question_answer

In a p-n-p transistor working as a common base amplifier current gain is 0.96 and emitter current is 7.2 mA. The base current is :

A) 0.2mA                                  

B) 0.29mA

C) 0.35 mA                               

D) 0.4 mA

Correct Answer: B

Solution :

Key Idea: Emitter current is sum of base current and collector current. The circuit diagram is as follows. The emitter-base junction is forward biased and base-collector junction is reverse biased. Also emitter current is sum of base current and collector current, i.e., \[{{i}_{E}}={{i}_{B}}+{{i}_{C}}\] and current gain is defined as the ratio of the change in collector current to the change in emitter current at a constant collector to base voltage,\[\alpha ={{\left( \frac{\Delta {{i}_{C}}}{\Delta {{i}_{E}}} \right)}_{{{V}_{CB}}}}\] Given,     \[\alpha =0.96,\,{{i}_{E}}=7.2\,mA\] \[\therefore \]               \[{{i}_{C}}=\alpha \times \Delta {{i}_{E}}\]               \[=0.96\times 7.2\]            \[{{i}_{C}}=6.91\,mA\] Hence, base current is \[{{i}_{B}}={{i}_{E}}-{{i}_{C}}\] \[{{i}_{B}}=7.2-6.91=0.29\,mA\]