Patent Publication Number: US-8983090-B2

Title: MEMS microphone using noise filter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     A claim for priority under 35 U.S.C. §119 is made to Korean Patent Application No. 10-2012-0103347 filed Sep. 18, 2012, in the Korean Intellectual Property Office, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The inventive concepts described herein relate to an MEMS microphone, and more particularly, relate to an MEMS microphone including a noise filter. 
     An MEMS microphone may include a microphone sensor and an MEMS microphone ASIC. A voltage booster in the ASIC may be supplied with a DC reference voltage, and may generate a sensor bias voltage by pumping the DC reference voltage using a charge pump. The charge pump may have a switched capacitor structure, in general. Thus, a noise included in the DC reference voltage may be also included in the sensor bias voltage. In this case, the microphone sensor may be affected directly by the noise included in the DC reference voltage. 
     A bias circuit included in the ASIC may be supplied with a reference current to generate a bias voltage of an amplifier. A differential amplifier can remove a bias noise. However, an amplifier of the MEMS microphone ASIC may not remove a noise due to impedance mismatching between a source follower and an input terminal. Thus, a noise included in the reference current may affect an output of the MEMS microphone. 
     A noise of a reference voltage/current generator may include a flicker noise and a thermal noise of a transistor. In general, the flicker noise may be reduced using a large-sized transistor, and the thermal noise may be reduced by increasing transconductance. However, since a size of a circuit implemented using the above-described methods is large, it is difficult to provide a small-sized MEMS microphone. Thus, there may be required a small-sized circuit capable of reducing a noise of a DC reference voltage and a noise of a reference current. 
     SUMMARY 
     One aspect of embodiments of the inventive concept is directed to provide an MEMS microphone which includes a reference voltage/current generator configured to generate a DC reference voltage and a reference current; a first noise filter configured to remove a noise of the DC reference voltage; a voltage booster configured to generate a sensor bias voltage using the DC reference voltage the noise of which is removed; a microphone sensor configured to receive the sensor bias voltage and to generate an output value based on a variation in a sound pressure; a bias circuit configured to receive the reference current to generate a bias voltage; and a signal amplification unit configured to receive the bias voltage and the output value of the microphone sensor to amplify the output value. The first noise filter comprises an impedance circuit; a capacitor circuit connected to an output node of the impedance circuit; and a switch connected to both ends of the impedance circuit. 
     In example embodiments, the impedance circuit comprises a first diode having a cathode connected to an input of the impedance circuit and an anode connected to an output of the impedance circuit; and a second diode having an anode connected to the input of the impedance circuit and a cathode connected to the output of the impedance circuit. 
     In example embodiments, the impedance circuit comprises a first MOS transistor having a source connected to an input of the impedance circuit, a drain connected to an output of the impedance circuit, and a gate connected to the output of the impedance circuit; and a second MOS transistor having a source connected to the output of the impedance circuit, a drain connected to the input of the impedance circuit, and a gate connected to the input of the impedance circuit. 
     In example embodiments, the first and second MOS transistors are either PMOS transistors or NMOS transistors. 
     In example embodiments, the impedance circuit comprises a first bipolar junction transistor having an emitter connected to an input of the impedance circuit, a collector connected to an output of the impedance circuit, and a gate connected to the output of the impedance circuit; and a second bipolar junction transistor having an emitter connected to the output of the impedance circuit, a collector connected to the input of the impedance circuit, and a gate connected to the input of the impedance circuit. 
     In example embodiments, the first and second bipolar junction transistors are either npn-type bipolar junction transistors or pnp-type bipolar junction transistors. 
     In example embodiments, the capacitor circuit includes one or more transistors. 
     In example embodiments, the capacitor circuit includes one or more ones of an MIM capacitor, a MOS capacitor, and a poly capacitor. 
     In example embodiments, at an initial operation, the switch is closed to charge the capacitor circuit. 
     In example embodiments, the MEMS microphone further comprises a second noise filter configured to remove a DC noise of the bias voltage and to provide the signal amplification unit with the bias voltage the DC noise of which is removed. 
     In example embodiments, the second noise filter is formed the same as the first noise filter. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The above and other objects and features will become apparent from the following description with reference to the following figures, wherein like reference numerals refer to like parts throughout the various figures unless otherwise specified, and wherein 
         FIG. 1  is a block diagram schematically illustrating an MEMS microphone according to an embodiment of the inventive concept. 
