Although it is heuristically well-known that a negative resistor can berealized with nullors and positive resistors, the minimal realization of it have notbeen discussed systematically in detail. In this paper, based upon the general fundamental properties of linear active networks, it is proved systematically that nullors not less than one (two) and positive resistors not less than three ( one, respectively ) are necessary to realize a negative resistor and there exist only two circuits for the topologically minimal realization disregarding the labelling of nullors and resistors.
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