Question

The V-I characteristic of a diode is shown in the figure. The ratio of forward to reverse bias resistance is:

• A. 10
• B. 10-6
• C. 106
• D. 100

Answer 1

Answer: (B)

10-6

Answer 2

The forward I-V characteristic is when the diode is forward-biased, and the anodewith respect to the cathode is positive. On the other hand, the reverse I-V characteristics are observed when the cathode is positive with respect to the anode.

Formula Used:

Forward bias resistance Rf is given as - $R_f=\frac{ΔV_f}{ΔI_f}$

• where, $ΔV_f$ is the difference between forward bias voltages,
• $ΔI_f$ is the difference between currents.

Reverse bias resistance $R_r$ is given as - $R_r=\frac{ΔV_r}{ΔI_r}$

• where, $ΔV_r$ is the difference between reverse bias voltages,
• $ΔI_r$ is the difference between currents.

Complete step-by-step answer:

The forward resistance offered by a diode in forward bias is very small. On the other hand, a diode offers very high reverse resistancewhen it is in reverse bias.

The formula for forward bias resistance $R_f$ is given as $R_f=\frac{ΔV_f}{ΔI_f}$→................(1)

• where, $ΔV_f$ denotes the difference between forward bias voltages.
• $ΔI_f$ is the difference between currents approximating particular voltages.

From the given diagram, the current ranges from 10 to 20 milli-Amperes. Therefore, $ΔI_f=20−10$

$⇒ΔI_f=10mA$

$⇒ΔI_f=10×10^{−3}A$

and the voltage ranges from 0.7 to 0.8 V.

$⇒ΔV_f=0.8−0.7$

$⇒ΔV_f=0.1V$

Substituting the values in equation (1)

$R_f=\frac{ΔV_f}{ΔI_f}$

$⇒R_f=\frac{0.1}{10×10^{−3}Ω}$

$⇒R_f=10Ω$

The formula for reverse bias resistance $R_r$ is given as:

$R_r=\frac{ΔV_r}{ΔI_r}$→................(2)

• where, $ΔV_r$ is the difference between reverse bias voltages
• $ΔI_r$ is the difference between currents corresponding to particular voltages.

Also according to the diagram, the reverse bias current will be

$R_r=\frac{ΔV_r}{ΔI_r}$

$⇒R_r=\frac{10}{10^{−6}}Ω$

$⇒R_r=10^7Ω$

Then, the ratio of forward bias to reverse bias resistance can be written as:

$Ratio=\frac{R_f}{R_r}$

$⇒Ratio=\frac{10}{10^7}Ω$

$∴Ratio=10^{−6}Ω$

Hence, option B is the correct answer.

Note: The formula for reverse bias resistance and forward bias resistance can be written with the help of Ohm’s Law. Ohm’s law is given by V = IR.

• where V is the voltage,
• I is the current
• R is the resistance.

Thus, resistance can be written as R = VI

Independently in terms of forward and reverse bias resistance, this equation becomes $R_r=\frac{ΔV_r}{ΔI_r}$

And $R_r=\frac{ΔV_r}{ΔI_r}$ respectively.