Patent ID: 7183879

Claim:
A duplexer comprising: a transmitting-end filter including an electric filter and having a predetermined pass frequency band; and a receiving-end filter including the electric filter and having a pass frequency band different from that of the transmitting end filter, wherein the electric filter includes: a piezoelectric resonator for obtaining a signal of a predetermined resonance frequency from a bulk wave, wherein the piezoelectric resonator comprises: a first electrode; a second electrode placed opposite to the first electrode; and a piezoelectric film disposed between the first electrode and the second electrode, in which the bulk wave propagates, the electric filter satisfying k 2 >(BP/0.6f), where k 2 is the square of an effective electromechanical coupling coefficient of the piezoelectric resonator, BP a target pass bandwidth of the electric filter, and f a center frequency of the target pass band of the electric filter.