Transistor switch

This device is a simplified transistor switch for system-level simulation. It is modeled as a voltage-controlled switch in series with a DC source which models the transistor forward voltage drop.

The voltage-controlled switch is a resistance whose value is controlled by the gate voltage relative to ground. It has the following transfer function (the y-axis is logarithmic): 

transistor switch transfer function

In comparison to models based on primitive SPICE transistor devices or to subcircuit-based models, this model typically results in a faster simulation and in improved convergence, at the expense of accuracy. It is intended for macro-level simulations where the importance of a transistor's basic on-off switching behavior dominates all other behaviors.

Sample circuit:

transistor switch example