# Multiple op-amps problem

1. Oct 5, 2008

### psycovic23

Hey,

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:

$$V_o = \frac {20K}{R_2}V_3 + V_2(\frac{20K}{R_2} + 1) = 10I_s R_x$$

where V1 is the input to the first op-amp, or:
$$V_1 = 100I_s$$

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

V2 is input to op-amp 2:
$$V_2 = (100+R_x)I_s$$

Looking at Vo, I can see that the gain for the second opamp should be $$(\frac{20K}{R_2} + 1)$$ and the first would be $$(\frac{20K}{R_1}+1)$$ 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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