Abstract:
A class D amplifier including a PWM circuit, a buffer amplifying circuit, a low-pass filter, and two current sources is provided. The PWM circuit transfers an analog signal into a PWM signal. The buffer amplifying circuit amplifies the PMW signal and generates an amplified signal. The low-pass filter will filter high frequency components out from the amplified signal and then transmit the filtered signal to a loading of the class D amplifier. The two current sources provide currents flowing into and out from a feedback node in the PWM circuit, respectively. The charging and discharging provided by the two current sources can generate a triangular signal for the PWM circuit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the invention 
         [0002]    The invention relates to a class D amplifier circuit. In particular, the invention relates to a PWM circuit in a class D amplifier. 
         [0003]    2. Description of the prior art 
         [0004]    The class AB amplifier and a class D amplifier and is both power saving and has better tone quality than the class A amplifier that provides lower distortion, but it has higher power consumption, and class B amplifier has lower power loss, but has crossover distortion, hence widely used in consumer stereo and audio-visual equipment at present. The amplifier&#39;s main difference is the class AB amplifier uses a linear action region to amplify signal by power transistor, a class D amplifier technology is used for the PWM to amplify signal. 
         [0005]    Please refer to  FIG. 1 .  FIG. 1  shows a basic block diagram of half-bridge class D amplifier. The class D amplifier comprises of an integrator  102 , a comparator  104 , a pre-driver  106 , a power amplifier circuit  108  and a low-pass filter  110 . As show in  FIG. 1 , an input end of the integrator is used in receiving an analog signal V 1 , and uses a resistor R couples to another input end and an output end of a power amplifier circuit. Analog signal V 1  generates a voltage V 2  after going through the integrator  102 . The integration result compares with a normal triangular wave by comparator  104 . The triangular wave signal V TRI  can be a sampling signal of the integrate result V 2 . Therefore, in order to get a better sampling result, the triangular wave V TRI  frequency is usually larger than the best high frequency of the analog audio signal V 1 . 
         [0006]    When V 2  voltage is higher than V TRI , the output result of the comparator  104  can be a high level pulse wave signal; When V 2  voltage is smaller than V TRI , the output result of the comparator  104  can be a low level pulse wave signal. The integrator  102  and the comparator  104  are usually called a PWM circuit. The original analog audio signal is able to transform into a PWM signal of digital type by the PWM circuit; the amplitude value of the original analog signal direct ratio to part of the high level of pulse width in the digital signal. 
         [0007]    Then the output voltage V 3  of the comparator  104  can be amplified by pre driver  106 , and not only drives a power amplifier circuit  108 , but also controls and turns on or off the power transistor M 1 , M 2 . The power amplifier circuit  108  is used for amplifying signal. The low-pass filter  110  connected to the output end of the power amplifier  108  comprises an inductance and a capacitor, and the low-pass filter  110  to filter a carrier wave in PWM signal, and return the wave of the analog audio signal. Therefore, class D amplifier provides the returned signal V 4  to drive the amplify  112 . Please refer to  FIG. 2 , which shows a wave sample of the V 1 , VTRI, V 3  and V 4 . 
         [0008]    Please refer  FIG. 3 .  FIG. 3  shows a full-bridge class D amplifier circuit. In the example, the integrator  32  transmitted the integration result into the comparator  33 A and  33 B respectively after receiving the differential audio signal Vin+ and Vin−, after both output signals of the comparator go through the pre driver ( 34 A,  34 B), the power amplifier circuit ( 35 A,  35 B), and low-pass filter ( 36 A,  36 B), can be an analog signal used in driver the amplifier  38 . The integrator  32  and pre driver  34 A,  34 B is also called a PWM circuit. As show in  FIG. 3 , the comparator  33 A and  33 B have an input end to receive the triangular signal V TRI , respectively. 
