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DESCRIPTION JPH0670397

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DESCRIPTION JPH0670397
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
piezoelectric speaker device such as a ceramic speaker, and more specifically to a piezoelectric
speaker suitable as a receiver for portable telephones and cordless telephones and a drive circuit
therefor On the improvement of
[0002]
2. Description of the Related Art A piezoelectric speaker 10 such as a ceramic speaker is
provided with electrodes 14 on the front and back of a vibrating body 12 such as ceramic, as
schematically shown in FIG. Is applied to vibrate the diaphragm (not shown) by the piezoelectric
effect to perform acoustic conversion. The equivalent circuit is represented by a capacitor C1 as
shown in FIG. 7B, and its capacity is, for example, about 0.08 μF.
[0003]
As a conventional drive circuit of such a piezoelectric type speaker, there are those shown in FIG.
6 and FIG. Describing from the example of FIG. 6, the terminal T1 to which a signal is inputted is
connected to the inverting input side of an operational amplifier (hereinafter referred to as "OP
amplifier") A1 via a resistor R1. The (1/2) VCC is applied to the non-inverting input side of the
OP amplifier A1, and the output side is connected to the inverting input side via the resistor R2.
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The output side of the OP amplifier A1 is connected to the terminal 5 of the piezoelectric speaker
10 via a resistor R3. The terminal 6 of the piezoelectric speaker 10 is grounded.
[0004]
This drive circuit uses the negative feedback amplifier circuit of a typical OP amplifier as it is,
and the signal input to the terminal T1 is amplified at an amplification factor determined by the
circuit constant and applied to the piezoelectric speaker 10 Be done.
[0005]
Next, an example of FIG. 7 will be described.
The terminal T2 to which a signal is input is connected to the inverting input side of one OP
amplifier A2 via a resistor R4. The output side of the OP amplifier A2 is connected to the
inverting input side of the other OP amplifier A3 via a resistor R5. The (1/2) VCC is applied to
the non-inverting input side of the OP amplifiers A2 and A3, respectively, and the output side is
connected to the inverting input side via the resistors R6 and R7. The output sides of the OP
amplifiers A2 and A3 are connected to the series circuit of the resistor R8 and the piezoelectric
speaker 10, respectively.
[0006]
In this drive circuit, the output of the OP amplifier A2 is input to the inverting input side of the
OP amplifier A3 by the resistor R5, and has a so-called BTL (Balanced Transformer Less)
connection configuration. The signal input to the terminal T2 is amplified on the positive phase
side by the OP amplifier A2 and on the negative phase side by the OP amplifier A3 and applied to
the piezoelectric speaker 10. According to this BTL connection configuration, a large output can
be obtained at a low power supply voltage.
[0007]
In the drive circuit for voltage-driving such a piezoelectric speaker 10, a resistor R3 or R8 is
inserted in series with the piezoelectric speaker 10 in order to improve low frequency
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characteristics. It is connected. On the other hand, in the piezoelectric element such as the
piezoelectric speaker 10, the output sound pressure is also determined by the magnitude of the
voltage generally applied.
[0008]
However, when such a series resistance is present, the voltage applied to the piezoelectric
speaker 10 itself is reduced, and a sufficient sound pressure can not be extracted. Furthermore,
in portable devices that use batteries, the power supply voltage is often as low as 3 to 4V. For
this reason, it is difficult to take out the amplifier output voltage necessary for driving the
speaker, and even if the efficiency is increased by performing the BTL connection shown in FIG. 7
or the like, it can not be said that the sound pressure of the piezoelectric speaker is insufficient.
[0009]
In order to improve such a disadvantage, it is desirable to have a circuit which has no series
resistance and which can guarantee the frequency characteristics of sound pressure. In addition,
it is also necessary to efficiently use a low power supply voltage.
[0010]
The present invention focuses on these points, and uses a current-driven BTL connection
amplifier to drive a piezoelectric speaker that is a capacitive load, and a structure that supports
the piezoelectric speaker as a current-driven BTL amplifier. An object of the present invention is
to provide a piezoelectric speaker device capable of obtaining sufficient sound pressure even
with a low power supply voltage.
