On Fri, 15 Apr 2011 18:05 -0400, "V G"  wrote:
> > It does.  The "gain" of a BJT (more specifically Hfe) is the collector
> > current divide by the base current.  If the base current is 1, then the
> > collector current is GAIN, and the emitter current is GAIN+1.  In your
> > case,
> > the collector current is the battery current, but what you measure with=
 a
> > low side current sense is the emitter current.  Note that the ratio of =
GAIN
> > to GAIN+1 approches 1 as the gain gets large.
> >
> >
> I was thinking of doing high side current sensing like the first diagram
> here: http://i.cmpnet.com/planetanalog/2009/02/C0363-Figure1.gif
>=20
> That's why I didn't understand all the talk about inaccuracies.

That's a simplified diagram, not a real schematic.

Your high side amp could easily have one percent base inaccuracy, plus
temperature and common mode error.

Typically one would not do high side sensing. If I did, it would be
because I did the math on it and found it to be worth it. It usually
isn't. A good monolithic diff amp is not cheap and making one that is
good still takes a decent opamp and some very good resistors.

In your application I'd sense on the low side. And since you don't need
low saturation voltage, you could even triple-darlington for your active
load. Or use a FET.

Cheers,

Bob


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