We have found the dynamic resistance graphically, but there is a basic definition in differential
calculus that states:
The derivative of a function at a point is equal to the slope of the tangent line drawn
at that point.
Equation (1.5), as defined by Fig. 1.26 , is, therefore, essentially finding the derivative of
the function at the Q -point of operation. If we find the derivative of the general equation
(1.2) for the semiconductor diode with respect to the applied forward bias and then invert
the result, we will have an equation for the dynamic or ac resistance in that region. That is,
taking the derivative of Eq. (1.2) with respect to the applied bias will result in