Circuit for sensor assembly, integrated circuit and microphone sensor assembly

Abstract

The present application relates to a circuit, an integrated circuit, and a microphone sensor assembly for a sensor assembly. The sensor assembly includes a capacitive sensor like a micro-electromechanical system microphone. The circuit includes: a first transistor having an input gate connectable to the capacitive sensor; a second transistor having an input gate connected to an output of the first transistor; a feed-forward circuit interconnecting a back gate of the second transistor with the output of the first transistor; and a filter circuit interconnecting the output of the first transistor with the input gate of the second transistor.

Description

Technical Field

[0001] The present disclosure relates generally to capacitive sensor assemblies (eg, MEMS microphones) and circuits for such sensor assemblies. Background Art

[0002] Hearing devices, such as hearing aids, true wireless stereo (TWS) earphones, and other hearables worn on or in the user's ears, typically employ one or more microphones and one or more sound-emitting acoustic transducers that convert the microphone signals into acoustic output signals. These and other applications require microphones with low power consumption, low noise, and low distortion. Capacitive microelectromechanical system (MEMS) microphones have replaced the electret microphones that were once primarily used in these and other applications because of their low cost, small size, and high sensitivity.

[0003] Capacitive microphones typically include a capacitive transducing element or motor (also referred to herein as a "capacitive sensor"), such as a MEMS die, coupled to a circuit that conditions the sensor signal before output to a host device such as a hearing device. The circuit typically includes a buffer amplifier for impedance matching and filters for frequency band shaping, as well as other components depending on the specific use case. Figure 2 A prior art two-stage microphone buffer circuit is illustrated, the circuit comprising a first CMOS transistor having a first CMOS transistor coupled to a capacitor motor C MOTOR and an output terminal V connected to the input terminal of the second CMOS transistor through a band shaping filter circuit. OUT1 The buffer circuit is the main source of power consumption and noise, and the signal-to-noise ratio (SNR) is limited by the current level and interstage filter components. Reducing the resistance of the filter will cause loading problems, such as slew rate or gain degradation.

[0004] The objects, features and advantages of the present disclosure will become more fully apparent to those of ordinary skill in the art upon careful consideration of the following detailed description and appended claims taken in conjunction with the following drawings. Summary of the invention

[0005] One aspect of the present invention relates to a circuit for a capacitive sensor assembly, the circuit comprising: a first transistor, the first transistor having an input gate connectable to a capacitive sensor; a second transistor, the second transistor having an input gate coupled to an output of the first transistor, the second transistor including a back gate; a feedforward circuit interconnecting the back gate of the second transistor with the output of the first transistor; and a filter circuit interconnecting the output of the first transistor and the input gate of the second transistor.

[0006] Another aspect of the present invention relates to an integrated circuit of a capacitive micro-electro-mechanical system microphone assembly, the integrated circuit comprising: a first transistor, the first transistor being configured as a source follower, the source follower comprising: an input gate, the input gate being connectable to the output of a capacitive micro-electro-mechanical system die; and a source, the source being connected to the output of the first transistor; a second transistor, the second transistor being configured as a source follower, the source follower comprising: an input gate, the input gate being connected to the output of the first transistor; a source, the source being connected to the output of the second transistor; and a back gate, the back gate being electrically connected to the output of the first transistor; an audio band filter circuit, the audio band filter circuit interconnecting the output of the first transistor and the input gate of the second transistor; a DC bias circuit, the DC bias circuit having a reference signal output, the reference signal output being connected to the input gate of the first transistor via a resistor component, the reference signal output of the DC bias circuit also being connected to the input gate of the second transistor via a resistor component, the integrated circuit having an input capacitance of 0.1 pF.

[0007] Yet another aspect of the present invention relates to a microphone sensor assembly, the microphone sensor assembly comprising: a housing having an external device interface and an acoustic port; a capacitive microelectromechanical system die, the capacitive microelectromechanical system die being disposed in the housing and acoustically coupled to the acoustic port; an integrated circuit, the integrated circuit being disposed in the housing and electrically coupled to contacts on the external device interface, the integrated circuit comprising: a first transistor having an input gate coupled to a first node of the capacitive microelectromechanical system die; a second transistor having an input gate coupled to an output terminal of the first transistor, the second transistor comprising a back gate; a feedforward circuit interconnecting the back gate of the second transistor with the output terminal of the first transistor; a filter circuit interconnecting the output terminal of the first transistor and the input gate of the second transistor; and a microelectromechanical system die bias circuit, the microelectromechanical system die bias circuit being coupled to a second node of the capacitive microelectromechanical system die. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 is a cross-sectional view of a generic capacitive sensor assembly.

