1. DC OR STATIC RESISTANCE
When diode is subjected to a dc voltage its operating point on the characteristic curve does not change with time. Hence the resistance of the diode can be found easily
rD =vD/iD

2. DC OR STATIC RESISTANCE
It may also be seen on the curve that the resistance in the reverse bias is quite high.

3. EXAMPLE
Determine the resistance level for the diode having characteristics as shown at
(1)ID =1mA
(2)ID=20mA
(3)VD= -10V

4. SOLUTION
From the curve
for ID =1mA ,VD=0.5volts
RD = 0.5/1m
=500 ohms

5. SOLUTION
(2)
for ID =20mA , VD = 0.8volts
RD = 0.8/20m
=40ohms.

6. SOLUTION
(3)
VD =-10V, ID= -1microA
RD = 10/10-6
= 10Mohms

7. THE CONSTANT VOLTAGE DROP MODEL
We use a vertical straight line to approximate the fast rising part of the exponential curve, as shown in the fig.

9. THE CONSTANT VOLTAGE DROP MODEL
The resulting model simply says that a forward conducting diode exhibits a constant voltage drop VD. The value of VD is usually taken to be as 0.7 volts in case of silicon diode. The constant voltage drop model can be represented by the equivalent model shown in the fig.

11. THE EFFECT OF THE FORWARD VOLTAGE DROP ON CIRCUIT ANALYSIS
To illustrate this effect consider this
circuit,

12. According to the KVL, the sum of the
component voltages in the circuit must be
equal to the applied voltage .By formula
Vs = VF + VR
if we consider VF =0.7 volts and arrange
this equation for VR then

13. VR = Vs – 0.7V
According to ohms law
IT = VR/R
Putting the value of VR from this equation
IT = Vs – 0.7/R1

14. Now by solving
IT = Vs – 0.7/R1
= 5 – 0.7/1
=4.3A

15. If we consider this diode to be ideal than
no voltage drop will occur across it
and we will get
Vs = VR
VF = 0
IT = 5mA

16. Example
Find voltage across R1.
Consider silicon diode so that 0.7 V drop is assumed across it.

17. By KVL
Vs=VR+Vf
VR=Vs-0.7
VR=6-0.7
=5.3 volts≈5 Volts.

18. THE CONSTANT VOLTAGE DROP MODEL
Diode current will remain at zero until the knee voltage is reached.
Once the applied voltage reaches the value of Vk , the diode turns on and the forward conduction occurs.

19. THE CONSTANT VOLTAGE DROP MODEL
(3) As long as the diode conducting, the value of VF is approximately equal to vK. In other words the value of VF is assumed to be 0.7volts regardless of the value of IF.

20. AC OR DYNAMIC RESISTANCE
rD = ΔvD/ ΔiD
Δshows a finite change in quantity. The resistance in the vertical rise region is small, where as ac resistance is much larger at low current levels.