WO2018106514A1

WO2018106514A1 - High impedance bias for microphones

Info

Publication number: WO2018106514A1
Application number: PCT/US2017/064037
Authority: WO
Inventors: Ben HOOMES; Andrew Fitting; Patricia Lee
Original assignee: Knowles Electronics, Llc
Priority date: 2016-12-05
Filing date: 2017-11-30
Publication date: 2018-06-14

Abstract

This disclosure provides methods, systems, and apparatuses, for a microelectromechanical (MEMS) acoustic microphone circuit. In particular, the circuit includes a high pass filter formed by a combination of a capacitive microphone and a bias resistor. The bias resistor can include a switched-capacitor resistor. The switched-capacitor resistor includes at least one capacitor and at least two switches switched at a switching frequency. The switched- capacitor resistor can include multiple stages, where each stage includes at least one capacitor and at least one switch. The cut-off frequency of the high-pass filter is a function of the number of stages. The cut-off frequency is also a function of the switching frequency.

Description

HIGH IMPEDANCE BIAS FOR MICROPHONES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/429,990, filed December 5, 2016, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

[0002] The present disclosure relates generally to microphones, and more particularly to biasing networks and noise reducing circuits for microphones.

BACKGROUND

[0003] The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art.

[0004] Several audio sensing applications include capacitive microphones, the capacitance of which changes as a function of incident audio energy. The capacitive microphones are appropriately biased such that the change in capacitance can be transformed into a

corresponding change in voltage or current associated with the microphone. In some instances, additional circuit elements such as resistors, filters, and amplifiers are used to process the voltage or current. The frequency response of the microphone can be based on the values and characteristics of the circuit elements selected. In some implementations, the values of the circuit elements can be extremely large for a desired frequency response of the microphone, such as a frequency response within the audible range. Circuit elements with extremely large values may be difficult to reliably realize in some process technologies.

Moreover, even if the desired values of the circuit elements were available, they may not provide the flexibility to dynamically change the frequency response of the microphone during operation to adapt to changing noise frequencies. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several embodiments in accordance with the disclosure and are, therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings.

[0006] Figure 1 shows a cross-sectional representation of an example of a micro-electromechanical systems (MEMS) transducer device in accordance with various implementations.

[0007] Figure 2 shows a representation of an electronic circuit used with a MEMS microphone in accordance with various implementations.

[0008] Figure 3 shows an representation of a frequency response of a high-pass filter formed by a MEMS microphone and a biasing resistor shown in Figure 2 in accordance with various implementations.

[0009] Figure 4 depicts an example switched capacitor resistor in accordance with various implementations.

[0010] Figure 5 shows control signals for controlling the states of the switches in the switched capacitor resistor shown in Figure 4 in accordance with various implementations.

[0011] Figure 6 depicts an example multi-stage switched capacitor resistor in accordance with various implementations.

[0012] Figure 7 shows another multi-stage switched capacitor resistor in accordance with various implementations.

[0013] Figure 8 shows yet another multi-stage switched capacitor resistor in accordance with various implementations.

[0014] Figure 9A shows a representation of an electronic circuit used with a MEMS microphone in accordance with various implementations. [0015] Figure 9B shows a representation of the change in the frequency response of a high- pass filter formed by a MEMS microphone and a switched capacitor resistor shown in Figure 9A in accordance with various implementations.

[0016] In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure.

DETAILED DESCRIPTION

[0017] The present disclosure describes devices and techniques to provide a high-pass filter in electronic circuits for use with capacitor microphones. In one or more embodiments, the electronic circuits include one or more bias resistors that set a cut-off frequency of the high- pass filter formed with the combination of the bias resistor and the capacitive microphone. In some embodiments, the bias resistors set a DC operating configuration for the microphone. The capacitive microphone can be implemented using MEMS technology or using an electret material. In other embodiments, other capacitive microphones technologies may be used.

[0018] In one or more embodiments, the bias resistor can be implemented using switched- capacitor resistors. The switched-capacitor resistor can include a combination of switches and capacitors having an effective resistance that is substantially equal to the desired bias resistance.

[0019] In one or more embodiments, multi-stage switched capacitor resistors can be used to implement the bias resistor. The multi-stage switched capacitor resistors can provide reduced sensitivity to parasitic effects by allowing the use of a relatively larger capacitor for a given value of effective resistance than that allowed by a single stage switched capacitor resistor. [0020] In one or more embodiments, the switching frequency of the switched capacitor resistor can be altered to change a cut-off frequency of the high-pass filter formed by the switched-capacitor resistor and the condenser microphone. Dynamically altering the cut-off frequency can enable in-operation mitigation of noise signals. For example, in situations such as in windy conditions or in musical concert venues, sound pressures at low frequencies can be very high, resulting in loss of desirable audio signal. Such loss may be attributable to saturation of analog front-end circuits, like an amplifier driven by the output of the microphone. By dynamically altering the cut-off frequency of the high pass filter to be above the frequencies at which undesired noise is generated, the saturation of the front-end circuits, and thereby loss of the desirable audio signals, can be avoided.

