How to avoid the saturation of the current amplifier?
If you solve the circuit, you'll find the reason why it's not stable: 

\$V_+\$ and \$V_-\$ being the positive and negative terminal of the opamp

$$ V_+ = V_{out} \frac{R_4}{R_3+R_4} $$
$$ V_- = (V_{out}-V_3) \frac{R_1}{R_1+R_2} + V_3 $$
$$ V_{out} = A (V_+ -V_-) $$

$$ V_{out} = A (V_{out} \frac{R_4}{R_3+R_4} - (V_{out}-V_3) \frac{R_1}{R_1+R_2} - V_3 ) $$

$$ V_{out} = A V_{out} (\frac{R_4}{R_3+R_4} - \frac{R_1}{R_1+R_2}) + A V_3 (\frac{R_1}{R_1+R_2} - 1) $$

$$ \frac{V_{out}}{A} - V_{out} (\frac{R_4}{R_3+R_4} - \frac{R_1}{R_1+R_2}) = V_3 (\frac{R_1}{R_1+R_2} - 1) $$

$$ \lim_{A \to \infty} \frac{V_{out}}{A} - V_{out} (\frac{R_4}{R_3+R_4} - \frac{R_1}{R_1+R_2}) = V_3 (\frac{R_1}{R_1+R_2} - 1) $$

$$ - V_{out} = V_3 \frac{ (\frac{R_1}{R_1+R_2} - 1) }{ (\frac{R_4}{R_3+R_4} - \frac{R_1}{R_1+R_2})  } $$

$$ - V_{out} = V_3 \frac{ R_2 }{ (\frac{R_4(R_1+R_2)}{R_3+R_4} - R_1 ) } $$

$$ - V_{out} = V_3 \frac{ R_2(R_3+R_4) }{ (R_4(R_1+R_2) - R_1(R_3+R_4) ) } $$

$$ V_{out} = - V_3 \frac{ R_2 R_3 + R_2 R_4 }{ R_4 R_2 - R_1 R_3 } $$