Abstract:
A surface mounted device (SMD) type biased condenser microphone includes two terminals for a surface mounting process. The SMD type biased condenser microphone includes a grounding terminal for connecting with an external circuit, a diaphragm/backplate set, one end of which is connected to the grounding terminal, for varying a capacity according to an intensity of sound pressure and converting sound into an electric signal, a DC-DC converter for providing a bias voltage so as to form an electrostatic field at one side of the diaphragm/backplate set, a buffer IC for amplifying the electric signal from the diaphragm/backplate set, and a decoupling capacitor for preventing the bias voltage output from the DC-DC converter from being directly applied to the buffer IC and transferring the electric signal from the diaphragm/backplate set to the buffer IC.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a biased condenser microphone, and more particularly, to an SMD (surface mounting device) type biased condenser microphone having two connecting terminals for a surface mounting.  
         [0003]     2. Description of the Related Art  
         [0004]     Generally, a condenser microphone includes a set of diaphragm and backplate provided with a capacitor (C), the capacitance of which is changed depending on a voltage bias factor and a sound pressure, and a junction field effect transistor (JFET) for buffering an output signal.  
         [0005]     As an example of such a condenser microphone, there is a biased condenser microphone in which a bias voltage is supplied from the outside to form an electrostatic field between the diaphragm and the backplate.  
         [0006]      FIG. 1A  is an equivalent circuit diagram of a conventional biased condenser microphone. A microphone capsule  10  including a buffer IC  14  and a variable condenser  12  in a microphone unit is connected through three terminals  16 - 1 ,  16 - 2  and  16 - 3  to an external circuit. The first terminal  16 - 1  is used to connect an output portion of the buffer IC  14  through a resistor R 1  to a power source Vdd and, at the same time, through a capacitor to a signal output portion, and the second terminal  16 - 2  is used to connect the buffer IC  14  to a grounding portion GND. Also, the third terminal  16 - 3  is used to supply a bias voltage to the microphone unit.  
         [0007]      FIG. 1B  is another equivalent circuit diagram of the conventional biased condenser microphone. The microphone capsule  10  including the buffer IC  14  and the variable condenser  12  in the microphone unit is also connected through three terminals  16 - 1 ,  16 - 2  and  16 - 3  to an external circuit. The first terminal  16 - 1  is used to supply a bias voltage through a resistor R 2  to the microphone unit and the second terminal is used to connect an output portion of the buffer IC  14  through a resistor R 1  to a power source Vdd and, at the same time, through a capacitor to a signal output portion. Also, the third terminal  16 - 3  connects the buffer IC  14  to the grounding portion GND.  
         [0008]     However, since the conventional biased condenser microphone is provided with at least three terminals such as the bias terminal, the power and output terminal and the grounding terminal so as to interface with the outside, there is a problem that a direction of the circular condenser microphone should be checked upon a surface mounting process. Further, since a separate voltage device for supplying the bias voltage has to be provided to the outside of the microphone, it is difficult to miniaturize the microphone. Furthermore, since it has poor compatibility with an electret condenser microphone (ECM), which is generally used for connection with an external circuit, there is another problem that includes providing a printed circuit board (PCB), which has to be separately designed.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, the present invention is directed to a biased condenser microphone that substantially obviates one or more problems due to limitations and disadvantages of the related art.  
         [0010]     An object of the present invention is to provide an SMD type biased condenser microphone that can improve compatibility with a conventional ECM by including a two-terminal type device using a decoupling capacitor, and solve the directional problem of the circular condenser microphone in the surface mounting process, and also form the electrostatic field by applying a voltage from the outside so as to be capable of maintaining a constant electric field even after reflow work thereby preventing loss of sensitivity.  
         [0011]     Another object of the present invention is to provide an SMD type biased condenser microphone in which a voltage pump IC having a built-in decoupling capacitor is mounted on a PCB of a microphone, and the voltage pump IC and a buffer IC are driven by a voltage supplied through a single voltage input terminal, and sensitivity can be adjusted by changing an intensity of electrostatic field between a diaphragm and a backplate according to an intensity of bias voltage amplified and transferred from an output terminal of the voltage pump IC.  
         [0012]     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided an SMD type biased condenser microphone, comprising a grounding terminal for connecting with an external circuit; a diaphragm/backplate set one end of which is connected to the grounding terminal, for varying a capacity according to an intensity of sound pressure and converting sound into an electric signal; a DC-DC converter for providing a bias voltage so as to form an electrostatic field at one side of the diaphragm/backplate set; a buffer IC for amplifying the electric signal from the diaphragm/backplate set; and a decoupling capacitor for preventing the bias voltage output from the DC-DC converter from being directly applied to the buffer IC and transferring the electric signal from the diaphragm/backplate set to the buffer IC.  
