Filter using piezoelectric resonator

A duplexer comprising:

• a transmission terminal;
• a reception terminal;
• an antenna terminal connected between the transmission terminal and the reception terminal;
• a transmission filter disposed between the transmission terminal and the antenna terminal;
• and
• a reception filter disposed between the reception terminal and the antenna terminal,
• wherein at least one of the transmission filter and the reception filter comprises:
• a series piezoelectric resonator part including at least one series piezoelectric resonator having a first electrode and a second electrode, the series piezoelectric resonator being connected in series between an input terminal and an output terminal;
• a parallel piezoelectric resonator part including at least two parallel piezoelectric resonators each having a first electrode and a second electrode, the parallel piezoelectric resonators being connected, via the first electrodes, in parallel to the series piezoelectric resonator and in a ladder configuration;
• an inductor part including inductors provided to at least two of the parallel piezoelectric resonators, respectively, the inductors each being connected between a ground and a respective one of the second electrodes of the parallel piezoelectric resonators;
• and
• an additional piezoelectric resonator part including at least one additional piezoelectric resonator including a first electrode and a second electrode, the additional piezoelectric resonator being connected between any two second electrodes of the parallel piezoelectric resonators having the inductors connected thereto.

A communication device comprising:

• an antenna;
• a transmission device;
• a reception device;
• and
• a duplexer, disposed at a connection portion of the antenna, the transmission device, and the reception device,
• wherein the duplexer comprises:
• a transmission terminal;
• a reception terminal;
• an antenna terminal connected between the transmission terminal and the reception terminal;
• a transmission filter disposed between the transmission terminal and the antenna terminal;
• and
• a reception filter disposed between the reception terminal and the antenna terminal,
• wherein at least one of the transmission filter and the reception filter comprises:
• a series piezoelectric resonator part including at least one series piezoelectric resonator having a first electrode and a second electrode, the series piezoelectric resonator being connected in series between an input terminal and an output terminal;
• a parallel piezoelectric resonator part including at least two parallel piezoelectric resonators each having a first electrode and a second electrode, the parallel piezoelectric resonators being connected, via the first electrodes, in parallel to the series piezoelectric resonator and in a ladder configuration;
• an inductor part including inductors provided to at least two of the parallel piezoelectric resonators, respectively, the inductors each being connected between a ground and a respective one of the second electrodes of the parallel piezoelectric resonators;
• and
• an additional piezoelectric resonator part including at least one additional piezoelectric resonator having a first electrode and a second electrode, the additional piezoelectric resonator being connected between any two second electrodes of the parallel piezoelectric resonators having the inductors connected thereto.