A question about op-amp

Discussion in 'General Electronics Chat' started by clockz, Oct 9, 2007.

  1. clockz

    Thread Starter New Member

    Oct 9, 2007
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    http://www.maxim-ic.com/appnotes.cfm/appnote_number/3390

    the first formula: V0/VIN=........(1+E1)-1.

    What is the meaning of E1? How to get the following E1 expression?

    I have checked some op-amp books but found nothing about the error term. Can anyone tell me what is the detailed meaning of error term E1 and how to calculate it?

    Many Thanks.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    The paper gives the calculation of E1 as: [​IMG]
    - which looks fairly detailed. It also appears as if this is a calculation specific to the application, so a generalized definition of E1 may not be forthcoming. This applies to termination losses driving coax.
     
  3. Distort10n

    Active Member

    Dec 25, 2006
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  4. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    I see one error in your derivation. The value you have for B (near the very bottom) is B = (Rf+Rg)/Rg, but it should be B = Rg/(Rf+Rg)
     
  5. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    Look on page 3. There is an expression for the gain if the opamp were ideal. Now compare that to the non-ideal gain expression on page 2 that has as its last term the parenthetical (1 + E1)^-1. The gain expression for a non-ideal opamp is very complicated. E1 is just what it takes so that if you multiply the ideal gain expression by (1 + E1)^-1 you will get the full, complicated, expression for the gain of a non-ideal opamp.

    The E1 expression is clearly incorrect, because if the opamp gain, Aop-amp, becomes zero, the E1 expression becomes infinite, and then the infinite term (1 + E1)^-1 divides the Vo/Vin expression, causing it to go to zero. But, plainly, if the opamp gain is zero, the input signal will find its way to the output through R1 and R2 and Rtheta in series (if Rool is not zero).
     
  6. Distort10n

    Active Member

    Dec 25, 2006
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    DOH! Good catch. I will have to update that soon.
     
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