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Old 23rd February 2009, 05:45 PM   #11
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Default Re: Re: Most of the time...

Quote:
Originally posted by wg_ski


That's not the op-amps fault.

Nope, and that was my point. I guess I should have been more clear. Most times I've had an op-amp go to one rail, it's because of a wiring or design mistake. Rarely have I had a bad part. Understand how op-amps work and circuit faults can be readily diagnosed.

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Old 23rd February 2009, 05:53 PM   #12
wg_ski is offline wg_ski  United States
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Yeah - I just assumed it was the usual noob mistake of why the output is sitting at -14 when both inputs are grounded.
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Old 23rd February 2009, 06:21 PM   #13
syn08 is offline syn08  Canada
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Quote:
Originally posted by AndrewT
why do some invert the output when driven hard?
Simplified, because an inverting gain stage may (depending on the toplogy) become non-inverting when the active device(s) saturate. Therefore, negative feedback becomes positive feedback - and the output latches.
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Old 23rd February 2009, 06:42 PM   #14
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I had TL074 went to the rails before. It was wired as Unity Gain Buffers for a distribution amp as in one of their Application Notes. Initially on power up, no problems. Left it on overnight. Next morning, output was at one of the rails. Apparently, the output DC offset crept towards the rail as the opamp temperature increases.
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Old 23rd February 2009, 06:59 PM   #15
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If one AC-couples the input without a DC-bias setting resistor after it something like that could happen... Input bias current of the opamp is pretty low so it might take a while though
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Old 23rd February 2009, 07:09 PM   #16
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Quote:
Originally posted by megajocke
If one AC-couples the input without a DC-bias setting resistor after it something like that could happen... Input bias current of the opamp is pretty low so it might take a while though
Pardon my ignorance, but what exactly does a DC bias setting resistor do?
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Old 23rd February 2009, 08:05 PM   #17
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Simple: they rail because the differential input voltage times the gain is higher that the supply voltage.
Now, the reason for that may be another story...

Nice try Jack!

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Old 23rd February 2009, 09:09 PM   #18
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Quote:
Originally posted by janneman
Simple: they rail because the differential input voltage times the gain is higher that the supply voltage.
Now, the reason for that may be another story...

Nice try Jack!

Jan Didden


Jan,

Your response was what I was trying to say...but you said it way better.

Thanks,

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