rail to rail

Discussion in 'General Electronics Chat' started by new2circuits, Sep 28, 2009.

  1. new2circuits

    Thread Starter Member

    Apr 22, 2009
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    HI

    Can someone please explain what is meant by "rail to rail" input and/or output as it relates to amplifiers ?

    Thanks.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Rail to Rail is the ability of a amp/driver to get close to both power supplies in it's full swing. Many circuit (like modern op amps) simply can't do it, they get (at best) to within 2V of either power supply voltage, be it ground and +Vcc, or ±V. Thing is, this can be important, that is wasted power and loss. If you have a 9V power supply, and you can only get within 2V of either rail, that means you're limited to a 5VP-P signal.

    Another device that can't do rail to rail is the 555. It comes close on the minus side, within .1-.2 volts, but the postive side is limited to 1.2-1.4V. Interestingly enough, the CMOS 555 and CMOS logic is a rail to rail system.
     
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    This probably refers to the capability of certain amplifier types to provide an output voltage swing from a value equal to (or very near to) either the negative supply line voltage or the positive supply line voltage.

    With respect to the input side it may mean the amplifier can accommodate an input range of the same negative or positive excursions as the supply voltage limits without damage to the device.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Rail to rail definately refers to an amplifier or device to get very close to both sides, near the power supply voltages. If it ain't both, it isn't rail to rail.
     
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Thanks Bill - It's my lousy English not my lack of appreciation of the concept. No dispute about it being both rails.
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    BTW Bill, do you know where did the concept of 'rails' came from - was it to do with the way the power supply lines were drawn on schematics? - i.e. something akin to railway tracks....?
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    That sounds as reasonable as anything I've heard. Not sure though.

    It could also have something to do with trains getting their electrical power from the rails. Just another thought. Like I tell the kids, the correct answer in this case is I don't know.
     
  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    A power supply rail or voltage rail refers to a single voltage provided by a power supply unit (PSU) relative to some understood ground. Although the term is generally used in electrical engineering, most people encounter it in the context of personal computer power supplies.
     
  9. new2circuits

    Thread Starter Member

    Apr 22, 2009
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    Thanks for the explanations guys. Much appreciated.
     
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