Common Mode Input Voltage

Looks like the LM321 is just a single version of LM324.

Depending on your feedback structure, going above 3.5V can cause various bizarre effects, snapping to negative or positive rail, or simply indeterminate.

I wouldn't try it as a unity gain buffer above 3.5V input.

If you need that performance, get a CMOS R-R I/O OpAmp. ...Jim Thompson

Not necessarily. Like Jim said, parts can do really oddball things when you exceed their input common-mode range. Some parts are better-behaved when you go out of the bounds (slowing down or losing gain). Some parts do the rail-snapping thing that Jim talks about.

Your best bet is to follow what the manufacturer specifies, and if they don't say _what_ the part does outside of bounds, _in writing_, then you should just not do that.

Yes. Basically, the common-mode voltage is the voltage limit on either input. Since it's an op-amp, it's presumed that the differential voltage will be close to zero, hence "common-mode voltage range" and "individual input voltage range" are pretty much synonymous.

Depends on OpAmp architecture. LM321 is NOT R-R, either input OR output.

LM321 basically handles 0-to-(Vcc-2*Vbe) input or output. ...Jim Thompson

That's a different spec. and will depend on how much current you draw. The spec sheet will also tell you how close the output can can get to either rail. (with perhasp more than one load resistance)

George H.

(Top posting fixed)

That's not a question of input common mode range. That's a question of output voltage range. Look at your data sheet, but I'm pretty sure the answer is "no".

If that's one of the ones I'm thinking of, it'll pull down to 0.2V, but only up to +V - 1.5 or something. In that case you can use a pull-up resistor to get it to pull up to +V, in a power-wasting and wimpy sort of way.