Abstract:
A power amplifier and a bridge circuit in a power amplifier, thereinto, the power amplifier includes a comparator, a bridge circuit and a low-pass filter. The comparator is adapted to receive a first analog signal, compare the first analog signal with a reference signal and output a square wave signal. The bridge circuit is adapted to amplify the square wave signal and output the amplified square wave signal. The low-pass filter is adapted to convert the amplified square wave signal into a second analog signal. The bridge circuit includes a first MEMS switch and a second MEMS switch. The first MEMS switch and the second MEMS switch turn on alternately when the polarity of the square wave changes, and output a first voltage signal or a second voltage signal respectively. The amplified square wave signal includes the first voltage signal and the second voltage signal output alternately. The present disclosure substitutes the MOS transistors in prior art with surface MEMS switches, so the power consumption, the size of devices and the manufacture costs all can be reduced.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the priority of Chinese Patent Application No. 201010114944.3, entitled “Power amplifier and bridge circuit in power amplifier”, and filed Feb. 25, 2010, the entire disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a power amplifier, and particularly relates to a power amplifier for amplifying audio frequency signal or other analog signals, and a bridge circuit in a power amplifier. 
         [0004]    2. Description of Prior Art 
         [0005]    As shown in  FIG. 1 , a conventional power amplifier includes a comparator  1 , a bridge circuit  2 , and a low-pass filter  3 . The bridge circuit  2  includes NMOS transistor  21  and PMOS transistor  22 . The gates of NMOS transistor  21  and PMOS transistor  22  are both connected to the output end of the comparator  1 . The source electrode of PMOS transistor  22  is connected to a high voltage signal HV, while the source electrode of NMOS transistor  21  is connected to ground. The bridge circuit  2  includes NMOS transistor  21  and PMOS transistor  22 . The drains of NMOS transistor  21  and PMOS transistor  22  are both connected to the input end of the low-pass filter  3 . During working, the comparator  1  compares a first analog signal (for example, a audio frequency signal) with a reference signal, and exports a square wave signal. The bridge circuit  2  realizes amplifying the audio frequency signal by activating the NMOS transistor  21  or the PMOS transistor  22 , according to the square wave signal. The output signal of the bridge circuit  2  is changed to an audio frequency signal (a second analog signal) by the low-pass filter  3 , and then exported to loudhailer  4 . 
         [0006]    Detailed descriptions regarding an audio power amplifier are disclosed in other documents such as Chinese patent application No. 200810166133.0 and No. 200810239524.0. The audio power amplifier disclosed in patent application No. 200810166133.0 includes a comparator and a full bridge output circuit, and the comparator exports a pulse width modulated square wave signal. The full bridge output circuit includes an inverter and four MOS transistors M 1 , M 2 , M 3  and M 4 , which amplifies the pulse width modulated square wave signal exported by the comparator. The audio power amplifier disclosed in patent application No. 200810239524.0 includes a pre-amplifier, an error amplifier, a comparator, a bridge circuit and a feedback circuit, wherein the bridge circuit and the pre-amplifier are composed of transistors. 
         [0007]    The circuits described above are all started by pulse signals activating the corresponding transistors. As we all know, transistors cost much power during working. What&#39;s more, manufacturing comparators and triangle wave generators which generate the reference signal adopts standard CMOS technology, while manufacturing PMOS and NMOS transistors adopts high voltage CMOS technology which is different from the standard CMOS technology. Therefore the comparators, triangle wave generators, PMOS and NMOS transistors mentioned above all have to adopt high voltage CMOS technology, which is complex, enlarges the area of all devices and increases the cost. 
       SUMMARY OF THE INVENTION 
       [0008]    An object of the present invention is to provide a power amplifier whose manufacturing cost is reduced, manufacturing process is simplified and power consumption is reduced. 
