If the ac resistance ( r d ) were determined at D = 2 mA, its value would be more than 5 Ω,
and if determined at 17 mA, it would be less. In between, the ac resistance would make the
transition from the high value at 2 mA to the lower value at 17 mA. Equation (1.7) defines
a value that is considered the average of the ac values from 2 mA to 17 mA.