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Problem with OP07 OPA.

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SPAL

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Hi,
I am facing a problem with this circuit. While the output is expected to be 125mv, very often it goes to -ve rail. I think, going by a few nA input bias current, a feed back resistance of 2.2M should be fine. Any suggestions please..View attachment 60299
Thanks
-SPal
 
hi & welcome.

Whats the source and waveform [AC/DC] of the 'difference' input to the OPA input 470K's
 
Hi
Input is DC coming from a sensor which is equivalent to 5Kx4 balanced bridge excited with +/- 5V.
However, this problem is also faced when nothing is connected at the input, R1 and R2 are open.
 
Every opamp has its allowed "input common-mode voltage range" listed on its datasheet. The TL084 has an allowed input common-mode voltage range that is from 4V above its negative supply voltage (an allowed input of at least +4V in your circuit) to 4V less than its positive supply voltage. But many of them have inputs that work as high as the positive supply voltage.

The TL081 is an old opamp with Jfet inputs. It has a problem called "Opamp phase Inversion" where the output suddenly goes high if an input voltage is outside the allowed input common-mode voltage range and your input voltages are too low so of course the output latches high.

The OP07 has an allowed input common-mode voltage range that is from 2V above its negative supply voltage (an allowed input of at least +2V in your circuit) to 2V less than its positive supply voltage. Again, your circuit will not work with it.

There are many opamps that have an allowed input common-mode voltage range that includes their negative supply voltage (0V in your circuit) like an MC34071 or a TLE2141.
 

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  • opamp phase inversion.PNG
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Request again advice on my OP07 circuit posted on 19 Jan 2012 . This behavior of output going -ve is not with all devices. When I change the chip (OP07 only) it becomes OK. I must have tried about 100 chips and in about 50% got this problem. Temped to think the chips are bad, though they are sourced from a reputed source. Before taking up with the issue with the opamp manufacturer I wanted to get opinion from others who would have more knowledge. Thanks
 
SPAL, the protocol in most (maybe all) forums is to start a new thread when you want to ask a question. What you have done is called hijacking.
To start a new thread, go General Electronics Chat, scroll down to the bottom, and click on "Create New Thread".


Posts moved to New Thread.

Moderation.
 
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Every opamp has its allowed "input common-mode voltage range" listed on its datasheet. The TL084 has an allowed input common-mode voltage range that is from 4V above its negative supply voltage (an allowed input of at least +4V in your circuit) to 4V less than its positive supply voltage. But many of them have inputs that work as high as the positive supply voltage.

The TL081 is an old opamp with Jfet inputs. It has a problem called "Opamp phase Inversion" where the output suddenly goes high if an input voltage is outside the allowed input common-mode voltage range and your input voltages are too low so of course the output latches high.

The OP07 has an allowed input common-mode voltage range that is from 2V above its negative supply voltage (an allowed input of at least +2V in your circuit) to 2V less than its positive supply voltage. Again, your circuit will not work with it.

There are many opamps that have an allowed input common-mode voltage range that includes their negative supply voltage (0V in your circuit) like an MC34071 or a TLE2141.
He's powering the op amp with +/-5V. Why shouldn't it work?
 
My answer was addressed to the other thread (it used a TL081 opamp) that has been cut into two pieces.

This "highjacked" thread uses an OP07 opamp. I have never used an OP07 so I don't know if its output goes low if its input common-mode voltage is beyond its allowed range.
 
My answer was addressed to the other thread (it used a TL081 opamp) that has been cut into two pieces.

This "highjacked" thread uses an OP07 opamp. I have never used an OP07 so I don't know if its output goes low if its input common-mode voltage is beyond its allowed range.
I guess your post was unnecessarily included in the thread move.
I can't find the original thread. Can someone post a link to it, please?
 
I guess your post was unnecessarily included in the thread move.
I can't find the original thread. Can someone post a link to it, please?

hi Ron,
I am trying to find the original Thread, if I do I will link it.

E.

EDIT:
Tried all the methods I know, unable to find the 2010 Thread.

Will give it some more thought.
 
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