[0009] Figure 2 is a prior art circuit diagram including a two-stage buffer amplifier combined with a capacitive sensor.

[0010] Figure 3 is a circuit diagram including a two-stage buffer amplifier combined with a capacitive sensor.

[0011] Figure 4 is an alternative circuit diagram including a two-stage buffer amplifier combined with a capacitive sensor.

[0012] Figure 5 is a qualitative illustration of the improvement in noise performance of a microphone using the circuit described herein compared to the prior art. DETAILED DESCRIPTION

[0013] The present disclosure generally relates to circuits for sensor assemblies including capacitive sensors. Such assemblies include microphones, vibration sensors, and pressure sensors, among other sensor assemblies. The capacitive sensor may be a micro-electromechanical system (MEMS) die, an electret, or as some other capacitive transducing component. The capacitance of a MEMS capacitive sensor is typically between 0.1 pF and 5.0 pF, but this range is only approximate, and other capacitive sensors may have more or less capacitance.

[0014] Figure 1The present invention is a universal sensor assembly 100 including a capacitive sensor 110 electrically coupled to a circuit 120, the circuit and the capacitive sensor are disposed in a housing 130, the housing including a cover 132 mounted on a base 134, the base having an external device interface having contacts 137 electrically coupled to the circuit. The external device interface may be a surface mount interface (e.g., Figure 1 ), or some other known or future interface technology for integration with a host device. Microphone and other sensor components include an acoustic port 138 located on the base or lid, where the acoustic port is acoustically coupled to a capacitive sensor as is generally known. However, a vibration sensor component, etc. may not have an acoustic port.

[0015] The circuit generally includes a two-stage buffer amplifier having an input terminal connected to the capacitive sensor and other components. For example, the circuit may also include a charge pump that can be connected to the capacitive sensor, and the charge pump is used for applications where the sensor requires a DC bias. Some capacitive sensors (such as electrets) do not require a bias, so some circuits do not include a charge pump. Depending on the intended application, the circuit may also include other circuit components not shown in the figure such as band shaping filters. The circuit in the digital sensor may include a delta-sigma analog-to-digital (A / D) converter circuit connected to the output terminal of the buffer amplifier, and the circuit in the microphone assembly may include a protocol interface circuit that formats the output signal for a specific protocol (such as PCM, PDM, SoundWire, etc.). The circuit may be one or more integrated circuits, such as an application specific integrated circuit (ASIC). In a typical sensor assembly, the circuit generally includes a power supply, a ground, an output terminal, and may also have other contacts, which may be connected to corresponding contacts on an external device interface by wire welding or other known or future interconnection techniques.

[0016] Figure 3 and Figure 4 A circuit 300 in combination with a capacitive sensor 302 is shown, wherein the circuit includes a two-stage buffer amplifier including a first transistor 310 and a second transistor 320. The first transistor includes an input gate 312 coupled to the capacitive sensor 302 and an output terminal V coupled to the second transistor. OUT1 , as further described herein. The first transistor and the second resistor may be transistors such as MOSFETs or JFETs. Figure 3 and Figure 4 In the embodiment, the first transistor is configured as a source-follower (also known as a common drain topology), wherein the source is connected to the output terminal V OUT1. The second transistor is similarly configured. In other implementations, however, one or both of the first transistor and the second transistor can be configured in different circuit topologies. Alternatively, the first transistor can be an operational amplifier and the second transistor can be an amplifier, such as a class AB source follower, etc. When the input capacitance of the buffer amplifier is less than the capacitance of the capacitive sensor, the attenuation of the input signal can be reduced. In one implementation, the buffer amplifier has an input capacitance on the order of 0.1 pF (e.g., 0.2 pF).