[0021] Figure 1 shows a cross-sectional representation of an example of a micro-electromechanical systems (MEMS) transducer device 100. The MEMS transducer device 100 includes a printed circuit board (PCB) 102, a substrate 104, a MEMS transducer 106, and a housing 150. While the following discusses the MEMS transducer device 100 including a MEMS transducer 106, it is understood that the transducer device 100 could instead include a non-MEMS transducer such as a capacitor based microphone (e.g., an electret microphone) or other capacitive sensors. The PCB 102 includes several interconnects 108 that carry electrical signals between various components (not shown) mounted on the PCB 102. For example, the interconnects 108 may carry signals between the MEMS transducer 106 and one or more components such as, for example, power supplies, capacitors, resistors, inductors, transistors, diodes, controllers, and processors. In one or more embodiments, the PCB 102 can be a multi-layer PCB having two or more planes of conductive materials patterned into

interconnects 108. One or more planes of conductive material of the PCB 102 can be connected to a supply voltage such as VDD or G D to provide a supply plane. The conductive materials are separated by non-conductive layers, which in some embodiments can include FR-4 material. In one or more embodiments, the conductive materials can include copper, aluminum, silver, or various metal alloys. The multi-layer PCB 102 permits the inclusion of circuitry, solder pads, ground pads, capacitance layers, and plated through holes integrated onto the single structure. The PCB 102 also defines a PCB opening 110 to allow sound energy to propagate to the MEMS transducer 106. [0022] The substrate 104 provides support and connectivity to the MEMS transducer 106. In one or more embodiments, the substrate 104 can be formed of various types of

semiconductor materials such as, but not limited to, silicon (Si), gallium arsenide (GaAs) and indium phosphide (InP). The substrate 104 defines a substrate opening 112, which is aligned with the PCB opening 110. The substrate opening 112 provides a path for sound energy to propagate to the MEMS transducer 106. In one or more embodiments, the substrate opening 112 may be formed by etching the bottom surface of the substrate 104 using etching processes such as wet etching or dry etching, although other suitable techniques may also be employed. The substrate opening 112 along with the PCB opening 110 define a bottom port of the MEMS transducer device 100 through which sound energy can propagate towards the MEMS transducer 106. In one or more embodiments, the MEMS transducer device 100 may use a front port, which is formed on the housing 150 over the MEMS transducer 106. In some such embodiments, the PCB 102 may be devoid of the PCB opening 110, and the substrate opening 112 forms a back volume. The back volume provides a pressure reference with which the pressure of the incident sound energy is measured. A binding layer 113 binds the substrate 104 to the PCB 102. In one or more embodiments, in which the substrate 104 is formed using a semiconductor material, the semiconductor material can be doped p-type or n-type. In one or more embodiments, the substrate 104 can be used to carry electrical signals for one or more components of the MEMS transducer device 100. For example, the substrate 104 can be electrically coupled to the diaphragm 114 and to a bonding pad, which provides electrical voltage and current to the diaphragm 114.

[0023] The MEMS transducer 106 includes a diaphragm 114, a support structure 116 and a back plate 118 supported over the diaphragm 114 by the support structure 116. The diaphragm 114 includes a conductive material, which, in one or more embodiments, can include copper, aluminum, or a metal alloy. In one or more embodiments, the diaphragm 114 also can include in insulating layer deposited on the conductive layer. The diaphragm 114 shape and dimensions, such as thickness, length, width, and/or diameter, can be selected based on desired frequency sensitivity. For example, the shape and dimensions of the diaphragm 114 can be selected such that a resonance frequency of the diaphragm is in the desired frequency range, such as, for example the audible frequency range and the ultrasonic frequency range. The support structure 116 is formed on the substrate 104 establishes a gap between the diaphragm 114 and the back plate 118. In one or more embodiments, the support structure 116 can be formed using a semiconductor-nitride material. The back plate 118 is shaped and sized to generally cover the diaphragm 114. The back plate 118 also includes several vents 120 to allow for air movement across the back plate 118. The backplate further includes a conductive layer 122 deposited on the side of the back plate 118 that faces the diaphragm 114. In one or more embodiments, the conductive layer 122 can be formed using metals such as copper, aluminum, or metal alloys. Motion limiting posts 124 restrict the movement of the diaphragm 114 towards the back plate 118 when the diaphragm 114 responds to incident sound energy. In one or more embodiments, the posts 124 can be distributed along the perimeter of the back plate 118.

[0024] The diaphragm 114 and the conductive layer 122 on the back plate 118 form a capacitor, the value of which is altered by the movement of the diaphragm 114 in response to sound energy. This change in the capacitance is measured to estimate the characteristics of the incident sound energy.

[0025] The housing 150 covers the entire MEMS transducer 106. In one or more embodiments, the housing 150 also can cover one or more electronic components that are disposed over the substrate. The housing 150 can be a bowl-shaped or cup-shaped structure that is inverted and affixed to the substrate 104. In one or more embodiments, the housing 150 can be formed using one or more materials such as, for example, metal, plastic, and rubber. As mentioned above, in one or more embodiments, the MEMS transducer device 100 may include a front port defined by the housing 150, instead of the bottom port defined by the substrate 104 and the PCB 102.

[0026] As discussed further below, the MEMS transducer device 100 is coupled to electrical circuitry that provides power to and processes electrical signals received from the MEMS transducer 106. In one or more embodiments, the electrical circuitry can include a power supply, such as a charge-pump or a battery, to provide voltage and current to the diaphragm 114, and voltage or current sensing circuitry coupled to the conductive layer 122 on the back plate 118. The electrical circuity also can include signal conditioning circuits, amplifiers, and digital processors for processing the sensed signals. [0027] During operation, sound energy enters the MEMS transducer device 100 via the back port defined by the PCB opening 110 and the substrate opening 112. The diaphragm 114 responds to the incident sound energy by moving in relation to the conductive layer 122. The resulting change in capacitance causes a change in voltage and/or current at the conductive layer 122, which change is detected by the electrical circuitry. In one or more embodiments, the MEMS transducer 106 can also be operated as a transmitter, where electrical signals received by the diaphragm 114 are transformed into corresponding pressure variations, which result in an audio signal.

