Audio output device

Abstract

To provide a sound output device capable of reducing component cost and manufacturing cost without the need for providing a resistor in series with a piezoelectric element.SOLUTION: An audio output device (1) includes a piezoelectric device (BZ), a control unit (10) that generates a PWM signal according to an audio signal, a FET as a switching device that performs a switching operation based on the PWM signal, a bridge circuit (30) configured using the FET in a lower arm, a DC power source (Vcc) that applies a DC voltage to the bridge circuit, and a gate drive circuit (20) that applies a predetermined gate-source voltage (Vgs) to the FET according to the PWM signal. a1 Vb1 a1, the on-resistance Ron of the FET under the operating condition a1 and a capacitor equivalent to the piezoelectric device BZ constitute a low pass filter.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an audio output device that outputs and reproduces audio using a piezoelectric element. [Background technology]

[0002] For example, Patent Document 1 describes an audio playback device that includes a piezoelectric element (piezoelectric buzzer) that outputs, as sound, the vibrations of the piezoelectric element that are generated by applying a signal voltage to the piezoelectric element, and a resistor that is connected in series to the piezoelectric element and that, together with a capacitor that is equivalent to the piezoelectric element, forms a low-pass filter. This makes it possible to effectively remove high-frequency components that cause noise, thereby achieving high-quality audio output. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-107752 Summary of the Invention [Problem to be solved by the invention]

[0004] In Patent Document 1, a resistor is inserted in series with the piezoelectric element, which increases the number of parts and increases costs. Also, when the frequency of the PWM signal changes, the frequency of the high-frequency component to be removed also changes, so the cutoff frequency of the low-pass filter needs to be changed, but each time this is done, the resistance value of the resistor needs to be changed, which requires a part change. [Means for solving the problem]

[0005] In order to solve the above problems, in the voice output device according to claim 1 of the present invention, a piezoelectric element that outputs vibrations generated by applying a voltage to a piezoelectric body as voice, a control unit that generates a PWM signal according to a voice signal, a FET as a switching element that performs a switching operation based on the PWM signal, a bridge circuit configured by using the FET for a lower arm, a DC power source that applies a DC voltage to the bridge circuit, and a gate drive circuit that applies a predetermined gate-source voltage Vgs to the FET according to the PWM signal. In a voice output device provided with the above components, as the gate-source voltage Vgs increases, the on-resistance Ron between the drain and source of the FET decreases, and in a characteristic in which the voltage Vb1 across both ends of the piezoelectric element changes with a maximum value, in an operating condition a1 that operates in a region A1 near the maximum value of the voltage Vb1, while the gate drive circuit performs the switching operation of the FET, the on-resistance Ron in the operating condition a1 and a capacitor equivalent to the piezoelectric element constitute a low-pass filter.

[0006] Further, in the voice output device according to claim 2, the bridge circuit is characterized in that the FETs are arranged in upper and lower arms to form a full bridge circuit.

[0007] Further, in the voice output device according to claim 3, the bridge circuit is characterized in that the FETs are arranged in upper and lower arms to form a half bridge circuit having one leg.

[0008] Further, in the voice output device according to claim 4, in the characteristic of the drain current Id and the drain-source voltage Vds of the FET, in an operating condition a2 in which the increase in the drain current Id saturates as the drain-source voltage Vds increases, and which is a pinch-off voltage Vp < Vds saturation region, the gate drive circuit performs the switching operation of the FET.

[0009] Further, in the voice output device according to claim 5, a piezoelectric element that outputs vibrations generated by applying a voltage to a piezoelectric body as voice, a control unit that generates a PWM signal according to a voice signal, a bipolar transistor as a switching element that performs a switching operation based on the PWM signal, a bridge circuit configured using the bipolar transistor in a lower arm, a DC power supply that applies a DC voltage to the bridge circuit, and a gate drive circuit that supplies a predetermined base current Ib to the bipolar transistor according to the PWM signal. In the voice output device provided with, as the base current Ib increases, the collector-emitter voltage Vce of the bipolar transistor decreases, and the voltage Vb2 across both ends of the piezoelectric element changes with a maximum value. In an operating condition b1 in a region B1 near the maximum value of the voltage Vb2, while the gate drive circuit performs the switching operation of the bipolar transistor, the behavior like the resistance between the collector and emitter of the bipolar transistor in the operating condition b1 and a capacitor equivalent to the piezoelectric element constitute a low-pass filter.

