Film acoustically-coupled transformers with two reverse c-axis piezoelectric elements
View Patent ↗Embodiments of an acoustically-coupled transformer have a first stacked bulk acoustic resonator (SBAR) and a second SBAR. Each of the SBARs has a lower film bulk acoustic resonator (FBAR) and an upper FBAR, and an acoustic decoupler between the FBARs. The upper FBAR is stacked atop the lower FBAR. Each FBAR has opposed planar electrodes and a piezoelectric element between the electrodes. The piezoelectric element is characterized by a c-axis. The c-axes of the piezoelectric elements of the lower FBARs are opposite in direction, and the c-axes of the piezoelectric elements of the upper FBARs are opposite in direction. The transformer additionally has a first electrical circuit connecting the lower FBAR of the first SBAR to the lower FBAR of the second SBAR, and a second electrical circuit connecting the upper FBAR of the first SBAR to the upper FBARs of the second SBAR.
1. An acoustically-coupled transformer, comprising:
a first stacked bulk acoustic resonator (SBAR) and a second SBAR, each SBAR comprising:
a lower film bulk acoustic resonator (FBAR) and an upper FBAR, the upper FBAR stacked atop the lower FBAR. each FBAR comprising opposed planar electrodes and a piezoelectric element between the electrodes, the piezoelectric element characterized by a c-axis,
an acoustic decoupler between the FBARs, in which the c-axes of the piezoelectric elements of the lower FBARs are opposite in direction and the c-axes of the piezoelectric elements of the upper FBARs are opposite in direction;
a first electrical circuit connecting the lower FBAR of the first SBAR to the lower FBAR of the second SBAR; and
a second electrical circuit connecting the upper FBAR of the first SBAR to the upper FBARS of the second SBAR.
2. The transformer of claim 1 , in which:
the transformer additionally comprises first terminals and second terminals;
the first electrical circuit additionally connects the lower FBARs to the first terminals; and
the second electrical circuit additionally connects the upper FRARs to the second terminals.
3. The transformer of claim 1 , in which:
the first electrical circuit connects the lower FBARs in series; and
the second electrical circuit connects the upper FBARs in series.
4. The transformer of claim 1 , in which:
the first electrical circuit connects the lower FBARs in one of anti-series, parallel and anti-parallel; and
the second electrical circuit connects the upper FBARs in one of anti-series, parallel and anti-parallel.
5. The transformer of claim 1 , in which the acoustic decouplers each comprise a layer of acoustic decoupling material.
6. The transformer of claim 5 , in which:
the piezoelectric material has an acoustic impedance; and
the acoustic decoupling material has an acoustic impedance less than the acoustic impedance of the piezoelectric material.
7. The transformer of claim 6 , in which the acoustic decoupling material has an acoustic impedance intermediate between the acoustic impendance of the piezoelectric material and the acoustic impedance of air.
8. The transformer of claim 5 , in which the acoustic decoupling material has an acoustic impedance in the range from about 1 Mrayl to about 16 Mrayl.
9. The transformer of claim 5 , in which the acoustic decoupling material comprises plastic.
10. The transformer of claim 5 , in which the acoustic decoupling material comprises polyimide.
11. The transformer of claim 5 , in which the acoustic decoupling material comprises poly(para-xylylene).
12. The transformer of claim 5 , in which:
the transformer has a pass band characterized by a center frequency; and
the layer of acoustic decoupling material has a nominal thickness equal to an odd integral multiple of one quarter of the wavelength in the acoustic decoupling material of an acoustic wave having a frequency equal to the center frequency.
13. The transformer of claim 1 , in which the acoustic decouplers each comprise a Bragg structure.
14. The transformer of claim 1 , in which one of the electrical circuits comprises a center tap.