Question

The current gain a of a transistor is $0.9.$ The transistor is connected to common base configuration. What would be the change in collector current when base current changes by $4\,mA$ ?

• A. 1.2 mA
• B. 12 mA
• C. 24 mA
• D. 36 mA

Answer 1

Answer: (D)

36 mA

Answer 2

The ratio of collector current $\left(I_{C}\right)$ to emitter current $\left(I_{e}\right)$ is known as current gain $(\alpha)$ of a transistor. Therefore, $\alpha=\frac{\Delta I_{c}}{\Delta I_{e}} \ldots$ (i) Also, emitter current is equal to sum of change of base current and collector current. Therefore, $\Delta I_{e}=\Delta I_{b}+\Delta I_{c} \ldots$ (ii) From Eqs. (i) and (ii), we get $\alpha=\frac{I_{c}}{\Delta I_{b}+\Delta I_{c}}$ Given, $\alpha=0.9,$ $\Delta I_{b}=4 m A$ $\therefore 0.9=\frac{I_{c}}{4+I_{c}}$ $\Rightarrow 0.9\left(4+I_{c}\right)=I_{c}$ $\Rightarrow 3.6+0.9 I_{c}=I_{c}$ $\Rightarrow 3.6=0.1 I_{c}$ $\Rightarrow I_{c}=36\, m A$