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close this bookAmplifier Teaching Aid (DED Philippinen, 86 p.)
close this folderLesson 2 - Bipolar Transistor
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Transistor currents

(see Fig. 2-4)

VBB forward biases the emitter diode, forcing the free electrons in the emitter to enter the base. The thin and lightly doped base gives almost all these electrons enough time to diffuse into the collector. These electrons flow through the collector, through RC, and into the positive terminal of the VCC voltage source. In most transistors, more than 95% of the emitter electrons flow to the collector, less than 5% flow out the external base lead.


Fig. 2-4: NPN Transistor

Recall Kirchhoff's current law:

=> IE = IC + IB


Fig. 2-5: Transistor currents

Because IB is very small, for circuit analysis, we can do the following approximation:

IC is equal to IE