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: