The reason that the output signal can leave a negative resistance through the same port that the input signal enters is that from transmission line theory, the AC voltage or current at the terminals of a component can be divided into two oppositely moving waves, the incident wave V I , which travels toward the device, and the reflected wave V R , which travels away from the device. A negative differential resistance in a circuit can amplify if the magnitude of its reflection coefficient Γ Γ , the ratio of the reflected wave to the incident wave, is greater than one. | Γ Γ | ≡ ≡ | V R V I | > 1 where Γ Γ ≡ ≡ Z N − − Z L Z N + Z L The "reflected" (output) signal has larger amplitude than the incident; the device has "reflection gain". The reflection coefficient is determined by the AC impedance of the negative resistance device, Z N ( j ω ω ) = R N + j X N , and the impedance of the circuit attached to it, Z L ( j ω ω ) = R L + j X L . If R N < 0 and R L > 0 then | Γ Γ | > 0 and the device will amplify. On the Smith chart, a graphical aide widely used in the design of high frequency circuits, negative differential resistance corresponds to points outside the unit circle | Γ Γ | = 1 , the boundary of the conventional chart, so special "expanded" charts must be used.