Question

The $I-V$ characteristic of a silicon diode is shown in the figure. Calculate the resistance of the diode at $I_b=15\text{mA}$.

• A. $20\Omega$
• B. $50\Omega$
• C. $30\Omega$
• D. $10\Omega$

Answer 1

The correct option is D $10\Omega$

Considering the diode characteristics as a straight line between $I=10\text{mA}$ to $I=20\text{mA}$ passing through the origin,

we can calculate the resistance using ohm's law

From the curve at $I=20\text{mA}\to V=0.8\text{V}$,
$I=10\text{mA}\to V=0.7\text{V}$

$R_{fb}=\frac{\Delta V}{\Delta I}$

$\Rightarrow R_{fb}=\frac{0.1\text{V}}{10\text{mA}}$

$\Rightarrow R_{fb}=10\Omega$

Hence, option $\left(\text{A}\right)$ is correct.

Why this question? Note: In a $I-V$ characteristic curve of a $p-n$ junction, the dynamic resistance at any point on the graph is the reciprocal of the slope at that point.