Demodulated signal generator for air pulse generators

Abstract

To provide a demodulation signal generator.SOLUTION: A demodulation signal generator is coupled to an air-pulse generator comprising a flap pair, and includes a resonance circuit. The resonance circuit produces a first demodulation signal and a second demodulation signal. The resonance circuit and the flap pair co-perform a resonance operation, so that the first demodulation signal and the second demodulation signal are generated via the co-performed resonance operation and have opposite polarity. The flap pair performs a differential movement to form an opening to perform a demodulation operation on a modulated air pressure variation.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This application relates to a drive circuit or a demodulation signal generator. More specifically, it relates to a drive circuit or a demodulation signal generator that can drive a flap pair to perform differential movement and has low power consumption.

Background Art

[0002] Speaker drive devices and back speakers are two major design challenges in the speaker industry. For conventional speakers, it is difficult to cover the entire audible frequency band from, for example, 20 Hz to 20 kHz. To generate high-fidelity sound with a sufficiently high sound pressure level (SPL), both the radiating / moving surface of conventional speakers and the volume / size of the back speaker are required to be sufficiently large.

[0003] Ultrasonic air pulse generators have been studied to generate air pulses or sounds that overcome the design challenges faced by conventional speakers. For ultrasonic air pulse generators including capacitive actuators, high power consumption is expected when operating at ultrasonic rates, which is not desirable for portable or consumer electronic devices.

[0004] Therefore, how to design a drive circuit for driving an ultrasonic air pulse generator with low power consumption is an important goal in this field.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

[0006] [Non-Patent Document 1] TutorialsPoint, Analog Communication ‐ DSBSC Modulation", captured by web.archive.org on 11 / 26 / 2020, retrieved on 12 / 11 / 2023 from the Internet URL: https: / / web.archive.org / web / 20201126095635 / https: / / www.tutorialspoint.com / analog_communication / analog_communication_dsbsc_modulation.htm [Overview of the project]

[0007] The main objective of this application is therefore to provide a demodulated signal generator that improves upon the shortcomings of the prior art.

[0008] One embodiment of the present invention discloses a demodulation signal generator coupled to an air pulse generator. The demodulation signal generator includes a first node coupled to a first flap and a second node coupled to a second flap, and a resonant circuit coupled to the first and second nodes, the resonant circuit configured to generate a first demodulation signal at the first node and a second demodulation signal at the second node. The air pulse generator includes a membrane structure, the membrane structure includes a flap pair, the flap pair includes a first flap and a second flap. The resonant circuit and the flap pair perform a resonant operation together, and as a result of the resonant operation performed together, the first demodulation signal and the second demodulation signal are generated. The first demodulation signal and the second demodulation signal have opposite polarities. The first flap receives the first demodulation signal and the second flap receives the second demodulation signal, and as a result, the flap pair performs differential motion. The differential motion is configured to form an opening in order to perform a demodulation operation in response to modulated air pressure fluctuations generated by the membrane structure.

[0009] These and other objects of the present invention will undoubtedly become apparent to those skilled in the art after reading the following detailed description of preferred embodiments shown in various figures and drawings. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 shows a schematic diagram of an air pulse generator according to one embodiment of the present invention. [Figure 2] Figure 2 shows a wiring scheme according to one embodiment of the present invention. [Figure 3] Figure 3 shows a schematic diagram of a demodulated signal generator according to one embodiment of the present invention. [Figure 4] Figure 4 shows a schematic diagram of a demodulated signal generator according to one embodiment of the present invention. [Figure 5] Figure 5 shows the timing diagram of the demodulated signal generator in Figure 4. [Figure 6] Figure 6 shows a schematic diagram of a demodulated signal generator according to one embodiment of the present invention. [Figure 7] Figure 7 shows the timing diagram of the demodulated signal generator in Figure 6. [Figure 8] Figure 8 shows a schematic diagram of a demodulated signal generator according to one embodiment of the present invention. [Figure 9] Figure 9 shows a schematic diagram of a detection-and-control circuit according to one embodiment of the present invention. [Modes for carrying out the invention]

[0011] In this invention, the term “coupled to” may mean a direct or indirect connection. “Component A is coupled to component B” may indicate that component A is directly connected to component B, or that component A is connected to component B via component C.

