Download - Wolfram Research

2.3 Circuits and Subcircuits 51
C
B
E
CM
B
E
IB
X
RB
C
beta*IB
E
B
E
IB
X
RB
beta*IB
R0
C
E
Figure 3.2: Different transistor small-signal models: simple (left), dynamic (right)
Both circuits have in common that they represent the same object, namely an NPN transistor. Thus,
they are two members of a subcircuit class we shall name NPNTransistor:
Name −> NPNTransistor
Within this class, each member must have a unique selector by which it can be identified. We will
denote the simple small-signal model on the left-hand side of Figure 3.2 by
and the one on the right-hand side by
Selector −> ACsimple
Selector −> ACdynamic
From now on we will adopt the notation name/selector for denoting a subcircuit definition. For
instance, we will refer to the model defined with Name −> NPNTransistor and Selector −> ACsimple
as NPNTransistor/ACsimple. Both subcircuits contain four nodes: B, C, E, and X, where B, C, and E
represent the transistor’s base, collector, and emitter terminal respectively. The node X is an internal
subcircuit node which is needed for properly connecting the controlling branch of the currentcontrolled
current source in series with the base resistor RB (see also Section 2.2.2). Therefore, we
declare only B, C, and E as port nodes of the subcircuits in order to make X invisible from the outside:
Ports −> {"B", "C", "E"}
Note that we use strings to denote the port nodes instead of symbols. Although this requires some
more typing this is the recommended way for specifying nodes: If we would have used symbols
in the above example, the symbol for the emitter node E could have been mixed up with the
Mathematica symbol E which denotes the exponential constant e.