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 (
PDC) from the bias current will be converted to AC power (
PAC) 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
PDC =
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
PAC =
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.