[0012] The contents of U.S. Patent Application No. 18 / 321,757 are incorporated herein by reference.

[0013] Figure 1 shows an air-pulse generator (APG) 1 according to one embodiment of the present application. The APG 1 can be used for sound reproduction or cooling applications and includes a film structure 10. The film structure 10 is configured to perform a modulation operation to generate an ultrasonic acoustic / air wave (UAW) according to an acoustic signal SS, and to perform a demodulation operation to generate an ultrasonic pulse array (UPA) according to the ultrasonic acoustic / air wave (UAW). The modulation operation is performed via common-mode movement of the film structure 10, and the demodulation operation is performed via differential-mode movement of the film structure 10. After the inherent low-pass filtering effects of the natural / physical environment and the human auditory system, the sound corresponding to the acoustic signal SS is reproduced.

[0014] As taught in No. 18 / 321,757, the membrane structure 10 comprises a flap pair 102. The flap pair 102 is driven to perform common-mode motion to generate an ultrasonic acoustic / airwave UAW in order to perform a modulation operation. The flap pair 102 is also driven to perform differential-mode motion (or simply differential motion) to perform a demodulation operation to generate an ultrasonic sonic pulse array (UPA) at a pulse rate (e.g., 192 kHz) according to the ultrasonic acoustic / airwave UAW.

[0015] The flap pair 102 comprises a first flap 101 and a second flap 103. Both flaps 101 and 103 are connected to a modulation signal generator 16 to receive a modulation signal SM and are driven to perform common-mode motion as well as modulation operation. Meanwhile, flaps 101 and 103 are connected to a demodulation signal generator 14 to receive a first demodulation signal +SV and a second demodulation signal -SV, respectively. The demodulation signals +SV and -SV generally have opposite polarity with respect to a given level, so that flaps 101 and 103 can perform differential motion as well as demodulation operation. Specifically, the differential motion is configured to form an opening 112 (shown in Figure 2) to perform demodulation operation against the membrane structure 10 or the modulated air wave or pressure fluctuation UAW generated by the flap pair 102.

[0016] The modulated signal SM has a modulation frequency, which is the pulse rate (e.g., 192 kHz). The demodulated signal +SV / -SV has a demodulation frequency. Due to the differential motion of the flap pair, the demodulation frequency may be half the pulse rate or half the modulation frequency (e.g., 96 kHz).

[0017] A detailed wiring scheme between the APG and the (demodulated) modulation signal generator is shown in Figure 2, with schemes 131-133 being shown. The APG 1 includes a first actuator 101A located on flap 101 and a second actuator 103A located on second flap 103. The actuators 101A / 103A include an upper electrode and a lower electrode. In one embodiment, the actuators 101A / 103A also include a piezoelectric layer, which may be formed of PZT (capacitive lead zirconate titanate), sandwiched between the upper and lower electrodes. In one embodiment, the demodulated signal generator may be coupled to one electrode of the actuator 101A / 103A, and the modulation signal generator may be coupled to the other electrode of the actuator 101A / 103A. For example, the demodulated signal generator may be coupled to the upper electrode of the actuator 101A / 103A, and the modulation signal generator may be coupled to the lower electrode of the actuator 101A / 103A. In one embodiment, the modulation signal generator and the demodulation signal generator may be coupled to at least one electrode of the actuator 101A / 103A.

