Concept:Because a diode has a non-linear I-V characteristic, its resistance varies with the operating point. Dynamic resistance represents the reciprocal slope of the forward I-V curve at a specified operating point.
Formula:$$r_d = \frac{\Delta V_f}{\Delta I_f} = \left( \frac{dI_f}{dV_f} \right)^{-1} \approx \frac{\eta V_T}{I_f}$$
Solution:- Dynamic (or AC) resistance is determined by taking the differential ratio \( \frac{\Delta V_f}{\Delta I_f} \) at a given point on the forward conduction curve.
- As the forward current \( I_f \) increases above the threshold knee voltage, the slope of the curve increases, causing dynamic resistance \( r_d \) to decrease to a few ohms.
Why other options are incorrect:- Option B: Static (DC) resistance is \( R_D = \frac{V}{I} \) using total values, not a ratio of forward voltage to reverse current.
- Option C: Dynamic resistance is a differential quantity, not the product of barrier and breakdown potentials.
- Option D: Resistance at zero bias is not dynamic operating resistance, which is defined at an active forward-biased operating point.
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.