Abstract:
A capacitor microphone includes: a capacitor microphone unit including a diaphragm that vibrates upon receiving sound waves and a fixed electrode arranged opposite to the diaphragm with a space therebetween; and a polarization voltage generating circuit that generates polarization voltage to be applied across the diaphragm and the fixed electrode. The polarization voltage generating circuit includes an oscillating circuit that alternately turns on and off DC power, coils to boost the voltage of the power alternately turned on and off, and a DC boosting circuit including a rectifying circuit that rectifies the boosted voltage. The coils are formed of two inductors that are electromagnetically coupled and are provided with an electromagnetic coupling adjusting unit with which the level of electromagnetic coupling between the two inductors is adjusted to adjust the polarization voltage.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a capacitor microphone including a polarization voltage generating circuit, and more particularly to a capacitor microphone including a polarization voltage generating circuit in which polarization voltage can be controlled. 
         [0003]    2. Description of the Related Art 
         [0004]    Capacitor microphones generally include a capacitor microphone unit mainly including a diaphragm that vibrates upon receiving sound waves and a fixed electrode arranged to be opposite to the diaphragm. A peripheral portion of the diaphragm is attached to a diaphragm holding ring with appropriate extension force applied to the diaphragm. A ring-shaped spacer is provided between the fixed electrode and a portion of the diaphragm attached to the diaphragm holding ring, thereby providing a minute space defined by the thickness of the spacer between the fixed electrode and the diaphragm. The diaphragm and the fixed electrode are incorporated in a unit casing of the microphone unit along with other elements such as an insulating base, a field effect transistor (FET) serving as an impedance converter, and a circuit board. The elements are positioned and fixed in the unit casing. 
         [0005]    The diaphragm is connected to, for example, the anode side of a power source for polarization via the diaphragm holding ring and the unit casing. The fixed electrode is connected to, for example, the cathode side of the power source for polarization via a leading terminal penetrating the insulating base to protrude therefrom. Thus, polarization voltage is applied across the diaphragm and the fixed electrode, whereby a capacitor is formed by the diaphragm and the fixed electrode. 
         [0006]    When the diaphragm vibrates upon receiving sound waves, the distance between the diaphragm and the fixed electrode changes and the capacity of the capacitor changes. This is output as a change in current across the diaphragm and the fixed electrode. Impedance of such an output is extremely high. Thus, an output from the impedance converter formed by the FET, which lowers the impedance, serves as an output from the microphone unit. 
         [0007]    The sensitivity of the capacitor microphone having the structure described above depends on the polarization voltage applied across the diaphragm and the fixed electrode. Higher polarization voltage is directly related to higher sensitivity. Therefore, polarization voltage is generated by increasing DC power voltage by a DC boost circuit, i.e., a DC-DC converter. Unfortunately, due to other circuits in the microphone, the DC-DC converter is only capable of receiving current of 1 mA or lower. Thus, the polarization voltage cannot be increased over a certain level. To further increase the polarization voltage, the DC-DC converter is provided with a rectifying circuit of a voltage multiplier configuration.  FIG. 5  exemplarily illustrates a polarization voltage generating circuit, included in a conventional capacitor microphone, in which a rectifying circuit of the DC-DC converter has a voltage multiplier configuration. 
         [0008]    The polarization voltage generating circuit illustrated in  FIG. 5  is configured as a DC-DC converter that has a power input terminal  10  connecting to a phantom power source. The polarization voltage generating circuit mainly includes an oscillating circuit  12  and a rectifying circuit  14  of a voltage multiplier configuration. The oscillating circuit  12  is mainly composed of a transistor TR. A variable resistor VR is connected between the power input terminal  10  and the collector of the transistor TR. A capacitor C 5  is connected between the power input terminal  10  and the emitter of the transistor TR. A resistor R is connected between the collector and the base of the transistor TR. A coil L 2  is connected between the emitter of the transistor TR and earth. The base of the transistor TR is connected to one end of a capacitor C 4  and the emitter of the transistor TR is connected to one end of a coil L 1 . The other ends of the capacitor C 4  and the coil L 1  are connected with each other. Thus, the capacitor C 4  and the coil L 1  are connected in series between the base and the emitter of the transistor TR. 
