Diodes can be defined as non-linear devices. That is we can not apply superposition to circuits containing diodes. By Ohms law, when a voltage is applied across a resistor, the current flows in proportion to the voltage and the proportionality factor is constant. The voltage- current (VI) characteristic for a resistor is shown in Figure 1and is expressed analytically by V = I*R, I=V/R
Figure 1: Resistor circuit and its V-I characteristics
The V-I characteristics of a diode is shown in figure 2 and is described analytically by the equation .
Where Is is the reverse saturation current, a small ( ≈ 10-9 A ) current will occur at the negative voltage region. q / KT is a constant dependents on temperature ≈ 1/0.026V at room temperature. VD is the voltage occurred across the diode and
Figure 2: Diode circuit and its V-I characteristics
For the non-linearity of a diode, consider the circuit shown in Figure 3
Figure 3: A diode circuit with 2 voltage sources.
When V1 and V2 are applied simultaneously, the voltage across D is -3 V and the resulting current is very close to -Is . If superposition is applied, we get V1 produces 2 V across D and I1 = 2190 ISV2 Produces -5 V across D and I2 = -Is . If we add these two currents, we will get I = 2189 Is. which is an incorrect current. Since the superposition principle does not apply, we can say that a diode is a non-linear element.