[0017] According to another aspect of the present disclosure, generally speaking, a first transistor of a two-stage buffer amplifier is coupled to a second transistor via a band-shaping filter circuit and a feed-forward circuit. In microphone applications, the filter circuit is an audio band filter. Figure 3 and Figure 4 In the example, the band-shaping filter circuit is a bandpass filter 330, which converts the output terminal V OUT1 Interconnected to the input gate 322 of the second transistor. Other filter circuits (eg, low pass filter, high pass filter, notch filter) may alternatively be used depending on the use case.

[0018] Figure 3 and Figure 4 A second transistor is also shown, which includes an output terminal V coupled to the first transistor through a feed-forward circuit 340. OUT1 In one implementation, the feed-forward circuit is a circuit between the back gate 324 of the second transistor and the output terminal V OUT1 Alternatively, the feedforward circuit may include a filter circuit or a signal processing component located between the back gate 324 of the second transistor and the output terminal V OUT1 between.

[0019] Biasing the second stage output buffer to the first stage output reduces the noise injected into the output at the passband frequency because the noise from the bandpass filter circuit is not included in the control path. The roll-off or slope of the band-shaping filter depends on the ratio of the transconductance (gm) of the metal / oxide / channel to the transconductance (gmbs) of the bulk / channel. The transconductance gm is generally much higher than the transconductance gmbs, so the overall transfer function can have a roll-off or slope above and below the corner frequency of the band-shaping filter.

[0020] The circuit may optionally include other circuit components at the input of the buffer amplifier. Figure 3 and Figure 4In, for example, a resistive component 316 including a reverse diode is connected to an input gate 312 of a buffer amplifier. Alternatively, the resistive component can be a reverse transistor or a conventional resistor. The resistive component combined with the capacitance of the capacitive sensor forms a high-pass filter. The corner frequency or cut-off frequency of the filter can be set by properly selecting the capacitance and resistance values ​​of these circuit components. For audio applications, the corner frequency can be set at the lower end of the human audible frequency range. In a microphone implementation in which the capacitance of the capacitive sensor is in the order of picofarads (pF), a resistance in the order of gigaohms will filter low-frequency noise. The corner frequency of other types of sensors will depend on the use case. For example, an ultrasonic sensor will have a higher corner frequency than an audio sensor, while a vibration sensor will have a lower cut-off frequency.

[0021] In implementations where the capacitive sensor requires a bias voltage, the circuit may also include a sensor bias circuit, such as a charge pump. Figure 3 and Figure 4 In FIG. 3 , a charge pump 314 is coupled to one node of the capacitive sensor 302, and another node of the sensor is coupled to the input 312 of the buffer amplifier. Capacitive sensors that do not require a bias do not require a charge pump.

[0022] According to another aspect of the present disclosure, the circuit further includes a DC bias circuit coupled to the buffer amplifier. Among other benefits, the DC bias circuit also improves the dynamic range of the circuit. Figure 4 , the DC bias circuit 350 includes a comparator 352 having a bias signal output 354 coupled to the input gate 312 of the first transistor through a resistive component 316. The bias signal output 354 is also coupled to the input gate 322 of the second transistor through a resistor 332. In this embodiment, the resistor 332 is also part of the filter circuit 330.

[0023] Figure 5 Demonstrates the implementation Figure 4 The noise performance of a condenser microphone with a circuit of FIG. 1 across the entire audio band from about 100 Hz to 10 kHz. The noise is characterized as a sound pressure level (SPL) in decibels (dB). The bar graph shows the improved noise performance across the audio band, with relatively consistent improvements from about 100 Hz to about 5 kHz and slightly declining improvements above 5 kHz. The noise improvement at higher frequencies is slightly less due to high frequency acoustic dominance of the noise. The high frequency noise improvement will track the noise improvement at lower frequencies in a microphone with non-acoustic (i.e., electronic) dominant high frequency noise. The relative noise improvement across most frequency bands is 1 dB to 2 dB, depending on the ratio of acoustic noise to electronic noise contributions.