         [0009]    To achieve the project, the present invention provides a power amplifier which comprises a comparator, a bridge circuit and a low-pass filter. The comparator receives a first analog signal, compares the first analog signal with a reference signal, and then exports a square wave signal. The bridge amplifies the square wave signal and exports the amplified square wave signal. The low-pass filter converts the amplified square wave signal into a second analog signal. The mentioned bridge circuit comprises a first MEMS switch and a second MEMS switch. The first MEMS switch and the second MEMS switch turn on alternately when the polarity of the square wave signal changes, and export a first voltage signal or a second voltage signal accordingly. The amplified square wave signal includes the first voltage signal and the second voltage signal which are exported in turns. 
         [0010]    Optionally, the first MEMS switch comprises a first electrode and a second electrode. The first electrode comprises a first conductor and a second conductor connected to the input end of the low-pass filter, and the second electrode comprises a first connecting conductor. The first electrode and the second electrode are configured to shift away from or close to each other according to the square wave signal, in order to connect the first conductor and the second conductor through the first connecting conductor. The second MEMS switch comprises a third electrode and a fourth electrode. The third electrode comprises a third conductor and a fourth conductor connected to the input end of the low-pass filter, and the fourth electrode comprises a second connecting conductor. The third electrode and the fourth electrode are configured to shift away from or close to each other according to the square wave signal, in order to connect the third conductor and the fourth conductor through the second connecting conductor, thereby outputting the second voltage signal which is put on the third conductor from the fourth conductor. 
         [0011]    Optionally, the first electrode further comprises a first basal plate and a first insulating layer set on the first basal plate, while the first conductor and the second conductor are on the surface of the first insulating layer. The second electrode further comprises a second polar plate and a second insulating layer, while the first connecting conductor and the second polar plate are arranged on the opposite surface of the second insulating layer. The first basal plate and the second polar plate are configured to shift away from or close to each other according to the square wave signal. The third electrode further comprises a second basal plate and a third insulating layer set on the second basal plate, while the third conductor and the fourth conductor are arranged on the surface of the third insulating layer. The fourth electrode further comprises a third polar plate and a fourth insulating layer, while the second connecting conductor and the third polar plate are arranged on the opposite surface of the fourth insulating layer. The second basal plate and the third polar plate are configured to shift away from or close to each other according to the square wave signal. 
         [0012]    Optionally, the output end of the comparator is connected to the second polar plate of the first MEMS switch and the third polar plate of the second MEMS switch, while the first basal plate and the second basal plate are of opposite polarity. Or the output end of the comparator is connected to the second polar plate of the first MEMS switch and the second basal plate of the second MEMS switch, while the first basal plate and the third polar plate are of opposite polarity. Or the output end of the comparator is connected to the first basal plate of the first MEMS switch and the third polar plate of the second MEMS switch, while the second polar plate and the second basal plate are of opposite polarity. Or the output end of the comparator is connected to the first basal plate of the first MEMS switch and the second basal plate of the second MEMS switch, while the second polar plate and the third polar plate are of opposite polarity. 
         [0013]    Optionally, the output end of the comparator is connected to the second polar plate of the first MEMS switch, and to the third polar plate of the second MEMS switch through an inverter, while the first basal plate and the second basal plate are of the same polarity. Or the output end of the comparator is connected to the second polar plate of the first MEMS switch, and to the second basal plate of the second MEMS switch through an inverter, while the first basal plate and the third polar plate are of the same polarity. Or the output end of the comparator is connected to the first basal plate of the first MEMS switch, and to the third polar plate of the second MEMS switch through an inverter, while the second polar plate and the second basal plate are of the same polarity. Or the output end of the comparator is connected to the first basal plate of the first MEMS switch, and to the second basal plate of the second MEMS switch through an inverter, while the second polar plate and the third polar plate are of the same polarity. 
