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Old 23rd February 2009, 02:17 AM   #1
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Default Why do opamps "rail" ?

Do opamps "rail" of their own volition, for what reason do they rail?
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Old 23rd February 2009, 05:47 AM   #2
kubeek is offline kubeek  Czech Republic
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Mostly because they want to.
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Old 23rd February 2009, 05:48 AM   #3
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Back in the day, hippies railed too. Except against the government, not to the supply.
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Old 23rd February 2009, 08:31 AM   #4
Bonsai is offline Bonsai  Taiwan
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Can you ask the question using engineering terms and them maybe you can ge t a proper answer. Do you mean why do they latch-up at eithe r th e + or - supply rail?
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Old 23rd February 2009, 12:39 PM   #5
wg_ski is offline wg_ski  United States
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Of course that's what he means. The simple answer is that they're not perfect. It takes a small (usually microvolt or millivolt) DC "signal" between the + and - inputs to get the output to zero. It's spec'ed as the input offset voltage. Multiply this number by the open loop gain, and you get a big number which is typically greater than the supply volatge and that's where it "tries" to go. It stops at the rail.

"Latch-up" is a seaprate phenomenon, and that requires overdrive.
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Old 23rd February 2009, 12:59 PM   #6
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Default Most of the time...

the go to one supply rail or the other because of a mistake in the input circuitry...and when you think about it, they are simply responding to the conditions at the input terminals.

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Old 23rd February 2009, 01:51 PM   #7
wg_ski is offline wg_ski  United States
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Default Re: Most of the time...

Quote:
Originally posted by SpeakerScott
the go to one supply rail or the other because of a mistake in the input circuitry...and when you think about it, they are simply responding to the conditions at the input terminals.

Scott
That's not the op-amps fault.
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Old 23rd February 2009, 05:00 PM   #8
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TL072 and similar opamps do strange things when input nears negative rail voltage...

http://www.dself.dsl.pipex.com/ampins/webbop/072.htm

New TI datasheets say "Latch up free operation" though so it might only apply to old parts and/or parts from other manufacturers.
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Old 23rd February 2009, 05:13 PM   #9
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why do some invert the output when driven hard?
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Old 23rd February 2009, 05:31 PM   #10
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Here's a TL072 or similar.

Node B sits at 2Vbe:s + the drop over Re above negative rail. If an input is driven negative enough so that the gate junction of one of the JFET:s lets the input steal current from the current source then there won't be enough voltage at B to turn on the vas and the output will go to the positive rail.

For example, if IN+ is connected to negative rail, S will be 1Vbe above negative rail. B will be at about the same voltage, which isn't enough to turn on the VAS so the output will go positive instead of negative as expected.

The TL0xx series opamps input common mode range includes positive rail though!
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