[0010] Further, in the voice output device according to claim 6, the bridge circuit is characterized in that the bipolar transistor is arranged in upper and lower arms to form a full bridge circuit.

[0011] Further, in the voice output device according to claim 7, the bridge circuit is characterized in that the bipolar transistor is arranged in upper and lower arms to form a half bridge circuit having one leg.

[0012] Further, in the voice output device according to claim 8, in the characteristics of the collector current Ic and the collector-emitter voltage Vce of the bipolar transistor, in a region B2 near the collector-emitter saturation voltage Vce(sat) where the increase in the collector current Ic saturates as the collector-emitter voltage Vce increases, and in an active region where Vce(sat) < Vce, the gate drive circuit performs the switching operation of the bipolar transistor. [Effects of the Invention]

[0013] According to the present invention, by using the on-resistance of an FET as the resistance that constitutes the low-pass filter, or by utilizing the characteristic that the collector-emitter of a bipolar transistor behaves like a resistor, there is no need to provide a resistor in series with the piezoelectric element, which reduces component and manufacturing costs and also makes it easy to change the resistance value. [Brief explanation of the drawings]

[0014] [Figure 1] System configuration diagram of one embodiment of the present invention [Figure 2] A driving circuit for an audio output device according to a first embodiment of the present invention [Figure 3] Graph showing changes in Vb1 and Ron with respect to Vgs in one embodiment of the present invention. [Figure 4] Graph showing change in Id with respect to Vds in one embodiment of the present invention. [Figure 5] A driving circuit for an audio output device according to a second embodiment of the present invention [Figure 6] A driving circuit for an audio output device according to a third embodiment of the present invention [Figure 7] A driving circuit for an audio output device according to a fourth embodiment of the present invention [Figure 8] Graph showing changes in Vb2 and Vce relative to Ib in one embodiment of the present invention. [Figure 9] Graph showing change in Ic relative to Vce in one embodiment of the present invention. [Figure 10] A driving circuit for an audio output device according to a fifth embodiment of the present invention [Figure 11] A driving circuit for an audio output device according to a sixth embodiment of the present invention DETAILED DESCRIPTION OF THE INVENTION

[0015] Next, an audio output device according to an embodiment of the present invention will be described with reference to the drawings.

[0016] A system configuration diagram of this embodiment is shown in Fig. 1. The audio output device 1 has a control unit 10, a gate drive circuit 20, a bridge circuit 30, and a piezoelectric element BZ.

[0017] The control unit 10 is configured, for example, by a microcomputer, and includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), etc. The control unit 10 also includes a storage unit 11. The storage unit 11 is configured by a flash ROM, and stores audio signal data 12. The control unit 10 generates a PWM signal (Pulse Width Modulation: PWM) based on the audio signal data 12 in the storage unit 11, and outputs the generated PWM signal to the gate drive circuit 20. Note that the audio signal data 12 does not have to be stored in the storage unit 11 within the control unit 10, and may be stored in an EEPROM (Electrically Erasable and Programmable Read Only Memory), which is an externally connected non-volatile memory.

[0018] The gate drive circuit 20 generates a predetermined gate drive signal in response to the input PWM signal. The gate drive signal is supplied to switching elements configured in a bridge circuit 30. The gate drive circuit 20 and the bridge circuit 30 will be described in detail later.

[0019] Piezoelectric elements BZ are composed of, for example, piezoelectric buzzers and piezoelectric speakers, and include components that vibrate and generate sound (audio) when a voltage is applied to a piezoelectric body (crystal, ferroelectric ceramics, etc.). Piezoelectric elements BZ are used in audio output devices such as the audio output of remote controls and the guidance voice of in-car navigation systems. Since the sound pressure level output by piezoelectric element BZ is proportional to the applied voltage (voltage across both ends of piezoelectric element BZ), the volume of piezoelectric element BZ can be increased by increasing the applied voltage.

[0020] 2 shows the configuration of a drive circuit for an audio output device 1 (first embodiment), which is composed of a bridge circuit 30 using switching elements Q1 and Q2 in the lower arm and a gate drive circuit 20 (20a, 20b). In this embodiment, field effect transistors (FETs) are used as the switching elements Q1 and Q2. The FET is a voltage-driven switching element, and performs on / off switching when a gate drive signal of voltage Vgs is applied between the gate and source in response to a PWM signal.

