English: Current-voltage (IV) diagram showing the I-V curve (characteristic curve) of an "active resistor" or negative linear resistance (red), a type of negative resistance. Active resistance obeys Ohm's law but the proportionality constant is negative ${\displaystyle R<0}$, so it's I-V curve is a straight line through the origin with negative slope. Devices with active resistance have both negative differential resistance and negative static resistance, equal and constant:

${\displaystyle R={\Delta v \over \Delta i}={v \over i}<0\,}$

Active resistance is created by amplifier circuits with positive feedback such as the negative impedance converter. Active resistor circuits are widely used in op amp active filters because they can create transfer functions that positive resistances cannot. In electronics active resistance is commonly just called positive feedback or regeneration.