Abstract:
A micro-speaker. The micro-speaker includes a first plate, a second plate, and a diaphragm. The first plate is biased to a first voltage. The second plate is biased to a second voltage. The diaphragm is positioned between the first plate and the second plate and is configured to receive a digital signal. The digital signal causes the diaphragm to cycle between fully displaced toward the first plate and fully displaced toward the second plate, creating air pressure pulses that mimic the digital signal.

Description:
BACKGROUND 
       [0001]    The invention relates to an electrostatic speaker, specifically a class D electro-static micro-speaker. 
         [0002]    Speakers produce sound waves by converting electrical signals into air pressure pulses. A classic dynamic loudspeaker uses a voice coil in a magnetic gap to move a cone and convert the electrical signals into air pressure pulses. A prior-art electrostatic speaker  100 , such as shown in  FIG. 1 , has a diaphragm  105  biased (by springs  110 ) at a midpoint within a usable gap  115 . An analog electric signal  120  (i.e., an audio signal) is applied to the diaphragm  105  which cyclically draws the diaphragm  105  toward from a back-plate  125  and allows the diaphragm  105  to return to a resting position (by the biasing force of the springs  110 ). 
         [0003]    In the prior-art electrostatic speaker  100 , the springs  110 , along with air that is moved (e.g., in the usable gap  115 ), tend to dampen the response of the electrostatic speaker  100 . The electrostatic speaker  100  also has numerous nonlinear mechanisms (e.g., mechanical stiffness which varies based on the position of the diaphragm  105 ) which affect the performance of the speaker  100 . In addition, the diaphragm is limited to only about 10% of its actual range to reduce linearity issues and to guard against “snap-in” (i.e., where the diaphragm latches in a fully extended position). Micro-speakers are a sub-set of electrostatic speakers, referring generally to the size of the electrostatic speaker. Micro-speakers are small speakers such as those found in cell phones. 
         [0004]    Class D amplifiers combine a desired output signal with a relatively high-frequency signal to generate a digital signal which can be amplified by switching power devices. The frequency of the high-frequency signal is typically chosen to be ten or more times the highest frequency of interest in the desired output signal. The digital signal includes both a low-frequency component (i.e., the desired output signal) and a high-frequency component. A passive low-pass filter is used to remove the high-frequency component, and recover the desired low-frequency output signal. 
       SUMMARY 
       [0005]    The class D micro-speaker of the invention provides flatter frequency response throughout the audio band, improved linearity and distortion performance, and greater sound pressure levels than the prior art electrostatic speakers  100 . The class D micro-speaker of the invention overcomes these issues by having only two states for a diaphragm—snapped up or snapped down (i.e., displaced 100% of its range). To achieve these states, the electrical input of the class D micro-speaker overdrives the core mechanical resonances to drive the diaphragm to geometrical stops, independent of the signal level. Because the class D micro-speaker travels to its full snapped up or snapped down position, it is using 100% of its range (compared to the prior art speaker&#39;s 10% range). This generates a much greater sound pressure level (SPL) output for an equivalently sized speaker, reducing the cost and size of the speaker. 
         [0006]    In one embodiment, the invention provides a micro-speaker. The micro-speaker includes a first plate, a second plate, and a diaphragm. The first plate is biased to a first voltage. The second plate is biased to a second voltage. The diaphragm is positioned between the first plate and the second plate and is configured to receive a digital signal. The digital signal causes the diaphragm to cycle between a first fully displaced position near the first plate and a second fully displaced position near the second plate, creating air pressure pulses that mimic the digital signal. 
         [0007]    In another embodiment the invention provides a method of producing sound waves. The method includes receiving an analog electric signal representative of the sound waves to be produced, generating a high-frequency signal, producing a pulse-width-modulated signal based on the analog electric signal and the high-frequency signal, amplifying the pulse-width-modulated signal, and applying the amplified pulse-width-modulated signal to a diaphragm of a speaker. The diaphragm is positioned between a first plate and a second plate. The first plate has a first electrical bias, and the second plate has a second electrical bias. The amplified pulse-width-modulated signal causes the diaphragm to cycle between a fully displaced position toward the first plate and a second fully displaced position toward the second plate, creating air pressure pulses that mimic the pulse-width-modulated signal. 
         [0008]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a diagram of a prior-art electrostatic speaker. 
           [0010]      FIG. 2  is a diagram of a class D micro-speaker. 
