Calculating the power dissipated by a transistor

[jigidyjensen]

When calculating the power dissipated by a npn transistor, Pq = (Vc)(Ic). Why don't we include the current in the bass as well? Why isn't it the following

Pq = (Vc)(Ic) + (Vb)(Ib)

??

 

[audioguru]

The base voltage and current are so small that they barely count.

 

[MikeMl]

You do. Its even slightly more complicated than that if the emitter is not at zero volts...

Look how LTSpice does it: Plot of the collector voltage, the collector current, and the power dissipated in the transistor as the base input voltage sweeps from 0 to 10V. Note the expression that calculates the transistor dissipation ( I didn't have to type it in; LTSpice knows it automatically).