[0028] Figure 2 shows a representation of an electronic circuit 200 used with a capacitor microphone. The electronic circuit 200 includes a condenser microphone 202, a bias resistor 204, an amplifier 206, and a signal processor 208. The microphone 202 can include any condenser transducer that can convert acoustic energy into an electrical signal. For example, in one or more embodiments, the MEMS transducer 106 discussed above in relation to Figure 1 or any other condenser transducer can be used to implement the microphone 202. The microphone 202 is represented as a variable capacitor with two terminals, one of which is coupled to a MEMS voltage supply V_MEMS, and the other is coupled to an input node 210. As discussed above in relation to the MEMS transducer 106, the diaphragm 114 and the conductive layer 122 form a capacitor, the value of which changes in response to incident acoustic energy. In one or more embodiments, the diaphragm of the microphone 202 can be electrically connected to the MEMS voltage supply V_MEMS- In some other embodiments, the conductive layer, instead of the diaphragm, can be electrically connected to the voltage supply V_MEMS- In one or more embodiments, the voltage V_MEMS can in the range of tens of volts, e.g., about 20 V to about 60 V, or about 40 V.

[0029] The varying capacitance of the microphone 202, in response to incident acoustic signals, causes a corresponding change in the impedance of the microphone 202, which, in turn, causes a change in the magnitude of current flow from the MEMS voltage supply V_MEMS to the input node 210. This change in the magnitude of current flow results in a

corresponding change in the voltage Vi_n at the input node 210.

[0030] The biasing resistor 204 is coupled between a biasing voltage supply V_BIAS and the input node 210. In one or more embodiments, the magnitude of the biasing voltage supply V_BIAS is less than the magnitude of the MEMS voltage supply V_MEMS- The biasing resistor 204 is used to set a DC operating point for the MEMS microphone 202. In addition, the biasing resistor 204, in combination with the capacitance of the MEMS microphone 202, contributes in setting a cut-off frequency of a high-pass filter formed by the biasing resistor 204 and the MEMS microphone 202. The high-pass filter attenuates the signal Vi_n provided to the amplifier 206 based on the magnitude of the capacitance of the MEMS microphone and the magnitude R_BIAS of the bias resistor 204. The frequency response of the high-pass filter is discussed further below in relation to Figure 3. In one or more embodiments, the biasing voltage supply V_BIAS can provide a voltage that is less than the voltage provided by the MEMS voltage supply V_MEMS- In one or more embodiments, the voltage provided by the biasing voltage supply V_BIAS can be of the order of a few millivolts, or about 0 V to about 500 mV, or about 250 mV.

[0031] The amplifier 206 amplifies the voltage Vi_n at the input node 210. In one or more embodiments, the amplifier 206 can include one or more of an operational amplifier (op- amp), a transconductance amplifier, a high gain amplifier, a high input impedance amplifier, a voltage follower, and a differential amplifier. In one or more embodiments, the amplifier 206 can be implemented using discrete components, such as, for example, transistors, resistors, capacitors, inductors, and diodes. In one or more embodiments, the amplifier 206 can be implemented in an integrated circuit. In some embodiments, the amplifier 206 can be disposed on the substrate 104 shown in Figure 1. For example, the amplifier 206 can be soldered to the substrate 104. In some other implementations, the amplifier 206 can be fabricated on the substrate 104. The output of the amplifier 206 is provided to the signal processor 208.

[0032] The signal processor 208 processes the output signal of the amplifier 206. In one or more embodiments, the signal processor can include a digital signal processor 208 for digitizing and processing the output signal of the amplifier 206. For example, in some such embodiments, the signal processor 208 includes an analog to digital converter (ADC) that converts the sensed acoustic signals into digital signals. In one or more embodiments, the digital signal processor 208 can include additional circuitry to process the digital acoustic signals. For example, the signal processor 208 can include a pulse density modulator (PDM) for generating PDM electrical signals corresponding to the acoustic signals sensed by the MEMS microphone 202. In other embodiments, the processor 208 can perform additional operations such as digital filtering and pulse code modulation. The signal processor 208 may also include a memory for storing data corresponding to electrical signals received from the amplifier 206 and for storing instruction code, which when executed by the digital signal processor 208, causes the digital signal processor 208 to perform the above-mentioned operations. In one or more embodiments, the signal processor 208 can include an analog signal processor in addition to, or instead of, the digital signal processor. The analog signal processor may include analog circuitry to perform various operations such as filtering, amplification, and modulation of electrical signals received from the amplifier 206. In one or more embodiments, the output of the signal processor 208 can be provided to additional processors or circuitry (not shown) for further processing.

[0033] Figure 3 shows a representation of a frequency response 300 of a high-pass filter formed by the MEMS microphone 202 and the biasing resistor 204 shown in Figure 2. The Y-axis represents the magnitude (dBV) (relative to 1 V) of the voltage Vi_n at the input node 210 and the x-axis represents the frequency (Hz). The frequency response 300 is

characterized by an increase in the magnitude of the voltage Vi_n at the input node 210 as the frequency increases until it reaches a constant magnitude V_m. In one or more embodiments, the magnitude V_m can be equal to about 0 dB. The rate of increase in the magnitude in relation to the frequency can indicate the order of the filter. For example, for a first order filter the magnitude increases at about 20 dB/decade increase in frequency; and for a second order filter, the magnitude increases at about 40 dB per decade increase in frequency. The frequency response 300 is also characterized by a cut-off frequency f_cut-off, which represents the cut-off frequency of the high-pass filter. In one or more embodiments, f_cut-off can be the frequency at which the magnitude is about 3 dB below the magnitude V_m.

