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Multiple op-amps problem

  1. Oct 5, 2008 #1

    I've been working on this problem for a while now to no avail. The question is to choose R1 and R2 such that Vo = 10IsRx.

    I've gotten as far as solving for each input voltage for the op amps and their accompanying output voltages, but I end up with a huge expression in terms of Is and Rx that I can't really simplify.

    Written out, this is what I have:

    [tex]V_o = \frac {20K}{R_2}V_3 + V_2(\frac{20K}{R_2} + 1) = 10I_s R_x[/tex]

    where V1 is the input to the first op-amp, or:
    [tex]V_1 = 100I_s[/tex]

    V3 is output of opamp 1, or:
    [tex](\frac{v_1}{R_1})20K + V_1[/tex]

    V2 is input to op-amp 2:
    [tex]V_2 = (100+R_x)I_s[/tex]

    Looking at Vo, I can see that the gain for the second opamp should be [tex](\frac{20K}{R_2} + 1)[/tex] and the first would be [tex](\frac{20K}{R_1}+1)[/tex] but I don't know what to put for those individual gains. Any advice? Thanks!
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

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