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'[EE] LM741 single supply operation.'
2017\12\02@065328 by James Burkart

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Over on reddit (r/AskElectronics) a guy was having troubles getting a
circuit to work. His problem was that he was using an LM741 with a single
5V supply and expecting it to operate near the rails. Everyone was helpful
and explains the 741's limitations and suggested another opamp. But I took
issue with what another redditer said, that the LM741 was not designed for
single supply operation. The following was the discussion that followed. Am
I just completely wrong?



Me: I wouldn't say that it's "not designed" for single supply operation.
Just gotta know that it's not a rail to rail opamp and so in single supply
operation it's output can never reach 0V, which happens to be it's negative
rail potential.

Him: It's actually not designed for operation from a single power supply.
Not my  opinion it's straight from the Fairchild specification sheet.  In
fact it kinda, sorta, operates with a single power supply but will not
perform as specified.

Me: Where in the Fairchild spec sheet does it say that?

Him: The operating conditions for all tabular data and for every graph
stipulates split supply operation.

You won't find a any reference to single supply operation on any spec sheet
for the 741 from any manufacturer.

A single supply op amp is specifically designed to have a common-mode range
which extends all the way to the ground.  Its output stage is usually
designed to swing close to ground.  The 741 does not incorporate either of
these design attributes.

Me: There is no ground pin on the 741 so the amp isn't even aware of where
ground is. Single supply or split supply, the thing works exactly how it's
supposed to. Obviously one needs to design the circuit to operate within
the capabilities of the components used, so if the designer needs the amp's
output to swing negative then he's obviously going to use a split supply.
But if he doesn't need the output of the amp to drop below 2V, or his
circuit requirements allows him to float his ground, then unless another
part of the circuit requires a split supply, there is no need to use a
split supply, and the 741 will work exactly to spec, assuming all else in
the circuit design is correct.

Him: If it's operating on a single power supply the negative pin is
connected to ground.  Common mode operation does not include ground so it
will not meet specification.  That said if you can keep the input within
the restricted common mode range, about 2V above ground, it will work.
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2017\12\02@071746 by James Burkart

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As I was re reading this email I noticed that he made some edits since he
first posted, specifically the last thing he mentions about it working.
After going back I saw that he added an edit to one of his posts.

Him: The operating conditions for all tabular data and for every graph
stipulates split supply operation.

You won't find a any reference to single supply operation on any spec sheet
for the 741 from any manufacturer.  EDIT: I checked the most recent (2015)
TI specification and it says "The LM741 can operate with a single or dual
power supply voltage."   All tabular data and graphs still stipulate split
supply operation and the common mode range still doesn't include ground
when a single supply is employed.

A single supply op amp is specifically designed to have a common-mode range
which extends all the way to the ground.  Its output stage is usually
designed to swing close to ground.  The 741 does not incorporate either of
these design attributes.


On Dec 2, 2017 4:53 AM, "James Burkart" <spam_OUTjamesTakeThisOuTspamburkartstudios.com> wrote:

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2017\12\02@074026 by Isaac M. Bavaresco

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One can always create a split-supply with a virtual ground using just
two resistors, and optionally with an op-amp voltage-follower if lower
impedance is needed.

You are not wrong and the guy is just using an autistic interpretation
of the datasheets.

Cheers,

Isaac




Em 02/12/2017 09:53, James Burkart escreveu:
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2017\12\02@074719 by Isaac M. Bavaresco

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To complement my previous reply:

You don't need a split supply if your signals have a DC offset that
bring them into the common  mode voltage range of the op-amp.

Also, if your input and output signals are AC-coupled then you don't
need a split supply also, even if they go below GND.

Cheers,

Isaac



Em 02/12/2017 10:17, James Burkart escreveu:
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2017\12\02@085758 by RussellMc

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I'd call it a draw OR a lack of clear definitions.

*IF* you deem single supply to mean "Input signals can be ground referenced
with more or less correct operation"  then the LM741 fails with a 2 or 3 V
out of range area wrt ground with a 30V single supply

If you deem single supply to mean "supply is 0V and +xxV and **signal**
ground is somewhere between these" then an LM741 can operate "single
supply" subject to common mode restrictions.

Ref:  Revised TI datasheet October 2015
https://media.digikey.com/pdf/Data%20Sheets/Texas%20Instruments%20PDFs/LM741_Series_Rev_July_2016.pdf

The data sheet mentions "common mode range" 7 times BUT only defines it for
a 30V (or +/- 15v) supply.
At 30V between + and - rails Vcm is +/- 13V typical and +/- 12v  min.
ie for design +/- 12v should be used - ie Vin cannot be taken with 3V of
either rail (or withn 2V typical).
The datasheet does not make it clear if this value alters with resucing
supply anmd, if so, whether it gets better or worse. .
In the absence of clarification Occam says to use the values iven - but
Murphy laughs.

That means that on a 6V total supply Vin allowable range is 0V and even at
2v dead band the allowable Vin is 2.5V +/- 0/5V with a 5V supply.

That the person using 5V supply "had trouble" is unsurprising. If they had
made it work it would have been remarkable.

Even "single supply" opamps that claim rail to rail operation often mean
"almost rail to rail" - especially on outputs.
Genuine RR in and out amps use internal power converters to produce
internal rails outside the supply rails.


     Russell





On 3 December 2017 at 01:17, James Burkart <jamesspamKILLspamburkartstudios.com> wrote:

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2017\12\14@091518 by Justin Richards

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For sometime I have wanted to get to the bottom of this.
Thanks for posting, it has provided some answers.

On 2 Dec 2017 21:58, "RussellMc" <EraseMEapptechnzspam_OUTspamTakeThisOuTgmail.com> wrote:

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