On 15/04/2011 16:59, V G wrote:
> I didn't know the gain worked like that. I assumed 10 mA into the base =
=3D
> 1000 mA pulled by the collector. I tested it on LTSpice and that's what I
> saw as well.
>

If the current gain is 100, then that is what the collector will sink,=20
but the emitter will sink 1010mA.
Check the Ie on LTSpice, compare it with the Ic and you will find the=20
difference is Ib.
If this Ib is flowing through a low side sense resistor then it will add=20
an error, as you are sensing the Ie but wanting to control Ic. This is=20
why a high side sense resistor would be better, but that requires a=20
differential amp, or use a load with less base current, like a Fet or a=20
Darlington.


> If I drive the base with 10 mA, why does the collector only pull 990 mA?
>
> Also, if I use a Darlington pair, should I use a small transistor as the
> first transistor and the large transistor for the second? Or should both =
of
> them be the same large transistor?
> To measure current, I'll just measure the voltage drop across the load
> resistor and adjust the base current according to that.

Where will the load resistor be?

--=20
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