Abstract:
Methods and system are described for cancelling interference in a microphone system. A positive bias voltage is applied to a first microphone diaphragm and a negative bias voltage is applied to a second microphone diaphragm. The diaphragms are configured to exhibit substantially the same mechanical deflection in response to acoustic pressures received by the microphone system. A differential output signal is produced by combining a positively-biased output signal from the first microphone diaphragm and a negatively-biased output signal from the second microphone diaphragm. This combining cancels common-mode interferences that are exhibited in both the positively-biased output signal and the negatively-biased output signal.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/038,097, filed on Sep. 26, 2013 and entitled “DIFFERENTIAL MICROPHONE WITH DUAL POLARITY BIAS,” which claims the benefit of U.S. Provisional Application No. 61/782,307, filed on Mar. 14, 2013 and entitled “DIFFERENTIAL MEMS MICROPHONE USING DUAL POLARITY BIAS,” the entire contents of both of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates to differential microphone systems. Differential microphones typically include two membranes. The signals detected by the two membranes are then processed to provide a desired output. For example, the two diaphragms can be arranged facing different directions and the differential signal used to cancel ambient noise (i.e., noise cancellation differential microphones). In some systems, the differential signal can also be used to determine the directionality of sound (i.e., from what direction did the sound originate). 
     SUMMARY 
     In one embodiment, the invention provides a differential microphone system including a first microphone diaphragm and a second microphone diaphragm. The first and second microphone diaphragms are positioned to receive acoustic pressure from the same direction at substantially the same amplitude. As such, the deflection of the first and second diaphragms caused by acoustic pressures applied to the microphone are substantially the same. A positive bias voltage is applied to the first diaphragm while a negative bias voltage is applied to the second diaphragm. A differential amplifier is configured to receive the positively-biased output signal from the first microphone diaphragm and the negatively-biased output signal from the second microphone diaphragm and to produce a differential output signal. 
     In some embodiments, the opposite biasing voltages applied to the first and second diaphragms causes the same diaphragm deflections to produce output signals that have the same magnitude but opposite polarity. In some embodiments, the microphone system is configured such that non-acoustic interference (for example, light interference) affects the positively-biased output signal and the negatively-biased output signal in the same way—the positively-biased and negatively-biased signals are both offset by the same magnitude and the same polarity. As such, when the positively-biased signal and the negatively-biased signal are combined to produce the differential signal, common-mode interference is cancelled and the differential signal more accurately represents the acoustic pressures applied to the first and second diaphragms of the microphone system. 
     In another embodiment, the invention provides a method of cancelling interference in a microphone system. A positive bias voltage is applied to a first microphone diaphragm and a negative bias voltage is applied to a second microphone diaphragm. The diaphragms are configured to exhibit substantially the same mechanical deflection in response to acoustic pressures received by the microphone system. A differential output signal is produced by combining a positively-biased output signal from the first microphone diaphragm and a negatively-biased output signal from the second microphone diaphragm. This combining cancels common-mode interferences that are exhibited in both the positively-biased output signal and the negatively-biased output signal. 
     In still another embodiment, the invention provides a microphone system that includes a first microphone diaphragm, a second microphone diaphragm and a differential amplifier. The two microphone diaphragms are arranged to receive acoustic pressures from the same direction at the same amplitude. However, a positive bias voltage is applied to the first diaphragm while a negative bias voltage is applied to the second diaphragm. The differential amplifier receives a positively-biased output signal from the first microphone diaphragm and a negatively-biased output signal from the second microphone diaphragm and produces a differential output signal by combining the positively-biased output signal and the negatively-biased output signal. The microphone system is configured such that acoustic pressure received from the first direction causes mechanical deflections of the first diaphragm and substantially identical mechanical deflections of the second diaphragm. Mechanical deflections of the first diaphragm produce the positively-biased output signal while mechanical deflections of the second diaphragm produce the negatively-biased output signal. The positively-biased output signal and the negatively-biased output signal caused by the mechanical deflections of the first and second diaphragms have substantially the same magnitude but opposite polarities. When, non-acoustic interference alters the positively-biased output signal and the negatively biased output signal, both the positively-biased output signal and the negatively-biased output signal are altered by substantially the same magnitude and polarity. Therefore, combining the positively-biased output signal and the negatively-biased output signal cancels the non-acoustic interference from the differential output signal. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is an overhead-cutaway view of a differential microphone system according to one embodiment. 
