The I– V characteristic of a p – n junction diode is shown in figure. Find the approximate dynamic resistance of the p – n junction when
a). a forward bias of 1 Volt is applied
b). a forward bias of  2 Volt is applied

a). The current at 1 Volt is 10 mA and at 1.2 Volt is 15 mA . The dynamic resistance in this region is
    $R=\frac{\Delta V}{\Delta i}=\frac{(1.2-1)\text{Volt}}{(15-10) \text{mA}}=\frac{0.2\text{Volt}}{5 \text{mA}}=40\Omega$
b). The current at 2 Volt is 400 mA and at 2.1 Volt it is 800 mA. The dynamic resistance is
   
$R=\frac{\Delta V}{\Delta i}=\frac{(2.1-2)\text{Volt}}{(800-400) \text{mA}}=\frac{0.1\text{Volt}}{400 \text{mA}}=0.25\Omega$