Question

A transistor operating in a common base configuration has forward current gain factor, $\alpha =0.99$. If the emitter current changes by $1\text{mA}$, then the changes in the base current will be

• A. $\text{100 mA}$
• B. $\text{0.01 mA}$
• C. $\text{0.99 mA}$
• D. $\text{99 mA}$

Answer 1

The correct option is B $\text{0.01 mA}$

Given:
Current gain, $\alpha =0.99$
Change in emitter current, $\Delta I_E=1\text{mA}$
Change in base current, $\Delta I_B=?$

We know, $\alpha =\frac{\Delta I_C}{\Delta I_E}$

$\Rightarrow \alpha =\frac{\Delta I_E-\Delta I_B}{\Delta I_E}[\because \Delta I_E=\Delta I_B+\Delta I_C]$

$\Rightarrow 0.99=\frac{1-\Delta I_B}{1}$

$\Rightarrow \Delta I_B=0.01\text{mA}$

Hence, $\left(B\right)$ is the correct answer.

$\begin{array}{c}\text{Why this question?}\\ \text{Caution: Formula of current gain depends}\\ \text{on the type of configuration of transistors.}\end{array}$