Question about a specific OpAmp circuit

Discussion in 'Homework Help' started by techie, Jan 2, 2008.

  1. techie

    Thread Starter New Member

    Jan 2, 2008
    4
    0
    Hello there , first time poster .
    I'm trying to find the Voutput of this circuit ( attachment ) but i think i am missing something .
    Using the superposition principle i begin by grounding V2 to find the effect V1 has at Vo and then i ll do the same for V2 and add them to find the total output but im left with 4 equations and 5 variables system that i just cant solve .
    Anyway these are the equations with V2 grounded :
    Using KCL at the 4 nodes ( a , b , c , d ) :

    a -> (v1-Va)/R1 = - (Vc - Va)/R2
    b-> Vb/R1= (Vd - Vb)/R2
    c-> (Vo-Vc)/R2 = (Vc- Va)/R2 + (Vc- Vd)/Rg
    d-> (Vc-Vd)/Rg =Vd/R2 + (Vd - Vb)/R2

    then we know that Va=Vb :

    a -> (v1-Va)/R1 = - (Vc - Va)/R2
    b-> Va/R1= (Vd - Va)/R2
    c-> (Vo-Vc)/R2 = (Vc- Va)/R2 + (Vc- Vd)/Rg
    d-> (Vc-Vd)/Rg =Vd/R2 + (Vd - Va)/R2

    Thats the 4 equations/5 variables system and i just cant find an expression of Vo only dependant on V1
    Some stuff i ve found : Vo = 2 ( Vc + Vd - Va )

    (Vc-Vd)/Rg = - R2*V1/R1*Rg
    So , is there something im missing ? (a current or voltage equation ? or some other method i could follow ?)

    When i first saw the circuit i thought thats an opamp with both positive and negative feedback with Rg setting the amount of possitive feedback . Is this right in any way ?

    Thanks for any tips/help/pointers beforehand and sorry for any grammatical errors i may have .
     
  2. Distort10n

    Active Member

    Dec 25, 2006
    429
    1
    Using Superposition would be the first step. Then I would use Thevenin to find the equivalent resistance of the 'T' type network.
     
  3. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    2,281
    326
    It's true that you have a 4 equations/5 variables system, but of the 5 variables only 4 are unknown; v1 is a given.

    I was able to solve the system and I get Vo = - V1*(2*R2*(R2+Rg))/(R1*Rg)
     
  4. techie

    Thread Starter New Member

    Jan 2, 2008
    4
    0
    correct , thank you ;D
     
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