[0021] In FIG. 2, the PWM signal applied to gate drive circuit 20a is PWM1 as the gate drive signal for FET Q1, and the PWM signal applied to gate drive circuit 20b is PWM2 as the gate drive signal for FET Q2. The magnitude of the gate-source voltage Vgs applied to Q1 is a voltage value obtained by dividing a reference voltage (e.g., 5 V) not shown by resistors R1 and R2 of gate drive circuit 20a. That is, a voltage Vgs=5 V×R1 / (R1+R2) is applied to Q1. The magnitude of Vgs, i.e., the magnitudes of resistors R1 and R2, is determined by the magnitude of on-resistance Ron, which will be described later. The same is true for resistors R5 and R6 in gate drive circuit 20b. The carrier frequency of gate drive signals PWM1 and PWM2 is, for example, 32 kHz.

[0022] A DC power supply Vcc (e.g., 15 V) is applied across the bridge circuit 30, which is ground GND, which is a reference potential. When FET Q1 is turned on, current flows through FET Q1 via resistor R4 and piezoelectric element BZ, and a voltage (positive voltage) is applied across piezoelectric element BZ. On the other hand, when FET Q2 is turned on, current flows through FET Q2 via resistor R3 and piezoelectric element BZ, and a voltage of the opposite polarity (negative voltage) is applied across piezoelectric element BZ. Resistors R3 and R4 are provided to limit the current flowing through FET Q2 and Q1, and to prevent a short circuit between the DC power supply Vcc and GND.

[0023] To reproduce sound using piezoelectric element BZ, it is necessary to reproduce the analog audio signal using a PWM signal with a duty cycle proportional to the amplitude of the original analog audio signal. Therefore, by applying a gate drive signal corresponding to the PWM signal to FET Q1, a voltage generated by a positive pulse signal is applied across piezoelectric element BZ. On the other hand, by applying a gate drive signal corresponding to the PWM signal to FET Q2, a voltage generated by a negative pulse signal is applied across piezoelectric element BZ. By periodically generating positive and negative pulse signals, the average voltage of the pulse signal becomes equivalent to the original analog signal, allowing it to be reproduced and output as sound. The period during which the positive and negative polarities occur depends on the period of the original analog signal.

[0024] 3 is a graph showing the change in the voltage Vb1 across the piezoelectric element BZ and the change in the on-resistance Ron of the FETs Q1 and Q2 with respect to the change in the gate-source voltage Vgs applied to the FETs Q1 and Q2 in the drive circuit of the audio output device 1. The graph also shows the waveform of the voltage across the piezoelectric element BZ at any of Vgs (a) to (e).

[0025] As shown in Figure 3, when the gate-source voltage Vgs is increased from 2.0 V to 3.0 V, the on-resistance Ron of FETs Q1 and Q2 decreases from over 8000 Ω to nearly 0 Ω. Also, the voltage Vb1 across piezoelectric element BZ increases from approximately 10 V, reaches a maximum value near approximately 25 V, and then decreases to approximately 20 V.

[0026] Here, we consider the change in the voltage waveform across piezoelectric element BZ in response to changes in gate-source voltage Vgs. As mentioned above, an analog audio signal can be reproduced by piezoelectric element BZ using a PWM signal, but because PWM signals contain harmonics, the audio will contain noise. Therefore, by cutting out the harmonic components with a low-pass filter before applying voltage to piezoelectric element BZ, it is possible to reproduce clear audio with good sound quality. If the voltage waveform applied to piezoelectric element BZ is equivalent to the original analog signal, audio reproduction will be high.

[0027] The present invention is characterized in that the resistance component of a switching element such as an FET is used as the resistance component constituting the low-pass filter, and the capacitance component of piezoelectric element BZ is used as the capacitance component. In other words, in the first embodiment, when FET Q1 switches, the low-pass filter is formed by the resistance component formed by the combination of resistor R4 and the on-resistance Ron of Q1, and the capacitance component of piezoelectric element BZ. Also, when FET Q2 switches, the low-pass filter is formed by the resistance component formed by the combination of resistor R3 and the on-resistance Ron of Q2, and the capacitance component of piezoelectric element BZ. This eliminates the need for a dedicated resistor to be connected in series with FETs Q1 and Q2 to form the low-pass filter, thereby reducing component and manufacturing costs.

