Abstract:
Traditionally, switching amplifiers (i.e., class-D and class-G) with negative supply rails had issues with direct current (DC) power loss, included large external capacitors, had a comparative reduction in efficiency, and oftentimes included separate power management circuits. Here, a class-D amplifier is provided with an output stage that provides negative supply voltages, positive supply voltages, and ground. Essentially, this amplifier provides some of the benefits of the conventional amplifiers without the drawbacks.

Description:
TECHNICAL FIELD 
     The invention relates generally to a class-D amplifier and, more particularly, to a class-D amplifier that uses a signal supply. 
     BACKGROUND 
     Currently, Texas Instruments Incorporated (TI) offers a number of audio amplifiers that a include DIRECTPATH™ architecture. Some features of this architecture are that an output direct current or DC blocking capacitor is not needed and that there is a negative supply rail. An example is TI&#39;s TPA6140A2, which is a class-G amplifier that uses a charge pump to invert the voltage (from a positive supply) to create a negative supply voltage so that the headphone amplifier output can be centered at 0V without the need for DC blocking capacitors. However, there are some tradeoffs associated with this architecture; namely, supplying a negative supply voltage requires several external components as well as it reduces efficiency. Additionally, these types of devices may have also need to have separate power management circuits (i.e., buck converters). Therefore, there is a need for a method or an apparatus that offers similar advantages without the drawbacks. 
     Some other conventional circuits are: U.S. Pat. No. 6,320,460; U.S. Pat. No. 6,753,729; U.S. Pat. No. 7,330,069; and U.S. Pat. No. 7,400,191. 
     SUMMARY 
     A preferred embodiment of the present invention, accordingly, provides an apparatus. The apparatus comprises an integrator that receives an input signal; a pulse width modulator (PWM) that is coupled to the integrator; a logic circuit coupled to the PWM; and an output stage having: a first node; a second node; a third node; a fourth node that is coupled to ground; a capacitor that is coupled between the first and second nodes; and a switch network that is coupled to the first node, the second node, the third node, the fourth node, and a supply rail, wherein the switch network is controlled by logic circuit so as to operate in first mode, a second mode, and a third mode, and wherein, in the first mode, the voltage supply charges the capacitor and the third node is grounded, and wherein, in the second mode, the capacitor provides a positive supply voltage to the third node, and wherein, in the third mode, the capacitor provides a negative supply voltage in the third mode. 
     In accordance with a preferred embodiment of the present invention, the switch network further comprises: a first switch that is coupled between the supply rail and the first node, wherein the first switch is closed in the first mode; a second switch that is coupled between the first node and the fourth node, wherein the second switch is closed in the third mode; a third switch that is coupled between the first node and the third node, wherein the third switch is closed in the second mode; a fourth switch that is coupled between the second node and the fourth node, wherein the fourth switch is closed in the first mode and the second mode; a fifth switch that is coupled between the second node and the fourth node, wherein the fifth switch is closed in the third mode; and a sixth switch that is coupled between the third node and the fourth node, wherein the sixth switch is closed in the first mode. 
     In accordance with a preferred embodiment of the present invention, the input signal is differential. 
     In accordance with a preferred embodiment of the present invention, the integrator further comprises: an amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal; and a first feedback capacitor that is coupled between the first input terminal and the first output terminal; and a second feedback capacitor that is coupled between the second input terminal and the second output terminal. 
     In accordance with a preferred embodiment of the present invention, the PWM further comprises: a first comparator having a first input terminal, a second input terminal, and an output terminal, wherein the first input terminal of the first comparator is coupled to the integrator, and wherein the output terminal of the first comparator is coupled to the logic circuit; a second comparator having a first input terminal, a second input terminal, and an output terminal, wherein the first input terminal of the second comparator is coupled to the integrator, and wherein the output terminal of the second comparator is coupled to the logic circuit; and a ramp generator that is coupled to the second input terminal of the first comparator and the second input terminal of the second comparator. 