[0018] Figure 3 shows a schematic diagram of a demodulation signal generator 14 according to one embodiment of the present application. The demodulation signal generator can be considered as a drive circuit configured to drive a pair of flaps to perform differential mode motion and demodulation operation. In addition to having nodes N101 and N103 connected to flaps 101 and 103, respectively, the demodulation signal generator 14 generally comprises a resonant circuit 140. The demodulation signal generator 14 generates a demodulation signal +SV for flap 101 via node N101 and a demodulation signal -SV for flap 103 via node N103. The resonant circuit 140 and the flap pair 102 (flaps 101 and 103) perform resonance operation, resulting in the generation of demodulation signals +SV and -SV with opposite polarities.

[0019] For example, FIG. 4 shows a schematic diagram of the demodulation signal generator 24 according to an embodiment of the present application. The demodulation signal generator 24 includes a resonance circuit 240, nodes N101, N103, and switches SW 1H , SW 1L , SW 2H , SW 2L and. The resonance circuit 240 may be an alternating module 242 or may include an alternating module. The alternating module 242 includes an inductor L and a switching unit SW ER . In the embodiment shown in FIG. 4, the switching unit includes a switch SW ER1 . The alternating module 242 is connected between the node N101 and the node N103.

[0020] FIG. 5 shows a timing diagram of the demodulation signal generator 24. The alternating module 242 is performed during the conduction period, for example, T12 or T21 shown in FIG. 5. After the conduction period T12, the demodulation signal +SV alternates from the high voltage level V H to the low voltage level V L , and after the conduction period T12, the demodulation signal -SV alternates from the low voltage level V L to the high voltage level V H . Similarly, after the conduction period T21, the demodulation signal +SV alternates from the low voltage level V L to the high voltage level V H , and after the conduction period T21, the demodulation signal -SV alternates from the high voltage level V H to the low voltage level V L . Therefore, after the conduction period T12 / T21, the voltage levels of the first demodulation signal +SV and the second demodulation signal -SV alternate. On the other hand, the demodulation signal +SV and the demodulation signal -SV can be regarded as having opposite polarities.

[0021] In FIGS. 4 and 5, the switches SW 1H and SW 1L are connected to the node N101, while the switches SW 2H and SW 2LIt is connected to node N103. Switch SW 1H and SW 2H is high voltage V H And, the switch SW 1L and SW 2L Low voltage V L To receive.

[0022] During period T1 (before continuity period T12), switch SW 1H and SW 2L It is conducted / ON, and the switch SW 1L and SW 2H This is a cutoff / off state, and as a result, the demodulated signal +SV is a high voltage V H And the demodulated signal - SV is a low voltage V L During period T1, no current flows through the replacement module 242, and the switching section SW ER It is off.

[0023] During period T2 (after the continuity period T12), switch SW 1H and SW 2L It is off, and the switch SW 1L and SW 2H It is ON, and as a result the demodulated signal +SV is low voltage V L And the demodulated signal -SV is high voltage V H During period T2, no current flows through the replacement module 242, and the switching section SW ER It is off.

[0024] During the continuity period T12, switch SW 1H SW 1L SW 2H SW 2L It is off, and the switching section SW ERIt is turned on. A current is formed from node N101 to node N103, thereby decreasing the demodulated signal +SV and increasing the demodulated signal -SV. Consequently, the electrical energy stored in the capacitance corresponding to actuator 101A is transferred to the capacitance corresponding to actuator 103A.

[0025] During the continuity period T21, similarly, switch SW 1H SW 1L SW 2H SW 2L It is off, and the switching section SW ER It is ON. A current is formed from node N103 to node N101, thereby increasing the demodulated signal +SV and decreasing the demodulated signal -SV. Consequently, the electrical energy stored in the capacitance corresponding to actuator 103A is transferred to the capacitance corresponding to actuator 101A.

[0026] Switch (SW in Figure 4) 1H SW 1L SW 2H SW 2L , and SW ER1 The on-off operation of the demodulation signal generator 24 (which means that) allows the demodulation signal generator 24 to generate demodulated signals +SV and -SV having waveforms shown at the top of Figure 5. Note that the demodulation signals +SV and -SV applied to the capacitive actuators 101A and 103A have low power consumption by the flap pair 102 and the demodulation signal generator 24 because, with the help of the resonant circuit 240, energy is reused between N101 and N103, even though the APG has many transitions within the period T12 / T21 while the APG is operating.

