Karamal,

You had better check your formula for Rd again. Using Schockley's diode equation, when I calculate the dynamic resistance by finding the derivative of the applied voltage Vd with respect to the saturation current Is, I see an exponential term in the denominator of the formula.

Let me understand what you are trying to do. You are applying a "small" voltage of 3 volts P-P across a diode whose DC voltage is probably less than 1 volt? If that AC voltage is small, what do you consider large?

Keep in mind that the dynamic resistance of a diode depends on characteristics of the diode and the voltage applied across its terminals. Therefore, if you change its operating point, you will change its dynamic resistance.

Ratch