TABLE OF CONTENTS Earlier, you were introduced to the concept of negative resistance ( R) and its inverse, negative conductance ( G = 1/ R). To review the facts, the negative resistance phenomena, also called negative differential resistance (NDR) , is a seemingly strange phenomena in which materials behave electrically contrary to Ohm s law. In ohmic materials (i.e., those that obey Ohm s law), we normally expect current to increase as electric potential increases. In these positive resistance materials, I = V/R. In negative resistance (NDR) devices, however, there might be certain ranges of applied potential in which current decreases with increasing potential.
In addition to the strange I-vs- V characteristic, certain other properties of negative resistance materials distinguish them from ohmic materials. First, in ohmic materials, we normally expect to find the voltage and current in-phase with each other, unless an inductive or capacitive reactance is also present. Negative resistance, however, causes current and voltage to be 180 out of phase with each other. This relationship is a key to recognizing negative-resistance situations.
In the resonant type of negative resistance circuit, depicted in Fig. 22-5, current pulses from the NDR device shock excite a high- Q LC tank circuit or resonant cavity into self-excitation. The oscillations are sustained by repetitive reexcitation. Unres- onant oscillator circuits use no tuning. They depend on the device dimensions and the average charge carrier velocity through the bulk material to...
