  The relation between passive and positive real systems has been extensively
studied in the literature. In this paper, we study their connection to the more
recently used notion of port-Hamiltonian descriptor systems. It is well-known
that port-Hamiltonian systems are passive and that passive systems are positive
real. Hence it is studied under which assumptions the converse implications
hold. Furthermore, the relationship between passivity, KYP inequalities and
a~finite available storage is investigated.