[0028] In the voltage waveform across piezoelectric element BZ in Figure 3, as it transitions from (e) to (a), the harmonic components are gradually attenuated, resulting in a smoother waveform. This is because, as the on-resistance Ron, which is the resistive component that makes up the low-pass filter, increases, the cutoff frequency Fc=1 / (2π·R0·C0) (R0: resistive component, C0: capacitive component) of the low-pass filter decreases, further attenuating the harmonic components. In other words, by operating piezoelectric element BZ in Figure 3(a) or (b), it is possible to reproduce audio with good sound quality.

[0029] On the other hand, the voltage Vb1 across the piezoelectric element BZ has a maximum value near the region A1 in Fig. 3, and the highest sound pressure level can be obtained in this region. In other words, by operating the piezoelectric element BZ in Fig. 3(b) or (c), it is possible to reproduce sound at a higher volume.

[0030] Therefore, when reproducing sound using piezoelectric element BZ, in order to reproduce sound with a good balance between sound quality and volume, the gate drive circuit should perform the switching operation of FETs Q1 and Q2 under conditions where the voltage Vb1 across piezoelectric element BZ is in region A1, where the voltage Vb1 across piezoelectric element BZ is near its maximum value. If the operating condition at this time is designated as a1, an example of the operating condition a1 in the first embodiment is the conditions in Figure 3(b), where the gate-source voltage Vgs is 2.27 V, the on-resistance Ron of FETs Q1 and Q2 is 1000 Ω, and the voltage Vb1 across piezoelectric element BZ is 24 V.

[0031] FIG. 4 is a graph showing the change in drain current Id with respect to the change in drain-source voltage Vds in the drive circuit of the audio output device 1, with the gate-source voltage Vgs of FETs Q1 and Q2 as a parameter. In the graph, Vp indicates the pinch-off voltage, which represents the boundary between the linear region and the saturation region of the FET. Under operating condition a1 in FIG. 3, the on-resistance Ron of FETs Q1 and Q2 is a relatively large value of 1000 Ω. In other words, because operation is required at a relatively large drain-source voltage Vds, operating condition a2, which includes operating condition a1, exists in the saturation region of the FET in FIG. 4. Operating condition a2 is a condition where the pinch-off voltage Vp is less than the saturation region of the drain-source voltage Vds.

[0032] 5 shows the configuration of a drive circuit of the audio output device 1 (second embodiment) which is configured with a full-bridge circuit 31 using switching elements Q1 to Q4 in both the upper and lower arms, and a gate drive circuit 21 (21a, 21b) in contrast to the first embodiment of FIG. 2. In this embodiment, FETs are used for the switching elements Q1 to Q4. The switching elements Q1 and Q3 in the upper arm are P-channel FETs, and the switching elements Q2 and Q4 in the lower arm are N-channel FETs.

[0033] The switching operation of FETs Q1 to Q4 in the second embodiment of FIG. 5 will now be described. When the gate drive signal PWM1 of the gate drive circuit 21a is H (e.g., 5V), FET Q2 is on and FET Q1 is off. On the other hand, when PWM1 is L (e.g., 0V), FET Q2 is off and FET Q1 is on. The same applies to the gate drive circuit 21b. Here, the gate drive signals PWM1 and PWM2 are provided so that they have opposite polarities, that is, when PWM1 is H or L, PWM2 is L or H. This alternates between a switching state in which FETs Q2 and Q3 are on and FETs Q1 and Q4 are off, and a switching state in which FETs Q2 and Q3 are off and FETs Q1 and Q4 are on. At this time, the voltage Vb1 across the piezoelectric element BZ is applied with varying duty ratios within a range of +15V to 0V to -15V. As a result, by periodically applying a pulse signal voltage of positive and negative polarity to the piezoelectric element BZ, the average voltage of the pulse signal becomes equivalent to the original analog signal, and can be reproduced and output as sound.

[0034] 5, the low-pass filter is configured using, as its resistance component, the on-resistance Ron of only the FETs Q1 and Q2 in the left leg, only the FETs Q3 and Q4 in the right leg, only the FETs Q1 and Q3 in the upper arm, only the FETs Q2 and Q4 in the lower arm, or all of the FETs Q1 to Q4. In this case, by appropriately setting the gate-source voltage Vgs of the FET that uses the on-resistance Ron to a value that becomes the desired resistance component of the low-pass filter, it becomes possible to set the cutoff frequency over a wide range, and the piezoelectric element BZ can reproduce sound with a better balance between sound quality and volume.

