Impedance transforming bulk acoustic wave baluns

A bulk acoustic wave (BAW) device comprising:

• a stacked bulk acoustic wave resonator (SBAR) comprising an acoustic decoupler between first and second film bulk acoustic resonators (FBARs),
• the first FBAR being resonant at the resonant frequency and comprising first and second planar electrodes abutting opposite sides of a first resonator volume free of any intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to a propagation axis normal to the first and second electrodes, the first FBAR having a first electrical impedance parallel to the propagation axis,
• the second FBAR being resonant at the resonant frequency and comprising third and fourth planar electrodes abutting opposite sides of a second resonator volume free of any intervening electrodes and containing piezoelectric material disposed for acoustic vibrations parallel to the propagation axis, the second FBAR having a second electrical impedance parallel to the propagation axis and different from the first electrical impedance.