[0011]
SUMMARY OF THE INVENTION One of the piezoelectric speaker devices according to the present
invention is to provide two sets of electrodes so that the capacity between the electrodes
becomes equal to the vibrating body of the piezoelectric speaker. In addition to the terminal
structure, the present invention is characterized by comprising a drive circuit including BTLconnected amplification means for amplifying and conducting an audio signal between the
electrodes of the first and second sets.
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[0012]
Another invention is a piezoelectric speaker device according to the above, wherein an audio
signal is amplified by a drive circuit of BTL connection which negatively feeds back the output of
the first amplification means to the second amplification means, and the amplified signal is
applied to a piezoelectric speaker. A drive circuit is connected between the output of the first
amplification means and the ground between a capacitor of a capacity corresponding to the
equivalent capacity of the piezoelectric speaker and a feedback resistor, and the voltage
generated in the feedback resistor is the first voltage. It is characterized in that a negative
feedback is made to the amplification means.
[0013]
According to the first aspect of the present invention, the piezoelectric speaker has a fourterminal structure in which two electrodes are respectively provided on the front and back of the
vibrating body, and a current drive type in which the amplifying means is BTL connected. Drive
by drive circuit.
Since the current drive is performed, it is not necessary to provide a series resistor for improving
the frequency characteristic, and the voltage is effectively applied to the piezoelectric speaker.
According to the second aspect of the invention, by passing a current equivalent to the load
current to the capacitor having the same capacity as the piezoelectric speaker which is the load,
the amplification means is subjected to current negative feedback corresponding to the load
current. A BTL drive circuit can be realized.
[0014]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the piezoelectric
speaker device according to the present invention will be described below with reference to the
attached drawings.
First, the first embodiment of the present invention will be described with reference to FIGS. FIG.
1 shows the piezoelectric speaker 20 of the first embodiment. In the figure, (A) is the front side
and (B) is the back side. Electrodes 24 and 26 are formed concentrically on the surface of the
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vibrating body 22, and electrodes 28 and 30 are formed concentrically on the back of the
vibrating body 22.
[0015]
Among these, a capacitor CA is formed between the electrodes 24 and 28, and a capacitor CB is
formed between the electrodes 26 and 30. The areas of the electrodes 24 to 30 are set such that
the capacitances of the capacitors CA and CB become equal, for example, 0.04 μF. The
relationship between the terminals 1 to 4 and the capacitors CA and CB is as shown in FIG.
[0016]
Next, the drive circuit of the first embodiment will be described with reference to FIG. In the
figure, a terminal TA to which an audio signal for driving a speaker is input is connected to the
non-inverting input side of one OP amplifier AA. Further, the terminal TB to which a
predetermined voltage (1/2) VCC is applied is connected to the non-inverting input side of the
other OP amplifier AB. The inverting input sides of the OP amplifiers AA and AB are connected by
a resistor RA.
[0017]
Next, the terminal 1 of the piezoelectric speaker 20 is connected to the output side of the OP
amplifier AA, and the terminal 2 is connected to the inverting input side of the OP amplifier AA
via a resistor RB. The terminal 4 of the piezoelectric speaker 20 is connected to the output side of
the OP amplifier AB, and the terminal 3 is connected to the inverting input side of the OP
amplifier AB via a resistor RC. Furthermore, a resistor RD is connected between the terminals 1
and 2 and a resistor RE is connected between the terminals 2 and 3. Further, a resistor RF is
connected between the terminals 3 and 4. The numerical values shown in the drawing are
examples of specific circuit constants.
[0018]
Next, the operation of the present embodiment configured as described above will be described.
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The audio signal input between the terminals TA and TB is amplified by the OP amplifiers AA and
AB, and the load is a capacitor CA, which is a load. It is applied to the resistor RE and the
capacitor CB. As a result, the piezoelectric speaker 20 is driven. The resistor RA is a negative
feedback resistor of the BTL connection, and the resistor RE is a resistor for taking out a voltage
proportional to the current flowing to the capacitors CA and CB. The resistors RD and RF are bias
resistors for obtaining the output amplifier output midpoint.
