The AC current gain ($\beta_{ac}$) of a transistor in a common emitter configuration is defined as the ratio of the change in collector current ($\Delta I_C$) to the change in base current ($\Delta I_B$) at a constant collector-emitter voltage.

Given: Change in base current, $\Delta I_B = 300 \text{ } \mu\text{A} - 100 \text{ } \mu\text{A} = 200 \text{ } \mu\text{A} = 0.2 \text{ mA}$ Change in collector current, $\Delta I_C = 20 \text{ mA} - 10 \text{ mA} = 10 \text{ mA}$

Current gain, $\beta_{ac} = \frac{\Delta I_C}{\Delta I_B}$ $\beta_{ac} = \frac{10 \text{ mA}}{0.2 \text{ mA}} = 50$

Therefore, the current gain is 50.