Abstract:
The present disclosure introduces a simple method and apparatus for a Class-D amplification and Pulse Width Modulation (PWM) of an input signal, such as a voice signal. The proposed circuits do not require reference input signals such as triangular signals; rather, the combination of the circuits&#39; self-oscillating device arrangements and the delay elements performs pulse width modulation at higher frequencies while producing less noise. Among other advantages, these circuits offer shorter response time, less distortion, better power supply ripple rejection, larger negative feedback, and simpler construction. The recommended Class-D amplifiers can be used with speakers and can have single-ended or differential input.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to audio signal processing and, in particular, to a system that includes a Class-D amplifier for audio signal amplification and other audio signal processing.  
       BACKGROUND  
       [0002]     A Class-D switching amplifier, which is often desirable for amplification of audio signals, is substantially similar to Class-A, Class-B, and Class-AB, with a major difference in the signals provided to the output stage. Instead of feeding the audio waveform directly to the output stage, Class-D amplifiers modulate the audio waveforms as on-off pulses using duty-cycle modulation methods such as Pulse Duty-Cycle Modulation (PDM) or Pulse Width Modulation (PWM), before feeding the signal to the output stage.  
         [0003]     By using transistors and semiconductors as switches rather than as linear amplifiers, the modulation stage rapidly switches the output stage on and off with the width, in the case of PWM, varying as a function of the audio signal. Subsequently, sound is recreated by filtering the signal—usually by low-pass filtering the switching signal—at the output, resulting in an amplified version of the analog input signal. Class-D amplifiers typically use triangular reference waveform as the comparison signal for modulation. In practice, high-frequency modulation is required to make a smooth waveform at the speaker. The switching scheme makes Class-D amplifiers more efficient and smaller in size, with less wasted heat energy and a smaller power supply. Class-D amplifiers are much more efficient than the nonswitching linear amplifiers.  
         [0004]     Existing Class-D amplifiers suffer numerous shortcomings in areas including modulation, feedback, distortion, power supply ripple rejection, response time, isolation, and last stage filtering. The triangular waveform, for modulation purposes, by itself is the cause of several problems in Class-D amplifiers such as superimposed high frequency noise, which is a source of distortion. Pulse transient damping issues, at frequencies above 1 kHz are another source of distortion in Class-D amplifiers.  
         [0005]     As a result of the above-mentioned problems and other identified disadvantages in the art, there is a need for an improved Class-D audio amplifier without the triangular or other reference input signal that can operate in the several hundred kilohertz range. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The foregoing aspects and many of the attendant advantages of the invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0007]      FIG. 1  is a schematic circuit diagram of a self-oscillating differential feedback Class-D amplifier, in accordance with an embodiment of the present invention.  
         [0008]      FIG. 2  is a schematic circuit diagram of a self-oscillating differential feedback Class-D amplifier, in accordance with another embodiment of the present invention.  
         [0009]      FIG. 3  is a schematic circuit diagram of a self-oscillating differential feedback Class-D amplifier, in accordance with yet another embodiment of the present invention.  
         [0010]      FIG. 4  is a schematic circuit diagram of a self-oscillating differential feedback Class-D amplifier, in accordance with yet another embodiment of the present invention.  
         [0011]      FIG. 5  is a schematic circuit diagram of a self-oscillating differential feedback Class-D amplifier, in accordance with yet another embodiment of the present invention.  
         [0012]      FIG. 6  is a schematic circuit diagram of a self-oscillating differential feedback Class-D amplifier, in accordance with yet another embodiment of the present invention.  
         [0013]      FIG. 7  is a schematic circuit diagram of a self-oscillating differential feedback Class-D amplifier, in accordance with yet another embodiment of the present invention.  
         [0014]      FIG. 8  is a graph of the test results of the self-oscillating differential feedback Class-D amplifier of  FIG. 1 .  
         [0015]      FIG. 9  is a flow diagram of the pulse width modulation and amplification method, in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]     Embodiments of a method and apparatus for a Class-D amplification and pulse Width Modulation (PWM) of an input signal, such as a voice signal, are described in detail herein. The proposed circuits do not require reference input signals, such as triangular signals. The combination of the circuits&#39; self-oscillating device arrangements and the delay elements performs PWM at higher frequencies than the traditional Class-D circuits, while producing less noise.  
