Q:     For the diode which conducts 1mA at a forward voltage drop of 0.7 Volts and which n =1. Determine the eq of the straight line tangent at I_D = 1mA

SOLUTION:

                                                Slope = 1/ r_d

                                                          = I /nV_T         //r_d = nV_T

                                                         = 1mA/1x25mV=1/25ohms

                                                On V_D axis it is

                                                       = 0.7 - 1m (25)

                                                       = 0.7 - 0.025= 0.675 V

                        Therefore  

                                    From the equation of straight line    y - y₁= m  (x-x₁)

                                                Where m is a slope = 1/25ohms

                                                 i_D = 1/25  (V_D -0.675)

                                                 V_D - 0.675 - i_D(25) =0

                                    That is the needed equation of straight line.