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BJT in Active Region (2)

  • Base Emitter junction is forward biased

  • Base Collector junction is reverse biased

transistor is acting as current controlled current source (iC is

controlled by

, and

=β

)

  • Since the base emitter junction is forward biased, from Shockley

equation

=

exp

1

7

Early Effect and Early Voltage

  • As reverse-bias across collector-base junction increases, width of the collector-base depletion layer increases and width of the base decreases (base-width modulation).

In a practical BJT, output characteristics have a positive slope in forward-

active region; collector current is not independent of

.

Early effect: When output characteristics are extrapolated back to point of

zero

, curves intersect (approximately) at a common point

=-

which lies between 15 V and 150 V. (VA is named the Early voltage)

  • Simplified equations (including Early effect):

=

e x p

1 +

1 +

β =β

=

β

e x p

8

4

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