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Question about op amp input bias current

  1. Aug 11, 2013 #1
    Hello,

    I have been researching operational amplifiers because I plan on using some in a circuit I wish to build, but have been stuck for awhile on the concept of the maximum current that can be handled by the non-inverting input. It appears as though the input bias current is the current necessary to cause the transistors within the op amp to function, and it seems logical to assume that the Input bias current (max) [ as is stated on the filter selection for these amplifiers http://www.mouser.com/Semiconductors/Amplifier-ICs/Operational-Amplifiers-Op-Amps/_/N-4h00g?FS=True [Broken] ] is the maximum current that can be used to cause the transistors to operate without risking damaging them.

    Is it correct to state that the maximum input bias current is the maximum current that can be given to the non-inverting input as shown on this reference? http://sub.allaboutcircuits.com/images/03034.png

    For clarification, this is not the maximum current that the operational amplifier can handle from what is used to power the op amp, but the signal that is being amplified by the op amp. Clarification on the matter would be greatly appreciated.

    Some additional details are that I plan on using non-inverting op amps with negative feed back to amplify the signal given from a line level audio source in order to drive LEDs.
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Aug 11, 2013 #2

    dlgoff

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    Operational amplifiers have differential inputs and the beauty is there is very little input current.

    http://en.wikipedia.org/wiki/Operational_amplifiers
     
  4. Aug 11, 2013 #3

    jim hardy

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    Input bias current is a necessary evil - an ideal op-amp would have none, as dlgoff's link showed.
    Bias current you notice is always a small number.


    Op-amps are tested to see how closely they approach perfection and the numbers are published so a designer can pick one suitable for his design that doesn't cost so much as the very high precision ones .
    An integrator for example would need an amp with very low bias current.

    The link you posted is a selection guide. You can expect that any individual op-amp will require less than the stated current for its type into or out of its input terminals to operate normally.

    Try TI's fabulous book "Op Amps for Everyone", they have graciously made it available as a pdf . Search on the title or www.ti.com/lit/an/slod006b/slod006b.pdf [Broken]

    I advise everyone to print themselves a copy and put it in a binder.
     
    Last edited by a moderator: May 6, 2017
  5. Aug 11, 2013 #4

    berkeman

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    No. The maximum input bias current is the most that the opamp will draw. It has nothing to do with damage. The inputs "draw" bias current; you do not "drive" current into the inputs.

    :smile:
     
  6. Aug 11, 2013 #5
    Thank you all for your replies and I shall investigate that .pdf, though is there typically any maximum current that is allowed through the non-inverting input? I have seen it explicitly stated on a few datasheets, though on others it does not mention such limits.
     
  7. Aug 11, 2013 #6

    jim hardy

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    Yes there is.

    When powered up most opamps present an electrically 'stiff' input, that is it's pretty high impedance so will not accept much more than bias current.
    When the opamp is powered down, or when the voltage applied to an input pin exceeds the supply voltage, current can flow into or out of the pin and wreck internal parts by overheating them.
    You'll usually see that under "Absolute maximum ratings" section.
    Some opamps are internally protected .

    Learning to read opamp datasheets http://www.ti.com/lit/an/sloa011/sloa011.pdf

    be aware there exist "Norton" opamps with inputs that accept current instead of voltage , eg lm3900.
    http://www.ti.com/lit/an/snoa653/snoa653.pdf
     
  8. Aug 12, 2013 #7

    meBigGuy

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    I think you need to understand that an ideal op-amp has zero input current. It doesn't matter what voltage you put on it, the input current is zero. In the real world a little current is required for the input devices to function. This is small for bipolar amplifiers, and much smaller for cmos amplifiers. What is of concern is the maximum voltage you can apply, either common mode or differential, and still expect proper operation and/or no damage.
     
  9. Aug 12, 2013 #8

    NascentOxygen

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    Puppetstring, I think you are mis-interpreting the meaning of this data figure. The bias current is more like an indication of how close to being ideal the OP-AMP is. For practical purposes in typical use, you can ignore it. It does not mean what you think it does.

    [Broken] The fact that you are asking this question makes me 99% confident that you can safely dismiss the input bias current figure from your thinking, and base your choice of OP-AMP on other performance data.

    But just to be certain, what is the circuit or application you have in mind?
     
    Last edited by a moderator: May 6, 2017
  10. Aug 12, 2013 #9
    Not to confuse the issue, but the above is true for a VFB op amp (voltage feedback). A good VFB op amp is intentionally designed for high input impedance. Superbeta bjt parts are used, or FET input parts (J- or MOS-) so that high current gain is achieved.

    If the 2 input bias currents are closely matched, which is generally the case, the error due to input bias current can be reduced by placing a resistor in the non-inverting input side, equal in ohmic value to the dc Thevenin equivalent resistance at the inverting input terminal. This will result in an output error equal to input offset current times feedback resistance value (dc). Input offset current is just the difference between the 2 inputs' bias currents.

    In general, the offset current is less than the bias currents because bjt parts used for input differential pair are on a common substrate, in close proximity, so that they track with temperature. The added resistor in the non-inverting input optimizes the dc performance by minimizing output errors due to bias current.

    The reason I mentioned VFB op amp topology at the start is because this trick does not work for CFB op amps (current feedback). That, however is for another thread. Since VFB is the most common op amp used, we will discuss it exclusively. BR.

    Claude
     
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