High impedance piezoelectric transducer

An improved bimorph piezoelectric transducer provides higher input impedance and allows operation at higher operating voltages. First and second piezoelectric elements have opposing electrode patterns which define a plurality of capacitors connected in series. This allows such transducers to be directly connected to high voltage audio distribution systems without the need for an impedance matching circuit.

BACKGROUND 
This invention relates to piezoelectric transducers and more specifically 
to such transducers which can provide a high input impedance by coupling a 
plurality of integral capacitors in series. 
Conventional bimorph voice range piezoelectric speakers have an input 
impedance of less than 100 ohms at a frequency of 1500 Hertz (Hz). A 
typical 70.7 volt audio distribution system is designed to accept speakers 
having an impedance of 500 ohms to 10,000 ohms which corresponds to power 
levels of 10 watts to 0.5 watts, respectively. Thus, a matching circuit or 
transformer is required to couple a conventional piezoelectric speaker to 
such an audio system. 
It is an object of this invention to provide a piezoelectric transducer 
having a higher input impedance which can be directly coupled to audio 
systems requiring such impedances. A further object of this invention is 
to provide a piezoelectric transducer capable of operating with sustained 
voltages greater than 20 volts.

DETAILED DESCRIPTION 
FIG. 1 shows a conventional bimorph driver 10 having piezoelectric elements 
12 and 14. Both major surfaces of these elements have conductive 
electrodes. The adjacent electrodes of elements 12 and 14 are connected 
together by a center vane 16 which is preferably made of a conductive 
woven mesh such as described in U.S. Pat. No. 4,078,160. The outside 
electrodes are connected together by an external wire to the positive 
terminal 18 of the driving voltage source; the negative terminal 20 of the 
source is connected to the center vane 16 and hence to the inside 
electrodes on the elements. 
FIG. 2 is a schematic of an equivalent circuit of the driver 10. Capacitors 
13 and 15 represent the capacitance C of elements 12 and 14, respectively. 
As is apparent capacitors 13 and 15 are connected in parallel and provide 
an equivalent total circuit capacitance of 2C. 
FIG. 3 illustrates a generally circular piezoelectric driver 22 according 
to the present invention which includes piezoelectric elements 24 and 26. 
The upper surface of element 24 has a center circular electrode area 30 
surrounded by an annular spaced-apart electrode 28 as shown in FIG. 5. As 
used herein, "annular" means the continuous center electrode 30 as well as 
ring electrode 28. The lower surface of element 24 has a center circular 
electrode area 32 that is smaller than electrode 30 and an annular 
electrode 34 which is wider than electrode 28 so that it opposes the 
latter and also overlaps a portion of electrode 30. These opposing 
electrodes define capacitors C1, C2 and C3 which are connected in series 
as shown by the equivalent circuit in FIG. 4. 
In the embodiment of driver 22 shown in FIG. 3, element 26 has the same 
electrode patterns as element 24. These elements are disposed so that 
adjacent surfaces have the same electrode patterns. Thus element 26 
defines series connected capacitors C4, C5 and C6 which are equal in 
capacitance to capacitors C3, C2 and C1, respectively. 
A center wafer 36 disposed contiguously between elements 24 and 26 consists 
of an nonconductive ring 38 and a spaced-apart center conductive portion 
40. A conductive woven mesh such as described in U.S. Pat. No. 4,078,160 
is suitable for portion 40. The same type of woven mesh except without 
being conductive is suitable for ring 38. The conductive portion 40 
provides electrical connection between the electrodes 32 and 41 which 
connects capacitors C3 and C4 as shown in FIG. 4. 
The driver 22 provides an equivalent capacitance of C/18 since the six 
series connected capacitors each have a capacitance of C/3. Because 
impedance is inversely proportional to capacitance, the impedance of 
driver 22 is eighteen times the impedance of a monomorph having a 
capacitance of C and thirty-six times the impedance of the bimorph driver 
10. Thus a piezoelectric transducer according to the present invention can 
provide a higher input impedance than conventional bimorph and monomorph 
transducers. 
The arrows in FIG. 3 between the electrodes defining the capacitor plates 
show the polarity of poling, i.e. the application of an initial voltage 
across the areas of the piezoelectric elements needed to ititalize it. 
This alternating polarity of poling is needed so that the alternating 
charges which will develop across each series capacitor will induce forces 
that each contribute to the same type of dimensional variation in each 
piezoelectric element. Of course, the dimensional variation in element 24 
will be opposite that of element 26 to enhance the flexure of the 
transducer. 
If it is desirable to maintain uniform poling along each piezoelectric 
element, separately formed capacitors without common electrodes on each 
element can be formed. In order to maintain the same polarity of 
capacitance charge relative to the poling polarity in series connected 
capacitors on a uniformly poled element, external wires or conductive 
feedthrough paths in the elements are needed to interconnect the bottom 
electrode in one capacitor to the top electrode in an adjacent capicator 
to form a daisy-chain of capacitors. The same electrical performance can 
be attained with uniformly poled elements but at the expense of more 
complex interconnections. 
FIG. 6 shows an audio distribution system such as could be used in a large 
building for paging. A public address amplifier or audio source 42 
typically drives an audio line 44 having an impedance of greater than 1000 
ohms, such as 5000 ohms, with a relatively high voltage audio signal such 
as 70 volts. A conventional piezoelectric driven speaker 46 has a typical 
impedance of less than 100 ohms and cannot be directly connected since it 
cannot withstand the high operating voltages present on the audio line 44. 
A transformer 48 provides a voltage step down for speaker 46 thereby also 
providing an impedance match. 
A speaker 50 having a piezoelectric driver 22 according to the present 
invention and a diaphragm 52 can be directly connected to line 44 since it 
has a compatible impedance and can operate at the higher voltages normally 
used in such audio distribution systems. By contrasting FIG. 4 with FIG. 2 
it will be seen that the total audio voltage applied will be present 
across capacitors 13 and 15 while only 1/6 of the total voltage will 
appear across each of capacitors C1-C6. Thus the present invention 
eliminates the need for a matching circuit or transformer. 
In the illustrative embodiment of the present invention electrode patterns 
were designed to form three capacitors on each piezoelectric element. It 
will be apparent to those skilled in the art that the present invention 
can employ two or more capacitors per element to achieve various impedance 
levels. Selecting an odd number of capacitors per element provides the 
advantage of allowing the capacitor formed in the center of the element to 
be internally connected to the opposing center formed capacitor. Thus the 
present invention contemplates N capacitors formed per piezoelectric 
element, where N is an integer greater than one and is preferably an odd 
integer. 
Forming a bimorph driver with two such elements in which the capacitors on 
the elements are connected in series allows higher impedances and 
operating voltages to be achieved. The capacitors could also be designed 
to have unequal capacitances and could have shapes other than annular. Of 
course, only one piezoelectric element could be used as a monomorph. 
Although an embodiment of the present invention has been described and 
shown in the drawings, the scope of the invention is defined by the claims 
which follow.