[0034] One approach to adjusting the frequency response 300 of the high-pass filter is to adjust the cut-off frequency f_cut-off- For example, if the application in which the MEMS microphone 202 is used operates in the audible frequency range, then it can be advantageous to set fcut-off to about 20 Hz, so that frequencies below 20 Hz are attenuated. In one or more embodiments, Equation (1) below describes the relationship between the cut-off frequency fcut-off, the value R_BIAS of the biasing resistor 204, and the value of the capacitance C_MEMS of the MEMS microphone 202:

[0035] In one or more embodiments, C_MEMS can represent a nominal capacitance of the MEMS microphone 202. In one or more embodiments, the nominal capacitance of the MEMS microphone 202 can represent the capacitance of the MEMS microphone 202 when no sound is present. In some other implementations, the nominal capacitance of the MEMS microphone 202 can be the capacitance of the MEMS microphone 202 under ambient sound conditions. In one or more embodiments, C_MEMS can represent an inherent capacitance of the MEMS microphone 202. Thus, for a desired cut-off frequency of about 20 Hz, the value of R_BIAS for a value of C_MEMS = 1 pF would be about 8 GQ. In one or more embodiments, the bias resistor 204 is implemented on a semiconductor chip. Implementing a resistor with such a large value can be difficult. In some embodiments, reverse biased diodes having low reverse leakage current values can be used to provide the high resistance value. However, maintaining the effective resistance of the reverse biased diode across wide voltage, temperature, and processes conditions can be difficult.

[0036] Moreover, using a diode can limit the ability to purposefully alter the cut-off frequency of the high-pass filter in a controlled manner. In one or more embodiments, noise caused by environmental conditions, such as wind or noise at concerts, may result in large, low frequency signals being generated by the MEMS microphone 202. These low frequency signals, if within the pass-band of the high-pass filter, may cause the amplifier 206 to saturate. Recovering the desired audio signal may be difficult when the amplifier 206 is saturated. One approach to attenuating low frequency noise signals is to move the cut-off frequency to be greater than the noise signal. However, as the effective resistance of the diode is not well controlled, the cut-off frequency cannot be easily adjusted during operation.

[0037] One approach to providing a high resistance value that can also be varied as desired is to use a switched capacitor resistor to implement the biasing resistor 204. The switched capacitor resistor not only allows implementation of large resistance values, but also allows resistance values to be changed during operation. [0038] Figure 4 depicts an example switch capacitor resistor 404. In one or more embodiments, the switched capacitor resistor 404 can be used to implement the biasing resistor 204 shown in Figure 2. The switched capacitor resistor 404 includes a first switch 406, a second switch 408 connected in series with the first switch 406, and a capacitor 410, one terminal of which is connected to a common node 412 between the first switch 406 and the second switch 410 and a second terminal of which is connected to ground. A first terminal of the first switch 406 can be connected to the biasing voltage supply VBIAS (shown in Figure 2) and a second terminal of the first switch is connected to the node 412. A first terminal of the second switch 408 can be connected to the node 412 and a second terminal of the second switch 408 can be connected to a node that provides an input voltage Vi_n to an amplifier. For example, the second terminal of the second switch 408 can be connected to the input node 210 shown in Figure 2. The states of the first switch 406 and the second switch 408 can be controlled by a controller 414. The controller 414 provides a first control signal 416 and a second control signal 418 to control the states of the first switch 406 and the second switch 408, respectively. As discussed below, the effective resistance of the switched capacitor resistor 404 can be selected based, in part, on the value of the capacitor 410 and the switching frequency of the first switch 406 and the second switch 408.

[0039] In one or more embodiments, the first switch 406 and the second switch 408 can be implemented using semiconductor switches on a semiconductor die. For example, MOS, PMOS, or CMOS (transmission gate) transistors can be used to implement the first switch 406 and the second switch 408. In some such embodiments, control signals to switch the switches ON or OFF can be provided to the gate terminal of the transistors. In one or more

embodiments, the capacitor 410 also can be implemented on a semiconductor chip using MOS technology. In one or more embodiments, the controller 414 can be implemented using a digital logic circuit such as, for example, a microcontroller and a processor. In one or more embodiments, the controller 414 can be implemented using a field programmable gate array, an application specific integrated circuit, or a combination thereof. In one or more embodiments, the controller 414 can be implemented in the signal processor 208 discussed above in relation to Figure 2. [0040] Figure 5 shows control signals for controlling the states of the switches in the switched capacitor resistor 404 shown in Figure 4. In particular, Figure 5 shows a first control signal 516 and a second control signal 518 (collectively referred to as control signals 500) that can be provided to the first switch 406 and the second switch 408 shown in Figure 4. The first control signal 516 and the second control signal 518 transition between two states: an ON state and an OFF state. In the ON state, the corresponding switch is switched ON, while in the OFF state the corresponding switch is switched OFF. In one or more embodiments, the first control signal 516 and the second control signal 518 can represent voltages provided to control terminals of their corresponding switches. Of course, in some other implementations, based on the type of switch and the manner in which its states are controlled, the control signal can be a current, a wireless signal, etc.

