op-amp difference amplifier

[electroboy]

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..

 

[Laplace]

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?

 

[electroboy]

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..

 

[Laplace]

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.