[0027] By applying the demodulated signals +SV and -SV to flaps 101 and 103, the flap pair 102 can perform differential motion.

[0028] In this application, the flap pair performing differential motion either 1) during a transient state / period, one flap moves in a first direction and the other flap moves in a second direction opposite to the first direction, or 2) during a steady state / period, one flap bends upward and the other flap bends downward.

[0029] The differential motion resulting from the application of the demodulated signal generator 24 satisfies both of the above conditions 1) and 2). Specifically, during the steady-state period T1 shown in Figure 5, the flap 101 is subjected to a high voltage V H It is driven to bend upward upon receiving the demodulated signal +SV as a low voltage V, and the flap 103 is driven to bend upward. L It is driven to bend downward upon receiving the demodulated signal-SV as a signal. During the steady state period T2, the flap 101 is driven to bend downward at a low voltage V L It is driven to bend downward upon receiving the demodulated signal +SV as a signal, and the flap 103 is driven to bend downwards when the high voltage V H It is driven to bend upward upon receiving the demodulated signal -SV. During the transient period T12 between T1 and T2, flap 101 moves in the -Z direction, and flap 103 moves in the +Z direction opposite to the -Z direction.

[0030] Furthermore, during the conduction / transition / transition periods T12 / T21, the inductor L in the resonant circuit 240 and the capacitance of the piezoelectric layer in actuators 101A / 103A perform LC (inductance-capacitance) resonance. The LC resonance conducts current from N101 to N103 during period T12, and then conducts current from N103 to N101 during period T21.

[0031] Figure 6 shows a schematic diagram of a demodulated signal generator 34 according to one embodiment of the present application. The demodulated signal generator 34 is similar to the demodulated signal generator 24, and therefore, the same components are indicated by the same reference numerals. Unlike the demodulated signal generator 24, the switching unit SW ERThe switch SW connected between the inductor L and node N103 is located within the alternating module 342 or resonant circuit 340 of the demodulated signal generator 34. ER2 It also has the following features. Here, the switch SW ER1 and SW ER2 The switching section SW ER This is configured, and the inductor L and the switching section SW ER This constitutes the replacement module 342. In this application, the switching unit SW ER When it is ON, switch SW ER1 and SW ER2 This means that both are on, and the switching section SW ER The fact that it is off means that the switch SW ER1 and SW ER2 This means that at least one of them is off.

[0032] In one embodiment, switch SW ER1 / SW ER2 This can be realized by a MOSFET (metal-oxide-semiconductor field-effect transistor) with body diodes BD1 / BD2, as shown at the bottom of Figure 6. Switch SW ER1 / SW ER2 is a switch SW ER1 / SW ER2 When the switch SW is off, the body diodes BD1 / BD2 are configured to be able to interrupt the current. In one embodiment, the anodes of the body diodes BD1 / BD2 may be coupled to nodes N101 / N103, and the cathodes of the body diodes BD1 / BD2 may be coupled to inductor L. In this case, the body diodes BD1 / BD2 are connected to the switch SW ER1 / SW ER2 When this is turned off, it blocks the current flowing from inductor L to N101 / N103.

[0033] For the two - switches alternating module 342, it is not necessary to turn off both switches simultaneously at the end of the conduction period. Instead, if the body diodes are properly configured, it is sufficient to turn off only one switch at a time and at the end of the conduction periods T12 / T21. For example, at the end of the conduction period T12, when the demodulation signal +SV decreases and the demodulation signal -SV increases (or when the voltage at node N101 is smaller than the voltage at node N103 at the end of the conduction period T12), switch SW ER1 is turned off and switch SW ER2 can remain on. In this case, the body diode BD1 of the configuration shown in FIG. 6 blocks the current from the inductor to node N101. Similarly, at the end of the conduction period T21, switch SW ER2 is turned off and switch SW ER1 can remain on.

