Operational Amplifier

Discussion in 'Homework Help' started by circuit2000, Feb 18, 2007.

  1. circuit2000

    Thread Starter Active Member

    Jul 6, 2006
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    1) Consider an inverting operational amplifier as shown in figure. Suppose R1= 1000 ohm & R2 = 10^6 ohm. It is said that the input impedance=R1=1000 ohm and the Output impedance= 0. But for an Op-Amp, the input impedance is infinite isn’t it?

    2) Consider a non inverting operational amplifier as shown in figure. Suppose R1= 3500 ohm & R2 = 15000 ohm. It is said that the input impedance=infinite. Why is it so? Here R2 is the feedback resistor.
    As I am unable to upload the image, I am providing a link to site that contains the diagram of both inverting & non inverting operational amplifier.

    http://webpages.ursinus.edu/lriley/ref/circuits/node5.html
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The only opamp that is said to have an infinite input impedance is an "ideal" opamp. Real opamps have very high input impedances however they are not infinite.

    Once you surround an opamp, real or ideal, with external components, the nature of the circuit is changed.​


    A definition for input impedance is "how much current does the opamp circuit draw from the output of the circuit driving it". In the case of the inverting opamp stage, it can be shown that the output of the driving circuit is loaded by the input gain resistor of the opamp stage. You will notice that the driving circuit is connected to the "virtual" ground present at the negative input of the opamp through R1. ​

    In the non-inverting case, the input impedance is equivalent to the input impedance of the real opamp. While very high, this impedance is not infinite.

    hgmjr
     
  3. circuit2000

    Thread Starter Active Member

    Jul 6, 2006
    33
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    They say that the input impedence of a Op-Amp is very high and when negative feedback is used, the impedence is increased even further. But isn't the feedback is negative only in the case of an inverting amplifier? So, it could be taken infinite only in the case of inverting amplifier. please correct me if I am wrong.
    Also, do u mean that R1 is the input impedence of the Op-Amp in both cases?
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    Actually, the since the voltage on the inverting pin is practically equal to the voltage on the noninverting pin, the input impedance in the noninverting case is equal to the open-loop input impedance multiplied by the loop gain (which can be anywhere from ~ a million on down).
     
  5. Distort10n

    Active Member

    Dec 25, 2006
    429
    1
    I have a derivation of this. I will have to sit down and work my magic on it. Negative feedback increases input impedance and decreases output impedance.

    The magic of feedback.
     
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