Concept:A varactor diode operates in reverse bias, where varying the reverse voltage \( V_R \) changes the depletion layer width \( W \), providing a voltage-tunable transition capacitance \( C_T \).
Formula:$$C_T(V_R) = \frac{C_0}{\left(1 + \frac{V_R}{V_0}\right)^n} \quad \text{where } n = \frac{1}{2} \text{ (abrupt junction) or } \frac{1}{3} \text{ (linearly graded)}$$
Solution:- Operating in reverse bias prevents forward conduction, leaving only the voltage-dependent transition capacitance.
- Increasing \( V_R \) widens the depletion layer, decreasing capacitance.
- This enables voltage-controlled frequency tuning in RF oscillators and phase-locked loops (PLLs).
Why other options are incorrect:- Option A: Forward bias causes heavy conduction, which swamps the capacitive effect with low dynamic resistance.
- Option B: Zero bias provides a fixed capacitance with no voltage control.
- Option C: Reverse breakdown conducts large current and risks damaging the device.
Quality & Fidelity Assurance:
Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.