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> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Dave Tweed
> Sent: 26 April 2010 14:44
> To: piclist@mit.edu
> Subject: Re: [EE] DAC with 2x gain - isnt working as expected
>=20
> alan smith wrote:
> > I'm thinking that using the LM324 will solve the gain problem, and
using
> > some sort of current boost on the output..any suggestions on a
simple
> > transistor circuit? The LM324 can source up to 40mA but they want to
do
> at
> > least 100mA. Thinking a NPN/PNP pair (2N3904/2n3906) inside the
feedback
> > loop might be the easiest way? They are rated for 200mA continuous,
so
> that
> > would provide the necessary drive.
> >
> > Thoughts?
>=20
> I think you'll lose your rail-to-rail output capability with any sort
of
> simple transistor circuit (e.g., emitter-follower). You won't be able
to
> drive the output any closer than one B-E drop above ground.

Possibly not useful for the OP since he's using a quad package, but
there is a slightly cunning way=20to boost output drive if you use single
op-amps.  By putting current sense resistors in the positive and
negative supply connections to the op-amp you can switch on your current
boosting transistors only when needed.

I think I saw this in a Nat Semi datasheet many years ago.

Mike

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