说明：  This paper analyses general active filters that can be applicable to EMI filter, active power filter, etc. It gives generalized equations presenting insertion losses and input impedances of various kinds of active filters, from which the requirements and limitations of active filters can be identified. Based on this analysis, the effectiveness of the active filters presented in several literatures is investigated. With the results of this study, an appropriate topology of an active filter for a specific application can be selected with a good rationale.