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Calculating the power dissipated by a transistor

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jigidyjensen

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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

Well-Known Member
Most Helpful Member
The base voltage and current are so small that they barely count.
 

MikeMl

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Most Helpful Member
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).
 

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