How to power an OpAmp with two batteries without mains ground

[jim hardy]

Thanks Don i downloaded and saved.

first one in that series, bottom of pp 1.7 is a note that applies to this thread. I would post an excerpt but it appears to disallow copy & paste.

old jim

 

[dlgoff]

first one in that series, bottom of pp 1.7 is a note that applies to this thread. I would post an excerpt but it appears to disallow copy & paste.

Yea, it's copyrighted and can't be reproduced without permission from Analog Devices. Good info to have on hand though.

 

[Rudinhoob]

The circuit in question's noninverting input voltage is undefined, or 'indeterminate'.

So right hand circuit suffers more from an undefined input than from lack of feedback.

I thought it's 0 it it's considered a reference node. This is how it's treated in circuit books, they show the non-inverting input for an inverting OpAmp configuration connected to a common node, using the "ground" symbol, and the same symbol used for the input voltage source's negative rail.

 

[sophiecentaur]

There is no AC feedback concerning the circuit in question either.

I guess that's true.

 

[Rudinhoob]

There is no AC feedback concerning the circuit in question either.

I guess that's true.

What AC are you talking about here?

Apparently my simple question could not be answered by "engineers" and when they felt incompetent they start farting crap polluting the thread and adding more confusion in attempt to hide their inability to answer a simple straightforward question.

Moderators, please ban me, I'm not going to open this useless forum anymore or read any replies to me. ;)

F.U

 

[jim hardy]

I thought it's 0 it it's considered a reference node. This is how it's treated in circuit books, they show the non-inverting input for an inverting OpAmp configuration connected to a common node, using the "ground" symbol, and the same symbol used for the input voltage source's negative rail.

In your left hand circuit you connected +input to a definite point, the power supply centertap.
In your right hand one you didn't, you left it to find its own level.
Simply calling it "reference" doesn't make the rest of the circuit respect it.

In the boooks, they don't usually show the power supply. Power supply is used to define 'circuit common' , usually power supply centertap.
For opamps like LM324 which is designed to operate with single supply, they often but not always use power supply negative instead.

I strongly recommend you look into the first article in that Analog Devices link posted by dlgoff.
It was written before op-amp fundamentals became so entrenched we forgot how to teach them. Too many cooks and all that...



old jim

 

[jim hardy]

What AC are you talking about here?

Apparently my simple question could not be answered by "engineers" and when they felt incompetent they start ... adding more confusion in attempt to hide their inability to answer a simple straightforward question.

Moderators, please ban me, I'm not going to open this useless forum anymore or read any replies to me. ;)

dont give up now you're soooo close to success..

 

[Averagesupernova]

What AC are you talking about here?

Apparently my simple question could not be answered by "engineers" and when they felt incompetent they start farting crap polluting the thread and adding more confusion in attempt to hide their inability to answer a simple straightforward question.

Moderators, please ban me, I'm not going to open this useless forum anymore or read any replies to me. ;)

F.U

That's a pretty gutsy comment coming from the guy asking the questions. The AC I was talking about is the AC coming from the signal generator you have drawn in your schematic. Plain and simple fact is there is no feedback of any kind in the circuit on the right since both inverting and non-inverting inputs will see the same thing fed back from the output. Op-amp inputs are differential, so when they are treated the same as in the circuit on the right, there is essentially NO FEEDBACK

 

[sophiecentaur]

That's a pretty gutsy comment coming from the guy asking the questions. The AC I was talking about is the AC coming from the signal generator you have drawn in your schematic. Plain and simple fact is there is no feedback of any kind in the circuit on the right since both inverting and non-inverting inputs will see the same thing fed back from the output. Op-amp inputs are differential, so when they are treated the same as in the circuit on the right, there is essentially NO FEEDBACK

"gutsy"? More like plain rude and juvenile (teenage, no doubt). Voting with his feet is no great loss to PF, I think.

 

[jim hardy]

hmm There's no path for current through the two resistors, as there is in left circuit.
So the opamp can't develop a voltage across its feedback to oppose the voltage source.

Good thinking guys.

I missed that. duhhh...
This old age ain't for sissies, I tell ya.