The diagrams illustrate how a biased negative differential resistance device can increase the power of a signal applied to it, amplifying it, although it only has two terminals. Due to the superposition principle the voltage and current at the device's terminals can be divided into a DC bias component ( V b i a s , I b i a s ) and an AC component ( Δ Δ v , Δ Δ i ). v ( t ) = V bias + Δ Δ v ( t ) i ( t ) = I bias + Δ Δ i ( t ) Since a positive change in voltage Δ Δ v causes a negative change in current Δ Δ i , the AC current and voltage in the device are 180° out of phase. This means in the AC equivalent circuit (right), the instantaneous AC current Δ i flows through the device in the direction of increasing AC potential Δ v, as it would in a generator. Therefore, the AC power dissipation is negative ; AC power is produced by the device and flows into the external circuit. P AC = Δ Δ v Δ Δ i = r diff | Δ Δ i | 2 < 0 With the proper external circuit, the device can increase the AC signal power delivered to a load, serving as an amplifier, or excite oscillations in a resonant circuit to make an oscillator. Unlike in a two port amplifying device such as a transistor or op amp, the amplified signal leaves the device through the same two terminals (port) as the input signal enters.