Multiplexer, radio-frequency front end circuit, and communication device

1. A multiplexer comprising:

a common terminal;

a first terminal;

a second terminal;

a first filter that is on a first path connected between the common terminal and the first terminal and that includes a plurality of acoustic wave resonators; and

a second filter that is on a second path connected between the common terminal and the second terminal and that has a pass band located at a higher frequency than a pass band of the first filter; wherein

the plurality of acoustic wave resonators include:

two or more series resonators on the first path; and

one or more parallel resonators on paths connected between nodes on the first path and ground;

a first series resonator that is closest to the common terminal among the two or more series resonators is connected to the common terminal without any of the parallel resonators interposed therebetween;

the plurality of acoustic wave resonators each include a substrate that exhibits piezoelectricity, an InterDigital Transducer (IDT) electrode including a pair of comb-shaped electrodes on the substrate, and reflectors each including one or more reflection electrode fingers; and

a total number of reflection electrode fingers of at least one of the first series resonator and a first parallel resonator that is closest to the common terminal among the one or more parallel resonators is smaller than a total number of reflection electrode fingers of at least one of a remainder of the plurality of acoustic wave resonators.

2. The multiplexer according to claim 1 ,

wherein the total number of reflection electrode fingers of at least one of the first series resonator and the first parallel resonator is smaller than a total number of reflection electrode fingers of each of the remainder of the plurality of acoustic wave resonators.

3. The multiplexer according to claim 1 ,

wherein the total number of reflection electrode fingers of the first series resonator and the total number of reflection electrode fingers of the first parallel resonator are smaller than the total number of reflection electrode fingers of at least one of the remainder of the plurality of acoustic wave resonators.

4. The multiplexer according to claim 3 ,

wherein the total number of reflection electrode fingers of the first series resonator and the total number of reflection electrode fingers of the first parallel resonator are smaller than a total number of reflection electrode fingers of each of the remainder of the plurality of acoustic wave resonators.

5. The multiplexer according to claim 1 , wherein

the substrate of the plurality of acoustic wave resonators includes:

a piezoelectric layer including the IDT electrode on one main surface thereof;

a high-acoustic-velocity support substrate in which an acoustic velocity of a propagating bulk wave is higher than an velocity of an acoustic wave propagating along the piezoelectric layer, and

a low-acoustic-velocity film that is between the high-acoustic-velocity support substrate and the piezoelectric layer and in which an acoustic velocity of a propagating bulk wave is lower than an acoustic velocity of an acoustic wave propagating along the piezoelectric layer.

6. The multiplexer according to claim 1 ,

wherein a frequency of a stop band response generated by the first filter is included in a frequency pass band of the second filter.

7. A radio-frequency front end circuit comprising:

the multiplexer according to claim 1 ;

and an amplification circuit that is connected to the multiplexer.

8. A communication device comprising:

an RF signal processing circuit that processes a radio-frequency signal transmitted or received by an antenna element; and

the radio-frequency front end circuit according to claim 7 , which transmits the radio-frequency signal between the antenna element and the RF signal processing circuit.

9. The radio-frequency front end circuit according to claim 7 , wherein

the amplification circuit includes a reception switch, a transmission switch, a low-noise amplifier circuit, and a power amplifier circuit.

10. The multiplexer according to claim 1 , wherein

the first filter is defined by a reception filter which uses a downlink frequency band in a low band as the pass band; and

the second filter is defined by a reception filter that uses a downlink frequency band in a high band as the pass band.

11. The multiplexer according to claim 1 , wherein

an impedance matching inductor is connected to an end of the first filter.

12. The multiplexer according to claim 1 , wherein

the second filter includes longitudinally-coupled surface acoustic wave resonators.

13. A multiplexer comprising:

a common terminal;

a first terminal;

a second terminal;

a first filter that is on a first path connected between the common terminal and the first terminal and that includes a plurality of acoustic wave resonators; and

a second filter that is on a second path connected between the common terminal and the second terminal and that has a pass band located at a higher frequency than a pass band of the first filter; wherein

the plurality of acoustic wave resonators includes:

one or more series resonators on the first path; and

two or more parallel resonators on paths connected between the first path and ground;

the two or more parallel resonators include a first parallel resonator that is located on a side where the common terminal is provided and a parallel resonator that is located on a side where the first terminal is provided as seen from a first series resonator that is closest to the common terminal among the one or more series resonators;

the plurality of acoustic wave resonators each include a substrate that exhibits piezoelectricity, an InterDigital Transducer (IDT) electrode including of a pair of comb-shaped electrodes on the substrate, and reflectors each including one or more reflection electrode fingers; and

a total number of reflection electrode fingers of at least one of the first parallel resonator and the first series resonator is smaller than a total number of reflection electrode fingers of at least one of a remainder of the plurality of acoustic wave resonators.

14. The multiplexer according to claim 13 ,

wherein the total number of reflection electrode fingers of the at least one of the first parallel resonator and the first series resonator is smaller than a total number of reflection electrode fingers of each of the remainder of the plurality of acoustic wave resonators.

15. The multiplexer according to claim 13 ,

wherein the total number of reflection electrode fingers of the first parallel resonator and the total number of reflection electrode fingers of the first series resonator are smaller than the total number of reflection electrode fingers of at least one of the remainder of the plurality of acoustic wave resonators.

16. The multiplexer according to claim 15 ,

wherein the total number of reflection electrode fingers of the first parallel resonator and the total number of reflection electrode fingers of the first series resonator are smaller than a total number of reflection electrode fingers of each of the remainder of the plurality of acoustic wave resonators.

17. A radio-frequency front end circuit comprising:

the multiplexer according to claim 13 ;

and an amplification circuit that is connected to the multiplexer.

18. A communication device comprising:

an RF signal processing circuit that processes a radio-frequency signal transmitted or received by an antenna element; and

the radio-frequency front end circuit according to claim 17 , which transmits the radio-frequency signal between the antenna element and the RF signal processing circuit.