By combining plural resonators using surface acoustic waves (SAW) or bulk acoustic waves (BAW) using piezoelectric materials, a filter element for high-frequency communication has been developed and put into practical use, which is characterized by passing only electrical signals in a specific frequency band. Further, in recent years, a boundary wave filter that mainly propagates along the boundary between a piezoelectric substrate and a medium formed thereon also has been developed. Such filter components using SAW or BAW have a small profile compared with other dielectric filters or ceramic filters and have steep roll-off characteristics. Therefore, such filters are suited to mobile communication components such as mobile phones that are required to be compact and to have a narrow bandwidth ratio.
A duplexer is a component that applies SAW or BAW filters thereto. A duplexer has transmitting/receiving functions, and is used in a wireless device in which a transmission signal frequency and a reception signal frequency are different from each other. An exemplary filter configuration using such SAW or BAW filters includes a ladder filter in which filters are connected in series-parallel in a ladder configuration. The ladder filter is low loss and has a superior power-handling performance, and hence is widely used in the duplexer. JP 2001-24476 A discloses a ladder filter that is formed by connecting FBARs (Film Bulk Acoustic Resonators) in a ladder configuration, which function similarly to the SAW filter and the like.
The input/output terminals of duplexers have, to date, been limited to single-ended terminals in most cases (see Patent Document 1). However, the future trend in a RF architecture is that an LNA (Low Noise Amplifier) connected downstream of the reception filter tends to be compatible with balanced input. This necessitates the output terminal of the reception filter of a duplexer connected to the LNA input terminal to be compatible with balanced output.