GATE 2020 EC – Question 54
In the voltage regulator shown below, $V_I$ is the unregulated input at 15 V. Assume $V_{BE}=0.7$ V and the base current is negligible for both the BJTs. If the regulated output $V_O$ is 9 V, the value of $R_2$ is ________ $\Omega$.
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Correct answer: 800
Explanation
The Zener sets the emitter of the control transistor at $3.3$ V, so its base is at $3.3+0.7=4.0$ V. This base is at the $R_1$-$R_2$ divider tap: $9\times\frac{R_2}{1\text{k}+R_2}=4$, so $9R_2=4000+4R_2$ and $R_2=800\ \Omega$.