Stability issues of Z+Z or Y+Y type cascade system

The stability issues of cascade systems have been studied for many years. Impedance ratio type criterions in the form of Z_o/Z_i or Z_i/Z_o have been proposed to solve these stability issues. However, existing ratio type criterions still have problems in some cascade systems. Terminal characteristic of single module is studied and the different types of cascade system are defined in this paper. It can be seen that the validity of conventional impedance ratio type criterion would be influenced by the right half plane zeros (RHZ) in terminal impedance of each module in the Z+Z type cascade system. Two improved criteria are proposed for the Z+Z type cascade system: one is improved ratio type criterion considering the RHZ in terminal impedance of each module; another one is sum type criterion which doesn't need the information about RHZ in terminal impedance of each module. The experiment results prove the validity of these two improved criterions.