Resonant Devices

The electrical impedance of a piezoelectric device is in reality more complicated than the simple capacitor representation generally employed in discussing non resonant devices. A more proper representation would be a capacitor representing the static capacitance of the piezo electric element, shunted by an impedance representing the mechanical vibrating system. In most non resonant devices, the latter impedance may be approximated by a capacitor. Therefore, we have a capacitor in parallel with a capacitor- hence the single capacitor representation.

In devices designed for operation at resonance, the impedance representing the mechanical system may become, at resonance, a resistance of relatively low value and this is shunted by the same static capacitance.

The shunt static capacitance generally is undesirable, whether the device is designed for operation at resonance or for broadband, below resonance operation. ln electrically driven devices, it shunts the driving amplifier or other signal source requiring that the source be capable of supplying extra current. In the case of mechanically driven devices, the static capacitance acts as a load on the active part of the transducer, reducing the electrical output.

In non-resonant devices, not much can be done about the shunt capacitance, except choose a piezoelectric material having maximum activity. In resonant devices, however, the static capacitance may be "neutralized " by employing a shunt or series inductor chosen to resonate with the static capacitance at the operating frequency. This is illustrated in Figure 10.

fig10 resonant device with static capacitance neutralized by inductor