Piezo
Ceramics Tutorial 6 of 15
Circuit
Considerations
To obtain
optimum performance from a piezoelectric device, the circuit to
which it is connected must have certain characteristics which are
dictated by the design of the device. In discussing this subject,
it is convenient to divide piezoelectric devices into two broad
categories non-resonant devices and resonant devices. Non resonant
devices are so named because they are designed to operate well
below resonance, or over a relatively large frequency range,
usually several octaves. A resonant device either operates at its
mechanical resonance or over a band of less than one octave around
this resonance.
Circuit
Considerations - Non-Resonant Mechanically Driven Devices
For most
practical circuit design purposes, a non-resonant piezoelectric
generator device, such as a microphone or accelerometer, together
with the load on the device, may be represented by the equivalent
circuit of Fig 9A,
where Ce
represents the capacitance of the piezo-electric element, Rc is
the shunt leakage resistance of the device, CL is the shunt load
capacitance, RL represents the load resistance and eL represents
the load potential.
The values
of the shunt leakage resistance and the capacitance of the
piezoelectric element are dependent on the dimensions of the
element and, in the case of leakage resistance, also upon the
dryness of the surface. Under normal operating conditions ,the
value of this resistance is many times greater than the normal
load resistance, ranging from several hundred megohms to many
thousands of megohms. Generally, except at very low frequencies,
the capacitances in the circuit have reactances very much lower
than the circuit resistances. Under these conditions, the circuit
reduces to the simple capacitive voltage divider circuit (Fig 9B).
It can be
seen from this circuit that the shunt load capacitance (CL) of
wiring, cable, and amplifier input does not bring about a relative
loss of high frequency response as in the case of resistive or
inductive sources shunted by capacitance loads, but instead
reduces the output at all frequencies (except very low frequencies
as discussed in the next paragraph).
By the
application of elementary network theory, the basic circuit can be
reduced to the equivalent circuit of Fig 9C which is useful for
analyzing the low frequency response. From this it can be seen
that when the combined reactance of the piezoelectric and circuit
capacitances in parallel approaches the combined resistance, the
low frequency response begins to fall off. This is analogous to
the situation existing in conventional RC-coupled amplifiers.
Circuit
Considerations - Non-Resonant Electrically Driven Devices
In the case
of non-resonant piezoelectric devices which are electrically
driven, the electrical impedance of the device may, for most
practical purposes, be considered to be purely capacitive. For all
frequencies well below the first mechanical resonance of the
device, the electromechanical relationships are such that the
displacement of the piezo electric element from its normal
position, at any instant, is directly proportional to the electric
charge applied at that instant.
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