The V-I characteristics of a silicon diode is given in Fig .Calculate the diode resistance in

forward bias at `V=+2V` and `V=+1V` and
reverse bias `V=-1V` and `V=-2V`.

Explanation

Forward bias diode resistance is
`R_(f)=(DeltaV)/(DeltaI)`
where `Delta V` and `Delta I` are small change in the voltage and current surrounding the desired values. From graph
At `V_(f)=+2V, DeltaV=2.2V -1.8V=0.4V`
`DeltaI=80mA-60mA=20mA=20xx10^(-3)A`
`:. R_(f)(at+2V)=(DeltaV)/(DeltaI)=(0.4V)/(20xx10^(-3)A)=20Omega`
At `V_(f)=+1V, DeltaV=1.2V-0.8V=0.4V`
`DeltaI=20mA-10mA=10mA=10xx10^(-3)A`
`R_(f)(at+1V)=(DeltaV)/(DeltaI)=(0.4V)/(10xx10^(-3))=40Omega`
In the reverse bias characteristic of p-n junction the non-linearity in V-I graph is very small. The slopes of V-I at -1 V and -2 V are nearly equal.
At `V_(r)=-1V, R_(r)(at-1V)=(-1V)/(-0.125xx10^(-6)A)`
`=8xx10^(6)Omega`
At `V_(r)=-2V, R_(r)(at-2V)=(-2V)/(-0.250xx10^(-6)A)`
`=8xx10^(6)Omega`