Physics Electronics PMDC Conceptual Practice
PMDC Verified Question 261 of 494
The dynamic conductance \( g_d \) of an ideal forward-biased PN junction diode conducting a steady forward current \( I_F \) at thermal voltage \( V_T \) is given by:
A
gd = IF^2 / VT
B
gd = VT / IF
C
gd = IF / (η VT)
D
gd = η VT × IF
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: gd = IF / (η VT)
Concept:

Dynamic conductance \( g_d \) is the reciprocal of dynamic resistance \( r_d \), representing the slope of the forward I-V characteristic: \( g_d = \frac{d I_F}{d V_F} \).

Formula:

$$g_d = \frac{1}{r_d} = \frac{d I_F}{d V_F} = \frac{I_F}{\eta V_T}$$

Solution:

  • Differentiating the diode equation \( I_F \approx I_0 e^{V_F / (\eta V_T)} \) gives \( g_d = \frac{d I_F}{d V_F} = \frac{I_F}{\eta V_T} \).


  • Dynamic conductance increases linearly with forward current \( I_F \).


Why other options are incorrect:

  • Option A: \( I_F^2 / V_T \) is dimensionally incorrect.
  • Option B: \( V_T / I_F \) defines dynamic resistance \( r_d \), the reciprocal of conductance.
  • Option D: Multiplying voltage by current gives power, not conductance.

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.

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