The current gain of a bipolar transistor in common emitter or common collector compared to common base configuration is:

In common emitter and common collector configurations, the input current is the base current, which is very small. The output current is either the collector current or emitter current. The ratio of these output currents to the tiny base current results in a high to very high current gain (typically between 50 and 500). In contrast, a common base configuration uses the large emitter current as the input and the slightly smaller collector current as the output. Since the collector current is always slightly less than the emitter current, the current gain (alpha) in a common base circuit is actually less than 1. Therefore, when compared to the common base configuration's fractional current gain, the current gains of both common emitter and common collector configurations are exceptionally high.