[0019]
As described above, according to the present embodiment, the piezoelectric speaker 20 has a
four-terminal structure, and is driven with a constant current by the BTL connection of the OP
amplifiers AA and AB. Therefore, the drive voltage can be favorably applied to the piezoelectric
speaker 20 without requiring a series resistor for improving the frequency characteristic. In
addition, since the drive circuit is in BTL connection, good sound pressure can be obtained even
in the case of a battery-powered portable device with a low voltage of the power supply.
[0020]
Next, FIG. 3 shows an example of modularization of the first embodiment as described above. The
piezoelectric speaker 20 is attached to a circular frame 32, and a stopper 34 is provided on the
outside of the frame 32. The resistors RD to RF in the drive circuit of FIG. 2 described above are
modularized as the mounting portion 36 inside the frame 32. Note that the OP amplifiers AA and
AB and the resistors RA to RC may be included in modularization. In this way, for example,
incorporation into a telephone handset can be easily performed.
[0021]
Second Embodiment Next, a second embodiment of the present invention will be described with
reference to FIG. In this embodiment, a piezoelectric speaker of the electrode structure shown in
FIG. 5 is used. In the figure, a terminal TC to which an audio signal is input is connected to the
inverting input side of one OP amplifier AC via a resistor RG. The output side of the OP amplifier
AC is connected to the inverting input side of the other OP amplifier AD via a resistor RH. (1/2)
VCC is applied to the non-inverting input side of each of the OP amplifiers AC and AD. The
resistors RI and RJ are resistors for output feedback of the OP amplifiers AC and AD, respectively.
The terminals 5 and 6 of the piezoelectric speaker 10 are connected to the output sides of the OP
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amplifiers AC and AD, respectively. Thus, in the present embodiment, the OP amplifiers AC and
AD have BTL connections.
[0022]
Further, in the present embodiment, a capacitor CC having a capacitance equivalent to the
equivalent capacitance C1 of the piezoelectric speaker 10 is connected in series with the
feedback resistor RK and the bias capacitor CD between the output side of the OP amplifier AC
and the ground. It is connected to the. A bias resistor RL is connected in parallel to the capacitor
CC.
[0023]
Next, the operation of the second embodiment configured as described above will be described.
This drive circuit is basically the same as the circuit of FIG. 7, and signal voltages of positive and
negative phases are applied to the piezoelectric speaker 10 by the OP amplifiers AC and AD.
Since the capacitor CC has the same capacity as the piezoelectric speaker 10 and is connected to
the ground, a current proportional to the current flowing to the piezoelectric speaker 10 which is
a load eventually flows to the capacitor CC.
[0024]
Since this current flows to the ground via the feedback resistor RK, a voltage corresponding to
this current is generated at the feedback resistor RK, and this voltage is fed back to the inverting
input side of the OP amplifier AC via the resistor RI. become. In this way, current negative
feedback equivalent to the current corresponding to the load current is applied to the OP
amplifier AC, and BTL circuit constant current drive is realized. The resistor R and the capacitor
CD are a resistor and a capacitor for DC biasing the OP amplifier AC.
[0025]
The present invention is not limited to the above embodiment, and various design changes can be
made to achieve the same function such as values of circuit constants and circuit configurations
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shown. Is also included in the present invention. For example, as the amplifier of the drive circuit,
standard ones as well as constant current ones are applicable.
[0026]
As described above, according to the piezoelectric speaker device according to the present
invention, the following (1) BTL connection amplification means for current drive for driving the
piezoelectric speaker which is a capacitive load: Since the piezoelectric speaker has a fourterminal structure corresponding thereto, it is possible to obtain sufficient sound pressure even
at a low power supply voltage. (2) A capacitor equal to the load capacitance is connected to the
output side of the BTL connection amplification means to flow a current equal to the load
current, and this is negatively fed back to the amplification means. There is an effect that
pressure can be obtained.
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