What is the load line equation for I-V Characteristic plot?

[red123]

What is the load line equation for I-V Characteristic plots?

Why is load line slope negative? According to Ohm's Law: i = V/R; so as voltage increases current increases then slope should be positive, right?

 

[NascentOxygen]

To examine the load line equation "properly", you must derive the algebraic equation for V_D in terms of load current, I. Do this from the point of view of the linear part of the circuit, without regard for the particular device (the device in this case we can see is a diode).

Using Ohm's Law, V_D = V_DD - ...

Re-arrange to compare with the equation for a straight line on an I vs. V plot, to find intercepts and slope.

 

[red123]

Oh, so...
V_D = V_DD - I×R

Re-arranging gets:
I = V_DD/R - V_D/R

Where V_DD/R is the I-intercept and -1/R is the slope.

 

[NascentOxygen]

Right. So onto this you can superimpose the I vs. V plot for any load (a non-linear diode, or even another resistor), and find the Q-point.

 

[CWatters]

Can be instructive to look at what happens when you either short or open circuit V_D..

Shorting V_D to ground means V_D = 0 and I is at a maximium = V_DD/R.

Open circuiting V_D by removing the diode gives minimium current I = 0 and V_D = V_DD.

Gives you two points on the load line graph.