Question

For a common emitter amplifier, current gain is 50. What is the current gain if it is working as common base amplifier?

• A. 1.02
• B. 0.98
• C. 98
• D. 1

Answer 1

The correct option is B 0.98

The common emitter amplifier can be represented schematically as:


The common base amplifier can be represented schematically as:


For common emitter, the current gain $\beta =\frac{i_C}{i_B}$
For common base, the current gain
$\alpha =\frac{i_C}{i_E}$

We know $i_E=i_B+i_C$
Dividing both sides by $i_C$
$\frac{i_E}{i_C}=\frac{i_B}{i_C}+\frac{i_C}{i_C}$
$\Rightarrow \frac{1}{\alpha }=\frac{1}{\beta }+1$

Given $\beta =50$
From the above expression,
$\alpha =\frac{\beta }{\beta +1}=\frac{50}{51}$
i.e $\alpha =0.98$