         FIG. 1B  is a cross-section side view of the differential microphone system of  FIG. 1A . 
         FIG. 2  is a schematic diagram of a differential biasing circuit for use with the microphone system of  FIG. 1A . 
         FIGS. 3A, 3B, and 3C  are graphs of a positively-biased output signal, a negatively-biased output signal, and a differential output signal produced by the microphone system of  FIG. 1A  in response to acoustic pressures on the diaphragms. 
         FIGS. 4A, 4B, and 4C  are graphs of a positively-biased output signal, a negatively-biased output signal, and a differential output signal produced by the microphone system of  FIG. 1B  in response to non-acoustic interference. 
         FIG. 5  is a schematic diagram of another differential biasing circuit for use with the microphone system of  FIG. 1A . 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
       FIG. 1A  illustrates a differential microphone system  100  that includes a CMOS-MEMS device layer  101 . A CMOS-MEMS device layer  101  is constructed primarily of silicon or other materials and includes both CMOS circuitry elements and one or more microelectromechanical structures (MEMS devices) formed directly within the CMOS-MEMS device layer  101 . The CMOS-MEMS device layer  101  of  FIG. 1A  includes first MEMS microphone diaphragm  103  and a second MEMS microphone diaphragm  105 . The two diaphragms  103 ,  105  are formed on the same package according to the same process. Therefore, acoustic pressures (i.e., sound) directed toward the top surface of the CMOS-MEMS device layer  101  cause substantially the same motion and deflection of the first membrane  103  and the second membrane  105 . 
       FIG. 1B  illustrates the same differential microphone system  100  from a cross-sectional perspective. A lid  107  is positioned above the CMOS-MEMS layer  101  to form an acoustic channel  109 . An opening (or acoustic port)  111  in the lid  107  allows acoustic pressures (sounds) to enter the acoustic channel  109  and cause mechanical deflection of the two diaphragms  103 ,  105 . As shown in  FIG. 1B , the diaphragms  103 ,  105  are arranged equidistant from the acoustic port  111 . Therefore, deflections of the diaphragms  103 ,  105  caused by acoustic pressures entering through the acoustic port  111  are substantially the same in both diaphragms. Below the CMOS-MEMS layer  101  is a back-volume component  113  that allows the diaphragms  103 ,  105  to move back and forth (or up and down) in response to the acoustic pressures. 
     Although the examples described herein refer to a CMOS-MEMS chip  101 , other constructions may include a MEMS device chip and a separate CMOS chip. In such constructions, the two diaphragms  103 ,  105  may be formed on the same chip (i.e., the MEMS chip). However, in still other constructions, the microphone system package can include two separate MEMS chips—one for each diaphragm—as long as the MEMS chips are arranged and manufactured such that acoustic pressures cause substantially the same deflection on both diaphragms. Furthermore, in other constructions, the position of the diaphragms relative to the opening  111  may be different than as illustrated in  FIG. 1B . For example, the diaphragms can be sized and arranged so that they are both positioned directly below the opening  111 . 
       FIG. 2  illustrates an example of a biasing circuit that uses an inverted transduction response to derive a differential audio signal from a dual-membrane microphone system such as illustrated in  FIGS. 1A and 1B . A positive bias voltage (+HV) is applied to a first terminal (node A) of the first microphone diaphragm  103 . The other terminal of the microphone diaphragm  103  is coupled to ground. The first microphone diaphragm  103  acts as a capacitor. As such, deflections of the diaphragm  103  caused by acoustic pressures change the capacitance between node A and ground. 
     A high-impedance device  201  is coupled between the positive bias voltage source and the first terminal of the microphone diaphragm  103  (i.e., node A). As used herein, a high-impedance device can include one or more electronic components designed to increase impedance between a voltage source and a mechanical or electronic component (i.e., the MEMS diaphragm  103 ). Another capacitor  203  is positioned between the first terminal of the diaphragm  103  and an output node A 1  of the biasing circuit for the first diaphragm  103 . 
