Piezo
Ceramics Tutorial 7 of 15
Circuit
Considerations - 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
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