AC equivalent circuit of a simple negative differential resistance connected to an external circuit, showing how it can amplify an applied AC signal.  The external circuit is represented as a current source ΔiS and load resistance R.    The negative resistance device is biased with a DC voltage (not shown) which moves its operating point into its negative resistance region, in which it has differential resistance Δv/Δi = -r.  Therefore it acts like a dependent current source (right) with a value of Δv/r.  Because in a negative differential resistance the instantaneous AC voltage Δv and current Δi are 180° out of phase, the direction of the current is out of the positive voltage terminal.  Therefore the negative resistance current Δi adds to the source current ΔiS through the resistor, increasing the output power.
Own work
Chetvorno