         [0013]     Therefore, the present invention can improve compatibility with a conventional ECM, and solve the directional problem of the circular condenser microphone in the surface mounting process, and also form the electrostatic field by applying a voltage from the outside so as to be capable of maintaining a constant electric field even after reflow work thereby preventing loss of sensitivity.  
         [0014]     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be ized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
         [0015]     It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings:  
         [0017]      FIGS. 1A and 1B  are circuit diagrams of conventional bias condenser microphones;  
         [0018]      FIG. 2  is a circuit diagram of an SMD type condenser microphone according to a first embodiment of the present invention;  
         [0019]      FIG. 3  is a cross-sectional view of the SMD type condenser microphone according to the first embodiment of the present invention;  
         [0020]      FIG. 4  is a perspective view of a connecting terminal of the condenser microphone of  FIG. 3 ;  
         [0021]      FIG. 5  is a circuit diagram of an SMD type condenser microphone according to a second embodiment of the present invention; and  
         [0022]      FIG. 6  is a cross-sectional view of the SMD type condenser microphone according to the second embodiment of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
         [0024]     Unlike in the conventional way of forming an electrostatic field by a forcibly injected electret, in an operational principle of the present invention, an electrostatic field is formed between a backplate and a diaphragm by applying a des voltage from the outside and outputting an electric signal corresponding to vibration of the diaphragm through a buffer IC.  
         [0025]     To supply the external power between the backplate and the diaphragm, the conventional microphone needs three external terminals, e.g., an external power supplying terminal, a signal outputting terminal and a grounding terminal. However, a condenser microphone of the present invention can be driven with two terminals.  
       First Embodiment  
       [0026]      FIG. 2  is a circuit diagram of an SMD type condenser microphone according to a first embodiment of the present invention, and  FIG. 3  is a cross-sectional view of the SMD type condenser microphone according to the first embodiment of the present invention.  
         [0027]     In the equivalent circuit according to a first embodiment of the present invention, as shown in  FIG. 2 , a diaphragm  206  and a backplate  210  are represented as a single variable condenser C 0  so that the diaphragm  206  is connected to a grounding portion GND and the backplate  210  is connected to a DC-DC converter  232 . A decoupling capacitor C 1  is connected between the DC-DC converter  232  and a buffer IC  240 . Herein, a voltage pump IC  230  is comprised of the DC-DC converter  232  and the decoupling capacitor C 1 , and the buffer IC  240  may include an FET, an amplifier or an analog-digital converter.  
         [0028]     Meanwhile, in an internal PCB circuitry, if necessary, a circuit for connecting capacitors or capacitors, resistors, etc., in series or parallel may be added to a basic component such as the voltage pump IC  230  and the buffer IC  240  in order to improve a characteristic with respect to EMI or ESD.  
         [0029]     In an SMD type biased condenser microphone according to a first embodiment of the present invention, as shown in  FIG. 3 , a sound inlet hole  202   a  is formed in a bottom surface. The diaphragm  206  integrally formed with a ring  204  is inserted into a cylindrical case  202 , one surface of which is opened. On the diaphragm  206 , there is provided a spacer  208  to secure a space between the backplate  210  and the diaphragm  206 . A cylindrical first base  212  made of an insulating material is disposed on the spacer  208 . The backplate  210  made of a metal plate is disposed at an inside portion of the first base  212  so as to be apart from the diaphragm by the spacer  208 . On the backplate  210 , there is provided a second base  214  made of a conductive material so as to electrically connect the backplate  210  with a circuit of a PCB  216 . The PCB  216  on which components (voltage pump IC, buffer IC, etc.) are mounted is disposed thereon, and then an end of the case  202  is curled or conformed to secure or seal the surface.  
         [0030]     Referring to  FIG. 3 , the backplate  210  is formed of a metal plate without a polymer series film so as to form the electret. The diaphragm  206  is formed of a metal film or formed by depositing a metal on one or both surfaces of an organic or inorganic film.  
         [0031]     Meanwhile, as shown in  FIG. 4 , connecting terminals  218 ,  220  are formed on an exposed surface of the PCB  216  so as to be protruded further than the curled surface of the case  202 , so that a microphone  200  can be attached to a main PCB (e.g., a PCB of a cellular phone) in an SMD type. To this end, as shown in  FIG. 4 , a circular terminal  220  for power and output connection Vdd/Out is formed at an inside portion, and a ring-shaped grounding terminal  218  is formed at the outside of the circular terminal  220  so as to be apart from the circular terminal  220  at a distance. The grounding terminal  218  is divided into three parts by three gas outlet grooves  222  for exhausting gas generated upon the SMD type attaching process.  
         [0032]     An operation of the biased condenser microphone according to the first embodiment of the present invention will be described in detail.  
