*The Basic Diode Theory :*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

is the dimensionless constant determined by the type of the diode (Silicon, Germanium, etc.)

**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 -

**. If superposition is applied, we get**

*I*s**V1**produces 2 V across D and

**Produces -5 V across D and I2 = -Is . If we add these two currents, we will get**

*I1 = 2190 ISV2**which is an incorrect current. Since the superposition principle does not apply, we can say that a diode is a non-linear element.*

**I = 2189 Is.**