The term "negative resistance" almost always means negative differential resistance r diff < 0 . Negative differential resistance devices have unique capabilities: they can act as one-port amplifiers, increasing the power of a time-varying signal applied to their port (terminals), or excite oscillations in a tuned circuit to make an oscillator. They can also have hysteresis. It is not possible for a device to have negative differential resistance without a power source, and these devices can be divided into two categories depending on whether they get their power from an internal source or from their port: