Graph of the current-voltage curve of a negative resistance oscillator| showing load lines.  It consists of a negative differential resistance such as a Gunn diode connected across a resonant circuit. A voltage source Vbias biases the diode into its negative resistance region, where it has resistance  Δv/Δv = -r.  The negative resistance cancels the positive resistance R in the circuit, exciting oscillations i(t).  DCL is the DC load line, which establishes the Q point.    SSL is the small signal load line, which is the operating condition when the oscillator first starts up and oscillations have a small amplitude.     As the oscillations grow the nonlinearity of the diode eventually limits the amplitude.  The sine wave extends into nonlinear regions near the edges of the negative resistance region, where the negative resistance is smaller, so the "average" negative resistance decreases until it is equal to the load resistance R, shown by LSL, the large signal load line.  The nonlinearity distorts the output sine wave at large amplitudes.
Own work
Chetvorno