James Burkart
2017-12-02 11:53:23 UTC
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.
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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