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I am new to analog electronics and I have been hearing this word from my seniors they always talk about Rail-Rail supply while working on some amplifiers and Opamp,also while calculating and designing a suitable pull-up network ,can anyone please explain what are significance of rail-rail supply.
All I know that is has to do something with the +ve supply and -ve supply,Can we just ground the other end ,
Why do we need this rail like structure?

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closed as off-topic by Leon Heller, Daniel Grillo, Nick Alexeev Aug 1 '15 at 5:40

  • This question does not appear to be about electronics design within the scope defined in the help center.
If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Here is a concise summary. And here is another. Would it not be better to ask the person using the term, or to look it up in a technical dictionary or electronics textbook, or search it on the Internet, rather than asking here as a first effort? You can certainly ask questions but the site is not really focused on teaching. \$\endgroup\$ – Oleksandr R. Jul 31 '15 at 13:21
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    \$\begingroup\$ Question should be closed - lack of research. \$\endgroup\$ – Leon Heller Jul 31 '15 at 15:09
  • \$\begingroup\$ what does lack of research mean? Oleksandr R. provided me some content.... \$\endgroup\$ – AminoAcid Jul 31 '15 at 15:42
  • \$\begingroup\$ @AminoAcid Yes, Oleksandr did. He probably have plugged in the title of your question into his favorite search engine. You should look up such things yourself. Otherwise the questions lacks research. \$\endgroup\$ – Nick Alexeev Aug 1 '15 at 5:39
  • \$\begingroup\$ Is asking question here not part of the "research" process? \$\endgroup\$ – Elgs Qian Chen Aug 28 '17 at 8:24
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Rail-to-rail means that the output of the op-amp can go all the way from V- to V+. Usually the output of non rail-to-rail op-amps can't reach negative or positive supply. For example if you apply a 5Vp-p sine wave to the AC-coupled input of a non rail-to-rail op-amp in unity gain configuration powered from +-2.5V, the output will be distorted (top and bottom of the waveform will be cut off). Some op-amps like LM358 are rated for "single supply operation". In practice this simply means that it's output can reach 0V when the negative supply is grounded.

Note that even if you ground the negative supply of the op-amp you might need to provide a virtual ground for the op-amp to operate correctly.

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  • \$\begingroup\$ Typically single-supply opamps also have a common-mode input range extending to ground or slightly below. This may or may not be true of rail-to-rail output opamps. \$\endgroup\$ – Oleksandr R. Jul 31 '15 at 13:31
  • \$\begingroup\$ what are their role in pull up resistors?,why use rail,rather simply ground them. \$\endgroup\$ – AminoAcid Jul 31 '15 at 14:06
  • \$\begingroup\$ @AminoAcid, I'm not sure which pull-up resistors you are referring to. A pull-up resistor might be used at the output of the op-amp when it's driving a load in order to minimize power dissipation. If you are using an op-amp as a comparator, you might use a pull-up resistor to set the threshold level. \$\endgroup\$ – v.m. Jul 31 '15 at 15:25
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The "rails" are just the supply lines. They're so-called because they can be represented like train tracks with the circuit inbetween. By convention, the most positive rail is usually drawn at the top and the most negative at the bottom.

Inputs

Op-amps generally have trouble working with voltages outside of their supply rails, or even too close to their supply rails. Some can still function with inputs a bit lower than the most negative supply rail (tens of mV usually) and a very few can function with inputs much higher than the most positive supply rail. The range over which the inputs are functional is called the "common mode range". Rail-to-rail input (RRI) op-amps can work with inputs anywhere between the supply rails and perhaps a bit beyond (that's a nasty requirement that causes compromises in the design- essentially you need two front ends and there is a transition between them). Some op-amps will work to the negative supply or a bit below but only to within a volt or two of the positive supply. For example, LM324. Some like the LM741 can only work within a volt or two of both supplies.

Output

The output can generally only go from one supply to the other, or less. Some op-amps, the so-called single supply type both have an input common mode range that includes the negative supply and have an output that can get very close to the negative supply (at least with a load connected between the output and the negative supply). They're thus suited for use with something like a single +5V supply, though you could also use a +/-5V supply if the chip can handle a 10V supply. Op-amps that have an output that can swing very close to both the supply rails (load dependent) are called rail-to-rail output (RRO). An example would be the AD8676, which is not RRI but is RRO.

Finally, op-amps that both have rail-to-rail input and rail-to-rail output are called RRIO.

Generally it's best to avoid having inputs and outputs go too close to the rails, even with a RRIO op-amp, and to carefully review the actual numerical datasheet limits on common mode range and output swing- with a jaundiced eye (sometimes the big print fibs a bit).

More reading here.

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  • \$\begingroup\$ Its said that single power supply Opamp are more portable,why are they more portable?,when dual supply can provide a better swing. \$\endgroup\$ – AminoAcid Jul 31 '15 at 16:01
  • \$\begingroup\$ I don't known what exactly is meant by 'portable' in this context. Do you have a link for this statement? \$\endgroup\$ – Spehro Pefhany Jul 31 '15 at 16:06
  • \$\begingroup\$ Single supply Opamp ti.com/lit/an/sloa030a/sloa030a.pdf \$\endgroup\$ – AminoAcid Jul 31 '15 at 16:07
  • \$\begingroup\$ Portable applications- battery powered. Extra supplies add bulk, cost, may use more power, performance requirements may not be as high as mains-powered so try to use a single supply. Trade-offs. There's no inherent reason dual supplies can't be used in portable devices if it's necessary. \$\endgroup\$ – Spehro Pefhany Jul 31 '15 at 16:10

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