Circuit Considerations
Circuit Considerations
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.