[0041] In one or more embodiments, such as the one shown in Figure 5, the first control signal 516 and the second control signal 518 are non-overlapping. That is, at no time are both the first control signal 516 and the second control signal 518 in the ON state. In one or more embodiments, control signals 500 are periodic. That is, the control signals 500 repeat the pattern of transitioning from ON state to the OFF state over time. In one or more

embodiments, the frequency of the first control signal 516 is same as the frequency of the second control signal 518. That is, the time period ti associated with the first control signal 516 is equal to the time period t₂ associated with the second control signal 518. In one or more embodiments, the duty cycle of the control signals 500 also can be the same. However, in some other embodiments, the duty cycle of the control signals 500 can be different while having the same frequency. In one or more embodiments, a lower bound to the ON time (the time for which the control signal is in the ON state) and the OFF time (the time for which the control signal is in the OFF state) of the control signals 500 can be a function of the time needed to sufficiently charge or discharge the capacitor 410 to a predetermined voltage level.

[0042] As mentioned above, the effective resistance of the switched capacitor resistor 404 shown in Figure 4 can be selected based, in part, on the value of the capacitor 410 and the frequency with which the first switch 406 and the second switch 408 are switched. Equation (2) below represents one approach in establishing a relationship between the effective resistance Re_ff of the switched capacitor resistor 404, the value C of the capacitor 410 and the frequency f_sw with which the first switch 406 and the second switch 408 are switched:

[0043] The relationship expressed in Equation (2) can be used to obtain large values R_eff (which, in turn, results in large values of R_BIAS)- For example, to obtain an effective resistance of about 8 GQ, a switching frequency of 50 kHz and a capacitor C of about 2.5 fF can be selected. Thus, for the switched capacitor resistor 404 shown in Figure 4 to provide an effective resistance of about 8 GQ, the capacitor 410 can be selected to have a value of about 2.5 fF, and the first switch 406 and the second switch 408 can be switched with non- overlapping control signals (i.e., non-overlapping ON states) having a time period of about 20

[0044] In one or more embodiments, the switching frequency f_sw can be selected to be outside a frequency band of interest. For example, in some embodiments, the frequency band of interest may be in the audible range (about 20 Hz to about 20 kHz). In some such embodiments, to reduce the impact of noise generated by the control signals 500 on the electrical signals generated by the MEMS microphone 202 and for that matter those generated by the electronic circuit 200, the switching frequency f_sw can be selected to be greater than highest audible frequency of interest (for example, about 20 kHz).

[0045] In one or more embodiments, for a desired effective resistance value, a higher switching frequency f_sw may reduce the highest value of the capacitor 410 to obtain the desired effective resistance. In some embodiments, implementing small values (of the order of femto-F and lower) of capacitor 410 may be challenging. In particular, because small valued capacitors may be difficult to implement accurately due to the impact of parasitic capacitance. Therefore, it is desirable to provide the switched capacitor resistor with a sufficiently large capacitor while still providing the desired large value of resistance.

[0046] Figure 6 depicts an example multi-stage switched capacitor resistor 604. The multistage switched capacitor resistor 604 includes a first set of m switches 606-1, 606-2 . . . 606- (m-1), and 606-m (collectively referred to as "the first set of switches 606"), a second set of / switches 608-1 . . . 608-1 (collectively referred to as "the second set of switches 608"), a set of n capacitors 610-1, 610-2 . . . 610-(n-l), and 610-n (collectively referred to as "the set of capacitors 610"), and a controller 614. The first set of switches 606 and the second set of switches 608 are arranged alternately in series between a bias voltage supply V_BIAS and the input voltage Vm. Each of the set of capacitors 610 is connected via a common node between one of the first set of switches 606 and one of the second set of switches 608 and ground. The controller 614 provides a first control signal 616 for controlling the first set of switches 606 and provides a second control signal 618 for controlling the second set of switches 608. Each of the switches in the first set of switches 606 receives the same first control signal 616, and each of the switches in the second set of switches 608 receives the same second control signal 618. In one or more embodiments, all of the first set of switches 606 are the same size and all of the second set of switches 608 are the same size. In one or more embodiments, all of the capacitors in the set of capacitors 610 have the same magnitude of capacitance C.

[0047] In one or more embodiments, the first control signal 616 and the second control signal 618 can be similar to the first control signal 516 and the second control signal 518 discussed above in relation to Figures 4 and 5. Thus, the first control signal 616 and the second control signal 618 are non-overlapping (i.e., non-overlapping ON states) and have the same frequency. As all of switches 606 receive the same control signal 616, all of switches in 606 maintain the same state at a given instant in time. Similarly, all of switches 608 maintain the same state at a given instant in time. Of course, as the control signals 616 and 618 are non-overlapping, at any given instant in time, no switch from the first set of switches 606 is in the ON state while a switch in the second set of switches 608 is also in the ON state.

[0048] The multi-stage switched capacitor resistor 604 is similar to the switched capacitor resistor 404 shown in Figure 4 in that the multi-stage switched capacitor resistor 604 also provides an effective resistance that is a function of the value of the capacitor C and the switching frequency of the constituent switches. However, the effective resistance provided by the multi-stage switched capacitor resistor 604 is also a function of the number of capacitors n in the set of capacitors 610. This relationship is expressed in Equation (3) below:

[0049] The effective resistance Re_ff is directly proportional to the number n of capacitors in the set of capacitors 610. As such, the size of the capacitors can be increased along with the number of capacitors to provide the same effective resistance. For example, to obtain an effective resistance of about 8 GQ, at a switching frequency of 50 kHz, and with n = 6, the value of the capacitor C can be equal to about 15 fF. This value of 15 fF is substantially larger than the 2.5 fF value needed in the switched capacitor resistor 404 shown in Figure 4. The larger capacitor reduces the risk of the impact of parasitic capacitance of the capacitors 610 on the value of the capacitance C, and improves the accuracy of the effective resistance

Reff-

[0050] In one or more embodiments, the number n of capacitors in the set of capacitors 610 can include an even number. In some such implementations, the first and last switch in the series of switches between the V_BIAS and the Vi_n terminals are controlled by the same control signal. For example, the multi-stage switched capacitor resistor 604 shown in Figure 6 may include an even number of capacitors 610 and where the first switch 606-1 and the last switch 606-m are controlled by the same control signal 616. In some other embodiments, the number n of capacitors in the set of capacitors 610 can include an odd number. In some such embodiments, the first switch 606-1 and the last switch 606-m can be controlled by non- overlapping control signals, for example, control signals 616 and 618, respectively.

