dc42:
When using a transistor as a switch, typically the base current drive provided is one-tenth of the collector current being switched. This is also the condition at which Vce(sat) is usually quoted on the datasheet.
Indeed this is usual for most medium-power switching transistors, but there are newer "superbeta" switching transistors around which fare a lot better (keeping high gain at high currents and getting much better saturation voltages at lower base drive) - ZTX851 is one I've used and this quotes min current gain of 100 at 2A collector current (for Vce=1V)... Also at 2A the Vce <= 0.15V if Ib=50mA (base current = 1/40th collector current).
Often this means for larger currents where you want 40mA base drive or more (too much for output pin) with a plain old switching transistor you can move to a super-beta device rather than a darlington and get much less voltage drop out (0.15V Vsat is very respectable at 2A, only 300mW dissipation)
[OK, 50mA is too much but with 30mA the device will drive 1A no sweat at all]