         [0033]     Referring again to  FIGS. 2 and 3 , according to the first embodiment of the present invention, the driving voltage Vdd is applied through the power and output terminal  220  of the PCB  216  to the buffer IC  240  and the voltage pump IC  230 , respectively. The applied driving voltage Vdd drives the buffer IC  240  and the voltage pump IC  230 . The DC-DC converter  232  of the voltage pump IC  230  converts the driving voltage Vdd into a DC bias voltage V B  amplified to a desired level. The bias voltage V B  is applied through the second base  214  to the backplate  210 . The grounding terminal  218  of the PCB  216  is commonly connected to the buffer IC  240  and the DC-DC converter  240 , and at the same time, connected through the case  202  and the ring  204  to the diaphragm  206 . Therefore, between the backplate  210  to which the bias voltage V B  is applied and the grounded diaphragm  206 , there is formed the capacitance C 0  and the electrostatic field by the bias voltage V B .  
         [0034]     In this situation, if the diaphragm  206  is vibrated according to external sound pressure, an electric signal is generated. The electric signal is transferred through the backplate  210  and the second base  214  to the buffer IC  240  of the PCB  216 , and amplified in the buffer IC  240 , and then output through the power and output terminal  220  of the PCB  216  to the outside. According to the present invention, in order for the DC bias voltage V B  output from the DC-DC converter  232  to be prevented from being directly applied to the buffer IC  240 , the decoupling capacitor C 1  is connected between an output portion of the voltage pump IC  230  and an input portion of the buffer IC  240 . The decoupling capacitor C 1  functions to prevent the DC bias voltage V B  from being directly applied to the buffer IC  240  and allow only the electric signal generated by the vibration of the diaphragm  206  to be passed to the buffer IC  240 , thereby separating the DC bias voltage V B  from the electric signal.  
       Second Embodiment  
       [0035]      FIG. 5  is a circuit diagram of an SMD type condenser microphone  500  according to a second embodiment of the present invention, and  FIG. 6  is a cross-sectional view of the SMD type condenser microphone  500  according to the second embodiment of the present invention.  
         [0036]     In comparison with the first and second embodiments of the present invention, since a structure of the second embodiment is entirely similar to that of the first embodiment except for relocation of the backplate  210  and the diaphragm  206 , the description of the same or similar parts will be omitted.  
         [0037]     Referring to  FIGS. 5 and 6 , in the circuit of the second embodiment compared with the equivalent circuit of the first embodiment, the variable condenser CO is equivalent to the backplate  210  and the diaphragm  206 , and the backplate  210  is connected to the grounding portion, and the diaphragm  206  is connected to the DC-DC converter  232 .  
         [0038]     That is, in the second embodiment, the driving voltage Vdd is applied through the power and output terminal Vdd/output  220  of the PCB  216  to the buffer IC  240  and the voltage pump IC  230 , respectively. The applied driving voltage Vdd drives the buffer IC  240  and the voltage pump IC  230 . The DC-DC converter  232  of the voltage pump IC  230  converts the driving voltage Vdd into the DC bias voltage V B  amplified to a desired level. The bias voltage V B  is applied through the second base  214  and the ring  204  to the diaphragm  206 . The grounding terminal  218  of the PCB  216  is commonly connected to the buffer IC  240  and the DC-DC converter  240 , and at the same time, connected through the case  202  to the backplate  210 . Therefore, between the grounded backplate  210  and the diaphragm  206  to which the bias voltage V B  is applied, there is formed the capacitance C 0  and the electrostatic field by the bias voltage V B .  
         [0039]     In this state, if the diaphragm  206  is vibrated according to external sound pressure, an electric signal is generated. The electric signal is transferred through the ring  204  and the second base  214  to the buffer IC  240  of the PCB  216 , and amplified in the buffer IC  240 , and then output through the power and output terminal  220  of the PCB  216  to the outside. According to the present invention, in order for the DC bias voltage V B  output from the DC-DC converter  232  to be prevented from being directly applied to the buffer IC  240 , the decoupling capacitor C 1  is connected between the output portion of the voltage pump IC  230  and an input portion of the buffer IC  240 . The decoupling capacitor C 1  functions to prevent the DC bias voltage V B  from being directly applied to the buffer IC  240  and allow only the electric signal generated by the vibration of the diaphragm  206  to be passed to the buffer IC  240 , thereby separating the DC bias voltage V B  from the electric signal.  
         [0040]     As described above, an SMD type biased condenser microphone according to the present invention can improve compatibility with the conventional ECM by forming into two-terminal (the power/output terminal and the grounding terminal type using the decoupling capacitor, and solve the directional problem of the circular condenser microphone in the surface mounting process, and further form the electrostatic field by applying the voltage from the outside so as to be capable of maintaining the constant electric field without the loss of sensitivity due to the reflow work at a high temperature. In addition, in the SMD type biased condenser microphone of the present invention, the voltage pump IC is mounted on a PCB of a microphone, and the voltage pump IC and a buffer IC are driven by the same voltage as that in conventional microphone, and the sensitivity can be adjusted according to the intensity of bias voltage amplified and transferred from the output terminal of the voltage pump IC.  
         [0041]     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.