[0024] According to one embodiment, the present disclosure relates to a circuit of a capacitive sensor assembly, the circuit comprising: a first transistor having an input gate connectable to a capacitive sensor; a second transistor having an input gate connected to an output of the first transistor; a feedforward circuit interconnecting a back gate of the second transistor with the output of the first transistor; and a filter circuit interconnecting the output of the first transistor with the input gate of the second transistor. The circuit can be used in combination with a capacitive microelectromechanical system (MEMS) die or other capacitive transducing components.

[0025] According to another embodiment, the present disclosure relates to an integrated circuit of a capacitive microelectromechanical system (MEMS) microphone assembly. The circuit includes: a first transistor, the first transistor is configured as a source follower, the source follower includes an input gate that can be connected to the output of the capacitive MEMS die, and a source connected to the output of the first transistor; a second transistor, the second transistor is configured as a source follower, the source follower includes an input gate connected to the output of the first transistor, a source connected to the output of the second transistor, and a back gate electrically connected to the output of the first transistor; an audio band filter circuit, the audio band filter circuit interconnects the output of the first transistor and the input gate of the second transistor; a DC bias circuit, the DC bias circuit has a reference signal output, the reference signal output is connected to the input gate of the first transistor through a resistor component, and the reference signal output of the DC bias circuit is also connected to the input gate of the second transistor through a resistor component. In an audio sensor (e.g., microphone) application, the transducer component has a capacitance between about 0.1 pF and about 5.0 pF, and the input capacitance of the circuit is on the order of 0.1 pF.

[0026] According to another embodiment, the present disclosure relates to a microphone sensor assembly, which includes a capacitive microelectromechanical system (MEMS) die and an integrated circuit disposed in a housing. The MEMS die is acoustically coupled to an acoustic port of the housing, and the integrated circuit is electrically coupled to contacts on an external device interface. The circuit includes: a first transistor having an input gate coupled to a first node of the capacitive MEMS die; a second transistor having an input gate coupled to an output of the first transistor, the second transistor including a back gate; a feedforward circuit interconnecting the back gate of the second transistor with the output of the first transistor; a filter circuit interconnecting the output of the first transistor and the input gate of the second transistor; and a MEMS die bias circuit coupled to a second node of the capacitive MEMS die.

[0027] In one implementation of any of these embodiments, the feed-forward circuit may be a direct electrical connection between the back gate of the second transistor and the output of the first transistor. Alternatively, the feed-forward circuit may include a filter circuit or a processing circuit between the back gate of the second transistor and the output of the first transistor.

[0028] In some implementations of any of these embodiments, the first transistor is configured as a source follower that includes a source connected to the output terminal of the first transistor, and the second transistor is configured as a source follower that includes a source connected to the output terminal of the second transistor.

[0029] In some implementations of any of these embodiments, the circuit includes a DC bias circuit having a reference signal output terminal, which is connected to the input gate of the first transistor through a resistance component, and the reference signal output terminal of the DC bias circuit is also connected to the input gate of the second transistor through a resistance component.

[0030] While the present disclosure and what is presently considered to be the best mode thereof has been described in a manner that identifies the inventors and enables one of ordinary skill in the art to make and use the same, it should be understood and appreciated that there are equivalents to the exemplary embodiments disclosed herein and that various modifications and variations may be made to these embodiments within the scope and spirit of the present disclosure, and that these modifications and variations are not limited to the exemplary embodiments described, but rather are limited by the appended claims.

Claims

1. A circuit for a capacitive sensor assembly, the circuit include: a first transistor having an input gate connectable to a capacitive sensor; a second transistor having an input gate coupled to an output terminal of the first transistor, the second transistor including a back gate; a feed-forward circuit interconnecting the back gate of the second transistor and the output terminal of the first transistor; as well as a filter circuit interconnecting the output terminal of the first transistor and the input gate of the second transistor, Wherein, the feedforward circuit bypasses the filter circuit.

2. The circuit for a capacitive sensor assembly according to claim 1, in, The feed-forward circuit is a direct electrical connection between the back gate of the second transistor and the output terminal of the first transistor.

3. The circuit for a capacitive sensor assembly according to claim 1, in, The filter circuit is an audio bandpass filter.

4. The circuit for a capacitive sensor assembly according to claim 1, The first transistor is configured as a source follower including a source coupled to the output terminal of the first transistor, The second transistor is configured as a source follower including a source coupled to an output terminal of the second transistor.