     Similarly, a negative bias voltage (−HV) is applied to the first terminal (node B) of the second diaphragm  105  while the second terminal of the diaphragm  105  is coupled to ground. A high-impedance device  205  is coupled between the source of negative bias voltage and the diaphragm  105  (i.e. node B). Another capacitor  207  is coupled between node B and an output node B 1  of the biasing circuit for the second diaphragm  105 . The output nodes A 1 , B 1  of both biasing circuits are coupled to ground each through another high impedance device ( 209  and  211 , respectively). The output node A 1  of the positively-biased diaphragm  103  is coupled to the positive terminal of a differential amplifier  213 . The output node B 1  of the negatively-biased diaphragm is coupled to the negative terminal of the differential amplifier  213 . As such, two oppositely-biased output signals are combined by the differential amplifier  213  to produce a differential output signal that represents the difference between the positively-biased signal and the negatively-biased signal at any given time. 
     As illustrated in  FIG. 3A , mechanical deflections of the first diaphragm  103  caused by acoustic pressures produce a positively-biased output signal (i.e., a voltage) at the output node A 1 . Due to the proportional relationship Q=C*V, voltage decreases as capacitance increases (e.g., due to movements/deflections of the diaphragm). The same mechanical deflections on the second diaphragm  103  produce a negatively-biased output signal (i.e., a voltage) at the output node B 1  as illustrated in  FIG. 3B . Although the mechanical deflections of both diaphragms are substantially identical, the opposite biasing voltages cause the output signals to have the same magnitude, but opposite polarities. The differential amplifier  213  combines the positively-biased output signal and the negatively-biased output signal to produce a differential output signal as shown in  FIG. 3C . Because of the opposite biases applied to the two diaphragms, deflections that cause the positively-biased output signal to “decrease” also cause the negatively-biased output signal to “increase” resulting in less potential difference in the combined differential signal. As such, the differential output signal is effectively an amplified version of both the positively-biased output signal and the negatively-biased output signal (which, themselves, are opposing representations of the same acoustic signal). 
     However, the output signal of a microphone can be affected by environmental factors other than acoustic pressures. For example, light contacting the microphone diaphragm can affect the output signal (i.e., the voltage). This interference can adversely affect the performance of a microphone and degrade the quality of the reproduced sound. The dual-polarity differential microphone system described above can reduce or eliminate the negative effect of such interference by cancelling common-mode interference that is exhibited on both the positively-biased output signal and the negatively-biased output signal. 
       FIG. 4A  illustrates the positively-biased output signal due only to non-acoustic interference (e.g., light interference without any acoustic pressure exerted on the diaphragm).  FIG. 4B  illustrates the effect of the same non-acoustic interference on the negatively-biased output signal. Although, as discussed above, the opposing bias voltages cause the same mechanical deflections to produce opposite output signals, non-acoustic interference affects both output signals the same way despite the opposite biasing voltages. As such, the magnitude and polarity of the positively-biased output signal due to non-acoustic interference is identical to the magnitude and polarity of the negatively-biased output signal. When the two signals are combined by the differential amplifier, the common-mode interference is effectively cancelled (as illustrated in  FIG. 4C ) and the differential output signal more accurately represents the acoustic pressures that cause deflections of the microphone diaphragms. 
       FIG. 3  illustrates an alternative construction of a differential biasing circuit that can be used to cancel common-mode interference. In this example, the positively-biased diaphragm is coupled a reference voltage V ref  through a high-impedance device  503  and coupled to the positive terminal of a differential amplifier  509 . A negatively-biased diaphragm  505  is similarly coupled to the reference voltage V ref  through another high-impedance device  507  and coupled to the negative terminal of the differential amplifier  509 . 
     A positive bias voltage (+HV) is applied to the opposite terminal (node A) of the first diaphragm  501  through yet another high-impedance device  511 . The diaphragm  501  is also coupled to ground at node A through a capacitor  513 . A negative bias voltage (−HV) is applied to the opposite terminal (node B) of the second diaphragm  505  through another high-impedance device  515 . The negatively-biased diaphragm  505  is also coupled to ground at node B through a second capacitor  517 . 
     Thus, the invention provides, among other things, a dual-polarity differential microphone system capable of cancelling common-mode interference caused by non-acoustic sources. Various features and advantages of the invention are set forth in the following claims.