In addition to the passive devices with intrinsic negative differential resistance above, circuits with amplifying devices like transistors or op amps can have negative resistance at their ports. The input or output impedance of an amplifier with enough positive feedback applied to it can be negative. If R i is the input resistance of the amplifier without feedback, A is the amplifier gain, and β β ( j ω ω ) is the transfer function of the feedback path, the input resistance with positive shunt feedback is R if = R i 1 − − A β β So if the loop gain A β β is greater than one, R i f will be negative. The circuit acts like a "negative linear resistor" over a limited range, with I–V curve having a straight line segment through the origin with negative slope (see graphs). It has both negative differential resistance and is active Δ Δ v Δ Δ i = v i = R if < 0 and thus obeys Ohm's law as if it had a negative value of resistance − R, over its linear range (such amplifiers can also have more complicated negative resistance I–V curves that do not pass through the origin).