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Opamp with CMIR down to 0V

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Newbie level 6
Sep 15, 2008
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I would like to create an opamp with a common mode input range down to 0V? It does not have to be rail to rail, but does have to operate down to 0V.

Is is possible to create such an opamp using, for example, a two stage opamp, with pmos diff pair? If not, can somebody please point me in the right direction?

Thanks in advance

There are many, many rail-rail I/O op amps out there
as commercial piece-parts. If you need both ends to be
at- (or over-) rail, you may end up with two follower-
buffered diff pairs (one N, one P) so that when one is
cut off the other is still working. Then the trick is in how
to cleanly combine the two, across the CM range.

Check out all the low voltage CMOS op amps from
Linear Tech, National, ADI and you may be able to
find a decent input stage concept schematic. Also the
IEEE JSSC pubs have had many such circuits shown,
over the years.

There should be no problem creating an opamp with PMOS input pairs that operate with an input of 0V.

Just make sure that it is a folded cascode type circuit whereby the drain of the PMOS input pair sees a current source (with VDsat across it) or even a resistor. If you use an NMOS current mirror active load, it is not going to work effectively.


I am using mcp604 rail to rail op-amp as instrumentation op-ampfor my appl'n.
it's result are satisfactory.

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