[0035] 6 shows a configuration of a drive circuit of the audio output device 1 (third embodiment) that is configured with a half-bridge circuit 32 configured with one leg, in which switching elements Q1 and Q2 are arranged in upper and lower arms, and a gate drive circuit 22, in contrast to the second embodiment in FIG. 5. In this embodiment, the switching element Q1 is a P-channel FET, and the switching element Q2 is an N-channel FET.

[0036] The switching operation of FETs Q1 and Q2 in the third embodiment of FIG. 6 is the same as that of FETs Q1 and Q2 in the second embodiment of FIG. 5. However, since there are no legs of FETs Q3 and Q4 in FIG. 5, +15 V is applied to piezoelectric element BZ when FET Q1 is on and FET Q2 is off, and 0 V is applied to piezoelectric element BZ when FET Q1 is off and FET Q2 is on. In other words, only one polarity voltage is applied to piezoelectric element BZ. Therefore, the voltage Vb1 across piezoelectric element BZ is applied with a varying duty ratio in the range of +15 V to 0 V. In this case, a low-pass filter is formed by utilizing the on-resistance Ron of FETs Q1 and Q2.

[0037] In the third embodiment shown in Fig. 6, the voltage Vb1 across the piezoelectric element BZ is half that of the second embodiment shown in Fig. 5, and therefore the sound pressure level is also half, resulting in a lower volume. However, the circuit configuration is simpler, which allows for reduced component costs.

[0038] FIG. 7 shows the configuration of a drive circuit of an audio output device 1 (fourth embodiment), which is composed of a bridge circuit 33 using switching elements Tr1 and Tr2 in the lower arm, and a gate drive circuit 23 (23a, 23b). In this embodiment, bipolar transistors (BJT: Bipolar Junction Transistor) are used as the switching elements Q1 and Q2. The transistors are current-driven switching elements, and perform on / off switching operations when a base current Ib is applied in response to a PWM signal. The circuit configuration of FIG. 7, which shows the fourth embodiment, is such that the FETs in FIG. 2 are replaced with transistors. Note that a description of the configuration equivalent to that of the first embodiment will be omitted.

[0039] 7, the magnitude of the base current Ib supplied to transistor Tr1 is a current value obtained by subtracting the base-emitter voltage Vbe of Tr1 from a reference voltage (e.g., 5V) not shown, and dividing the result by resistor R2 of gate drive circuit 23a. That is, a current value Ib=(5V-Vbe) / R2 is supplied to Tr1. The same applies to the base current Ib of transistor Tr2 in gate drive circuit 23b.

[0040] 8 is a graph showing the change in the voltage Vb2 across the piezoelectric element BZ and the collector-emitter voltage Vce of the transistors Tr1 and Tr2 with respect to the change in the base current Ib applied to the transistors Tr1 and Tr2 in the drive circuit of the audio output device 1. The graph also shows the waveform of the voltage across the piezoelectric element BZ at any of Ib values ​​(a) to (d).

[0041] 8, when the base current Ib is increased from 0 A to 1.4 mA, the collector-emitter voltage Vce of transistors Tr1 and Tr2 decreases from approximately 16 V to nearly 0 V. In addition, the voltage Vb2 across piezoelectric element BZ increases from approximately 10 V, reaches a maximum value near approximately 26 V, and then decreases to approximately 22 V.

[0042] Here, the change in collector-emitter voltage Vce with respect to changes in base current Ib follows a similar trend to the change in on-resistance Ron with respect to changes in gate-source voltage Vgs shown in FIG. 3. Furthermore, the change in the voltage waveform across piezoelectric element BZ with respect to changes in base current Ib follows a similar trend to the change in the voltage waveform across piezoelectric element BZ with respect to changes in gate-source voltage Vgs shown in FIG. 3. Meanwhile, as the voltage waveform across piezoelectric element BZ transitions from (d) to (a), the harmonic components are gradually attenuated, resulting in a smoother waveform. This indicates that the resistance component constituting a low-pass filter increases with a decrease in base current Ib. These results show that the collector-emitter pairs of transistors Tr1 and Tr2 behave like resistors, and that the magnitude of this resistance increases with a decrease in base current Ib. By utilizing this characteristic, a low-pass filter can be constructed in the fourth embodiment using transistors, as in the first embodiment using FETs, by utilizing the resistor-like behavior between the collector and emitter and the capacitance component of piezoelectric element BZ. As a result, the harmonic components of the voltage Vb2 across the piezoelectric element BZ are attenuated, making it possible to reproduce sound with good sound quality.