           [0011]      FIG. 3  is a schematic/block diagram of a circuit for driving the micro-speaker of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    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 following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
         [0013]      FIG. 2  shows a construction of a class D micro-speaker  200 . The speaker  200  includes a diaphragm  205 , made of an electrically conductive material, supported by a polysilicon  210 . One or more structures (e.g., a housing, a MEMS structure, etc.) supports the elements of the speaker  200  including the polysilicon  210 . The diaphragm  205  is positioned approximately midway between a back-plate  215  and a top-plate  220 , with an air gap  225  between the diaphragm  205  and each plate  215  and  220 . In some constructions, the diaphragm  205  is positioned closer to one of the back-plate  215  or the top-plate  220 . Both the back-plate  215  and the top-plate  220  include openings  230  to allow air in the air gaps  225  to escape/enter the speaker  200  when the diaphragm  205  is moved toward the top-plate  220  or toward the back-plate  215 . The back-plate  215  is held at a ground potential, while the top-plate  220  is held at a relatively high voltage level (compared to CMOS voltage levels, e.g., 10 to 50 volts DC). The voltage level required for the top-plate  220  is dictated by a number of design characteristics (e.g., desired SPL, costs, etc.) and the size of the air gap  225 . The larger the gap  225 , the higher the voltage needed to snap the diaphragm  205  toward and away from the top-plate  225 . 
         [0014]    In operation, the diaphragm  205  receives a signal  235  (i.e., a digital signal) that cycles the diaphragm  205  between a positive voltage and ground. When the signal  235  applied to the diaphragm  205  is a positive voltage, the diaphragm  205  is drawn toward the back-plate  215 , causing the diaphragm  205  to snap into a fully displaced position  240  toward or near the back-plate  215 . Snapping the diaphragm  205  into this position  240  forces air out of the air gap  222  through the openings  230  in the back-plate  215 , and draws air into the air gap  225  through the openings  230  in the top-plate  220 . The fully displaced position  240  depends on the play in the polysilicon  210 , but does not extend to contact the back-plate  215 . Similarly, when the diaphragm  205  receives a ground potential from signal  235 , it is drawn toward the top-plate  220 , snapping into a fully displaced position  245  toward or near the top-plate  220 . Snapping the diaphragm  205  into this position  245  forces air out of the air gap  225  through the openings  230  in the top-plate  220 , and draws air into the air gap  222  through the openings  230  in the back-plate  215 . Forcing the air out of the air gaps  220  and  225  generates air pressure pulses (e.g., a sound wave). In some embodiments, physical stops are used to limit the travel of the diaphragm  205  toward the top-plate  220  and the back-plate  215 . 
         [0015]      FIG. 3  shows a block diagram of a circuit  300  for providing the digital signal  235  to the diaphragm  205  of the class D micro-speaker  200 . The circuit  300  includes a ramp generator  305 , a comparator  310 , and a voltage driver  315 . The ramp generator  305  produces a triangle wave having a high-frequency relative to the human audible range (i.e., the high-frequency signal). Because the human audible range is between about 20 Hz and 20 kHz, the frequency of the triangle wave is preferably 200 kHz or more. The triangle wave is input into the comparator  310 . The comparator  310  also receives an analog electric signal  320  (i.e., the audio signal) to be converted to a sound wave (i.e., the output of the speaker  200 ). The output of the comparator  310  is a pulse-width-modulated signal (e.g., a class D signal). The class D pulse-width-modulated signal is a digital signal that incorporates the sound information from the electric signal (i.e., a low-frequency component) and the high-frequency of the triangle wave (i.e., a high-frequency component). The class D pulse-width-modulated signal is provided to the voltage driver  315 . The voltage driver  315  amplifies the class D pulse-width-modulated signal to a voltage level sufficient to fully displace the diaphragm  205 . The diaphragm  205  cycles between being fully displaced toward the top-plate  220  and fully displaced toward the back-plate  215 , based on the class D pulse-width-modulated signal. The diaphragm  205  is thus able to displace a relatively large amount of air, producing a sound wave having an SPL much greater than an equivalently sized prior art electrostatic speaker  100 . 
         [0016]    The sound wave (i.e., the air pressure pulses) produced by the diaphragm  205  mimics the digital signal  235 , and contains both the audio component and the high-frequency component. In some embodiments, the high-frequency component of the sound wave is filtered by a construction of a housing of the speaker  205 . In other embodiments, the human ear is relied upon to filter out the high-frequency components of the sound wave. 
         [0017]    In some constructions, the frequency of the triangle wave is used to control the volume of the speaker  200 . The higher the frequency, the more times the diaphragm  205  will cycle between being displaced toward the top-plate  220  and the back-plate  215  during positive cycles of the audio signal. Because the diaphragm  205  moves the same volume of air each time it cycles, the higher frequency will move a larger volume of air increasing the SPL (i.e., the volume of the speaker  200 ). 
         [0018]    The class D micro-speaker of the invention is especially well suited for small speaker applications such as ear buds and cell phones. 
         [0019]    Various features and advantages of the invention are set forth in the following claims.