An NPN transistor is used in common emitter configuration as an...

$loadresistace = outputresistace{R}_{L} = 1kΩ = 1000Ω$
$inputvoltage = \Delta{V}_{i} = 10mV = 10 \times {10}^{- 3}V$
$inputcurrent = basecurrent = \Delta{i}_{B} = 15\muA = 15 \times {10}^{- 6}A$
$outputcurrent = collectorcurrent = \Delta{i}_{C} = 3mA = 3 \times {10}^{- 3}A$
$\Delta{i}_{B}{R}_{i} = \Delta{V}_{i} = 0.67kΩ$
${R}_{i} = \dfrac{10 \times {10}^{- 3}}{15 \times {10}^{- 6}} = 0.67kΩ$
Voltage gain $= \dfrac{\Delta{I}_{C}{R}_{L}}{\Delta{V}_{i}} = \dfrac{3 \times {10}^{- 3} \times 1000}{10 \times {10}^{- 3}} = 300$