[0043] Here, when playing back sound using piezoelectric element BZ, in order to play back sound with a good balance between sound quality and volume, the gate drive circuit should perform the switching operation of transistors Tr1 and Tr2 under conditions where the voltage Vb2 across piezoelectric element BZ is in region B1 near its maximum value, as shown in Fig. 8. If the operating condition at this time is designated as b1, an example of the operating condition b1 in the fourth embodiment is the conditions in Fig. 8(b), where the base current Ib is 0.2 mA, the collector-emitter voltage Vce of transistors Tr1 and Tr2 is 1.9 V, and the voltage Vb2 across piezoelectric element BZ is 26 V.

[0044] FIG. 9 is a graph showing the change in the collector current Ic with respect to the change in the collector-emitter voltage Vce, taking the base currents Ib of the transistors Tr1 and Tr2 as parameters. Also, Vce(sat) indicates the collector-emitter saturation voltage. In the Ic-Vce characteristics of the transistors shown in FIG. 9, when Vce < the collector-emitter saturation voltage Vce(sat), it is the saturation region, and when the collector-emitter saturation voltage Vce(sat) < Vce, it is the active region. Also, under the operating condition b1 in FIG. 8, the collector-emitter voltage Vce of the transistors Tr1 and Tr2 is 1.9 V, and the switching operation is in the active region side from Vce(sat). That is, the switching operation point of the transistors in FIG. 9 is the region B2 near the collector-emitter saturation voltage Vce(sat), and it is the operating condition b2 where the active region is such that Vce(sat) < Vce.

[0045] FIG. 10 shows the configuration of the drive circuit of the audio output device 1, which is composed of a full-bridge circuit 34 using the switching elements Tr1 to Tr4 for both the upper and lower arms, and gate drive circuits 24(24a, 24b), for the fourth embodiment of FIG. 7 (the fifth embodiment). In this embodiment, the switching elements Tr1 to Tr4 use transistors. The switching elements Tr1 and Tr3 of the upper arm are pnp-type transistors, and the switching elements Tr2 and Tr4 of the lower arm are npn-type transistors. FIG. 10, which is the fifth embodiment, has a circuit configuration in which FETs are changed to transistors with respect to FIG. 5, which is the second embodiment. The description of the configuration equivalent to the second embodiment is omitted.

[0046] 10, the low-pass filter is configured using the resistor-like behavior of only the transistors Tr1 and Tr2 in the left leg, only the transistors Tr3 and Tr4 in the right leg, only the transistors Tr1 and Tr3 in the upper arm, only the transistors Tr2 and Tr4 in the lower arm, or all the transistors Tr1 to Tr4. In this case, by appropriately setting the base current Ib of the transistors that behave like resistors so as to configure the desired resistor components of the low-pass filter, it becomes possible to set the cutoff frequency over a wide range, and the piezoelectric element BZ can reproduce sound with a better balance between sound quality and volume.

[0047] 11 shows a configuration of a drive circuit of the audio output device 1 (sixth embodiment) that is configured with a half-bridge circuit 35 configured with one leg, in which switching elements Tr1 and Tr2 are arranged in upper and lower arms, and a gate drive circuit 25, in contrast to the fifth embodiment of FIG. 10. In this embodiment, the switching element Tr1 uses a pnp transistor, and the switching element Tr2 uses an npn transistor.

[0048] The switching operation of transistors Tr1 and Tr2 in the sixth embodiment of FIG. 11 is the same as that of transistors Tr1 and Tr2 in the fifth embodiment of FIG. 10. However, since the legs of transistors Tr3 and Tr4 in FIG. 10 are not present, +15V is applied to piezoelectric element BZ when transistor Tr1 is on and transistor Tr2 is off, and 0V is applied to piezoelectric element BZ when transistor Tr1 is off and transistor Tr2 is on. In other words, only one polarity voltage is applied to piezoelectric element BZ. Therefore, the voltage Vb2 across piezoelectric element BZ is applied with a varying duty ratio in the range of +15V to 0V. In this case, a low-pass filter is formed by utilizing the resistor-like behavior of transistors Tr1 and Tr2.

