Question:
When the forward bias voltage in a semiconductor diode is changed from 0.8 V to 1.0 V, the forward current changes by 2.0 mA. The forward bias resistance of the diode will be :
When the forward bias voltage in a semiconductor diode is changed from 0.8 V to 1.0 V, the forward current changes by 2.0 mA. The forward bias resistance of the diode will be :
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Whenever "change in voltage" or "change in current" is mentioned in diode problems, immediately think of dynamic resistance (\( r = \Delta V / \Delta I \)).
Always ensure current is converted from milliamperes (mA) to Amperes (A) before dividing.
Always ensure current is converted from milliamperes (mA) to Amperes (A) before dividing.
Updated On: Sep 14, 2026
- \( 200 \ \Omega \)
- \( 175 \ \Omega \)
- \( 100 \ \Omega \)
- \( 125 \ \Omega \)
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The Correct Option is C
Solution and Explanation
Concept:
• A semiconductor diode is a non-linear device, meaning it does not obey Ohm's Law in a simple linear fashion.
• Instead of a static resistance, we define its resistance for small signal changes as "dynamic resistance" or "AC resistance".
• Dynamic resistance \( r_d \) is defined as the ratio of a small change in applied voltage (\( \Delta V \)) to the corresponding small change in current (\( \Delta I \)).
• The formula is mathematically expressed as \( r_d = \frac{\Delta V}{\Delta I} \).
Step 1: Identify the changes in voltage and current
The initial forward bias voltage is \( V_1 = 0.8 \text{ V} \).
The final forward bias voltage is \( V_2 = 1.0 \text{ V} \).
The change in voltage is \( \Delta V = V_2 - V_1 = 1.0 - 0.8 = 0.2 \text{ V} \).
The corresponding change in forward current is given as \( \Delta I = 2.0 \text{ mA} \).
Convert this current to Amperes for standard SI unit calculations: \( \Delta I = 2.0 \times 10^{-3} \text{ A} \).
Step 2: Calculate the dynamic resistance
Substitute the values of \( \Delta V \) and \( \Delta I \) into the dynamic resistance formula:
\[ r_d = \frac{\Delta V}{\Delta I} \]
\[ r_d = \frac{0.2}{2.0 \times 10^{-3}} \]
Multiply the numerator and denominator by 1000 to remove the power of 10:
\[ r_d = \frac{0.2 \times 1000}{2.0} \]
\[ r_d = \frac{200}{2.0} \]
\[ r_d = 100 \ \Omega \]
Step 3: Conclusion
The forward bias resistance (dynamic resistance) of the diode is \( 100 \ \Omega \).
This explicitly matches option (C).
• A semiconductor diode is a non-linear device, meaning it does not obey Ohm's Law in a simple linear fashion.
• Instead of a static resistance, we define its resistance for small signal changes as "dynamic resistance" or "AC resistance".
• Dynamic resistance \( r_d \) is defined as the ratio of a small change in applied voltage (\( \Delta V \)) to the corresponding small change in current (\( \Delta I \)).
• The formula is mathematically expressed as \( r_d = \frac{\Delta V}{\Delta I} \).
Step 1: Identify the changes in voltage and current
The initial forward bias voltage is \( V_1 = 0.8 \text{ V} \).
The final forward bias voltage is \( V_2 = 1.0 \text{ V} \).
The change in voltage is \( \Delta V = V_2 - V_1 = 1.0 - 0.8 = 0.2 \text{ V} \).
The corresponding change in forward current is given as \( \Delta I = 2.0 \text{ mA} \).
Convert this current to Amperes for standard SI unit calculations: \( \Delta I = 2.0 \times 10^{-3} \text{ A} \).
Step 2: Calculate the dynamic resistance
Substitute the values of \( \Delta V \) and \( \Delta I \) into the dynamic resistance formula:
\[ r_d = \frac{\Delta V}{\Delta I} \]
\[ r_d = \frac{0.2}{2.0 \times 10^{-3}} \]
Multiply the numerator and denominator by 1000 to remove the power of 10:
\[ r_d = \frac{0.2 \times 1000}{2.0} \]
\[ r_d = \frac{200}{2.0} \]
\[ r_d = 100 \ \Omega \]
Step 3: Conclusion
The forward bias resistance (dynamic resistance) of the diode is \( 100 \ \Omega \).
This explicitly matches option (C).
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