         [0014]    Optionally, the first electrode further comprises a first basal plate and a first insulating layer set on the first basal plate. A first polar plate, the first conductor and the second conductor are on the surface of the first insulating layer. The second electrode further comprises a second polar plate and a second insulating layer. The first connecting conductor and the second polar plate are arranged on the opposite surface of the second insulating layer. The first polar plate and the second polar plate are configured to shift away from or close to each other according to the square wave signal. The third electrode further comprises a second basal plate and a third insulating layer set on the second basal plate. A fourth polar plate, the third conductor and the fourth conductor are on the surface of the third insulating layer. The fourth electrode further comprises a third polar plate and a fourth insulating layer. The second connecting conductor and the third polar plate are arranged on the opposite surface of the fourth insulating layer. The fourth polar plate and the third polar plate are configured to shift away from or close to each other according to the square wave signal. 
         [0015]    Optionally, the output end of the comparator is connected to the second polar plate of the first MEMS switch and the third polar plate of the second MEMS switch, while the first polar plate and the fourth polar plate are of opposite polarity. Or the output end of the comparator is connected to the first polar plate of the first MEMS switch and the fourth polar plate of the second MEMS switch, while the second polar plate and the third polar plate are of opposite polarity. Or the output end of the comparator is connected to the second polar plate of the first MEMS switch and the fourth polar plate of the second MEMS switch, while the first polar plate and the third polar plate are of opposite polarity. Or the output end of the comparator is connected to the first polar plate of the first MEMS switch and the third polar plate of the second MEMS switch, while the second polar plate and the fourth polar plate are of opposite polarity. 
         [0016]    Optionally, the output end of the comparator is connected to the first polar plate of the first MEMS switch, and to the fourth polar plate of the second MEMS switch through an inverter, while the second polar plate and the third polar plate are of the same polarity. Or the output end of the comparator is connected to the first polar plate of the first MEMS switch, and to the third polar plate of the second MEMS switch through an inverter, while the second polar plate and the fourth polar plate are of the same polarity. Or the output end of the comparator is connected to the second basal plate of the first MEMS switch, and to the fourth polar plate of the second MEMS switch through an inverter, while the first polar plate and the third polar plate are of the same polarity. Or the output end of the comparator is connected to the second polar plate of the first MEMS switch, and to the third polar plate of the second MEMS switch through an inverter, while the first polar plate and the fourth polar plate are of the same polarity. 
         [0017]    To the accomplishment of the foregoing, the present invention further provides a bridge circuit in a power amplifier, and the bridge circuit comprises a first MEMS switch and a second MEMS switch. The first MEMS switch receives a square wave signal and a first voltage signal, and the second MEMS switch receives a square wave signal and a second voltage signal. The first MEMS switch and the second MEMS switch turn on alternately according to the polarity of the square wave signal, and then output a first voltage signal and a second voltage signal. The amplified square wave signal is composed of the first voltage signal and the second voltage signal which are output alternately. 
         [0018]    Optionally, the output end of the comparator is connected to the second polar plate of the first MEMS switch, and to the third polar plate of the second MEMS switch, while the first basal plate and the second basal plate are of opposite polarity. Or the output end of the comparator is connected to the second polar plate of the first MEMS switch and the second basal plate of the second MEMS switch, while the first basal plate and the third polar plate are of opposite polarity. Or the output end of the comparator is connected to the first basal plate of the first MEMS switch and the third polar plate of the second MEMS switch, while the second polar plate and the second basal plate are of opposite polarity. Or the output end of the comparator is connected to the first basal plate of the first MEMS switch and the second basal plate of the second MEMS switch, while the second polar plate and the third polar plate are of opposite polarity. 
         [0019]    Compared with the prior art, the present invention substitutes the transistors with MEMS switches whose on-off are under the control of a comparator, and the invention uses surface MEMS technology, thereby the power consumption is reduced. What is more, MEMS switches, comparators and low-pass filters all can adopt CMOS technology, that is to say, they can be overlapped, which can reduce the size of all components and the manufacture costs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  shows the structure of the prior power amplifier; 
           [0021]      FIG. 2  shows one functional block diagram of the power amplifier of the present invention; 
           [0022]      FIG. 3  shows the structure of the power amplifier according to the first embodiment of the present invention; 
           [0023]      FIG. 4  shows the structure of the power amplifier according to the second embodiment of the present invention; 
           [0024]      FIG. 5  shows another functional block diagram of the power amplifier of the present invention; 
           [0025]      FIG. 6  shows the structure of the power amplifier according to the third embodiment of the present invention; 
           [0026]      FIG. 7  shows the structure of the power amplifier according to the fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0027]    Referring to  FIG. 2 , a power amplifier of the present invention includes: a comparator  1 , a bridge circuit  2 , and a low-pass filter  3 . 