         [0009]    With the explanations previously, both the half-bridge and the full-bridge class D amplifier need a triangular wave to be the standard of the PWM. In the prior art, the triangular wave signal usually is generated from a special circuit outside of the class D amplifier. As known by those skilled in the art, the amplitude and duty cycle of the triangular wave should be quite exact to avoid the inaccuracy of the voice output from the amplifier. Therefore, the triangular wave generation circuit usually is quite complex. 
         [0010]    In addition, when the complex triangular wave generation circuit is integrated with the class D amplifier in one single chip, it occupies a large area of the chip, and the cost for producing said chip will be largely increased. 
       SUMMARY OF THE INVENTION 
       [0011]    To solve the aforementioned problem, a scope of the invention is to provide a solution to generate a triangular wave for a class D amplifier. An embodiment according to the invention is a class D amplifier comprises a PMW circuit, a buffer amplifying circuit, a low-pass filter, a first current source, a second current source and a control circuit. The PMW circuit is used for transferring an analog signal into a PWM signal. The buffer amplifying circuit is used for amplifying the PMW signal and generating an amplified signal. The low-pass filter is applied for filtering a high frequency component out from an amplified signal to generate a filtered signal and then transmitting the filtered signal to a load of the class D amplifier. Both the first current source and the second source are electrically connected to a feedback node in the PMW circuit. The first current source is applied for providing a first current flowing into the feedback node, and the second current source is applied for providing a second current flowing out from the feedback node. The control circuit is used for selectively turning on or off the first current source and the second current source. 
         [0012]    Another embodiment according to the invention is also a class D amplifier. The class D amplifier comprises a PWM circuit, a buffer amplifying circuit, a low-pass filter, a first current source, a second current source and a control circuit. 
         [0013]    The PWM circuit of the embodiment is used for transferring a differential analog signal into a differential PWM signal. The buffer amplifying circuit is used for amplifying the differential PWM signal and generating a differential amplified signal. The low-pass filter is applied for filtering a high frequency component out from the differential amplified signal to generating a differential filtered signal and then transmitting the differential filtered signal to a loading of the class D amplifier. Both the first current source and the second current source are electrically connected to a feedback node in the PMW circuit. The first current source is used for providing a first current flowing into the feedback node, and the second current source is used for providing a second current flowing out from the feedback node. The control circuit is applied for selectively turning on or off the first current source and the second current source. 
         [0014]    The class D amplifier according to the invention applies the current sources to periodically charge and discharge the feedback node in the PWM circuit; it is equal to subtract a specific triangular wave signal from the original integrated result in the PWM circuit. Compared to the prior arts, there is no need for the half-bridge or full-bridge class D amplifier according to the invention to have a complex triangular generation circuit, so as to largely save chip area and hardware cost. 
         [0015]    The advantage and spirit of the invention may be understood by the following recitations together with the appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE APPENDED DRAWINGS 
         [0016]      FIG. 1  shows the basic block diagram of the half-bridge class D amplifier. 
           [0017]      FIG. 2  shows an example of signal wave in the class D amplifier. 
           [0018]      FIG. 3  shows the full-bridge class D amplifier circuit. 
           [0019]      FIG. 4(A)  and  FIG. 4(B)  show the half-bridge class D amplifier in an embodiment according to the invention. 
           [0020]      FIG. 5(A)  shows the full-bridge class D amplifier in an embodiment according to the invention.  FIG. 5(B)  shows the full-bridge class D amplifier in another embodiment according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Please refer to  FIG. 4(A) ,  FIG. 4(A)  shows a half-bridge class D amplifier of an embodiment according to the invention. The class D amplifier comprises a PWM circuit  402 , a buffer amplifying circuit  404 , a low-pass filter  406 , a first current source  408 , a second current source  410  and a control circuit  412 . In the embodiment, the PWM circuit  402  comprises an integrator  402 A and a comparator  402 B, and the buffer amplifying circuit  404  comprises a pre driver  404 A and a power amplifying circuit  404 B. 