[0034] The timing of switches SW ER1 and SW ER2 can be referred to FIG. 7. Turning on one switch (either SW ER1 or SW ER2 ) at a time reduces the toggling rate of each switch and thereby can further reduce the power consumption of the switches in the alternating module 342.

[0035] Note that switches SW ER1 and SW ER2 are not limited to being arranged on both sides of the inductor L as shown in FIG. 6. In one embodiment, switches SW ER1 and SW ER2 can be arranged on the same side of the inductor L (not shown), where the body diodes BD1 / 2 can still block the current flowing through there when switch SW ER1 / 2 is off. Switches SW ER1 and SW ER2As long as it is coupled between nodes N101 and N103, it is within the scope of this application.

[0036] In addition to LC resonances used by the 240 / 340 resonant circuits, CMOS (complementary metal oxide semiconductor)-MEMS (microelectromechanical system) resonances / oscillations can also be used to generate the demodulated signal ±SV.

[0037] Figure 8 shows a schematic diagram of a demodulated signal generator 44 according to one embodiment of the present application. The demodulated signal generator 44 includes a resonant circuit 440, which may be a start-up circuit 442 or may include a start-up circuit. The resonant circuit 440 or the start-up circuit 442 is coupled to flaps 101 and 103 and utilizes the resonant characteristics of flaps 101 / 103 to generate a demodulated signal ±SV. This can be considered as performing resonant operation together with the flap pair 102. The start-up circuit 442 includes a transimpedance amplifier 444, a detection control circuit 446, and a variable gain amplifier (VGA) 448.

[0038] The transimpedance amplifier 444 includes a first input terminal coupled to flap 101 to receive current i+, and a second input terminal coupled to flap 103 to receive current i-. The transimpedance amplifier 444 generates an output signal Vo according to the currents i+ and i-. As shown in Figure 8, the transimpedance amplifier 444 includes an operational amplifier and a feedback resistor and feedback capacitor coupled between the input and output terminals.

[0039] The detection control circuit 446 is coupled to the output terminal of the transimpedance amplifier 444. The detection control circuit 446 is configured to perform a detection operation according to the signal Vo. Here, the detection operation may be amplitude detection, phase detection, frequency detection, or a combination thereof.

[0040] The detection control circuit 446 controls the VGA 448 within the closed loop 46 so that the activation circuit 442 satisfies the Barkhausen criterion: that is, the loop gain is greater than or equal to 1, and the loop phase is 0 or equal to an integer multiple of 2π. Thus, the flap pair 102 and the resonant circuit 440 co-perform CMOS-MEMS resonance / oscillation.

[0041] In one embodiment, the detection control circuit 446 may include an amplitude control circuit 4461 (shown in Figure 9), which in turn can control the VGA 448 to form an automatic gain control (AGC) loop to satisfy the Barkhausen criterion.

[0042] In one embodiment, the detection control circuit 446 may perform a frequency detection operation to track the resonant frequencies of the flaps 101 / 103. The frequency detection operation may be performed by applying a frequency-sweep test signal. In one embodiment, the detection control circuit 446 may include a phase-locked loop (PLL) circuit 4462 (shown in Figure 9). The PLL circuit 4462 is configured to perform a frequency detection operation to track the resonant frequencies of the flaps 101 / 103 or the flap pair 102.

[0043] In one embodiment, a loop filter (not shown in Figure 8) may be included to avoid unwanted oscillations.

[0044] The loop 46, formed by the flap pair 102 and the resonant circuit 440, performs self-sustaining oscillation. Under self-sustaining oscillation, the displacement of flaps 101 / 103 can be amplified by a factor of Q. Note that Q is the quality factor (Q) value of the mechanical resonant modes of 101 and 103. Therefore, to maintain the desired displacement, the input signal amplitudes of +SV and -SV can be reduced by a factor of Q, and the power can be reduced by Q. 2 It is reduced significantly, by a factor of two.