         [0009]    The coils L 1  and L 2  are electromagnetically inductively coupled by, for example, being wound around a common core. The oscillating circuit  12  alternately oscillates. The coils L 1  and L 2  are electromagnetically inductively coupled. The turn ratio between the coils L 1  and L 2  is approximately 1 to 8. Thus, DC voltage of about 5 V input through the power input terminal  10  is boosted to AC voltage of about 40 V. This AC voltage is converted into high DC voltage by the rectifying circuit  14  of a voltage multiplier configuration described below. The current flowing through the oscillating circuit  12  can be adjusted by the variable resistor VR to obtain appropriate polarization voltage as described below. 
         [0010]    The rectifying circuit  14  of a voltage multiplier configuration includes four diodes D 1  to D 4  and three capacitors C 1  to C 3 . The four diodes are connected in series in the forward direction between the earth and an output terminal  16  in order of D 4  to D 1  from the earth to the output terminal  16 . Thus, the anode of the diode D 4  is connected to the earth while the cathode of the diode D 1  is connected to the output terminal  16 . The capacitor C 1  is connected between a connection point of the diodes D 2  and D 3  and the output terminal  16 . The capacitor C 2  is connected between connection points of the diodes D 1  and D 2  and the diodes D 3  and D 4 . The capacitor C 3  is connected between a connection portion of the diodes D 3  and D 4  and a connection portion of the capacitor C 4  and the coil L 1 . Thus, the rectifying circuit  14  has a voltage tripler configuration. Therefore, the AC voltage of about 40 V from the oscillating circuit  12  is boosted to DC voltage of about 100 to 120 V. The DC voltage obtained by the boosting is output from the output terminal  16  as polarization voltage of the capacitor microphone directly or after being smoothed by a smoothing circuit (not illustrated). 
         [0011]    Japanese Patent Application Publication H9-121533 discloses a polarization voltage generating circuit similar to that illustrated in  FIG. 5 . 
         [0012]    A conventional capacitor microphone having the polarization voltage generating circuit as illustrated in  FIG. 5  obtains appropriate polarization voltage by adjusting current flowing through the transistor TR for oscillation using the variable resistor VR, as described above. Operating condition of the oscillating circuit  12  fluctuates because the current flowing through the transistor TR is variable. Therefore, the current that can be supplied to the polarization voltage generating current cannot be specified or is limited. Thus, individual difference in consumption current of the microphone may be produced and the output voltage from the output terminal  16  may fluctuate. The same problem occurs when other conditions such as coupling level of the coils L 1  and L 2  change. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention is made in view of the problem of the conventional technique and an object of the present invention is to provide a capacitor microphone that allows the stabilization of the polarization voltage output by eliminating the individual difference in current consumption of the oscillating circuit included in a polarization voltage generating circuit while allowing the polarization voltage to be adjusted by adjusting the electromagnetic coupling of two inductors (coils) in the oscillating circuit. 
         [0014]    A capacitor microphone according to an aspect of the present invention includes: a capacitor microphone unit including a diaphragm that vibrates upon receiving sound waves and a fixed electrode arranged opposite to the diaphragm with a space therebetween; and a polarization voltage generating circuit that generates polarization voltage to be applied across the diaphragm and the fixed electrode. The polarization voltage generating circuit includes an oscillating circuit that alternately turns on and off DC power, coils that are electromagnetically coupled to each other to boost the voltage of the power alternately turned on and off, and a DC boosting circuit including a rectifying circuit that rectifies the boosted voltage. The coils are formed of two inductors that are electromagnetically coupled and are provided with an electromagnetic coupling adjusting unit with which the level of electromagnetic coupling between the two inductors is adjusted to adjust the polarization voltage. 