     In accordance with a preferred embodiment of the present invention, an apparatus is provided. The apparatus comprises an audio source that generates an audio signal; a class-D amplifier having: a first capacitor that is coupled to the audio source so as to receive the audio signal; an integrator is coupled to the first capacitor; a pulse width modulator (PWM) that is coupled to the integrator; a logic circuit coupled to the PWM; and an output stage having: a first node; a second node; a third node; a fourth node that is coupled to ground; a capacitor that is coupled between the first and second nodes; and a switch network that is coupled to the first node, the second node, the third node, the fourth node, and a supply rail, wherein the switch network is controlled by logic circuit so as to operate in first mode, a second mode, and a third mode, and wherein, in the first mode, the voltage supply charges the capacitor and the third node is grounded, and wherein, in the second mode, the capacitor provides a positive supply voltage to the third node, and wherein, in the third mode, the capacitor provides a negative supply voltage in the third mode; and a speaker that is coupled to the third node. 
     In accordance with a preferred embodiment of the present invention, an apparatus is provided. The apparatus comprises a supply rail; a first node; a second node; a third node; a fourth node that is coupled to ground; a first direct current (DC) blocking capacitor that receives a first portion of an input signal; a second DC blocking capacitor that receives a second portion of the input signal; an amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein the first input terminal of the amplifier is coupled to the first DC blocking capacitor, and wherein the second input terminal of the amplifier is coupled to the second DC blocking capacitor; a first feedback capacitor that is coupled between the first input terminal and the first output terminal; a second feedback capacitor that is coupled between the second input terminal and the second output terminal; a first comparator having a first input terminal, a second input terminal, and an output terminal, wherein the first input terminal of the first comparator is coupled to the first output terminal of the amplifier; a second comparator having a first input terminal, a second input terminal, and an output terminal, wherein the first input terminal of the second comparator is coupled to the second output terminal of the amplifier; a ramp generator that is coupled to the second input terminal of the first comparator and the second input terminal of the second comparator; a logic circuit coupled to the output terminals of the first and second comparators; an output capacitor that is coupled between the first and second nodes; a first switch that is coupled between the supply rail and the first node, wherein the first switch is closed by the logic circuit in a first mode; a second switch that is coupled between the first node and the fourth node, wherein the second switch is closed by the logic circuit in a second mode; a third switch that is coupled between the first node and the third node, wherein the third switch is closed by the logic circuit in a third mode; a fourth switch that is coupled between the second node and the fourth node, wherein the fourth switch is closed by the logic circuit in the first mode and the third mode; a fifth switch that is coupled between the second node and the fourth node, wherein the fifth switch is closed by the logic circuit in the second mode; and a sixth switch that is coupled between the third node and the fourth node, wherein the sixth switch is closed by the logic circuit in the first mode. 
     In accordance with a preferred embodiment of the present invention, the apparatus further comprises an audio source that is coupled to the first and second DC blocking capacitors so as to provide the input signal. 
     In accordance with a preferred embodiment of the present invention, the apparatus further comprises a speaker that is coupled to the third node. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram of an example of a class-D amplifier in accordance with a preferred embodiment of the present invention; and 
         FIG. 2  is a system that employs the class-D amplifier of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     Referring to  FIG. 1  of the drawings, the reference numeral  100  generally designates an example of a class-D amplifier in accordance with a preferred embodiment of the present invention. Amplifier  100  generally comprises direct current (DC) blocking capacitors C 1  and C 2 , input resistors R 2  and R 3 , an integrator  102 , a pulse width modulator (PWM)  104 , logic circuit  106 , and output stage  108 . The integrator  102  generally comprises amplifier  110 , feedback capacitors C 3  and C 4 , and resistors R 1  and R 4  PWM  104  generally comprises comparators  112  and  114  and ramp generator  116 , and output stage  108  generally comprises output capacitor C 5  (which may be a signal capacitor or multiple capacitors) and switches S 1  through S 6 . Additionally, supply rail VDD is typically coupled to a battery, a cell, or another power source. 
