English: 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

${\displaystyle Z_{N}(j\omega )=R_{N}+jX_{N}\,}$

where ${\displaystyle R_{N}<0}$, and a circuit connected to it modeled as a linear impedance

${\displaystyle Z_{L}(j\omega )=R_{L}+jX_{L}\,}$

The voltage at the terminals of the negative resistance can be divided into an incident wave ${\displaystyle V_{I}}$ moving toward the negative resistance, and a reflected wave ${\displaystyle V_{R}}$ 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

${\displaystyle |\Gamma |={\bigg |}{V_{R} \over V_{I}}{\bigg |}={\bigg |}{Z_{N}-Z_{L} \over Z_{N}+Z_{L}}{\bigg |}>1\,}$