An equivalent circuit for a device is a circuit that represents the device behaviour. Usually, the equivalent circuit is made up of a number of resistors and voltage cells.
A diode equivalent circuit (as shown in the Fig 1.9) may be drawn by observing its behaviour in forward and reverse bias conditions.
In forward bias, diode has fixed voltage VF and negligible resistance. In this case, diode is replaced by a single voltage cell with voltage VF.
In reverse bias, diode has maximum resistance. In this case, diode is replaced by a single dynamic resistor with value rd.
An ideal diode is also involved to show the direction of the forward current.