     In operation, amplifier  100  receives an analog input signal IN and generates an output signal OUT (which generally has values of “0” or ground, “−1” or negative supply voltage, and “+1” or positive supply voltage.) To accomplish this, the input signal IN (which is generally differential, but may also be single ended) is provided to capacitors C 1  and C 2  and resistors R 2  and R 3 , where capacitors C 1  and C 2  operate to substantially remove DC components from input signal IN. Additionally, capacitors C 1  and C 2  are optional components, which are not necessary for amplifier  100  to operate. The integrator  102  then integrates the signal from capacitors C 1  and C 2  and resistors R 1  and R 2 . Each of comparators  112  and  114  receives a portion of the (differential) integrated signal from integrator  102  and compares its portion to a ramp signal from ramp generator  116 . The comparison results from comparators  112  and  114  can then be used by logic circuit  106  to generate control signals for switches S 1  through S 6 . 
     In particular, there are three modes of operation. In a first mode, logic circuit  106  closes switches S 1 , S 4 , and S 6 , which couples the output node N 4  and node N 2  to node N 3  (which is coupled to ground) and couples node N 1  to the supply rail VDD. Thus, output capacitor C 5  can be charged to the voltage on supply rail VDD, while a “0” is output from node N 4 . In a second mode, logic circuit  106  closes switches S 3  and S 4  to provide a positive supply voltage or “+1” at node N 4  because the positive plate of capacitor C 5  (plate charged to the voltage on supply rail VDD) is coupled to node N 4 , while the opposite, negative plate of C 5  is coupled to ground. Alternatively, for a second mode, switches S 1  and S 3  can be closed to provide a positive supply voltage or “+1” at node N 4  directly from supply rail VDD. In a third mode, logic circuit  106  closes switches S 2  and S 5  to provide a negative supply voltage or “−1” to output node N 4  because the positive plate of capacitor C 5  is coupled to ground while the negative plate of capacitor C 5  is coupled to node N 4 . Alternatively, five switches may be used to accomplish substantially the same result. 
     As a result of this configuration, large external capacitors (on the order of about 1 μF to about 47 μF) used for conventional circuits to generate a negative rail voltage can be eliminated. Additionally, there is a reduction in the DC power loss, and there is the efficiency in increased over conventional amplifiers. In particular, simulation results show a 40-50% increase over the TI&#39;s TPA6140 when delivering 1 mW at 16Ω. Additionally, simulation results have also shown a total harmonic distortion of THD of about 0.2% for about 1 mW delivered at 1 kHz for a supply voltage of 3.6V on rail VDD, and a load resistance of about 32Ω. 
     There are some limitations, however, with this amplifier  100 . Capacitor C 5  is not a true voltage source (like a battery or power supply) because it has a relatively short discharge time. This short discharge time limits the output pulse width through node N 4 , but if the control loop within logic circuit  106  is sufficiently fast and capacitor C 5  is sufficiently large, the rapid discharge time of capacitor C 5  should not significantly affect performance. For example, using an internal clock frequency of about 1 MHz with a capacitance of about 1 μF is generally sufficient. Additionally, the output duty cycle is generally limited to about 50% because the logic circuit  106  generally converts the simple differential PWM modulation into an SE ternary PWM modulation. 
     Turning now to  FIG. 2 , an example of a system  200  that employs amplifier  100  can be seen. Generally, system  200  is a portable media device (i.e., mp3 player) that generates an audio signal from an audio source  202 . Amplifier  100  receives this audio signal from source  202  and provides an amplified signal through output N 4  to a speaker  204  (i.e., headphones). Each of the audio device  202  and amplifier  100  are powered by an onboard power cell  206  (i.e., battery). 
     Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.