlobalParameters list will be used for all instances for which
no local temperature value is given.
Defaults
Defaults−>{param
−>value , param
−>value , † † † }
specify default values for model parameters
Format of the Defaults argument.
With the Defaults keyword, you can specify default values for model parameters which have been
left unbound in a model reference. For example, with the settings