Diode Piecewise-Linear, Simplified and Ideal Equivalent Circuits

The most important question of Semi Conductor Physics, Junction Diode Characteristics in Electronic Devices and Circuits; Explain about the Diode Piecewise-Linear, Simplified and Ideal Equivalent Circuits?, is being answered here.

Diode Equivalent Circuits:

An equivalent circuit is a combination of elements properly chosen to best represent the actual terminal characteristics of a device, system, or such in a particular operating region. In other words, once the equivalent circuit is defined, the device symbol can be removed from a schematic and the equivalent circuit inserted in its place without severely affecting the actual behavior of the system. The result is often a network that can be solved using traditional circuit analysis techniques.

Piecewise-Linear Equivalent Circuit:

One technique for obtaining an equivalent circuit for a diode is to approximate the characteristics of the device by straight-line segments, as shown in Fig. 1.31. The resulting equivalent circuit is naturally called the piecewise-linear equivalent circuit. It should be obvious from Fig. 1.31 that the straight-line segments do not result in an exact duplication of the actual characteristics, especially in the knee region. However, the resulting segments are sufficiently close to the actual curve to establish an equivalent circuit that will provide an excellent first approximation to the actual behavior of the device. The ideal diode is included to establish that there is only one direction of conduction through the device, and a reverse-bias condition will result in the opencircuit state for the device. Since a silicon semiconductor diode does not reach the conduction state until V_D reaches 0.7 V with a forward bias (as shown in Fig. 1.31), a battery V_T opposing the conduction direction must appear in the equivalent circuit as shown in Fig. 1.32. The battery simply specifies that the voltage across the device must be greater than the threshold battery voltage before conduction through the device in the direction dictated by the ideal diode can be established. When conduction is established the resistance of the diode will be the specified value of r_av.

Figure 1.31 Defining the piecewise-linear equivalent circuit using straight-line segments to approximate the characteristic curve.

The approximate level of r_av can usually be determined from a specified operating point on the specification sheet. For instance, for a silicon semiconductor diode, if I_F _ 10 mA (a forward conduction current for the diode) at V_D _ 0.8 V, we know for silicon that a shift of 0.7 V is required before the

Simplified Equivalent Circuit:

For most applications, the resistance r_av is sufficiently small to be ignored in comparison to the other elements of the network. The removal of r_av from the equivalent circuit is the same as implying that the characteristics of the diode. Under dc conditions has a drop of 0.7 V across it in the conduction state at any level of diode current.

Ideal Equivalent Circuit:

Now that r_av has been removed from the equivalent circuit let us take it a step further and establish that a 0.7-V level can often be ignored in comparison to the applied voltage level. In this case the equivalent circuit will be reduced to that of an ideal diode as shown in Fig. 1.34 with its characteristics.

Fig 1.34.: Ideal Diode Characteristics

Diode Piecewise-Linear, Simplified and Ideal Equivalent Circuits