         [0028]    Referring to  FIG. 2 , the comparator  1  receives a first analog signal, compares the first analog signal with a reference signal, and then exports a square wave signal. The first analog signal refers in particular to an audio signal in all the embodiments hereinafter. 
         [0029]    Referring to  FIG. 2  and  FIG. 3 , the bridge circuit includes: an inverter  23 , a first MEMS switch  24  and a second MEMS switch  25 . 
         [0030]    The inverter  23  is connected to the output end of the comparator  1 . 
         [0031]    Referring to  FIG. 2  and  FIG. 3 , the first MEMS switch  24  connected to the output end of the comparator  1  receives a first voltage signal, which is a high voltage HV in this embodiment. The first MEMS switch  24  is also connected to the input end of the low-pass filter  3 . 
         [0032]    Specifically, the first MEMS switch  24  comprises a first electrode  241  and a second electrode  242 . The first electrode  241  comprises a first basal plate  2411 , a first insulating layer  2412 , a first conductor  2413  and a second conductor  2414 . The first basal plate  2411  receives a first control signal (not shown in the figures), which controls the polarity of the first basal plate  2411 . The first insulating layer  2412  is upon the first basal plate  2411 , and the first conductor  2413  and the second conductor  2414  are arranged upon the first insulating layer  2412  with some interval. The first conductor  2413  is connected to the high voltage HV, and the second conductor  2414  is connected to the input end of the low-pass filter  3 . The second electrode  242  is connected to the input end of the comparator  1 . To be specific, the second electrode  242  comprises a first connecting conductor  2421 , a second insulating layer  2422 , a second polar plate  2423 , the second polar plate  2423  is connected to the input end of the comparator  1 , and the first connecting conductor  2421  and the second polar plate  2423  are arranged upon the opposite surface of the second insulating layer  2422 . 
         [0033]    Referring to  FIG. 3 , the input end of the inverter  23  is connected to the output end of the comparator  1 . The second MEMS switch  25  connected to the output end of the inverter  23  receives a second voltage signal, which is a ground signal, in this embodiment. The second MEMS switch  25  is also connected to the input end of the low-pass filter  3 . 
         [0034]    Specifically, the second MEMS switch  25  comprises a third electrode  251  and a fourth electrode  252 . The third electrode  251  comprises a third conductor  2511  and a fourth conductor  2512 . The third conductor  2511  is connected to the second voltage signal, and the fourth conductor  2512  is connected to the input end of the low-pass filter  3 . The third electrode  251  further comprises a second basal plate  2513  and a third insulating layer  2514 . The third insulating layer  2514  is upon the second basal plate  2513 , and the third conductor  2511  and the fourth conductor  2512  are arranged upon the third insulating layer  2514  with some interval. The second basal plate  2513  receives a second control signal (not shown in the figures), which controls the polarity of the second basal plate  2513 . The fourth electrode  252  is connected to the output end of the inverter  23 . To be specific, the fourth electrode  252  comprises a second connecting conductor  2521 , a third polar plate  2322  and a fourth insulating layer  2523 . The third polar plate  2522  is connected to the output end of the inverter  23 . The second connecting conductor  2521  and the third polar plate  2522  are arranged upon the opposite surface of the fourth insulating layer  2523 . 
         [0035]    What is more, the first MEMS switch  24  and the second MEMS switch  25  both comprise a spring arm  26 . The spring arm  26  of the first MEMS switch  24  is set on the first electrode  241  and connected to the second electrode  242 , so the first electrode  241  and the second electrode  242  can shift away from or close to each other. The spring arm  26  of the second MEMS switch  25  is set on the third electrode  251  and connected to the fourth electrode  252 , so the third electrode  251  and the fourth electrode  252  can shift away from or close to each other. 
