[SOLVED] Rail to rail op-amp versus simple op-amp

[mrinalmani]

What is the difference between rail to rail op-amp and a simple op-amp. As far as I know rail-rail amplifiers can work with input voltages very close to the power rails. The question is... can a simple op-amp not work close to rail voltages. If not, what irregularities can be expected as the input approaches the rail? (A hardware implementation of a NON rail-rail op-amp gave incorrect results, but after replacing it with a rail-rail op-amp, the results were ok... but why?)
Thank you

 

[barry]

There are different types of rail-to-rail opamps: some are only R-R on the output, some are R-R on both input and output. Many non-R-R opamps can go all the way to ground on the input, but not to the positive rail. You need to look at the specific op-amp to determine what it can do; they're all different. Some will just saturate if the input goes outside the allowable range; some will exhibit phase-reversal (the output starts going in the wrong direction).

 

[mrinalmani]

Phase reversal! Yes exactly.... this indeed happens, but the reason is beyond my comprehension. How can this be theoretically explained?
And I have also given up finding the reason as to why a precision rectifier's output bounces back during the negative cycle. All books give methods to correct it but I have not yet found why it actually occours..
Please help

 

[FvM]

"Simple op-amp" is a rather vague term. Each OP as different properties, also rail-to-rail OPs. It would be reasonable to discuss the problem referring a specific circuit and OP types. Phase reversal has happened with a number of classical OPs, but it has become rare with recent types.

 

[erikl]

Re: Phase reversal

Phase reversal! Yes exactly.... this indeed happens, but the reason is beyond my comprehension. How can this be theoretically explained?

G00GLE finds the explanation easily, s. here.

 

[mrinalmani]

This was a great attachment! Thanks a lot...
By "simple" I meant the ones whose datasheet do not mention rail-rail capability.