5. A circuit for a capacitive sensor assembly according to claim 4, the circuit having an input capacitance in the order of 0.1 pF.

6. The circuit for a capacitive sensor assembly according to claim 5, wherein the circuit further comprises: include: a sensor bias circuit, the sensor bias circuit being connectable to the capacitive sensor; and a DC bias circuit having a reference signal output terminal, the reference signal output terminal being coupled to an input gate of the first transistor through a resistance component, and the reference signal output terminal being further coupled to an input gate of the second transistor through a resistance component. 7 . The circuit for a capacitive sensor assembly of claim 6 , the first transistor comprising a back gate coupled to a source of the first transistor.

8. The circuit for a capacitive sensor assembly according to claim 6, in, The filter circuit is an audio band filter.

9. The circuit for a capacitive sensor assembly of claim 8, the circuit being an integrated circuit die.

10. The circuit for a capacitive sensor assembly of claim 9, the circuit in combination with a capacitive MEMS die having a capacitance between 0.1 pF and 5.0 pF.

11. An integrated circuit for a capacitive micro-electromechanical system microphone assembly, the integrated circuit include: A first transistor configured as a source follower, the source follower comprising: an input gate connectable to an output terminal of the capacitive MEMS die; and a source connected to an output terminal of the first transistor; a second transistor configured as a source follower, the source follower comprising: an input gate coupled to the output terminal of the first transistor; a source coupled to the output terminal of the second transistor; and a back gate electrically coupled to the output terminal of the first transistor; an audio band filter circuit interconnecting an output terminal of the first transistor and an input gate of the second transistor; a DC bias circuit, the DC bias circuit having a reference signal output terminal, the reference signal output terminal being connected to the input gate of the first transistor through a resistance component, the reference signal output terminal of the DC bias circuit being further connected to the input gate of the second transistor through a resistance component, The integrated circuit has an input capacitance in the order of 0.1 pF.

12. The integrated circuit of the capacitive MEMS microphone assembly according to claim 11, in, The back gate of the second transistor is directly coupled to the output terminal of the first transistor.

13. The integrated circuit of the capacitive MEMS microphone assembly of claim 11, the integrated circuit combined with the capacitive MEMS die having a capacitance between 0.1 pF and 5.0 pF.

14. A microphone sensor assembly, the microphone sensor assembly include: a housing having an external device interface and an acoustic port; a capacitive microelectromechanical system die disposed in the housing and acoustically coupled to the acoustic port; an integrated circuit disposed in the housing and electrically coupled to contacts on the external device interface, the integrated circuit comprising: a first transistor having an input gate coupled to a first node of the capacitive microelectromechanical system die; a second transistor having an input gate coupled to an output terminal of the first transistor, the second transistor including a back gate; a feed-forward circuit interconnecting the back gate of the second transistor with an output terminal of the first transistor; a filter circuit interconnecting an output terminal of the first transistor and an input gate of the second transistor; and a MEMS die bias circuit coupled to a second node of the capacitive MEMS die, Wherein, the feedforward circuit bypasses the filter circuit.

15. The microphone sensor assembly according to claim 14, in, The feed-forward circuit is a direct electrical connection between the back gate of the second transistor and the output of the first transistor.

16. The microphone sensor assembly according to claim 14, The first transistor is configured as a source follower including a source coupled to an output terminal of the first transistor, The second transistor is configured as a source follower including a source coupled to an output terminal of the second transistor.

17. The microphone sensor assembly of claim 16, the capacitive MEMS die having a capacitance between 0.1 pF and 5.0 pF.

18. The microphone sensor assembly according to claim 17 further comprises a DC bias circuit, wherein the DC bias circuit has a reference signal output terminal, wherein the reference signal output terminal is connected to the input gate of the first transistor through a resistance component, and the reference signal output terminal of the DC bias circuit is also connected to the input gate of the second transistor through a resistance component.

19. The microphone sensor assembly of claim 18, the microphone sensor assembly being a microphone and the filter circuit being an audio band filter.

20. The microphone sensor assembly of claim 19, the feed-forward circuit being a direct electrical connection between the back gate of the second transistor and an output of the first transistor.

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