Bulk resistance

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?