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'[EE] Linear regulator pass transistor'
2011\12\05@062244 by Electron

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Howdy,
to get more current out of a linear regulator, is a NPN pass transistor (collector
to VReg input, base to VReg output, and emitter to circuit) a good idea? From my
experiments it introduced a voltage drop... is it me or it's an intrinsic and
unavoidable defect of this simple solution?

Thanks,
Mario

2011\12\05@065028 by Michael Rigby-Jones

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Hi Mario,

An emitter follower will always introduce a Vbe drop, and it will spoil
the regulation.  A much better solution is to use a PNP transistor as
described in many of the 3 terminal regulator application notes e.g.

http://www.onsemi.com/pub_link/Collateral/MC7800-D.PDF

Note that this will defeat the current limiting built into the
regulator, so if this is important you will need a slightly more complex
circuit which is also described in this datasheet.

Mike

> {Original Message removed}

2011\12\05@070737 by RussellMc

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> An emitter follower will always introduce a Vbe drop, and it will spoil
> the regulation.  A much better solution is to use a PNP transistor as
> described in many of the 3 terminal regulator application notes e.g.
>
> http://www.onsemi.com/pub_link/Collateral/MC7800-D.PDF

What he says.
Note also that the LM317 is more flexible in this role due to it's
lower minimum Vout and the LM317 datasheets often contain sample
circuit.


            Russell

2011\12\05@102207 by Electron

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At 13.06 2011.12.05, you wrote:
>> An emitter follower will always introduce a Vbe drop, and it will spoil
>> the regulation.  A much better solution is to use a PNP transistor as
>> described in many of the 3 terminal regulator application notes e.g.
>>
>> http://www.onsemi.com/pub_link/Collateral/MC7800-D.PDF
>
>What he says.
>Note also that the LM317 is more flexible in this role due to it's
>lower minimum Vout and the LM317 datasheets often contain sample
>circuit.

Thank You both for the precious insights.

Cheers,
Mario

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