Hi
Every element that is modeled in two form by scientists, one macro model, the other micro model; in all fields of knowledge such as mechanics, electronics, etc;
In electronics, when you have a small variation in input signal (as input) of an element (as system), since your system can be modeled for every different situation (bias), so it shows a different behavior to this input signal!
when a specific amount of current flows in your diode, if this current (or input voltage) be changed very little, you may calculate new current or voltage across your diode just estimating your diode as a linear element, so ΔV=(dynamic resistance of the diode)rd . ΔI!
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Regards,
SAZ