Difference between \$r_d\$ and \$r_D\$ in diodes?
the dynamic resistance will, for an abrupt junction diode, be 26 ohms/current(milliAmps). Thus reac (my term) is 26 ohms at 1mA, 260 ohms at 0.1 mA, etc. This equation also holds for bipolar transistors when you know the emitter (or collector) current. Simply scale up by the Beta, and you have the low-frequency Rin. (Miller Effect will eat up signal charge at high frequencies).

The Rd likely is the bulk resistance of the silicon, apart from the junction region (microns thick), and becomes apparent at "high currents", where the expected exponential behavior transitions into a "linear" behavior.