By ignoring 
and 
we can define a simplified AC model
for the transistor (perfect transistor) that is independent of
the circuit configuration:
There is no PN voltage drop in these equations since it is a DC effect.
Since 
is typically 100 times larger than 
we can make the
approximation 
.
An ideal transistor is defined such that 
and hence
.
When the effects of 
cannot be ignored, we can use the perfect
transistor model as described by the two equations above.
On a circuit diagram can be added directly to the ideal
transistor symbol.
The ideal transistor model has the following two working rules:
).
They differ only by a constant DC potential 
.
and 
).