Basic Op-Amp Question

Discussion in 'General Electronics Chat' started by blackstars56, Feb 14, 2011.

  1. blackstars56

    Thread Starter New Member

    Feb 14, 2011
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    Hi,

    From what I understand from Op-Amps, the + and - input terminals have to ideally have zero current, with infinite resistance. Doesn't that render the Op-Amp useless since no current runs through it? silly question, but i'm confused lol

    thanks
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    You are talking an ideal op amp, but you have the basics down. Op amps compare the two voltages, and the output swings accordingly. If the positive input is more positive then the negative input then the output swings positive. Basically an op amp (in linear mode) tries to make the voltages on both inputs the same.

    Volume 3 - Chapter 8: OPERATIONAL AMPLIFIERS

    There are many devices that try to use only voltage for the input, and as little current as possible (such as a digital voltmeter). Numbers as low as picoamps or less are routine. A digital voltmeter usually has a 10MΩ input, while modern op amps use much less.

    µa = X10^-6
    pa = X10^-9
    na = X10^-12
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    Many opamps have FET transistors at the inputs that use no input current.
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Bill, you have the two swapped.
    µA = 10^-6 A
    nA= 10^-9 A
    pA = 10^-12 A
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Almost no current, TL071 datasheet states input bias current as 200pA max, so that is input impedance of 5 giga Ohm.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    A FET has no input bias current. It has a very low amount of input leakage current that increases when the temperature increases.
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    @blackstars56 : What makes you think that an op-amp would need to draw any input current in the first place? In many cases such current is detrimental, which is one reason why the FET input amplifiers mentioned above may be preferred.

    Having a negligible input current does not prevent the op-amp from delivering output current.
     
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