The operating point of a diode moves from one region to another the resistance of the diode will also change due to the nonlinear shape of the characteristic curve.
DC or Static Resistance
The application of a dc voltage to a circuit containing a semiconductor diode will result in an operating point on the characteristic curve that will not change with time
By Ohm’s Law we have;
The resistance levels in the reverse-bias region will naturally be quite high. Since ohmmeters typically employ a relatively constant-current source, the resistance determined will be at a preset current level (typically, a few milliamperes).
AC or Dynamic Resistance
If a sinusoidal rather than a dc input is applied, the situation will change completely. The varying input will move the instantaneous operating point up and down a region of the characteristics and thus defines a specific change in current and voltage
With no applied varying signal, the point of operation would be the Q -point appearing on Figure below, determined by the applied dc levels. The designation Q-point is derived from the word quiescent , which means “still or unvarying.”