         FIG. 2  is a block diagram schematically illustrating a noise filter according to an embodiment of the inventive concept. 
         FIGS. 3A to 3E  are diagrams illustrating an impedance circuit of  FIG. 2  according to embodiments of the inventive concept. 
         FIG. 4A  is a graph illustrating a sound level on an MEMS microphone to which DC noise filters  150  and  160  are not applied. 
         FIG. 4B  is a graph illustrating a sound level on an MEMS microphone to which DC noise filters  150  and  160  are applied. 
         FIG. 5  is a circuit diagram illustrating a microphone sensor and a signal amplification unit including a noise filter according to an embodiment of the inventive concept. 
         FIG. 6  is a block diagram schematically illustrating a voltage booster and a noise filter according to an embodiment of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will be described in detail with reference to the accompanying drawings. The inventive concept, however, may be embodied in various different forms, and should not be constructed as being limited only to the illustrated embodiments. Rather, these embodiments are provided as examples so that this disclosure will be thorough and complete, and will fully convey the concept of the inventive concept to those skilled in the art. Accordingly, known processes, elements, and techniques are not described with respect to some of the embodiments of the inventive concept. Unless otherwise noted, like reference numerals denote like elements throughout the attached drawings and written description, and thus descriptions will not be repeated. In the drawings, the sizes and relative sizes of layers and regions may be exaggerated for clarity. 
     It will be understood that, although the terms “first”, “second”, “third”, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the inventive concept. 
     Spatially relative terms, such as “beneath”, “below”, “lower”, “under”, “above”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” or “under” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary terms “below” and “under” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the inventive concept. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Also, the term “exemplary” is intended to refer to an example or illustration. 
     It will be understood that when an element or layer is referred to as being “on”, “connected to”, “coupled to”, or “adjacent to” another element or layer, it can be directly on, connected, coupled, or adjacent to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to”, “directly coupled to”, or “immediately adjacent to” another element or layer, there are no intervening elements or layers present. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this inventive concept belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and/or the present specification and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
       FIG. 1  is a block diagram schematically illustrating an MEMS microphone according to an embodiment of the inventive concept. 
     Referring to  FIG. 1 , an MEMS microphone  100  may include a microphone sensor  110 , a reference voltage/current generator  120 , a voltage booster  130 , a signal amplification unit  140  and a noise filter  150 . In example embodiments, the constituent elements  110 ,  120 ,  130 ,  140 , and  150  may be built in a chip to be formed of an Application Specific Integrated Circuits (ASIC). 
     The microphone  110  may be a sensor which generates an electric signal based on a vibration of an input sound wave or ultrasonic wave. The microphone  110  may operate in a DC bias manner as a condenser type. The microphone  110  may include an electrode layer. A gap of the electrode layer may vary according to a sound pressure. The electrode layer may have a characteristic of a variable capacitor having a capacitance value varied according to the gap of the electrode layer. For example, in the case that a sound pressure varies at a state where a sensor bias voltage Vc is applied to the microphone  110 , a capacitance value of the microphone  110  may vary. In this case, an output value Vc_out of the microphone  110  may vary. The output value Vc_out may be transferred to the signal amplification unit  140 . 
     The reference voltage/current generator  120  may generate a DC reference voltage Vref and a reference current Iref necessary for the MEMS microphone  100 . In example embodiments, the DC reference voltage Vref and the reference current Iref generated by the reference voltage/current generator  120  may be supplied to the noise filter  150 . The reference current Iref generated by the reference voltage/current generator  120  may be supplied to a bias circuit  141 . 
     The voltage booster  130  may receive the DC reference voltage Vref from the noise filter  150  to generate the sensor bias voltage Vc. A noise of the DC reference voltage Vref may be removed by the noise filter  150 . 
     The signal amplification unit  140  may include the bias circuit  141 , a noise filter  160 , and an amplifier  142 . The bias circuit  141  may receive the reference current Iref from the reference voltage/current generator  120  to generate an amplifier bias voltage to be supplied to the amplifier  132 . 
     The noise filter  160  may remove a noise of the amplifier bias voltage provided from the bias circuit  141 . The noise filter  160  will be more fully described with reference to  FIG. 2 . 