         [0036]    Referring to  FIG. 3 , the working process of the power amplifier in this embodiment is as follows. 
         [0037]    Ordinarily, the first analog signal (an audio signal) is a sine wave, and the reference signal is a triangular wave, which is generated by a triangular wave generator. The comparator  1  compares the audio signal with the reference signal, and then outputs a square wave signal. The square wave signal is composed of alternate a third voltage and a fourth voltage, and the third voltage is higher than the fourth voltage. That the polarity of the square wave signal is positive means the square wave signal is the third voltage, and that the polarity of the square wave signal is negative means the square wave signal is the fourth voltage. In this embodiment, the polarity of the first control signal put on the first basal plate  2411  of the first MEMS switch  24  and the polarity of the second control signal put on the second basal plate  2513  of the second MEMS switch  25  are the same, for example, the polarity is negative (the first control signal and the second control signal both are the fourth voltage). 
         [0038]    The comparator  1  compares the audio signal with the reference signal, and then outputs a square wave signal. When the polarity of the square wave signal is positive, the polarity of the second polar plate  2423  is positive too. Because the polarity of the second polar plate  2423  and the first basal plate  2411  are opposite to each other, static pull-in working on the second polar plate  2423  draws it near to the first conductor  2413  and the second conductor  2414  of the first electrode  241 , in this way, the first conductor  2413 , the second conductor  2414 , and the first connecting conductor  2421  will bring into contact with each other and be activated, whereby the high voltage HV can be transmitted into the low-pass filter  3 . Moreover, the polarity of the square wave signal is changed to negative after running through the inverter  23 , as well as the polarity of the third polar plate  2522 . Because the polarity of the third polar plate  2522  and the second basal plate  2513  are the same, the third polar plate  2522  will not move to the second basal plate  2513 , so the second connecting conductor  2521 , the third conductor  2511  and the fourth conductor  2512  will not be activated. When the polarity of the square wave signal is negative, the polarity of the second polar plate  2423  is negative too, that is to say, the polarity of the second polar plate  2423  and the first basal plate  2411  are the same, thereby they will repel each other, and the second basal plate  2513  will depart from the second polar plate  2423 . Moreover, the polarity of the square wave signal is changed to positive after running through the inverter  23 , as well as the polarity of the third polar plate  2522 . Because the polarity of the third polar plate  2522  and the second basal plate  2513  are opposite to each other, the third polar plate  2522  will move to the third conductor  2511  and the fourth conductor  2512 , in this way, the third conductor  2511  and the fourth conductor  2512  will be connected through the second connecting conductor  2521 , and the ground signal will be transmitted to the input end of the low-pass filter  3 . Thereby, the input end of the low-pass filter  3  will receive alternate HV and ground signal because of the output of the comparator  1 . That is to say, the amplitude value of the square wave signal which is output by the comparator  1  is amplified to HV after running through the first MEMS switch  24  and the second MEMS switch  25 . The square wave signal will be changed to an amplified audio signal (the second analog signal), and then transmitted to the input end of the loudhailer  4 . 
         [0039]    In addition, when the first control signal put on the first basal plate  2411  of the first MEMS switch  24  and the second control signal put on the second basal plate  2513  of the second MEMS switch  25  is positive (for example, they are the third voltage), the system can get the same effect. Simply the difference is as follows. When the square wave signal output by the comparator  1  is positive, the second MEMS switch  25  turns on, while the first MEMS switch  24  turns off. When the square wave signal output by the comparator  1  is negative, the first MEMS switch  24  turns on, while the second MEMS switch  25  turns off. 
