Base Width Modulation | Electronics Device and Circuits | EC

by / Monday, 18 September 2017 / Published in Technical Corner

Real Life Application:
In active region BJT will act as amplifier and we have assumed that the current gain alpha and beta will be constants with respect to applied voltage, which will not be valid in practicality because as we will apply ac signal in input then voltage in the BJT will keep on changing. Collector Base junction will also vary so depletion region will also vary.

Explanation:
1. Vcb is increasing: As voltage across collector base junction will increase, depletion width in base will also increase. Due to which effective area in base will keep on reducing. So recombination in base region will decrease, due to which collector current will increase and base current will reduce. So Alpha and Beta will increase. In this case minority carriers which are get injected from emitter to base will reach to collector junction early. Due to this phenomenon is also known as “Early Effect”.

2. Vcb is decreasing: As voltage across collector base junction will decrease, depletion width in base will also decrease. Due to which effective area in base will keep on increasing. So recombination in base region will increase, due to which collector current will reduce and base current will increase. So Alpha and Beta will increase.

This variation of effective base with the variation of reverse bias Collector to Base voltage is known as “Base with Modulation”.
If the output current is varying then output resistance will also vary as early voltage as

Ro=Va/Ic

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