In common emitter mode of transistor, the d.c. current gain is 20, the emitter current is 7 mA. The collector current is

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Given the d.c. current gain in common emitter mode, $β = 20$ , and the emitter current, $I_{E} = 7$ mA.

The relation between the common base current gain $α$ and common emitter current gain $β$ is given by:

$α = \frac{β}{1 + β}$

Substituting the value of $β$ :

$α = \frac{20}{1 + 20} = \frac{20}{21}$

We also know that $α = \frac{I _{C}}{I _{E}}$ , where $I_{C}$ is the collector current.

$I_{C} = α I_{E}$

$I_{C} = \frac{20}{21} \times 7$

$I_{C} = \frac{20}{3}$ mA

Answer: $\frac{20}{3}$ mA

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