     The amplifier  132  may receive the output signal Vc_out from the microphone sensor  110  to amplify the output signal Vc_out. The amplified signal may be transfer to another device as an output signal Vout of the MEMS microphone  100 . 
     The noise filter  150  may remove a noise of the DC reference voltage Vref provided from the reference voltage/current generator  120 . The noise-free DC reference voltage Vref may be provided to the voltage booster  130 . 
       FIG. 2  is a block diagram schematically illustrating a noise filter according to an embodiment of the inventive concept. 
     In example embodiments, a noise filter  160  of  FIG. 1  may have the same configuration as that of a noise filter  150  of  FIG. 2 . Below, it is assumed that an input of the noise filter  150  is referred to as a first node n 1  and an output of the noise filter  150  is referred to as a second node n 2 . 
     Referring to  FIG. 2 , the noise filter  150  may include an impedance circuit  151 , a capacitor circuit  152 , and a switch  153 . The impedance circuit  151  may be a circuit having a large impedance value. For example, the impedance circuit  151  may be formed of a back-to-back diode, a back-to-back diode-connected MOSFET, a back-to-back diode-connected BJT, or the like. This will be more fully described with reference to  FIGS. 3A to 3E . 
     One end of the capacitor circuit  152  may be connected to the second node n 2 . In example embodiments, the capacitor circuit  152  may be a circuit having at least one capacitor connected to the second node n 2 . For example, the capacitor circuit  152  may include a plurality of transistors. The capacitor circuit  152  can be formed of a MOS capacitor. Alternatively, the capacitor circuit  152  may be formed of capacitors (e.g., an MIM capacitor, an MOM capacitor, a poly capacitor, etc.) provided at an integrated circuit process. 
     The switch  153  may be used to connect the first and second nodes n 1  and n 2  such that voltages on the first and second nodes n 1  and n 2  are equalized in rapid time. The switch  153  may reduce a delay at an initial operation of the noise filter  160 . For example, both ends of the switch  153  may be connected to the first and second nodes n 1  and n 2 , respectively. At an initial operation of the noise filter  150 , the switch  150  may be closed to charge the capacitor circuit  152 . An impedance value of the impedance circuit  151  may be larger than that of the switch  153 . 
       FIGS. 3A to 3E  are diagrams illustrating an impedance circuit of  FIG. 2  according to embodiments of the inventive concept. In example embodiments, an impedance circuit  151  may be formed of one of circuits illustrated in  FIGS. 3A to 3E . 
     Referring to  FIG. 3A , the impedance circuit  151  may include first and second diodes D 1  and D 2 . A cathode of the first diode D 1  and an anode of the second diode D 2  may be connected to a first node n 1 , and an anode of the first diode D 1  and a cathode of the second diode D 2  may be connected to a second node n 2 . 
     Referring to  FIGS. 3B and 3C , the impedance circuit  151  may include first and second transistors T 1  and T 2 . The first and second transistors T 1  and T 2  may be formed of p-type or n-type MOS transistors. A drain and gate of the second transistor T 2  and source of the first transistor T 1  may be connected to the first node n 1 . Gate and drain of the first transistor T 1  and a source of the second transistor T 2  may be connected to the second node n 2 . 
     Referring to  FIGS. 3D and 3E , the impedance circuit  151  may include third and fourth transistors T 3  and T 4 . The third and fourth transistors T 3  and T 4  may be formed of npn-type or pnp-type bipolar junction transistors. An emitter of the third transistor T 3  and collector and base of the fourth transistor T 4  may be connected to the first node n 1 . Collector and base of the third transistor T 3  and an emitter of the fourth transistor T 4  may be connected to the second node n 2 . 
     The impedance circuit  151  formed using circuits described with reference to  FIGS. 3A to 3E  may have a large impedance value. 
       FIGS. 4A and 4B  are graphs illustrating a noise reduction effect of an MEMS microphone  100  according to an embodiment of the inventive concept. 
     In  FIGS. 4A and 4B , an X-axis may indicate a frequency, and a Y-axis may indicate a sound level.  FIG. 4A  is a graph illustrating a sound level on an MEMS microphone to which DC noise filters  150  and  160  are not applied.  FIG. 4B  is a graph illustrating a sound level on an MEMS microphone to which DC noise filters  150  and  160  are applied. 