         [0040]    Some variations to this embodiment of the present invention are as follows. The first control signal is put on the first basal plate  2411 , the output end of the comparator  1  is connected to the second polar plate  2423 , the second control signal is put on the third polar plate  2522 , and the output end of the inverter  23  is connected to the second basal plate  2513 . Or, the first control signal is put on the second polar plate  2423 , the output end of the comparator  1  is connected to the first basal plate  2411 , the second control signal is put on the second basal plate  2513 , and the output end of the inverter,  23  is connected to the third polar plate  2522 . Or, the first control signal is put on the second polar plate  2423 , the output end of the comparator  1  is connected to the first basal plate  2411 , the second control signal is put on the third polar plate  2522 , and the output end of the inverter  23  is connected to the second basal plate  2513 . 
         [0041]    In this embodiment, when the polarity of the first control signal and the second control signal both are negative, while the output signal of the comparator is positive, the first MEMS switch  24  will be switched on and the second MEMS switch  25  will be switched off. However, if the output signal of the comparator is negative, the first MEMS switch  24  will be switched off and the second MEMS switch  25  will be switched on. Therefore, the first MEMS switch  24  and the second MEMS switch  25  will just be switched on when the polarity of the square wave signal is opposite. 
         [0042]    In conclusion, the present invention substitutes the transistors with surface MEMS switches whose on-off are under the control of a comparator, the power consumption is reduced. What is more, MEMS switches, comparators and low-pass filters all can adopt CMOS technology, that is to say, MEMS switches, comparators and low-pass filters can be overlapped, which can reduce the size of all components and the manufacture costs. 
         [0043]    Referring to  FIG. 4 , it is structural sketch of the power amplifier according to the second embodiment of the present invention. In this embodiment, the power amplifier comprises: a comparator  1 , a bridge circuit  2 , and a low-pass filter  3 . The difference between this embodiment and the first embodiment is as follows: the structure of a first MEMS switch  34  and a second MEMS switch  35  in this embodiment is different from the structure of the first MEMS switch  24  and the second MEMS switch  25  in the first embodiment. The first MEMS switch  34  comprises a first electrode  341  and a second electrode  342  which are arranged opposite to each other. The first electrode  341  comprises a first polar plate  3411  which receives a first control signal, a first conductor  3412  which connects to a high voltage HV, and a second conductor  3413  which connects to the input end of the low-pass filter  3 . In this embodiment, the first electrode  341  further comprises a first basal plate  3414  and a first insulating layer  3415 . The first basal plate  3414  is a semiconductor substrate such as silicon substrate, and the first insulating layer  3415  is a silicon dioxide layer. The first insulating layer  3415  is formed on the first basal plate  3414 . The first polar plate  3411 , the first conductor  3412  and the second conductor  3413  are formed on the first insulating layer  3415 . In this embodiment, the second electrode  342  comprises a first connecting conductor  3421 , a second polar plate  3422  and a second insulating layer  3423 . The second polar plate  3422  connects to the output end of the comparator  1 . The second polar plate  3422  and the first connecting conductor  3421  are formed on the opposite surface of the second insulating layer  3423 . 
         [0044]    Referring to  FIG. 4 , the second MEMS switch  35  in the second embodiment comprises a third electrode  351  and a fourth electrode  352  which are arranged opposite to each other. The third electrode  351  comprises a fourth polar plate  3511  which receives a second control signal, a third conductor  3512  which connects to the ground, and a fourth conductor  3513  which connects to the input end of the low-pass filter  3 . In this embodiment, the third electrode  351  further comprises a second basal plate  3514  and a third insulating layer  3515 . The second basal plate  3514  is a semiconductor substrate such as silicon substrate, and the third insulating layer  3515  is a silicon dioxide layer. The third insulating layer  3515  is formed on the second basal plate  3514 . The fourth polar plate  3511 , the third conductor  3512  and the fourth conductor  3513  are formed on the third insulating layer  3515 . In this embodiment, the fourth electrode  352  comprises a second connecting conductor  3521 , a third polar plate  3522  and a fourth insulating layer  3523 . The third polar plate  3522  connects to the output end of the inverter  3 . The third polar plate  3522  and the second connecting conductor  3521  are formed on the opposite surface of the fourth insulating layer  3523 . In this embodiment, the first MEMS switch  34  and the second MEMS switch  35  both comprises a spring arm  36 , and it makes that the fourth electrode  352  shifts away from or close to the third electrode  351 , or the second electrode  342  shifts away from or close to the first electrode  341 , possible. 
