input current into ideal opamp in an inverter!!

Discussion in 'Homework Help' started by as21462, Dec 30, 2006.

  1. as21462

    Thread Starter Member

    Dec 29, 2006
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    Hi again,..I think I solved the last problem,..now its another new problem,...how can I measure the input current into the opamp in an inverting amplifier?!....It has been said that simly placing a marker on the input terminal of ideal opamp does not give a right answer.
    The input vomltage is a sine wave by 1kHz and Vp-p 1 volt.
    ---I was thinking it should be zero,...how can I get this zero value,if it has to be!
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    If you are referring to the hypothetical "ideal" opamp, then you should be aware that such an opamp is endowed with an infinite input impedance looking into both its negative and positive input terminals. That being the case then the input of the "ideal" opamp will act like an open circuit. It then follows that the current flowing into or out of the input is for all practical purposes zero.

    hgmjr
     
  3. as21462

    Thread Starter Member

    Dec 29, 2006
    22
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    but we are asked to show this currents for suitable vaules of Rf/Rin....!!!!!!
    How can I even show this zero,..putting a marker at positive or negative terminal of the opamp doesnt give zero,....
     
  4. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
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    In the case of real op amps an important point should be noted: there are differences in the input impedance of the op amp dependant on whether the op amp input stage (within the op amp circuitry) is bipolar or FET based - op amps with MOSFETs as part of the op amp input stage approach the ideal input impedance characteristics discussed by hgmjr above because the differential inputs (negative and positive) drive the MOSFET gates which are isolated from the FET channel.

    Dave
     
  5. as21462

    Thread Starter Member

    Dec 29, 2006
    22
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    how can I understand if the ideal opamp of orcad capture is working byy MOSFET or fet or,....others?!
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    It sounds like your difficulty is getting the circuit simulation software you are using to give you a value for the current flowing into or out of the negative opamp terminal.

    One way to overcome this is to introduce a dummy resistor, say 1 ohm, between the junction of Rf and Rin and the negative input terminal of the opamp. Then you can set the simulator to display the current flowing in this dummy 1 ohm resistor. In an "ideal" opamp model, this current will be zero.

    hgmjr
     
  7. as21462

    Thread Starter Member

    Dec 29, 2006
    22
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    it gives me an error of "less than 2 connections at this node"........!!!!!!!!!!!!!!1
     
  8. Distort10n

    Active Member

    Dec 25, 2006
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    Unless I missed it, post your circuit.
     
  9. as21462

    Thread Starter Member

    Dec 29, 2006
    22
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    this is an inverter amplifier by an ideal opamp.......
    maybe I should get the current going throw Rf and also the current passing throw Rin and subtract them together, then it would be zero,and it would ggive me the current going to the negative terminal of the opamp, do you think its the answer?!
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    That seems like a reasonable method to me.

    hgmjr
     
  11. as21462

    Thread Starter Member

    Dec 29, 2006
    22
    0
    I dont know how to attach the circuit to here,..thats an opj file,,and it gives me an errror of not valid file!!!!
     
  12. Distort10n

    Active Member

    Dec 25, 2006
    429
    1

    No. The currents summing together at the summing junction would be 0A. Since it is an ideal amplifier 0A will flow into the inverting (and non-inverting) inputs.

    You need a real op-amp and test the input bias current. Op-amps For Everyone has an input bias current test circuit. Other test circuits use a buffer in the feedback loop of a DUT and measurements are taken off the feedback.

    To post use PRINT SCREEN from the keyboard, cut n' paste in MS PAINT and save as a .BMP or cut n' paste into a .DOC.
     
  13. as21462

    Thread Starter Member

    Dec 29, 2006
    22
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    so,....this is the circuit.....
     
  14. Distort10n

    Active Member

    Dec 25, 2006
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    Is this what you are trying to show?
     
  15. as21462

    Thread Starter Member

    Dec 29, 2006
    22
    0
    yes,...exactly,...I can show this,..but I dont know if this is the right way to show the input current to the amplifier or not....!!!
     
  16. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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  17. Distort10n

    Active Member

    Dec 25, 2006
    429
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    ONOZ!! TeH end of teH worldz!

    Had to say it.

    If you want to show that the input current is 0A and that is what the assignment is I would say yes; however, the circuit I posted does not represent a real world test circiut for input bias currents. Use the one in Op-Amps For Everyone which is not the only test circuit either.
     
  18. as21462

    Thread Starter Member

    Dec 29, 2006
    22
    0
    Thanks everyone,..I think I got it,..
    I will send you the other problems:D
     
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