Patent ID: 7414495

Claim:
A balanced/unbalanced type coupled FBAR filter for performing conversion between a balanced signal and an unbalanced signal using FBARs, the balanced/unbalanced type coupled FBAR filter comprising: a first vibration section including an FBAR; a second vibration section including an FBAR and located parallel to the first vibration section; and a common vibration guaranteeing section, having the first and second vibration sections provided thereon, for commonly guaranteeing a vibration of the first vibration section and a vibration of the second vibration section so as to couple the two vibrations; wherein: the first vibration section includes: a first lower electrode; a first upper electrode located to face the first lower electrode; and a part of a piezoelectric thin film interposed between the first lower electrode and the first upper electrode; wherein either the first lower electrode or the first upper electrode is used as an input/output terminal; the second vibration section includes: a second lower electrode; a third lower electrode separated from the second lower electrode by an insulating area; a second upper electrode located to face a part of the third lower electrode; a third upper electrode located to face a part of the second lower electrode and separated from the second upper electrode by an insulating area; and another part of the piezoelectric thin film interposed between the second and third lower electrodes and the second and third upper electrodes; wherein the second lower electrode and the second upper electrode are both used as an input/output terminal; an area size of the first lower electrode is generally equal to a total area size of the second and third lower electrodes and the area for isolating the second and third lower electrodes; an area size of the first upper electrode is generally equal to a total area size of the second and third upper electrodes and the area for isolating the second and third upper electrodes; and a part of the second lower electrode and a part of the second upper electrode are located to face each other.