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

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

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

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

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

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

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.

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.

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

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

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

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

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

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

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

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.

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.

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.