A common example of an "active resistance" circuit is the negative impedance converter (NIC) shown in the diagram. The two resistors R 1 and the op amp constitute a negative feedback non-inverting amplifier with gain of 2. The output voltage of the op-amp is v o = v ( R 1 + R 1 ) / R 1 = 2 v So if a voltage v is applied to the input, the same voltage is applied "backwards" across Z , causing current to flow through it out of the input. The current is i = v − − v o Z = v − − 2 v Z = − − v Z So the input impedance to the circuit is z in = v i = − − Z The circuit converts the impedance Z to its negative. If Z is a resistor of value R , within the linear range of the op amp V S / 2 < v < − − V S / 2 the input impedance acts like a linear "negative resistor" of value − − R . The input port of the circuit is connected into another circuit as if it was a component. An NIC can cancel undesired positive resistance in another circuit, for example they were originally developed to cancel resistance in telephone cables, serving as repeaters.