β is current gain of transistor in common emitter configuration. Common emitter current gain beta (β) = Ic/Ib as in common emitter configuration input current isIb and output current is Ic. α is large signal current gain of transistor in common base configuration α = (Ic –Ico) / (IE-0). Alpha typically varies from 0.9 to 0.995.If we neglect reverse saturation current Ico then beta can be represented in terms of alpha, β= α / (1- α). In common collector configuration output is taken at emitter terminal, hence common collector current gain is given γ = IE/IB (input current is IB and output current is IE). γ is also called emitter injection ratio.