In this presentation we will do the load line analysis of PN junction diode. We use load line in graphical analysis of nonlinear electronic circuits. Now what do we mean by nonlinear electronic circuits? These are the circuits having nonlinear VI characteristics for example diode and transistor.
Here you can see VI characteristics for diode and it is nonlinear and we have nonlinear characteristics because they do not follow the Ohm's law and because of this we have nonlinear characteristics. Load line of diode represents the constraints other part of the circuit place on diode. So by using load line we can see how other part of the circuit place constraints. on the diode.
In this circuit we have p-n junction diode and the voltage across the diode is Vd. RL is the load resistance and V is the external voltage source. The negative terminal is connected to the n side and the positive terminal is connected to the p side.
Id is the current in this circuit and we can easily use we can easily use KVL Kirchhoff's voltage law. So by applying KVL by applying KVL Kirchhoff's voltage law we have V minus IDRL V minus IDRL minus VD minus VD equals to 0. So this is what we have by using the KVL and if we rearrange this if we rearrange this it is V equals to IDRL plus VD and let's say This is equation number 1 and I want to calculate the diode current ID when VD is equal to 0. So let's do it quickly. When VD is equal to 0, we have V equals to ID RL plus 0 by using equation number 1 and this will give us ID equal to V by RL.
So this is the value of Id when Vd is equal to 0. I will also show this in VI characteristics. When Vd is equal to 0, the current is equal to V by RL. This is the value of Id when Vd is equal to 0. In the same way, I will calculate Vd when Id is equal to 0. So let's do this. When Id is equal to 0, we have V equal to 0 multiplied by RL plus VD so we can say that VD is equal to V. So VD is equal to V when ID is equal to 0. When ID is 0 VD is equal to V. Now we have two points. First point is V by RL 0. This is the coordinates of point and the second point is 0 V. And if we combine the two points we will have a straight line.
like this and this straight line is called as the load line of the diode and this is the diode characteristics and we have an intersection. We have an intersection and this intersection is called as Q point or operating point. Operating point and this operating point is also called as Q point.
Q ascent point and it is equal to the voltage is Vdq the operating voltage and the operating current is Idq. So we have Idq and Vdq as the operating point of the diode. and the operating point is nothing but the intersection of load line and the diode characteristics.
This is the load line and we can easily find out load line by using the Kirchhoff's voltage law and we have the characteristics and the point at which they intersect is called as the operating point. Now we will try to find out the slope of the load line. This is the load line and we will try to find out the slope.
Let's do it quickly. From equation number one we have V equals to ID. RL plus VD. Now I will divide both the sides by RL and this will give me V by RL equals to ID plus VD by RL or we can write it as or ID is equal to minus VD by RL plus V by RL.
And if you compare this equation, if we compare this equation with Y equals to mx plus c then you will find the intercept the intercept c is equal to v by rl this is the intercept c and the slope is equal to minus 1 by rl the slope or m is equal to minus 1 by rl we have negative slope and if you see the plot you will find the load line is also having the negative slope vd is the x axis and ID is the y-axis. So we have y equals to 1 minus RL, this is M, VD is x and C is V by RL. So we can say that on changing RL, the load resistance RL, the Q point will also change.
Because on changing RL, slope will change and when slope changes, the Q point will also change. For example, If we increase RL, we have a new load line like this and the operating point will now shift.