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

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?

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NascentOxygen
Staff Emeritus
To examine the load line equation "properly", you must derive the algebraic equation for VD 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, VD = VDD - ......

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

Oh, so...
VD = VDD - I×R

Re-arranging gets:
I = VDD/R - VD/R

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

NascentOxygen
Staff Emeritus
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
Homework Helper
Gold Member
Can be instructive to look at what happens when you either short or open circuit VD..

Shorting VD to ground means VD = 0 and I is at a maximium = VDD/R.

Open circuiting VD by removing the diode gives minimium current I = 0 and VD = VDD.

Gives you two points on the load line graph.