         [0017]     In the following description, some specific details, such as example values for the circuit components, are presented to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.  
         [0018]     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the uses of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.  
         [0019]      FIG. 1  is a schematic circuit diagram of a self-oscillating differential feedback Class-D amplifier, in accordance with an embodiment of the present invention. A circuit  100  comprises two comparators  101  and  102 , a number of resistors and capacitors, and two external low-pass filters  103  and  104  to recreate the original signal before it enters a speaker  105 . In this embodiment an input jack  106  is an example of any possible input signal to this circuit. For the purpose of explanation, the input signal ranges between 0 to 1 volt, while V cc  is 24 volts.  
         [0020]     Furthermore, in this embodiment a resistance R 12  enhances the gain of the amplifier. It is known to the one skilled in the art that R 12  is not necessary for the operation of this circuit. A resistance R 8  and a capacitance C 9  also are not vital to the basic operation of the amplifier—they merely match an input resistance R 20  and a capacitance C 25  to prevent the audible clicking and popping sound during the turn-on and turn-off.  
         [0021]     A capacitor C 11  has a major role in delay creation in the feedback path of the comparators and consequently in the generation and control of the oscillation frequency of the amplifier. The inherent internal delays of the comparators  101  and  102 , or their hysteresis, are also a key factor in the self-oscillation of the amplifier and its frequency. Resistors R 3 , R 5 , R 7 , R 17 , R 19 , and R 21  form the basic blocks of the feedback circuitry or the feedback element of the two comparators  101  and  102 , and, in conjunction with the R 12 , they produce the gain of the amplifier.  
         [0022]     To demonstrate the self-oscillation of the circuit, a point in time may be assumed when the input to the circuit is a constant voltage, the voltage at an input 3  of the comparator  101  rises over its input 2 , and the voltage at an input 2  of the comparator  102  rises over its input 3 . After a time equal to the internal delay of the comparators—assuming the same time delay—an output 1  of the comparator  101  changes from low to high and the output 1  of the comparator  102  changes from high to low. At this instance the capacitor C 11  starts discharging and subsequently charging to the reverse polarity.  
         [0023]     As the C 11  goes through this polarity inversion, it lowers the voltage at the input 3  of the comparator  101  and the input 2  of the comparator  102  while raising the voltage at the input 2  of the comparator  101  and the input 3  of the comparator  102  until the voltage at the input 3  of the comparator  101  is lower than its input 2  and the voltage at the input 2  of the comparator  102  is lower than its input 3 , at which time the comparators are triggered and their outputs switch after a time delay. Once the outputs of the comparators switch, the entire process reverses and the discharging and charging of the capacitor C 11  will cause yet another output switch. As evident from this process, aside from the internal delay of the comparators or their hysteresis behavior, the speed of charging and discharging of the C 11 , or in other words the capacitance of the C 11 , controls the speed of switching.  
         [0024]     The PWM operation of the circuit  100  is as follows. Because the input voltage to the amplifier circuit is capacitively coupled to the feedback loop of the comparators, its variations will be imposed on the naturally varying voltage differences of the inputs to the comparators described above. For example, if at a point in time the input 3  of the comparator  101  is decreasing, an increasing input to the amplifier circuit will oppose its decrease for the entire duration of time such input to the amplifier circuit is rising. Such phenomenon will delay the switching of the comparators&#39; outputs in proportion to the rise of the said input, and results in pulse width modulation of the comparators&#39; outputs. In another embodiment of this invention, op-amps may be substituted for comparators. One skilled in the art realizes that op-amps can be configured to replace comparators in different embodiments of the present invention.  
         [0025]      FIG. 2  is a schematic circuit diagram of a self-oscillating differential feedback Class-D amplifier, in accordance with another embodiment of the present invention. A circuit  200  in  FIG. 2  is analogous to the circuit  100  of  FIG. 1  and bears like numbers. More specifically, the main difference between the circuit  100  and the circuit  200  is the absence of the R 12  resistor in the circuit  200 . The behavior of the circuit  200  is similar to the behavior of the circuit  100  while offering a lower gain.  
         [0026]      FIG. 3  is a schematic circuit diagram of a self-oscillating differential feedback Class-D amplifier, in accordance with yet another embodiment of the present invention. A circuit  300  in  FIG. 3  is analogous to the circuit  100  of  FIG. 1  and bears like numbers. More specifically, the main difference between the circuit  100  and the circuit  300  is the absence of the R 8  resistor and the C 9  capacitor in the circuit  300 . The behavior of the circuit  300  is similar to the behavior of the circuit  100 , except for a clicking or popping sound which circuit  300  may make whenever the circuit is turned off or on.  
