On the unconditional stability of N-port networks

This work reviews the topic of two-port unconditional stability (US) and fixes some misconceptions still common among high-frequency designers, notwithstanding the broad, relevant literature. Ohtomo's test for network stability is then presented in a synthetic form and linked to US conditions. Finally, this approach is extended to N-port networks to obtain two sets of US conditions, which are intuitive generalizations of results well known in the two-port case. These conditions represent a necessary complement to the few works which have discussed three-port US up to now, since they only focus on the geometrical part of the problem, omitting to explicitly take care of Rollett's proviso and its implications on US.