The circuit of is the simplest of diode configurations. We are going to replace the Approximate model of an Diode to a Real Diode in Nature.
The diode characteristics are placed on the same set of axes as a straight line defined by the parameters of the network. The straight line is called a load line because the intersection on the vertical axis is defined by the applied load R . The analysis is therefore called load-line analysis. The intersection of the two curves will define the solution for the network and define the current and voltage levels for the network.
What actually happens?
The effect of the “pressure” established by the DC supply is to establish a conventional current in the direction indicated by the clockwise arrow. The fact that the direction of this current has the same direction as the arrow in the diode symbol reveals that the diode is in the “on” state and will conduct a high level of current. The polarity of the applied voltage has resulted in a forward-bias situation. With the current direction established, the polarities for the voltage across the diode and resistor can be superimposed. The polarity of VD and the direction of ID clearly reveal that the diode is indeed in the forward-bias state, resulting in a voltage across the diode in the neighborhood of 0.7 V and a current on the order of 10 mA or more.
On applying Kirchhoff’s Law to the circuit we have,
+E – VD – VR = 0
or
E = VD + VR
The two variables of , VD and ID , are the same as the diode axis variables on the graph. This similarity permits plotting ) on the same characteristics . The intersections of the load line on the characteristics can easily be determined if one simply employs the fact that anywhere on the horizontal axis ID = 0 A and anywhere on the vertical axis VD = 0 V. If we set VD = 0 V in the equation and solve for ID , we have the magnitude of ID on the vertical axis. Therefore, with VD= 0 V, the equation becomes
E = VD + IDR
= 0 V + IDR and
ID = E/R | VD =0 V.
If we set ID = 0 A and solve for VD , we have the magnitude of VD on the horizontal axis. Therefore, with ID= 0 A, the equation becomes
E = VD + IDR
= VD + (0 A)R and
VD = E | ID =0 A
A straight line drawn between the two points will define the load line as depicted in the graphical representation . Change the level of R (the load) and the intersection on the vertical axis will change. The result will be a change in the slope of the load line and a different point of intersection between the load line and the device characteristics. We now have a load line defined by the network and a characteristic curve defined by the device. The point of intersection between the two is the point of operation for this circuit. By simply drawing a line down to the horizontal axis, we can determine the diode voltage VDQ, whereas a horizontal line from the point of intersection to the vertical axis will provide the level of IDQ. The current ID is actually the current through the entire series configuration. The point of operation is usually called the quiescent point (abbreviated “ Q – point”) to reflect its “still, unmoving” qualities as defined by a DC network.