[0051] In some embodiments, the acceptable number n of capacitors in the set of capacitors 610 can be limited by the increase in the noise which is associated with an increase in the number n of capacitors. The increase in noise can be due to each capacitor aliasing thermal noise from a preceding capacitor.

[0052] Figure 7 shows another multi-stage switched capacitor resistor 704. The multi-stage switched capacitor resistor 704 has reduced sensitivity to parasitic capacitance as compared to that of the multi-stage switched capacitor resistor 604 shown in Figure 6. The multi-stage switched capacitor resistor 704 includes a two stages: a first stage 760-1 and a second stage 760-2. The first stage 760-1 includes a first capacitor 710-1, a left terminal of which is coupled to a first left switch 706L and the right terminal of which is coupled to a first right switch 706R. Similarly, the second stage 706-2 includes a second capacitor 710-2, a left terminal of which is coupled to a second left switch 706L and the right terminal of which is coupled to a second right switch 706R. The first capacitor 710-1 of the first stage 760-1 is coupled to the bias voltage supply V_BIAS via an input switch 756-1. The second capacitor 710-2 of the second stage 760-2 is coupled to the input port (V_m) via an output switch 756-2. The two stages 760-1 and 760-2 are separated by a series switch 758, such that the right terminal of the first capacitor 710-1 is coupled to one terminal of the series switch 758 and the left terminal of the second capacitor 710-2 is coupled to the other terminal of the series switch 758.

[0053] The multi-stage switched capacitor resistor 704 also includes a controller 714 that generates a first control signal 716 and a second control signal 718. The controller 714 can be similar to the controller 414 discussed above in relation to Figure 4. Moreover, the first control signal 716 and the second control signal 718 can be similar to the first control signal 516 and the second control signal 518 discussed above in relation to Figures 4 and 5. In particular, the first control signal 716 and the second control signal 718 can be non- overlapping control signals (i.e., non-overlapping ON states) that control the ON and OFF states of the switches in the multi-stage switched capacitor resistor 704. In addition, the frequency of the first control signal 716 can be the same as the frequency of the second control signal 718. The first control signal 716 is provided to both the input switch 756-1 and the output switch 756-2. In contrast, the series switch 758 is controlled by the second control signal 718. Further, the first left switch 706L in the first stage 760-1 and the second right switch 706R in the second stage 760-2 are controlled by the first control signal 716, while the first right switch 706R in the first stage 760-1 and the second left switch 706L in the second stage 760-2 are controlled by the second control signal 718.

[0054] The effective resistance provided by the multi-stage switched capacitor resistor 704 also can be represented by Equation (3) discussed above in relation to the multi-stage switched capacitor resistor 604 shown in Figure 6. That is, the effective resistance R_eff is equal to the ratio of the number n (=2) of stages in the multi-stage switched capacitor resistor 704 shown in Figure 7 to a product of the frequency f_sw of the first and the second control signals 716 and 718 and the value C of the first and second capacitors 710-1 and 710-2. [0055] Figure 8 shows yet another multi-stage switched capacitor resistor 804. The multistage switched capacitor resistor 804 is similar to the multi-stage switched capacitor resistor 704 shown in Figure 7, but includes n stages. For example, the multi-stage switched capacitor resistor 804 includes a first stage 760-1, a second stage 760-2 . . . up to an n^th stage 760-n between the input switch 756-1 and the output switch 756-2. Each stage, similar to the first and the second stages 760-1 and 760-2 discussed above, includes a capacitor 710, a left switch 706L and a right switch 706R. Each stage is separated by a series switch 758. For example, the first stage 760-1 and the second stage 760-2 are separated by a first series switch 758-2, the second stage 760-2 and the third stage (not shown) are separated by a second series switch 758-2, and so on.

[0056] The controller 714 provides the first control signal 716 and the second control signal 718. The left switch 706L of every odd-numbered stage is controlled by the first control signal 716, while the left switch 706L of every even numbered stage is controlled by the second control signal 718. Similarly, the right switch 706R of every odd numbered stage is controlled by the second control signal 718 and the right switch 706R of every even numbered stage is controlled by the first control signal 716. Furthermore, the every odd numbered series switch 758 is controlled by the second control signal 718, while every even numbered series switch 758 is controlled by the first control signal 716. Moreover, the input switch 756-1 and the output switch 756-2 are controlled by the first control signal 716.

[0057] In some embodiments, such as the one shown in Figure 8, the multi-stage switched capacitor resistor 804 can include an even number of stages. In some such embodiments, the input switch 756-1 and the output switch 756-2 are controlled by the same first control signal 716.

[0058] The effective resistance of the multi-stage switched capacitor resistor 804 also can be expressed by Equation (3) discussed above. That is, the effective resistance R_eff is equal to the ratio of the number n of stages (760-1 to 760-n) in the multi-stage switched capacitor resistor 804 to a product of the frequency f_sw of the first and the second control signals 716 and 718 and the value C of the capacitors (710-1 to 710-n) in each stage. [0059] As discussed above, the effective resistance Reff of the switched capacitor resistors shown in Figures 4, 6, 7, and 8 is a function of the switching frequency f_sw of the first and the second control signals. In some embodiments, the switching frequency f_sw can be

dynamically changed by the controller to achieve the desired cut-off frequency of the high- pass filter formed by the switched capacitor resistor and the MEMS microphone 202 (Figure 2). That is, the switching frequency f_sw can be selected to obtain the desired value of the bias resistance that results in the desired cut-off frequency.

