Q:     Determine the AC resistance for the curve indicated at

                                                            (i) I_D = 2mA

                                                            (ii) I_D = 25mA

SOLUTION:

                                    (1) Selecting a swing of pulse minus 2mA

                                                           I_D = 4mA and V_D =.76V

                                    Also    I_D = 0mA , V_D =0.65V

                                                             Δ I_D =4 - 0 = 4mA

                                                             Δ V_D =0.76 -0.65 =.11V

                                                                 r_D = 0.11/4m= 27.5 ohms

  (2) Select a swing of pulse minus 5mA

                             ΔI  = 30 - 20 = 10mA

                                                            ΔV_D = 0.8 -0.78 = 0.02V

                                                               r_D = 0.02/10= 10 ohms