     Referring to  FIG. 4A , an output noise N 1  of an MEMS microphone to which the noise filters  150  and  160  are not applied may have a noise characteristic of −20 dB/dec. The noise N 1  may be a noise generated by a reference voltage/current generator  120 . In comparison with an A-weighting level  200 , the noise N 1  of the MEMS microphone  100  may have a noise area N 1   —   a . The larger the noise area N 1   —   a , the larger distortion of a signal. The A-weighting level  200  may be one of characteristics of a weighting network used in a sound level meter, and may indicate a characteristic of a sound proximate to an audible frequency of a human. 
     Referring to  FIG. 4B , an output noise N 2  of an MEMS microphone to which the noise filters  150  and  160  are applied may have a noise characteristic of −40 dB/dec. As illustrated in  FIG. 4B , compared with the A-weighting level  200 , the output noise N 2  of the MEMS microphone to which the noise filters  150  and  160  are applied may not have a noise area. Thus, it is possible to output an output signal Vout having a reduced noise. 
     With an embodiment of the inventive concept, it is possible to provide an MEMS microphone having an improved noise characteristic by reducing DC noises of a DC reference voltage and a reference current. 
       FIG. 5  is a circuit diagram illustrating a microphone sensor and a signal amplification unit including a noise filter according to an embodiment of the inventive concept. An impedance circuit  161  of  FIG. 5  may be formed of a circuit illustrated in  FIG. 3B . However, the inventive concept is not limited thereto. 
     Referring to  FIG. 5 , a signal amplification unit  140  may include a bias circuit  141 , a source follower  142   a , an operational amplifier  142   b , a feedback circuit  142   c , an impedance circuit  161 , a capacitor circuit  162 , and a switch  163 . 
     The bias circuit  141  may generate a bias voltage necessary for the source follower  142   a  and the operational amplifier  142   b  based on a reference current Iref. A noise of the bias voltage may be reduced by the impedance circuit  161  and the capacitor circuit  162 , and the noise-free bias voltage may be applied to the source follower  142   a  and the operational amplifier  142   b . The capacitor circuit  152  may include a metal oxide semiconductor capacitor using a transistor. 
     The microphone  110  may be supplied with a sensor bias voltage Vc, and may provide an output value Vc_out to the source follower  142   a  based on a variation in a sound pressure. The source follower  142   a , the operational amplifier  142   b , and the feedback circuit  142   c  may amplify the output value Vc_out based on the bias voltage to output an output signal Vout as an amplification result. 
     Since a noise component included in the reference current Iref is reduced by the impedance circuit  161  and the capacitor circuit  162 , the output signal Vout may not be affected by a noise component included in the reference current Iref. 
     In example embodiments, at an initial operation of the signal amplification unit  140 , the switch  163  may be closed such that the capacitor circuit  162  is charged with the bias voltage in rapid time. 
       FIG. 6  is a block diagram schematically illustrating a voltage booster and a noise filter according to an embodiment of the inventive concept. 
     Referring to  FIG. 6 , an impedance circuit  151  may include PMOS transistors. The impedance circuit  151  may have an impedance value larger than that of a switch  153 . The impedance circuit  151  may be connected to one end of the capacitor circuit  152 . An output of the impedance circuit  151  may be connected to the voltage booster  130 . The capacitor circuit  152  may be formed of a MOS capacitor using an NMOS transistor. The switch  153  may selectively connect both ends of the impedance circuit  151 . 
     A noise of a DC reference voltage Vref provided from a reference voltage/current generator  120  may be reduced by the impedance circuit  151  and the capacitor circuit  152 , and a noise-free DC reference voltage may be provided to the voltage booster  130 . At an initial operation of the noise filter  150 , the switch  153  may be closed such that the capacitor circuit  152  is charged with the DC reference voltage Vref in rapid time. 
     With the above description, it is possible to provide an MEMS microphone having an improved noise characteristic. Further, it is possible to implement a small-sized and low-power MEMS microphone using a semiconductor element. 
     Although not shown in figures, noise filters according to an embodiment of the inventive concept may reduce DC noises of a bias voltage of a signal amplification unit and a sensor bias voltage, and may be also connected to an output of the signal amplification unit or an output of a microphone sensor to remove a DC noise. Thus, it is possible to provide an MEMS microphone the noise characteristic of which is further improved. 
     While the inventive concept has been described with reference to exemplary embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. Therefore, it should be understood that the above embodiments are not limiting, but illustrative.