         [0015]    The electromagnetic coupling adjusting unit may be: formed of a magnetic material provided across the two inductors and the level of electromagnetic coupling between the two inductors may be adjusted by adjusting the position of the magnetic material with respect to the two inductors. 
         [0016]    The electromagnetic coupling adjusting unit may be formed of an electromagnetic inductive coupling coil provided across the two inductors and the level of electromagnetic coupling between the two inductors may be adjusted by adjusting the position of the electromagnetic inductive coupling coil with respect to the two inductors. 
         [0017]    A current regulative element may be provided between a DC power input terminal and the oscillating circuit. 
         [0018]    A constant voltage element may be provided between the DC power input terminal and the oscillating circuit. 
         [0019]    The oscillating circuit in the polarization voltage generating circuit alternately turns on and off the DC power. The voltage of the power that is turned on and off is boosted by the two inductors electromagnetically coupled to each other. The boosted voltage is rectified by the rectifying circuit into DC voltage. The DC voltage is applied across the diaphragm and the fixed electrode as polarization voltage. The level of electromagnetic coupling between the two inductors can be adjusted to adjust the polarization voltage. The adjustment of the level of electromagnetic coupling between the two inductors causes no change of the operation condition of the oscillating circuit. Therefore, current supplied to the polarization voltage generating circuit is stabilized and thus, polarization voltage to be generated is stabilized. 
         [0020]    The electromagnetic coupling adjusting unit is formed of a magnetic material provided across the two inductors. The level of electromagnetic coupling between the two inductors is adjusted by adjusting the position of the magnetic material with respect to the two inductors. This structure allows the polarization voltage to be adjusted easily. 
         [0021]    The polarization voltage can be adjusted easily also when the electromagnetic coupling adjusting unit is formed of an electromagnetic inductive coupling coil provided across the two inductors and the level of electromagnetic coupling between the two inductors is adjusted by adjusting the position of the electromagnetic inductive coupling coil with respect to the two inductors. 
         [0022]    The polarization voltage to be generated can be further stabilized by providing a current regulative element or a constant voltage element between the DC power input terminal and the oscillating circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a circuit diagram of an example of a polarization voltage generating circuit used in a capacitor microphone according to the present invention; 
           [0024]      FIG. 2  is a plan view of an example of an adjusting unit for electromagnetic coupling between two inductors in the polarization voltage generating circuit; 
           [0025]      FIGS. 3A and 3B  are each a plan view of a modification of the adjusting unit for electromagnetic coupling between two inductors; 
           [0026]      FIG. 4  is a front view schematically illustrating the example of the adjusting unit for electromagnetic coupling between two inductors; and 
           [0027]      FIG. 5  is a circuit diagram of an example of a polarization voltage generating circuit used in a conventional capacitor microphone. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    An embodiment of a capacitor microphone according to the present invention is described below with reference to some of the accompanying drawings. Note that the feature of the present invention lies in the structure of a polarization voltage generating circuit and therefore the description is given mainly thereto. Elements similar to those in the conventional example illustrated in  FIG. 5  are given the same reference numerals. 
         [0029]    In  FIG. 1 , a polarization voltage generating circuit is configured as a DC-DC converter including a power input terminal  10  connecting to a phantom power source. The polarization voltage generating circuit mainly includes an oscillating circuit  12 , a rectifying circuit  14  of a voltage multiplier configuration, a current regulative diode CRD, and a Zener diode ZD. The oscillating circuit  12  is mainly composed of a transistor TR that is an active element. The power input terminal  10  is connected to the collector of the transistor TR via the current regulative diode CRD so that constant current of approximately 1 mA flows through the transistor TR. The Zener diode ZD is connected between the collector of the transistor TR and earth so that a constant voltage is applied to the collector of the transistor TR. The Zener diode ZD and a capacitor C 6  are connected in parallel. 