         [0045]    Referring to  FIG. 4 , the working process of this embodiment and the first embodiment are similar to each other. It is illustrated briefly thereafter. 
         [0046]    When the first MEMS switch  34  and the second MEMS switch  35  works, the polarity of the first control signal put on the first polar plate  3411  and the polarity of the second control signal put on the fourth polar plate  3511  are the same. 
         [0047]    When the polarity of the output signal of the comparator  1  is opposite to the first polar plate  3411 , the polarity of the first polar plate  3411  is also opposite to the second polar plate  3422 . The static pull-in between the first polar plate  3411  and the second polar plate  3422  draws the first connecting conductor  3421  near to the first conductor  3412  and the second conductor  3413 , in this way, the high voltage HV connected to the first connecting conductor  3421  can be transmitted into the input end of the low-pass filter  3 . Moreover, because of the inverter  23 , the polarity of the fourth polar plate  3511  and the third polar plate  3522  are the same, the fourth electrode  352  will not move to the third electrode  351 , so the second connecting conductor  3521 , the third conductor  3512  and the fourth conductor  3513  will not be activated, and the ground signal will not be transmitted to the input end of the low-pass filter  3 . 
         [0048]    Similarly, When the polarity of the output signal of the comparator  1  and the first polar plate  3411  are the same, the second electrode  342  of the first MEMS switch  34  will not move to the first electrode  341 , then the high voltage HV will not be transmitted to the input end of the low-pass filter  3 . The second electrode  352  of the second MEMS switch  35  will move to the first electrode  351  because of static pull-in, whereby the third conductor  3512  and the fourth conductor  3513  will be connected through the second connecting conductor  3521 , and the ground signal GND will be transmitted to the input end of the low-pass filter  3 . Again and again, the input end of the low-pass filter  3  receives a square wave signal whose amplitude value is HV because of the output signal of the comparator  1 . In other words, the square wave signal output by the comparator  1  is amplified to a square wave signal whose amplitude value is HV after transmitting through the first MEMS switch  34  and the second MEMS switch  35 . Next the amplified square wave signal is amplified and converted into an audio signal by the low-pass filter  3 , and then transmitted to the loudhailer  4 . 
         [0049]    Likewise, some variations to this embodiment of the present invention are as follows. The first control signal is put on the first polar plate  3411 , the output end of the comparator  1  is connected to the second polar plate  3422 , the second control signal is put on the third polar plate  3522 , and the output end of the inverter  23  is connected to the fourth polar plate  3511 . Or, the first control signal is put on the second polar plate  3422 , the output end of the comparator  1  is connected to the first polar plate  3411 , the second control signal is put on fourth polar plate  3511 , and the output end of the inverter  23  is connected to the third polar plate  3522 . Or, the first control signal is put on the second polar plate  3422 , the output end of the comparator  1  is connected to the first polar plate  3411 , the second control signal is put on the third polar plate  3522 , and the output end of the inverter  23  is connected to the fourth polar plate  3511 . These variations have the same effect. 
         [0050]    It is understandable that the first MEMS switch  34  and the second MEMS switch  35  used in a power amplifier can reduce the power consumption and the percentage area of the devices. 
         [0051]    Referring to  FIG. 5 , the present invention is not necessarily comprising an inverter. The difference between the power amplifier in this solution and the power amplifier shown in  FIG. 2  is that the bridge circuit has different structure. In this solution, the bridge circuit  2  comprises a first MEMS switch  24  and a second MEMS switch  25 . The first MEMS switch  24  is under the control of a first control signal, connected to the output end of a comparator  1 , a low-pass filter  3 , and a high voltage HV. The second MEMS switch  25  is under the control of a second control signal, connected to the output end of a comparator  1 , a low-pass filter  3 , and a ground signal GNU. The first MEMS switch  24  and the second MEMS switch  25  switch on alternately when the polarity of the first control signal or the second control signal is opposite to the polarity of the output signal of the comparator  1 , and output the high voltage HV or the ground signal GND accordingly to the low-pass filter  3 . Then the low-pass filter  3  converts the HV or GND signal into an audio signal and transmits the audio signal to a loudhailer  4 . 
