On 10-Jan-02 Alan B. Pearce wrote:
>>In this application I would
>>probably go for a darlington pair
>
> Be very careful trying to saturate a darlington transistor, as the
> saturation voltage is at least one Vbe (for the second transistor) + the Vce
> of the first transistor.

Yes, I've had problems with that in the past when the collector voltage of the
2nd device falls close to ground the 1st device can no longer provide transistor
action and provide the drive current to the 2nd device.  I seem to remember (it
was a few years ago) that something odd happens as the first device starts to
act as just a diode (be junction) and thus the current gain of the pair falls
and thus you need to provide more base current than you expect.

Peter.

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