English: Animated diagram illustrating how a negative differential resistance device amplifies an AC signal. A negative differential resistance is a two-terminal electronic component in which an increase in applied voltage across the component results in a decrease in electric current through the component. This contrasts with a positive resistance, in which the current increases with the voltage due to Ohm's law. Examples of negative resistance devices are the tunnel diode and the Gunn diode. If a time-varying voltage biased with a DC voltage is applied to it, as shown here, DC power (P_DC) from the bias current will be converted to AC power (P_AC) to amplify the applied signal.

The key point is that as the voltage and thus the electric field across the device increases (big arrows) the current of charge carriers (plusses) decreases (slows down). The arrows show the total voltage v and current i, and the AC voltage Δv and current Δi, throughout the cycle. The DC voltage and current always have the same sign, so P_DC = i•v > 0, the device consumes DC power. However, as can be seen, the AC voltage and current have opposite signs; they are 180° out of phase. Therefore P_AC = Δv•Δi < 0. This means AC power flows out of the device into the external circuit; adding to the power delivered to the load. The AC power comes from the DC power consumed.