What exactly is the mode of behavior when railing an op amp?

There is no general answer. Read the individual data sheet.

--
Rich Webb     Norfolk, VA

"Stretto"

.... Phil

"Rich Webb"

** Problem is, data sheets rarely mention what happens when the common mode range is exceed or an input is taken above or below a supply rail.

The TL071 series is famous for phase reversal - but TI figure that is for you to find out.

.... Phil

You're quite correct. My guidance above should be reworded as "Read the individual data sheet, hope that they characterize behavior outside the rails (or spec'd common mode range), and be suspicious if they don't." !-)

--
Rich Webb     Norfolk, VA

"Sticky rails" simply means that the compensation capacitor of the overdriven OpAmp has been charged beyond the linear operating region. So there is a recovery time before the OpAmp "un-sticks". ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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I love to cook with wine.     Sometimes I even put it in the food.

Hence the Hysteresis effects.

Jamie

Your terminology is a little fuzzy. We refer to "railing an opamp" as driving its output as far as it will go, which is usually close to the power supply rails, volts to millivolts depending on the amp. Exceeding the input common-mode range is a different issue.

Most opamps will "wind up" internally, overcharging the compensation node, so may take some time to recover once the inputs return to where the output should come off the rail. The recovery time could be nanoseconds to literally seconds.

Railing an opamp doesn't hurt it. Phase inversion latchup is uncommon these days; an LM709 did it (the NPNs would zener) and a 741 didn't, but I don't know of any modern opamps that do.

Some opamps, like HFA1130, are designed specifically to clamp at some defined output voltages and not wind up. Current-mode opamps tend to recover fast.

John

"John Larkin"

** So the Texas TL071 & 081 series are not modern ?

Nor the Natsemi LF351 etc series ?

These are widely used devices, sold in huge numbers.

... Phil

Thirtysomething years ago one of the other engineers in the group had an opamp (LF156) spec'd for recovery time from railing. It took a $2 opamp up to $25. It was a test they really didn't want to do because it was so slow.

I didn't say that. I said I didn't know of any modern opamps that have phase inversion latch up.

One of the first bullets on TI's TL071 data sheet says "Latch-Up Free Operation"

Hell, maybe 1978 qualifies as "modern." About as much as 18 nv/rthz qualifies as "low noise."

How can an opamp (with a 16-year old datasheet!) be modern and discontinued at the same time?

Besides, the bottom of page 6 says it won't latch.

John

"John Larkin" "Phil Allison"

** The OP metioned the phase inversion problem.

So you did not know that the TL071 series are famous for it?

**Nor the Natsemi LF351 etc series ?

** ST sell them - many as you like.

** The exact same section mentions the phase inversion problem.

My god, you are one LYING PIECE OF SHIT !!!

.... Phil

Only audio techs think that TL071 is famous.

But the datasheet says it doesn't latch.

John

There was one chopper opamp, Intersil I think, that took seconds to come off a rail.

John

yes, one good reason to have higher bus rails than needed and use a back to back zener, (-) feed back to insure it does not exceed the desired levels and prevent this type of hysteresis behavior..

jamie

s

Phase reversal and sticking are different issues. Phase reversal is due to input stage design. Sticking is generally due to devices down stream from the input stage.

You can make the design somewhat less likely to stick with diode clamps in critical locations, dependent on topology. What you want to do is make the amplifier clip gracefully, i.e. in a manner where the loops gain is reduced at extreme signal swing, but not enough to completely break the feedback path.

Oh, and please spare me all the cursing.

"Phil Allison" "Stretto"

Phase reversal and sticking are different issues.

** And my link goes along way to answering the former.

See what it is called ??

Fuckwit.

... Phil