         [0022]    The integrator  402 A is composed of an operational amplifier, two resisters and a capacitor. As show in  FIG. 4(A) , the positive input end of the operational amplifier  402 C connected to a ground. The resistor R 1  is connected between the negative input end and an input end of the operational amplifier  402 C, wherein the input end is used for receiving the analog signal Vin. The capacitor C 1  is connected between the negative input end and the output end of the operational amplifier  402 C. The resister R 2  is connected between the negative input end of the operational amplifier  402 C and the output end of the power amplifying circuit  404 B. Additionally, the positive input end and the negative input end of the comparator  402 B are connected to the output end and the ground of the integrator  402 A respectively. 
         [0023]    The PWM circuit  402  is applied for transferring the analog signal Vin into a PWM signal. As show in  FIG. 4(A) , both the first current source  408  and the second current source  410  are electrically connected to a feedback node in the PWM circuit  402  (i.e. the negative input end of the operational amplifier  402 C). The first current source  408  is applied for providing a first current flowing into the feedback node, and the second current source  410  is used for providing a second current flowing out form the feedback node. The control circuit  412  is used for selectively turning on or off the first current source  408  and the second source  410 . 
         [0024]    For example, the control circuit  412  can alternatively turn on the first current source  408  and the second current source  410 , so that the current sources can periodically charge and discharge the feedback node. The presence of the capacitor C 1 , charging and discharging the feedback node (i.e. charging and discharging the capacitor C 1 ) will result in an increase or decrease of triangular wave voltage at the output end of the operational amplifier  402 C, that is equal to add a triangular wave voltage to the original result of integration. The amplitude and the cycle time of the triangular wave voltage can be controlled by controlling the current value and turn on delay time of the first current source  408  and the second current source  410 . 
         [0025]    If the absolute value of the first current is substantially equal to that of the second current, the average potential of said triangular wave voltage will almost be zero, and the positive wave and the negative wave of said triangular wave voltage is substantially symmetrical. Since there is no specific phase relationship between the triangular wave voltage and the original result of integration, the increase and decrease of voltage at the output end of the operational amplifier  402 C can be equal to the subtraction of a triangular wave voltage from the original result of integration. Additionally, the difference between the phase of the triangular wave voltage as a subtrahend and that of the triangular wave voltage as an addend is 180 degree. 
         [0026]    As mentioned previously, the comparator  104  in  FIG. 1  is applied for comparing the magnitude of V 2  and V TRI . In other words, the comparator  104  is used for comparing the difference between the ground potential and the voltages V 2  and V TRI . When the difference between V 2  and V TRI  is larger then zero, the output voltage of the comparator  104  is at high level. 
         [0027]    In the circuit structure of the invention, the output end of the integrator  402 A generates the result of subtracting a triangular wave voltage from the original result of integration. Therefore, the comparator  402 B compares the output voltage with the ground voltage, which is equal to compare the original result of integration with said triangular wave voltage. Herewith, the PWM circuit  402  according to the invention does not provide the triangular wave signal to the integrator  402 A which still has effect of transferring the analog signal Vin into a PWM. Then, the PWM signal generated by the PWM circuit  402  is sent to a pre driver  404 A and a power amplifying circuit  404 B and generates an amplified signal. After the low-pass filter  406  is filtering a high frequency component out from the amplifying signal and then transmitting the filtered signal to a loading  48  of the class D amplifier (For example, an amplifier). 
         [0028]    In this embodiment, the low-pass filter  406  is composed of an inductance and a capacitor, the power amplifying circuit  404 B is composed of power CMOS. In practical applications, the PWM circuit  402 , the power amplifying circuit  404 B and the low-pass filter  406  are not limited by this embodiment. There are still other possible structures for the PWM circuit  402 , the power amplifying circuit  404 B and the low-pass  406 . 
