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GATE 2017 EC – Question 22

Analog Circuits · Op-amp Circuits · 1 mark · Multiple choice

For the operational amplifier circuit shown, the output saturation voltages are $\pm 15$ V. The upper and lower threshold voltages for the circuit are, respectively,

An op-amp with $V_{in}$ on the inverting side and a positive feedback network on the non-inverting input. The output $V_{out}$ goes through a $10\ \text{k}\Omega$ resistor to the non-inverting node, which is connected through a $5\ \text{k}\Omega$ resistor to a 3 V source.
  1. +5 V and -5 V
  2. +7 V and -3 V
  3. +3 V and -7 V
  4. +3 V and -3 V

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Correct answer: (B) +7 V and -3 V

Explanation

The voltage at the non-inverting input is $V_+ = 3 + (V_{out} - 3)\frac{5}{15} = 2 + \frac{V_{out}}{3}$. When the output is $+15$ V this is $7$ V, which is the upper threshold. When the output is $-15$ V it is $-3$ V, which is the lower threshold.