         [0030]    A capacitor C 5  is connected between the collector and the emitter of the transistor TR. A resistor R is connected between the collector and the base of the transistor TR. A coil L 2  is connected between the emitter of the transistor TR and the earth. The base of the transistor TR is connected to one end of a capacitor C 4 . The emitter of the transistor TR is connected to one end of a coil L 1 . Other ends of the capacitor C 4  and the coil L 1  are connected with each other. Thus, the capacitor C 4  and the coil L 1  are connected in series between the base and the emitter of the transistor TR. 
         [0031]    The coils L 1  and L 2  are wound around different cores as illustrated in  FIGS. 2 and 4 . The cores of the coils L 1  and L 2  are electromagnetically inductively coupled with each other with, for example, a wiring pattern formed on a circuit board  20  serving as an electromagnetic inductive coupling coil or with magnetic fluxes from the cores of the coils L 1  and L 2  being exchanged therebetween. The oscillating circuit  12  alternately oscillates with the emitter output from the transistor TR returning to the base of the transistor TR via the coil L 1  and the capacitor C 4 . The turn ratio between the coils L 1  and L 2 , which are electromagnetically-inductively coupled, is approximately 1 to 8. Thus, DC voltage of about 5 V input through the power input terminal  10  is boosted to AC voltage of about 40 V. This AC voltage is converted into high DC voltage by the rectifying circuit  14  of a voltage multiplier configuration described below. The inductances of the coils L 1  and L 2  are about 1 mH and 120 μH, respectively. The two coils L 1  and L 2 , i.e., the two inductors have an electromagnetic coupling adjustment unit with which the level of electromagnetic coupling therebetween is adjusted. Thus, the polarization voltage applied across the diaphragm and the fixed electrode of the capacitor microphone can be adjusted. The electromagnetic coupling adjustment unit is described in detail below. 
         [0032]    The rectifying circuit  14  of a voltage multiplier configuration includes four diodes D 1  to D 4  and three capacitors C 1  to C 3 . The four diodes are connected in series between the earth and an output terminal  16  in order of D 4  to D 1  from the earth to the output terminal  16 . Thus, the anode of the diode D 4  is connected to the earth while the cathode of the diode D 1  is connected to the output terminal  16 . The capacitor C 1  is connected between a connection point of the diodes D 2  and D 3  and the output terminal  16 . The capacitor C 2  is connected between connection points of the diodes D 1  and D 2  and the diodes D 3  and D 4 . The capacitor C 3  is connected between a connection portion of the diodes D 3  and D 4  and a connection portion of the capacitor C 4  and the coil L 1 . Thus, the rectifying circuit  14  has a voltage tripler configuration. Therefore, the AC voltage of about 40 V from the oscillating circuit  12  is boosted to DC voltage of 100 to 120 V. The DC voltage obtained by the boosting is output from the output terminal  16  as polarization voltage for the capacitor microphone directly or after being smoothed by a smoothing circuit (not illustrated.) 
         [0033]    A specific example of the electromagnetic coupling adjustment unit is described below with reference to  FIGS. 2 ,  3 A, and  3 B. The electromagnetic coupling adjustment unit is a magnetic material or an electromagnetically inductively coupling coil provided across the two inductors, i.e., the coils L 1  and L 2 . The level of electromagnetic coupling is adjusted by adjusting the position of the magnetic material or the electromagnetic inductive coupling coil with respect to the coils L 1  and L 2 . The circuit board  20  incorporated in the microphone is illustrated in  FIG. 2 . The DC boost circuit as illustrated in  FIG. 1  is installed in the circuit board  20  and the coils L 1  and L 2  are installed while being apart from each other with an appropriate distance. The electromagnetic coupling adjustment unit in  FIG. 2  is formed of an electromagnetic inductive coupling coil  22  of a single turn. 
         [0034]    As is also illustrated in  FIG. 4 , the electromagnetic inductive coupling coil  22  is an oval endless coil having a size large enough to include the two coils L 1  and L 2  arranged while being apart from each other for a certain distance. When the electromagnetic inductive coupling coil  22  is moved closer to the two coils. L 1  and L 2 , the electromagnetic coupling between the coils L 1  and L 2  is canceled out, whereby the polarization voltage output from the output terminal  16  of the polarization voltage generating circuit illustrated in  FIG. 1  is decreased. Alternatively, the winding direction or the direction of connection of the two coils L 1  and L 2  can be so set that the polarization voltage increases when the electromagnetic inductive coupling coil  22  is moved closer to the two coils L 1  and L 2 . 