[0045] It should be noted that Figure 8 shows only one embodiment of the oscillation initiation circuit. The demodulated signals +SV and -SV may, but are not limited to, be generated according to any type of oscillation initiation circuit or frequency detection circuit. As long as resonance / oscillation is formed between the flap pair and the resonant circuit, it is within the scope of the present invention.

[0046] Furthermore, the demodulation signal generator 44 may include a phase shifter 41 coupled between flaps 101 and 103. The phase shifter 41 can be set to have a phase difference of 180° between the demodulation signal +SV and the demodulation signal -SV, so that the demodulation signal +SV and the demodulation signal -SV have opposite polarities. By applying the demodulation signals +SV and -SV to flaps 101 and 103, the flap pair 102 can perform differential motion. Here, the differential motion resulting from the application of the demodulation signal generator 44 satisfies condition 1) described in the paragraph above.

[0047] In addition, the demodulated signal generator 44 may include a frequency multiplier 43, which is coupled to a start circuit 442 and receives the output signal SV_out of the start circuit 442. The frequency multiplier 43 is configured to double the frequency of SV_out in order to generate an output signal SM_ref, thereby generating a modulated signal SM according to the signal SM_ref. Here, the frequency of the output signal SM_ref is twice the frequency of the output signal SV_out, and consequently, if the modulation frequency is the pulse rate, the demodulated frequency is half the pulse rate or half the modulation frequency.

[0048] Note that resonant operation is performed together by resonant circuits 240 / 340, and the flap pair utilizes the electrical resonance of the components there, while resonant operation is also performed together by resonant circuit 440, and the flap pair utilizes the mechanical resonance of the components there.

[0049] One of the greatest advantages of a demodulated signal generator that utilizes resonance to generate a demodulated signal ±SV is the reduction in power consumption. Resonant operation is not limited to the LC or CMOS-MEMS resonances described above. It is within the scope of this application that a demodulated signal generator with a resonant circuit may utilize any resonance to generate a demodulated signal generator of the opposite polarity.

[0050] In short, this application utilizes a resonant circuit to perform a resonant operation using a flap pair to generate a demodulated signal ±SV with opposite polarity. Here, the resonant operation may be an LC resonance or a CMOS-MEMS resonance satisfying Barkhausen.

[0051] Those skilled in the art will readily understand that numerous modifications and changes relating to the devices and methods can be made while maintaining the teachings of the present invention. Accordingly, the above disclosure should be construed as being limited only by the boundaries and bounds of the appended claims.

Claims

1. A demodulated signal generator coupled to an air pulse generator, A first node connected to the first flap, and a second node connected to the second flap, A resonant circuit, coupled to the first node and the second node, configured to generate a first demodulated signal at the first node and a second demodulated signal at the second node, Includes, The air pulse generator includes a membrane structure, the membrane structure includes a pair of flaps, and the pair of flaps includes a first flap and a second flap. The resonant circuit and the flap pair perform a resonant operation together, and as a result of the resonant operation performed together, the first demodulated signal and the second demodulated signal are generated. The first demodulated signal and the second demodulated signal have opposite polarities. The first flap receives the first demodulation signal, and the second flap receives the second demodulation signal, and as a result, the flap pair performs differential motion. The differential motion is configured to form an opening in order to perform a demodulation operation against modulated air pressure fluctuations generated by the membrane structure. The aforementioned resonant circuit includes a starting circuit, The resonance is generated by the activation circuit and the flap pair. Demodulated signal generator.

2. The aforementioned resonant circuit includes an alternating module, The aforementioned replacement module is configured to run during the conduction period, The first voltage level of the first demodulated signal and the second voltage level of the second demodulated signal alternate after the conduction period. The demodulated signal generator according to claim 1.

3. The aforementioned replacement module includes an inductor and a switching unit, coupled between the first node and the second node. The demodulated signal generator according to claim 2.

4. The demodulated signal generator includes a first switch, a second switch, a third switch, and a fourth switch. The first switch and the second switch are connected to the first node, and the third switch and the fourth switch are connected to the second node. The demodulated signal generator according to claim 3.

