At 10:18 AM 15/04/2011, you wrote:
>Olin is assuming that you connect the battery to the collector of the
>BJT, control voltage to the base, and the emitter to ground through a
>sense resistor. The error comes from the fact that whatever current
>the base terminal draws will ALSO flow through the emitter. In other
>words, Ie=3DIc+Ib.
>
>The only way you could really avoid this (and still use a BJT) is to
>either use a BJT with a sufficiently-high current gain that the base
>current is negligible OR put the sense resistor in the collector lead.
>However, the latter option will require a differential amplifier to
>measure the sense resistor since it is no-longer ground referenced.

Or use a high side sensor at the battery V+, or an isolated
Hall sensor, or subtract the base current with op-amps etc.
etc. etc.

Or use the "hybrid darlington", as I mentioned earlier:


                                     o
                                     |
                                     |
                             +-------+
                             |       |
                             |       |
            N-channel     ||-+       |
            MOSFET        ||<-       |
                    o-----||-+       |
                             |     |/
                             +-----|    NPN BJT
                            .-.    |>
                            | |      |
                         R  | |      |
                            '-'      |
                             |       |
                             |       |
                             +-------+
                                     |
                                     |
                                     o

Pick R such that the worst case leakage through the MOSFET develops
only a couple hundred mV.

Note that the MOSFET may need a heat sink as well.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



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