AC equivalent circuit of a circuit with a negative differential resistance device like a Gunn diode or a neon lamp.  It consists of a negative resistance device which may have reactance
ZN(jω)=RN+jXN{\displaystyle Z_{N}(j\omega )=R_{N}+jX_{N}\,}where RN&lt;0{\displaystyle R_{N}&lt;0}, and a circuit connected to it modeled as a linear impedance 

ZL(jω)=RL+jXL{\displaystyle Z_{L}(j\omega )=R_{L}+jX_{L}\,}The voltage at the terminals of the negative resistance can be divided into an incident wave VI{\displaystyle V_{I}} moving toward the negative resistance, and a reflected wave VR{\displaystyle V_{R}} moving away from it.  The reflection coefficient is the ratio of the incident to the reflected wave.  The signature of a negative resistance is that the magnitude of its reflection coefficient is greater than one

|Γ|=|VRVI|=|ZN−ZLZN+ZL|&gt;1{\displaystyle |\Gamma |={\bigg |}{V_{R} \over V_{I}}{\bigg |}={\bigg |}{Z_{N}-Z_{L} \over Z_{N}+Z_{L}}{\bigg |}&gt;1\,}
Own work
Chetvorno