5. In the first period prior to the aforementioned conduction period, the first switch and the fourth switch are conductive, and the second switch and the third switch are closed. In the second period following the aforementioned conduction period, the first switch and the fourth switch are disconnected, and the second switch and the third switch are connected. The demodulated signal generator according to claim 4.

6. During the first or second period, the switching unit is shut off. The demodulated signal generator according to claim 5.

7. The first switch and the third switch receive the first voltage, The second switch and the fourth switch receive the second voltage. The demodulated signal generator according to claim 4.

8. The switching unit is A fifth switch and a sixth switch are connected between the first node and the second node, The demodulated signal generator according to claim 6.

9. During the aforementioned conduction period, the fifth switch and the sixth switch are made conductive. The demodulated signal generator according to claim 8.

10. The fifth switch includes a first body diode having a first anode coupled to the first node, The sixth switch includes a second body diode having a second anode coupled to the second node, During the conduction period, the first voltage at the first node decreases, and the second voltage at the second node increases. At the end of the conduction period, the first voltage is less than the second voltage, and the fifth switch is turned off. The demodulated signal generator according to claim 8.

11. During the first period, the sixth switch is shut off. During the second period, the fifth switch is shut off. The demodulated signal generator according to claim 8.

12. The aforementioned air pulse generator generates multiple air pulses at a pulse rate, The plurality of air pulses are generated according to the modulated air pressure fluctuations, The demodulation frequency of the first demodulated signal is half of the pulse rate. The demodulated signal generator according to claim 1.

13. The startup circuit includes a transimpedance amplifier, The transimpedance amplifier includes a first input terminal coupled to the first flap and a second input terminal coupled to the second flap. The transimpedance amplifier is configured to receive a first current corresponding to the first flap and a second current corresponding to the second flap. The transimpedance amplifier outputs a first output signal according to the first current and the second current. The first demodulated signal and the second demodulated signal are generated in accordance with the first output signal of the transimpedance amplifier. The demodulated signal generator according to claim 1.

14. The aforementioned startup circuit further, A detection control circuit, which is coupled to the output terminal of the transimpedance amplifier and configured to perform a detection operation in accordance with the first output signal of the transimpedance amplifier, A variable gain amplifier (VGA) is controlled by the detection control circuit, and the first demodulated signal is generated according to the second output signal of the variable gain amplifier. A demodulated signal generator according to claim 13, including the above.

15. The detection control circuit includes an amplitude control circuit, An automatic gain control (AGC) loop is formed by the amplitude control circuit and the VGA. The demodulated signal generator according to claim 14.

16. The detection control circuit includes a phase-locked loop (PLL) circuit. The PPL circuit is configured to perform a frequency detection operation in order to track the resonant frequency of the first flap. The demodulated signal generator according to claim 14.

17. The transimpedance amplifier includes an operational amplifier, The demodulated signal generator according to claim 13.

18. The transimpedance amplifier includes a feedback resistor coupled between the input terminal and output terminal of the operational amplifier. The demodulated signal generator according to claim 17.

19. The transimpedance amplifier includes a feedback capacitor coupled between the input terminal and output terminal of the operational amplifier. The demodulated signal generator according to claim 17.

20. The demodulated signal generator further, The phase shifter is coupled to the first flap and the second flap, The phase shifter sets the phase difference between the first demodulated signal and the second demodulated signal to 180°. The demodulated signal generator according to claim 1.

21. The demodulated signal generator further, The frequency multiplier includes a frequency multiplier which is coupled to the activation circuit and configured to receive a third output signal from the activation circuit and generate a fourth output signal, The frequency of the fourth output signal is twice the frequency of the third output signal. The modulated signal is generated according to the fourth output signal, The modulated signal is configured to drive the membrane structure, and as a result, the membrane structure generates the modulated air pressure fluctuations. The demodulated signal generator according to claim 1.

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