GATE 2014 EC (EC1) – Question 47
In the voltage regulator shown, the op-amp is ideal; the BJT has V_BE = 0.7 V and β = 100. Zener voltage is 4.7 V. For regulated output 9 V, find R (Ω).
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Correct answer: 1092 to 1094
Explanation
Negative feedback makes the ideal op-amp inputs equal, so the divider midpoint equals the4.7V zener reference. Its current is(9−4.7)/1000=4.3mA, with no op-amp input current. Therefore R=4.7/0.0043=**1093.023Ω**. The series BJT supplies the load but does not change the divider ratio.