         [0027]     It is also possible to configure the self-oscillating differential feedback Class-D amplifier as depicted in  FIG. 4 , which is yet another embodiment of the present invention. A circuit  400  in  FIG. 4  is analogous to the circuit  100  of  FIG. 1  and bears similar numbers. More specifically, the main difference between the circuit  100  and the circuit  400  is the absence of the R 12  and R 8  resistors and the C 9  capacitor in the circuit  400 . The behavior of the circuit  400  is similar to the behavior of the circuit  100 , except for a lower gain and a clicking or popping sound which circuit  400  may make whenever the circuit is turned off or on. Suitable values for the resistors and the capacitors in  FIG. 4  would be known by those skilled in the art based on the description of the embodiments provided herein.  
         [0028]      FIG. 5  is a schematic circuit diagram of a self-oscillating differential feedback Class-D amplifier, in accordance with yet another embodiment of the present invention. A circuit  500  in  FIG. 5  is analogous to the circuit  100  of  FIG. 1  and bears like numbers. More specifically, the main difference between the circuit  100  and the circuit  500  is the replacement of the external capacitors C 2  and C 22  of circuit  100  by a single capacitor C 23  in the circuit  500 , which is in parallel with the load. The behavior of the circuit  500  is similar to the behavior of the circuit  100 .  
         [0029]      FIG. 6  is a schematic circuit diagram of a self-oscillating differential feedback Class-D amplifier, in accordance with another embodiment of the present invention. A circuit  600  in  FIG. 6  is analogous to the circuit  100  of  FIG. 1  and bears like numbers. More specifically, the main difference between the circuit  100  and the circuit  600  is the absence of the DC-decoupling capacitor C 25  in the circuit  600  and the addition of a capacitor C 12  in parallel with the resistor R 12 . This circuit gives the user a choice if, for example, the input is a single-ended signal with no DC component. The behavior of the circuit  600  is similar to the behavior of the circuit  100 .  
         [0030]      FIG. 7  is a schematic circuit diagram of a self-oscillating differential feedback Class-D amplifier, in accordance with yet another embodiment of the present invention. A circuit  700  in  FIG. 7  is analogous to the circuit  100  of  FIG. 1  and bears like numbers. More specifically, the main difference between the circuit  100  and the circuit  700  is the absence of the DC-decoupling capacitor C 25 , connection of the input jack to the feedback loop such that it creates a differential input, and the addition of a capacitor C 12  in parallel with the resistor R 12 . This circuit gives the user a choice of, for example, using the circuit as a class-D differential amplifier or using it with a single-ended input signal. The behavior of the circuit  700  is similar to the behavior of the circuit  100 .  
         [0031]      FIG. 8  is a graph of the test results of the self-oscillating differential feedback Class-D amplifier of  FIG. 1 . A graph  802  of  FIG. 8  depicts the behavior of the circuit at point A, and a graph  804  depicts its behavior at point B of the circuit  100 . The test has been performed with a V cc  of 24 volts and a duty cycle of 50%.  
         [0032]      FIG. 9  is a flow diagram of the pulse width modulation and amplification method, in accordance with an embodiment of the present invention. At step  901  an input signal, such as a voice signal, is received. At step  902 , using two comparators and a feedback circuitry, a self-oscillating arrangement is created wherein one comparator&#39;s output signal is the complement of the other comparator&#39;s output signal, while oscillating. At step  903  the input signal is coupled, for example capacitively, to the feedback loop of the above-mentioned self-oscillating arrangement. At step  904  the natural pulse width of the comparators&#39; output signals is changed as a function of the input signal, through the imposition of the input signal on the feedback signals. At step  905  the outputs of the two comparators are passed through low-pass filters, and subsequently at step  906  a load is driven by the filtered output signals.  
         [0033]     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.  
         [0034]     For instance, while specific component values and voltage supply values are provided herein, it is to be appreciated that these values are for the sake of illustration and explanation. Various embodiments of the invention may utilize values that are different from what is specified herein.  
         [0035]     These modifications can be made to the invention in light of the above-detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.  
         [0036]     From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.