GATE 2021 EC – Question 19
For the circuit with an ideal OPAMP shown in the figure, $V_{REF}$ is fixed.
If $V_{OUT}=1$ volt for $V_{IN}=0.1$ volt and $V_{OUT}=6$ volt for $V_{IN}=1$ volt, where $V_{OUT}$ is measured across $R_L$ connected at the output of this OPAMP, the value of $R_F/R_{IN}$ is
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Correct answer: (D) 5.555
Explanation
The output is $V_{OUT}=\left(1+\frac{R_F}{R_{IN}}\right)V_+\pm\frac{R_F}{R_{IN}}V_{IN}$, where $V_+$ is fixed by $V_{REF}$ and the divider. Taking the two readings and subtracting eliminates $V_+$: $\frac{R_F}{R_{IN}}(1-0.1)=6-1=5$, so $\frac{R_F}{R_{IN}}=\frac{5}{0.9}=5.555$.