Why sum bulk and dynamic resistance for AC signals on diodes

[Godwin Kessy]

I have been going through the ac signals coupled by a capacitor to a dc network.. Using superposition principle.. for solution it was shown that both bulk resistance and dynamic resistances for the diodes used are required... Why is this happening.. Since to what i know bulk resistance is approximate to dynamic resistance... having dynamic resistance no need of the bulk resistance.. Yet they sum-up both of them to get what is called Total dynamic resistance.. Am I giving a challenge hear... Or there is something wrong with me.. or the materials am using?

 

[anorlunda]

See this other PF thread https://www.physicsforums.com/threads/comparison-between-bulk-and-ac-resistances-of-a-diode.644218/