         [0029]    Please refer to  FIG. 4(B) ,  FIG. 4(B)  shows a sample of the current source. As shown in  FIG. 4(B) , the first current source  408  is a PMOS (M 3 ) and the second current source  410  is a NMOS. Both gate terminals of the two transistors are connected to a control circuit  412 . When the control circuit  412  outputs a low level voltage signal, the transistor M 4  will be turned off and the transistor M 3  will be turned on to provide a current flow into the feedback node. When the control circuit  412  outputs a high level voltage signal, the transistor M 3  will be turned off and the transistor M 4  will be turned on and provide a current flow out from the feedback node. 
         [0030]    Please refer to  FIG. 5(A) ,  FIG. 5(A)  shows a full-bridge class D amplifier of an embodiment according to the invention. The full-bridge class D amplifier is suited for the input signal in a differential signal and the circuit usually the same used for modulating, amplifying and filtering the reverse or forward input signal. 
         [0031]    In this embodiment, the integrator  502 A,  502 B are receiving differential audio signal Vin+ and Vin− respectively and then transmitting the result of integration to comparator  504 A and  504 B respectively. After the output signal of the two comparators have passed through the pre driver ( 506 A,  506 B), the power amplifying circuit ( 508 A,  508 B) and the low-pass filter ( 510 A,  510 B) can be differential analog signals used for driving the amplifier  530 . 
         [0032]    The method of using the current source to charge and discharge the feedback node according to the invention is also applied to the full-bridge class D amplifier. As shown in  FIG. 5(A) , the first current source  514  and the second current source  516  are applied for using in the integrator  502  A; the third current source  518  and the fourth current source  520  are applied for using in the integrator  502  B. The control circuit  512  is applied for selectively turning on or off the four current sources, so at to achieve the efficiency of subtracting a specific triangular wave from the original result of the integration of the integrator  502 A,  502 B. As a result of every units show in  FIG. 5(A)  similar to every module show in  FIG. 4(A) , so no repeat again. 
         [0033]    Please refer to  FIG. 5(B) ,  FIG. 5(B)  shows another embodiment according to the invention which is a full-bridge class D amplifier. The difference between this embodiment and the one before is the class D amplifier comprises less current source in this embodiment. As shown in  FIG. 5(B) , both the first current source  514  and the second current source  516  are electrically connected to the negative input end of the integrator  502 A,  502 B respectively. The same previous embodiment, so as to suit to selectively control the time of turning on or off of the current sources can have an effect of the differential analog signal transfer into the PWM signal. 
         [0034]    In theory, the difference between the phases of the analog differential signals, which were inputted into the class D amplifier, is 180 degrees. Accordingly, the difference between the phases of the analog differential signals applied for driving the load should also be 180 degrees. 
         [0035]    As described previously, the triangular wave signal is applied for sampling the result of the integration of reference signal in the PWM circuit. When the integrator  502 A and  502 B using the same current source (i.e. so that the two integrators use the same sample signal) reduce the phase mismatch of the class D amplifier. 
         [0036]    The low-pass filter circuit  406  as show in  FIG. 5(A)  and  FIG. 5(B)  both comprise an inductance and a capacitor, the power amplifier circuit comprises a power CMOS. The PWM circuit, the power amplifier circuit and the low-pass filter circuit are not limited by this embodiment. Other possible structures for the PWM circuit exist, the power amplifier circuit and the low-pass filter. 
         [0037]    As the example and previous explanations stated, The class D amplifier according to the invention is used to the current source making a cycle charging and discharging to the feedback node in the PWM circuit equal to the original integrate result remove by a specially triangular signal in PWM circuit. Compared to the prior arts, the half-bridge or full-bridge class D amplifier according to the invention all do not have a triangular generation circuit, therefore the invention can save much chip space and hardware cost. 
         [0038]    With the example and explanations above, the features and spirits of the invention will be hopefully well described. Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teaching of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.