Once clarified what "load line" exactly means...
https://www.researchgate.net/post/What_is_actually_a_load_line_and_can_this_concept_be_presented_in_a_more_attractive_way_to_students
...now it only remains to clarify what a "generator line" means. This term is used much less frequently but for us it is of interest, because obviously it is connected in some way with the term of "load line".
For the purposes of the grapho-analyitical solution, we reduce any electric circuit to an equivalent network of three generalized "elements" - two "resistors" (E1 and E2) and one voltage source V. E1 and E2 can contain whatever elements - linear, nonlinear, passive, active... in the simple case of a voltage divider, they are simply resistors. If the common point between E2 and the negative terminal of V is grounded, E1 will act as an upper "pull-up" element, and E2 - as a lower "pull-down" element.
In this arrangement, the upper element E1 can be combined with the source V to obtain in total two parts - a "generator" and a "load". As they are connected to each other, the voltage across them and the current through them are the same. So, we can superimpose their IV curves on the same coordinate system - the IV curve of the upper part that is shifted from the origin with V and the IV curve of the lower part that begins from the origin. Depending on the corresponding elements, these characteristics can be lines or any other curves. The intersection (operating) point represents the instant magnitudes of the current and voltage (the equilibrium)... and is the solution...
Now we can change whatever in this arrangement... in the upper, in the lower or in both parts... and will see how the one, other or both IV curves move, rotate or both... When change something in one of the parts, its IV (scanning) curve moves/rotates and the intersection point slides along the other (scanned) curve... thus drawing it...
So the question is, "What is the load line in this arrangement... and what - the generator line?"
Maybe we should first to answer, "What is the load and what - the generator here?"
Josef, Erik, Glen and Aparna, let's depict the initial situation:
There are three elements in series represented by two intersecting lines. Each of quantites assosiated with the corresponding element, can be varied... as well the place of the ground.
The questions are: "What is the generator and what - the load here? Accordingly, what is the generator line and what - the load line?"
Well, maybe it would be useful to consider some cases...
Imagine in the arrangement below, we change the voltage E. As a result, the composed E-R1 IV curve moves horizontally (translates)... and the operating point slides along the R2 IV curve of the resistor R2.
So, we can conclude the composed E-R1 IV curve is the generator line while R2 is the load line... R1 is the generator internal resistance and R2 is the load.
Now imagine we vary the resistance R1 keeping constant the voltage E. As a result, the composed E-R1 IV curve rotates around the point E on the X coordinate... and the operating point slides again along the R2 IV curve.
Since we change again something related with the generator, it seems we can conclude the composed E-R1 IV curve is the generator line and R2 is the load line as before?
Now let's change something related with resistance R2 (by moving the slider of the lower resistor). Its R2 IV curve rotates around the origin of the coordinate system... and the operating point now slides along the composed E-R1 IV curve.
So we can conclude the opposite - the R2 IV is the generator line while the composed E-R1 IV curve is the load line... as though we have combined R2 + E as a generator with varying internal resistance?
The general conclusion is that "generator line" and "load line" are relative concepts. Somehow the one of the lines can be а "generator line" and the other - a "load line"; in other cases we see the opposite.
Vasile,
Why do we additionally complicate this simple electrical concept by involving the negative feedback principle?
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