[0060] In one or more embodiments, to achieve a desired effective resistance Reff, the effective number n of capacitors can be dynamically adjusted as an alternative to, or in addition to, adjusting the switching frequency f_sw. Referring to the switched capacitor resistor 604 shown in Figure 6, one or more switches can be maintained in the ON state to reduce the effective number n of capacitors. For example, switches 606-1 and 608-1 can be maintained in the ON state to reduce the effective number of capacitors to (n-l). In some embodiments, an additional switch can be used to connect the capacitor 606-1 to Vi_n, to reduce the effective number of capacitors to {n-l). Similarly, maintaining the switches 606-1, 608-1, and 606-2 in the ON state can result in the effective number of capacitors to be equal to (n-2). In some such implementations, the controller 614 can provide individual controls signals to each of the first set of switches 606 and the second set of switches 608, so that a selected number of switches can be maintained in the ON state, while the remainder of the switches can receive the controls signals 616 and 618 as discussed above. The effective number of capacitors can be similarly adjusted in the switched capacitor 804 shown in Figure 8. For example, in one or more embodiments, one or more stages 760 can include a shunt switch connected across each of a selected number of capacitors 710. To exclude a stage, the shunt capacitor of that stage can be switched ON while the left switch 706L and the right switch 706R of that stage can be switched OFF. That is, to remove the stage 706-1, the shunt switch across the capacitor 710-1 is switched ON, while the left switch 706L and the right switch 706R are switched OFF. As a result, the effective number of capacitors determining the effective resistance of the switched capacitor 804 would be equal to ( -l).

[0061] Figure 9A shows a representation of an electronic circuit 900 used with a MEMS microphone 202. The electronic circuit 900 is similar to the electronic circuit 200 discussed above in relation to Figure 2. However, the resistance R i_as of the bias resistor 904 in the electronic circuit 900 shown in Figure 9 can be dynamically changed. Any one, or

combination of, the switched capacitor resistors discussed above in relation to Figures 4-8 can be used to implement the bias resistor 904. The resistance R i_as of the bias resistor 904 can be changed by changing the switching frequency f_sw of the control signals provided to the switched capacitor resistors shown in Figures 4-8 and/or by changing the effective number n of capacitors in the switched capacitor resistors shown in Figures 4-8.

[0062] Figure 9B shows a representation of the change in the frequency response of a high- pass filter formed by the MEMS microphone 202 and the switched capacitor resistor 904 shown in Figure 9A. A first frequency response 950-1 represents the frequency response at a first switching frequency f_swl, while the frequency response 950-2 represents the frequency response at a second switching frequency f_sw2 of the switched capacitor resistor 904.

Assuming, for example, that initially the switching frequency of the switched capacitor resistor 904 is set at f_swi, which results in the frequency response 950-1 having a cut-off frequency of fcut-off- 1· Note that the cut-off frequency f_cut-off can be determined using Equations (l)-(3). Also assume that a noise signal having a center frequency at f_noise is received by the electronic circuit 900. To attenuate the noise signal, the controller of the switched capacitor resistor 904 can increase the switching frequency to a frequency f_sw2, which results in a cutoff frequency f_cut-off-2 that is greater than the center frequency f_noise of the noise signal. That is, the increase in the switching frequency f_sw reduces the value of the switched capacitor resistor 904, which, in turn, increases the cut-off frequency of the high-pass filter formed by the switched capacitor resistor 904 and the MEMS microphone 202. In one or more

embodiments, if noise is not a primary concern, the cut-frequency can be decreased to increase the bandwidth of the high-pass filter. In one or more embodiments, the effective number n of the capacitors in the switched capacitor resistor can be adjusted in addition to or instead of changing the switching frequency f_sw to achieve the desired cut-off frequency f_cut-off- In one or more embodiments, the cut-off frequency f_cut-off can be adjusted to between about 15 Hz to about 25 Hz or to about 20 Hz. In some other implementations, the cut-off frequency fcut-off can be adjusted to between about 45 Hz to about 55 Hz or to about 50 Hz. In one or more embodiments, the cut-off frequency f_cut-off can be adjusted to be up to about 800 Hz. [0063] In one or more embodiments, the controller shown in Figures 4, 6-8 can be configured to switch ON or OFF all control signals based on whether the MEMS microphone 202 is being powered. For example, the controller may switch OFF all the control signals when the MEMS microphone 202 does not receive any power from the MEMS voltage supply VMEMS-

[0064] The herein described subject matter sometimes illustrates different components contained within, or connected with, different other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as "associated with" each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being "operably connected," or "operably coupled," to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being "operably couplable," to each other to achieve the desired functionality. Specific examples of operably couplable include but are not limited to physically mateable and/or physically interacting components and/or wirelessly interactable and/or wirelessly interacting components and/or logically interacting and/or logically interactable components.

[0065] With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

[0066] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). [0067] It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two

recitations," without other modifiers, typically means at least two recitations, or two or more recitations).

[0068] Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B." Further, unless otherwise noted, the use of the words "approximate," "about," "around," "substantially," etc., mean plus or minus ten percent.

