Impedance matching and parasitic capacitor resonance of FBAR resonators and coupled filters

Transformers are used in many types of electronic device to perform such functions as transforming impedances, linking single-ended circuitry with balanced circuitry or vice versa and providing electrical isolation. However, not all transformers have all of these properties. For example, an autotransformer does not provide electrical isolation.

Transformers operating at audio and radio frequencies up to VHF are commonly built as coupled primary and secondary windings around a high permeability core. Current in the windings generates a magnetic flux. The core contains the magnetic flux and increases the coupling between the windings. A transformer operable in this frequency range can also be realized using an optical-coupler. An opto-coupler used in this mode is referred to in the art as an opto-isolator.

In transformers based on coupled windings or opto-couplers, the input electrical signal is converted to a different form of energy (i.e., a magnetic flux or photons) that interacts with an appropriate transforming structure (i.e., another winding or a light detector), and is re-constituted as an electrical signal at the output. For example, an opto-coupler converts an input electrical signal to photons using a light-emitting diode. The photons pass through an optical fiber or free space that provides isolation. A photodiode illuminated by the photons generates an output electrical signal from the photon stream. The output electrical signal is a replica of the input electrical signal.

At UHF and microwave frequencies, coil-based transformers become impractical due to such factors as losses in the core, losses in the windings, capacitance between the windings, and a difficulty to make them small enough to prevent wavelength-related problems. Transformers for such frequencies are based on quarter-wavelength transmission lines, e.g., Marchand type, series input/parallel output connected lines, etc. Transformers also exist that are based on micro-machined coupled coils sets and are small enough that wavelength effects are unimportant. However such transformers have issues with high insertion loss. In addition, coil-based transformers have very wide passbands which does not allow for significant filtering function.

All the transformers just described for use at UHF and microwave frequencies have dimensions that make them less desirable for use in modem miniature, high-density applications such as cellular telephones. Such transformers also tend to be high in cost because they are not capable of being manufactured by a batch process and because they are essentially an off-chip solution. Moreover, although such transformers typically have a bandwidth that is acceptable for use in cellular telephones, they typically have an insertion loss greater than 1 dB, which is too high.

Opto-couplers are not used at UHF and microwave frequencies due to the junction capacitance of the input LED, non-linearities inherent in the photodetector, limited power handling capability and insufficient isolation to give good common mode rejection.

A film acoustically-coupled transformer (FACT) shown in FIG. 1 . A FACT transformer has a first and a second stacked bulk acoustic resonator (SBAR 1 , SBAR 2 ). Each SBAR has a stacked pair of film bulk acoustic resonators (FBARs) and an acoustic decoupler between the FBARs. Each of the FBARs has opposed planar electrodes and a layer of piezoelectric material between the electrodes. A first electrical circuit connecting one of the FBARs of SBAR 1 to one of the FBARs of SBAR 2 and a second electrical circuit connecting the other of the FBARs of SBAR 1 to the other of the FBARs of SBAR 2 . The first electrical circuit connects the respective FBARs in parallel and second electrical circuit connects the respective FBARs in series. This embodiment has a nominal 1:1 impedance transformation ratio between the first and second electrical circuits. In the first stack, the parasitic capacitance across the acoustic decoupler can be high thus lowering the insertion loss and affecting the differential performance adversely.

A film acoustically-coupled transformer (FACT) shown in FIG. 2 . A FACT transformer has a first and a second stacked bulk acoustic resonator (SBAR 1 , SBAR 2 ). Each SBAR has a stacked pair of film bulk acoustic resonators (FBARs) and an acoustic decoupler between the FBARs. Each of the FBARs has opposed planar electrodes and a layer of piezoelectric material between the electrodes. A first electrical circuit connecting one of the FBARs of SBAR 1 to one of the FBARs of SBAR 2 and a second electrical circuit connecting the other of the FBARs of SBAR 1 to the other of the FBARs of SBAR 2 . The first electrical circuit connects the respective FBARs in anti-parallel and second electrical circuit connects the respective FBARs in series. This embodiment has a 1:4 impedance transformation between the first and the second electrical circuits. Similar to FIG. 1 , the first stack has a high parasitic capacitance across the acoustic decoupler that adversely affects performance. In addition, the required crossover in connectivity introduces series, shunt parasitic resistance and capacitance.

In an embodiment, a film acoustically-coupled transformer (FACT) has a first and a second stacked bulk acoustic resonator (SBAR 1 , SBAR 2 ). Each SBAR has a stacked pair of film bulk acoustic resonators (FBARs) and an acoustic decoupler between the FBARs. Each of the FBARs has opposed planar electrodes and a layer of piezoelectric material between the electrodes. A first electrical circuit connecting one of the FBARs of SBAR 1 to one of the FBARs of SBAR 2 and a second electrical circuit connecting the other of the FBARs of SBAR 1 to the other of the FBARs of SBAR 2 . The first electrical circuit connects the respective FBARs in parallel and second electrical circuit connects the respective FBARs in series. A shunt inductor is included from the reception path to ground.

FIG. 1 illustrates a prior art solution.

FIG. 2 illustrates a prior art solution.

FIG. 3 illustrates an embodiment of the present invention.

FIG. 4 illustrates an embodiment of the present invention.

FIG. 5 illustrates an embodiment of the present invention.

FIG. 6 illustrates an embodiment of the present invention.

Each of the disclosed embodiments includes shunt inductance in the reception path. The shunt inductance fulfills three distinct functions simultaneously which are impedance matching, parasitic capacitance resonance and electrostatic device (ESD) protection. The first two functions, impedance matching and parasitic capacitance resonance, if realized separately, by two different inductors, would require much larger inductors with additional insertion loss and area. The parallel combination of these two inductors allows for our shunt inductor to be significantly smaller. The reduced size of the shunt inductor greatly improves its effect as an ESD protector.

FIG. 3 illustrates an embodiment of the present invention 10 A. A film acoustically-coupled transformer (FACT) has a first and a second stacked bulk acoustic resonator (SBAR 1 , SBAR 2 ) 12 1 , 12 2 . Each SBAR 12 1 , 12 2 has a stacked pair of film bulk acoustic resonators (FBAR 1 . FBAR 2 ) and an acoustic decoupler between the FBARs. Each of the FBARs has opposed planar electrodes and a layer of piezoelectric material between the electrodes. A first electrical circuit connecting one of the FBARs of SBAR 1 12 1 to one of the FBARs of SBAR 2 12 2 and a second electrical circuit connecting the other of the FBARs of SBAR 1 12 1 to the other of the FBARs of SBAR 2 12 2 . The first electrical circuit connects the respective FBARs in parallel and second electrical circuit connects the respective FBARs in series. A shunt inductor 14 is included from V in to ground.

This embodiment has a nominal 1:1 impedance transformation ratio between the first and second electrical circuits, but the shunt inductor allows for a controlled change in this transformation ratio by lowering the total impedance of the electrical circuit at which it is placed.

FIG. 4 illustrates an embodiment of the present invention. The SBARs 12 1 , 12 2 are connected as shown in FIG. 3 . A first shunt inductor 14 A is included from V out + to ground. A second shunt inductor 14 B is included from V out − to ground.

FIG. 5 illustrates an embodiment of the present invention. The SBARs 12 1 , 12 2 are connected as shown in FIG. 3 . A shunt inductor 14 is included from the V out + to V out − .

FIG. 6 illustrates an embodiment of the present invention. The SBARs 12 1 , 12 2 are connected as shown in FIG. 3 . A shunt inductor 14 connects from the serially connected FBARs to ground.