The `V to I` characteristic of a silicon diode is shown in the figure. Calculate the resistance of the diode at (a) `I_(D)=15mA` and (b) `V_(D)=-10V`.

Explanation

Considering the diode characteristics as a straight line between I = 10 mA to I = 20 mA passing through the origin, we can calculate the resistance using Ohm.s law.
(a) From the curve, at `I_(2)=20mA` for `V_(2)=0.8V, I_(1)=10mA` for `V_(1)=0.7V`
`therefore` Resistance of forward bias,
`r_(fb)=(DeltaV)/(DeltaI)`
`therefore r_(fb)=(V_(2)-V_(1))/(I_(2)-I_(1))`
`=(0.8-0.7)/((20-10)xx10^(-3))=(0.1xx10^(3))/(10)`
`therefore r_(fb)=10Omega`
(b) From the curve at `V=-10V, I=-1muA`
`therefore` Resistance of reverse bias,
`r_(rb)=(V)/(I)`
`therefore r_(rb)=(-10)/(-1xx10^(-6))`
`=1xx10^(7)Omega`