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op-amp difference amplifier

Discussion in 'Electronics Homework Help' started by electroboy, Jan 20, 2011.

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  1. electroboy

    electroboy

    2
    0
    Jan 20, 2011
    please help me in designing a simple opamp difference amplifier where my inputs are v1, v2, v3 and my output should be av1+bv2-cv3 and a,b,c are any values .. i am doing it on my own interest.. i have tried it in many ways .. but i cant get my desired output... please help...

    i have attached my circuit.. ignore the values in it
    i got the eqn..

    v0=(R2/R1){ [ (1+R1/R2)/(1+R4/R5+R4/R3) ]V3 + [ (1+R1/R2)/(1+R3/R5+R3/R4) ]V2 - V1 }

    i am not getting my desired ratios here.. am i correct in this approach?

    THANKS IN ADVANCE ..this is my first post.. pls forgive me if anything is wrong..
     

    Attached Files:

  2. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    Letting Vi be the voltage at the op amp inputs, there are 2 node equations:

    (Vi-V2)/R3 + (Vi-V3)/R4 + Vi/R5 = 0

    (Vi-V1)/R1 + (Vi-Vo)/R2 = 0

    Substituting to eliminate Vi gives:

    ((V1/R1 + Vo/R2)/(1/R1 + 1/R2))(1/R3 + 1/R4 +1/R5) = V2/R3 + V3/R4

    Solving for Vo gives:

    Vo= -V1(R2/R1)+(V2(R4R5(1+R2/R1))+V3(R3R5(1+R2/R1)))/(R3R4+R3R5+R4R5)

    Did you start with the same node equations? I can't tell whether your Vo expression is the same as the one I derived. What desired ratios are you looking for?
     
  3. electroboy

    electroboy

    2
    0
    Jan 20, 2011
    thank you sir... i want output in format av3+bv2-cv1 , in which i should be able to manipulate for any values of a,b,c... (i.e) for eg, if i want 2v3+3v2-v1 .. the circuit should be able to give the output in the same ratios... will the circuit suit my requirement ? please help..
     
  4. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    If you will notice, my expression for Vo is in the form:
    Vo= -cV1+bV2+aV3 where (if my derivation is correct),

    c= (R2/R1)
    b= (R4R5(1+R2/R1))/(R3R4+R3R5+R4R5)
    a= (R3R5(1+R2/R1))/(R3R4+R3R5+R4R5)

    Your task, after verifying the derivation, is to select values for R1, R2, R3, R4, & R5 that will make c=1, b=3, a=2, for your example. Good luck, it may not be possible to have "any values of a,b,c" since this circuit would normally be used in a configuration where a=b=c.
     
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