Question

In an $n-p-n$ transistor, the collector current is $28$ mA. If $80%$ of electrons reach the collector, its base current in mA is

• A. 35
• B. 24
• C. 14
• D. 7

Hint:

The emitter current is always the sum of the collector and base currents: $I_e = I_c + I_b$.

Answer 1

The Correct Option is A

Concept:
In a transistor: \[ I_E = I_C + I_B \] where:

• $I_E$ = emitter current
• $I_C$ = collector current
• $I_B$ = base current

Also, common-base current gain is: \[ \alpha=\frac{I_C}{I_E} \] It represents the fraction of emitter electrons reaching the collector.
Step 1: Given data
Collector current: \[ I_C=28\ \text{mA} \] Percentage of electrons reaching collector = $80%$ \[ \alpha=0.80 \]
Step 2: Find emitter current
Using: \[ \alpha=\frac{I_C}{I_E} \] \[ I_E=\frac{I_C}{\alpha} \] \[ I_E=\frac{28}{0.80} \] \[ I_E=35\ \text{mA} \]
Step 3: Find base current
Using: \[ I_E=I_C+I_B \] \[ I_B=I_E-I_C \] \[ I_B=35-28 \] \[ I_B=7\ \text{mA} \]
Step 4: Final Answer
The base current is: \[ \boxed{7\ \text{mA \]