         [0035]    The direction to which the electromagnetic inductive coupling coil  22  is moved for adjusting, the level of electromagnetic coupling between the coils L 1  and L 2  can be arbitrarily set. The position of the electromagnetic-coupling adjustment coil  22  with respect to the coils L 1  and L 2  can be adjusted by moving the electromagnetic coupling adjustment coil  22  in the center axis direction of the cores of the coils L 1  and L 2  (vertical direction as viewed in  FIG. 4 ) or the direction orthogonal to the center axis direction (the direction orthogonal to the plane of the paper, i.e., the horizontal direction as viewed in  FIG. 4 .) 
         [0036]    An electromagnetic inductive coupling coil  24  as illustrated in  FIG. 3A  may be used instead of the electromagnetic inductive coupling coil  22  illustrated in  FIG. 2 . The electromagnetic inductive coupling coil  24  is formed by twisting the electromagnetic inductive coupling coil  22  once, thereby intersecting at the center portion in the longitudinal direction. The respective looped portions on both sides of the intersecting portion are opposed to the two coils L 1  and L 2 . When the electromagnetic inductive coupling coil  24  is moved closer to the two coils L 1  and L 2 , the electromagnetic coupling between the coils L 1  and L 2  is strengthened to increase the polarization voltage output from the output terminal  16  of the polarization voltage generating circuit illustrated in  FIG. 1 . Also with the electromagnetic inductive coupling coil  24 , the winding direction or the direction of connection of the two coils L 1  and L 2  can be so set that the polarization voltage decreases when the electromagnetic coupling adjustment coil  24  is moved closer to the two coils L 1  and L 2 . The direction to which the electromagnetic coupling adjustment coil  24  is moved can be arbitrarily set. 
         [0037]      FIG. 3B  illustrates an example of an electromagnetic coupling adjustment unit formed by a plate-like magnetic material  26 . An area of the magnetic material  26  is large enough to include the two coils L 1  and L 2  as viewed in a planer direction. As the magnetic material  26  is moved closer to the two coils L 1  and L 2 , the electromagnetic coupling between the coils L 1  and L 2  is canceled out, whereby the polarization voltage output from the output terminal  16  of the polarization voltage generating circuit illustrated in  FIG. 1  is decreased. 
         [0038]    In the embodiment described above, the polarization voltage can be adjusted by adjusting the level of electromagnetic coupling between the two inductors, i.e., the coils L 1  and L 2 . The adjustment of the level of electromagnetic coupling between the coils L 1  and L 2  for adjusting the polarization voltage causes no change in the operating condition of the oscillator  12 . Thus, the current to be supplied to the polarization voltage generating circuit can be stabilized and thus, polarization voltage to be generated can be stabilized. 
         [0039]    In addition, in the above-described embodiment, the current regulator diode CRD is connected between the DC power input terminal  10  and the oscillating circuit  12 . Thus, the current to be supplied to the oscillating circuit  12  and thus, polarization voltage to be generated can be stabilized further. 
         [0040]    Moreover, the Zener diode ZD is connected between the DC power input terminal  10  and the oscillating circuit  12 . Thus, operating voltage of the oscillating circuit  12  is stabilized and thus, polarization voltage to be generated can be stabilized even further. 
         [0041]    As described above, the individual difference in consumption current of the oscillating circuit  12  of the polarization voltage generating circuit can be eliminated. 
         [0042]    In the illustrated embodiment, DC voltage is input to the power input terminal  10  from the phantom power source. Alternatively, a battery incorporated in the microphone can also be used as the power source. 
         [0043]    The rectifying circuit  14  of a voltage multiplier configuration may multiply a supplied voltage by a factor arbitrarily set. The factor may be two or three or more.