[0049] In the sixth embodiment, the voltage Vb2 across the piezoelectric element BZ is half that of the fifth embodiment, so the sound pressure level is also half, resulting in a lower volume. However, the circuit configuration is simpler, which allows for reduced component costs.

[0050] Furthermore, the other configurations used in the present embodiment are presented as examples and are not intended to limit the scope of the invention, and the invention can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, and are also included in the inventions described in the claims and their equivalents. [Explanation of symbols]

[0051] 1...Audio output device 10...Control unit 11...Storage section 12...Audio signal data 20 (20a, 20b), 21 (21a, 21b), 22, 23 (23a, 23b), 24 (24a, 24b), 25...Gate drive circuit 30, 33...Bridge circuit 31, 34...Full bridge circuit 32, 35...Half-bridge circuit Vcc: DC power supply, GND: Ground BZ...Piezoelectric element Q1, Q2, Q3, Q4... Field effect transistors (FETs) Tr1, Tr2, Tr3, Tr4...Bipolar transistors (transistors)

Claims

1. A piezoelectric element that outputs sound by applying a voltage to the piezoelectric body and generating vibrations. a control unit that generates a PWM signal in response to an audio signal; an FET as a switching element that performs a switching operation based on the PWM signal; a bridge circuit configured using the FET in a lower arm; a DC power supply that applies a DC voltage to the bridge circuit; a gate drive circuit that applies a predetermined gate-source voltage Vgs to the FET in response to the PWM signal, In the characteristic where, as the gate-source voltage Vgs increases, the drain-source on-resistance Ron of the FET decreases and the voltage Vb1 across the piezoelectric element changes to have a maximum value, Under operating conditions a1 in which the gate drive circuit operates in an area A1 near the maximum value of the voltage Vb1, the gate drive circuit performs the switching operation of the FET, while The audio output device is characterized in that a low-pass filter is formed by the on-resistance Ron under the operating condition a1 and a capacitor equivalent to the piezoelectric element.

2. 2. The audio output device according to claim 1, wherein the bridge circuit is a full bridge circuit in which the FETs are arranged in upper and lower arms.

3. 2. The audio output device according to claim 1, wherein the bridge circuit is a half-bridge circuit having one leg and the FETs arranged in upper and lower arms.

4. The audio output device according to any one of claims 1 to 3, characterized in that the gate drive circuit performs the switching operation of the FET under operating condition a2, in which the characteristics of the drain current Id and the drain-source voltage Vds of the FET are in a saturation region of pinch-off voltage Vp<Vds, where the increase in the drain current Id saturates as the drain-source voltage Vds increases.

5. A piezoelectric element that outputs sound by applying a voltage to the piezoelectric body and generating vibrations. a control unit that generates a PWM signal in response to an audio signal; a bipolar transistor as a switching element that performs a switching operation based on the PWM signal; a bridge circuit configured using the bipolar transistor in a lower arm; a DC power supply that applies a DC voltage to the bridge circuit; a gate drive circuit that applies a predetermined base current Ib to the bipolar transistor in response to the PWM signal, In the characteristic where the collector-emitter voltage Vce of the bipolar transistor decreases with an increase in the base current Ib, and the voltage Vb2 across the piezoelectric element changes to have a maximum value, Under operating conditions b1 in a region B1 near the maximum value of the voltage Vb2, the gate drive circuit performs the switching operation of the bipolar transistor, while An audio output device characterized in that a low-pass filter is formed by the resistor-like behavior between the collector and emitter of the bipolar transistor under the operating condition b1 and a capacitor equivalent to the piezoelectric element.

6. 6. The audio output device according to claim 5, wherein the bridge circuit is a full bridge circuit in which the bipolar transistors are arranged in upper and lower arms.

7. 6. The audio output device according to claim 5, wherein the bridge circuit is a half-bridge circuit having one leg and the bipolar transistors arranged in upper and lower arms.

8. The audio output device according to any one of claims 5 to 7, characterized in that the gate drive circuit performs the switching operation of the bipolar transistor in a region B2 near a collector-emitter saturation voltage Vce(sat) where an increase in the collector current Ic saturates as the collector-emitter voltage Vce increases, and under operating condition b2 where the active region is Vce(sat)<Vce, in the characteristics of the collector current Ic and the collector-emitter voltage Vce of the bipolar transistor.

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