[0069] The foregoing description of illustrative embodiments has been presented for purposes of illustration and of description. It is not intended to be exhaustive or limiting with respect to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosed embodiments. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. A microphone assembly comprising:

a capacitive transducer having a supply voltage terminal and a second terminal, the supply voltage terminal being coupled to one of a back-plate and a diaphragm and the second terminal being coupled to the other of the back plate and the diaphragm;

an amplifier coupled to the second terminal of the capacitive transducer; and a switched-capacitor resistor having a bias voltage terminal and a second terminal coupled to the second terminal of the capacitive transducer, wherein the switched-capacitor resistor includes at least one capacitor and at least two switches, wherein a resistance value of the switched-capacitor resistor depends on a capacitance value of the at least one capacitor and a switching frequency of the at least two switches.

2. The microphone assembly of claim 1, wherein the capacitive transducer includes a microelectromechanical systems (MEMS) microphone.

3. The microphone assembly of claim 1, wherein a combination of the capacitive transducer and the switched-capacitor resistor forms a high pass filter having a cut-off frequency that is a function of a capacitance of the capacitive transducer and the resistance value of the switched-capacitor resistor.

4. The microphone assembly of claim 1, wherein the at least two switches includes a first switch and a second switch connected in series between the bias voltage terminal of the switched-capacitor resistor and the second terminal of the switched-capacitor resistor, and wherein one terminal of the at least one capacitor is coupled to a node common to both the first switch and the second switch.

5. The microphone assembly of claim 1, further comprising a controller configured to generate a first control signal for controlling a state of a first switch of the at least two switches and a second control signal for controlling a state of a second switch of the at least two switches, wherein the first control signal has a same frequency as the second control signal.

6. The microphone assembly of claim 5, wherein each of the first control signal and the second control signal transition between an ON state and an OFF state, wherein the ON state of the first control signal does not overlap with the ON state of the second control signal.

7. The microphone assembly of claim 1, wherein the at least one capacitor and the at least two switches form a plurality of stages coupled in series between the bias voltage terminal and the second terminal of the switched-capacitor resistor, wherein each stage includes a capacitor having a left terminal and a right terminal, a left switch coupled to the left terminal, and a right switch coupled to the right terminal.

8. The microphone assembly of claim 7, wherein the resistance value of the switched- capacitor resistor is a function of the number of the plurality of stages.

9. The microphone assembly of claim 7, wherein states of left switches of all odd- numbered stages of the plurality of stages are controlled by a first control signal, and states of right switches of all even-numbered stages of the plurality of stages are controlled by a second control signal distinct from the first control signal but having a same frequency as that of the first control signal.

10. The microphone assembly of claim 7, further comprising a plurality of series switches, where each of the plurality of series switches is coupled between two adjacent stages of the plurality of stages, wherein states of all odd-numbered serial switches of the plurality of switches are controlled by the first control signal, and all even-numbered serial switches of the plurality of switches are controlled by the second control signal.

11. The microphone assembly of claim 7, wherein the switched-capacitor resistor includes an input switch coupled between the bias voltage terminal of the switched-capacitor resistor and a first of the plurality of stages, and an output switch coupled between the second terminal of the switched-capacitor resistor and a last stage of the plurality of stages.

12. An acoustic microelectromechanical (MEMS) microphone assembly comprising: a MEMS sensor having an inherent capacitance;

an amplifier having an input coupled to an output of the MEMS sensor; and a switched-capacitor resistor coupled to the input of the amplifier, the switched- capacitor resistor having an effective resistance that depends on a capacitance value and a switching frequency of the switched-capacitor resistor,

wherein the capacitance of the MEMS sensor and the switched-capacitor resistor form a high pass filter at the input of the amplifier, and

wherein an electrical signal generated at the output of the MEMS sensor, in response to an acoustic input, is filtered by the high pass filter before the electrical signal is applied to the input of the amplifier.

13. The microphone assembly of claim 12, wherein the switched-capacitor resistor includes at least one capacitor and at least two switches.

14. The microphone assembly of claim 13, wherein the high pass filter has a cut-off frequency below 50 Hz.

15. The microphone assembly of claim 14, wherein the high pass filter has a cut-off frequency below 20 Hz.

16. A method in a microelectromechanical (MEMS) assembly having a MEMS transducer and an electrical circuit, the method comprising:

generating an electrical signal at an output terminal of the MEMS transducer in response to an acoustic input;

filtering the electrical signal with a high pass filter formed by an inherent capacitance of the MEMS transducer and an effective resistance of a switched-capacitor resistor; and applying the filtered electrical signal to an input of an amplifier of the electrical circuit.

17. The method of claim 16, wherein the switched-capacitor resistor includes a capacitor and first and second switches, the method further comprising switching the first and second switches of the switched-capacitor resistor at a common switching frequency.

18. The method of claim 16, wherein the switched-capacitor resistor includes a capacitor and first and second switches, further comprising controlling a cut-off frequency of the high pass filter by controlling the switching frequency of the switched-capacitor resistor with the electrical circuit, wherein the cut-off frequency of the high pass filter can be adjusted to filter varying levels of low frequency noise at the input of the MEMS transducer.

19. The method of claim 16, wherein the switched-capacitor resistor includes a capacitor and first and second switches, further comprising providing the effective resistance by repeatedly transitioning the first and second switches between respective ON states and OFF states, wherein the ON state of the first switch does not overlap with the ON state of the second switch.

20. The method of claim 16, wherein the switched-capacitor resistor includes a plurality of stages coupled in series, wherein each stage includes a stage capacitor having a left terminal and a right terminal, a left switch coupled to the left terminal and a right switch coupled to the right terminal, further comprising providing the effective resistance by adjusting an effective number stages from the plurality of stages.