Band-pass filter using film bulk acoustic resonator
View Patent ↗A band-pass filter has a ladder-type circuit including first and second terminals whose characteristic impedances are Z 0 , and series elements and shunt elements disposed between a first terminal and a second terminal, each of the series elements and shunt elements containing a film bulk acoustic resonator. Assuming that characteristic impedance of any one of the series elements is Z 1 and that characteristic impedance of any one of the shunt elements is Z 2 , the characteristic impedances Z 0 , Z 1 , and Z 2 have a relation of 1<(Z 1 /Z 0 )<2, preferably 1.3<(Z 1 /Z 0 )<1.7, and 0.5<(Z 2 /Z 0 )<1, preferably 0.6<(Z 2 /Z 0 )<0.8.
1. A band-pass filter having a ladder-type circuit including first and second terminals whose characteristic impedances are Z 0 , and series elements and shunt elements disposed between a first terminal and a second terminal, each of the series elements and shunt elements containing a film bulk acoustic resonator, wherein assuming that characteristic impedance of any one of the series elements is Z 1 and that characteristic impedance of any one of the shunt elements is Z 2 , the characteristic impedances Z 0 , Z 1 , and Z 2 have a relation of 1<(Z 1 /Z 0 )<2 and 0.5<(Z 2 /Z 0 )<1.
2. The band-pass filter according to claim 1 , wherein the characteristic impedances Z 0 , Z 1 , and Z 2 have a relation of 1.3<(Z 1 /Z 0 )<1.7 and 0.6<(Z 2 /Z 0 )<0.8.
3. The band-pass filter according to claim 1 , wherein the film bulk acoustic resonator comprises a substrate having a via-hole of substantially vertical taper angle so as to penetrate the substrate from one surface to the other surface thereof; and a stack containing a piezoelectric layer and a first and second electrodes sandwiching the piezoelectric layer, the stack is held on the one surface of the substrate, and a portion of the stack functions as a vibration portion facing the via-hole.