         [0052]    Referring to  FIG. 6 , how the invention adopts MEMS switches realizing power amplifying without an inverter is illustrated hereafter. The first MEMS switch  24  and the second MEMS switch  25  still adopt the structure shown in  FIG. 3 , but the opposite of the first control signal and the second control signal are opposite to each other. The first control signal is put on the first basal plate  2411  of the first MEMS switch  24 . The second control signal is put on the second basal plate  2513  of the second MEMS switch  25 . The working principle is the same as the power amplifier with an inverter. 
         [0053]    Likewise, some variations to this embodiment of the present invention are as follows. The first control signal is put on the first basal plate  2411 , the output end of the comparator  1  is connected to the second polar plate  2423 , the second control signal is put on the third polar plate  2522 , and the output end of the comparator  1  is connected to the second basal plate  2513 . Or, the first control signal is put on the second polar plate  2423 , the output end of the comparator  1  is connected to the first basal plate  2411 , the second control signal is put on the second basal plate  2513 , and the output end of the comparator  1  is connected to the third polar plate  2522 . Or, the first control signal is put on the second polar plate  2423 , the output end of the comparator  1  is connected to the first basal plate  2411 , the second control signal is put on the third polar plate  2522 , and the output end of the comparator  1  is connected to the second basal plate  2513 . The working process is similar to the first embodiment. 
         [0054]    Referring to  FIG. 7 , how the invention adopts MEMS switches realizing power amplifying without an inverter is illustrated hereafter. The first MEMS switch  34  and the second MEMS switch  35  still adopt the structure shown in  FIG. 4 , but the polarity of the first control signal and the second control signal are opposite to each other. The first control signal is put on the first polar plate  3411  of the first MEMS switch  34 . The second control signal is put on the fourth polar plate  3511  of the second MEMS switch  35 . The working principle is the same as the power amplifier with an inverter. 
         [0055]    Likewise, some variations to this embodiment of the present invention are as follows. The first control signal is put on the first polar plate  3411 , the output end of the comparator  1  is connected to the second polar plate  3422 , the second control signal is put on the third polar plate  3522 , and the output end of the comparator  1  is connected to the fourth polar plate  3511 . Or, the first control signal is put on the second polar plate  3422 , the output end of the comparator  1  is connected to the first polar plate  3411 , the second control signal is put on the fourth polar plate  3511 , and the output end of the comparator  1  is connected to the third polar plate  3522 . Or, the first control signal is put on the second polar plate  3422 , the output end of the comparator  1  is connected to the first polar plate  3411 , the second control signal is put on the third polar plate  3522 , and the output end of the comparator  1  is connected to the fourth polar plate  3511 . The working process is similar to the first embodiment. 
         [0056]    In addition, the MEMS switch can be formed with other types of structures, such as three-electrode MEMS switches. In this three-electrode scheme, the middle electrode is a movable part, and the polarity of the up electrode is opposite to the down electrode. If different voltages are put on the middle electrode, the up and down electrode will generate an attractive force and a repelling force, and perform switching on-off function by this means. In a word, this kind of switch also can achieve the purpose that using MEMS switches realize power amplifying. The detailed working principle is the same as adopting the first MEMS switch  24  and the second MEMS switch  25 . In the end, although the above descriptions take an audio signal as example, the present invention is also fit for other analog signals. 
         [0057]    Although the present invention has been disclosed as above with reference to preferred embodiments thereof but will not be limited thereto. Those skilled in the art can modify and vary the embodiments without departing from the spirit and scope of the present invention. Accordingly, the scope of the present invention shall be defined in the appended claims.