Abstract:
A Class-D amplifier arrangement is disclosed that implements an auxiliary feedback loop and a primary feedback loop. The auxiliary feedback loop operates upon an input signal when the Class-D amplifier arrangement is operating under a power-up condition and a power-down condition so that a modulated signal is confined within the auxiliary feedback loop during the power-up condition and the power-down condition. The confinement of the modulated signal within the auxiliary feedback loop during the power-up condition and the power-down condition diverts transient signals coupled onto the modulated signal from an output device. The primary feedback loop operates upon the input signal when the Class-D amplifier arrangement is operating under a normal condition so that the modulated signal is introduced to the output device during the normal condition.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The invention generally relates Class-D audio amplifiers. 
         [0003]    2. Related Art 
         [0004]    In general, Class-D amplifiers achieve high efficiency and dynamic range. However, they are susceptible to artifact noise, such as pop and click noise, which occurs during amplifier power up and power down. This artifact noise occurs due to the sudden application (during power up) of amplifier DC offset voltage and pulse-width modulation (PWM) pulses to terminals of a speaker to which the amplifier is connected. Similarly, this artifact noise occurs due to the sudden removing (during power down) of amplifier DC offset voltage and the PWM pulses from the speaker terminals. The sudden application and/or removal of the amplifier DC offset voltage and the PWM pulses to the terminals of the speaker generates a transient pulse that discharges through the speaker which causes an undesirable clicking or a pop sound. 
         [0005]    Class-D amplifiers are increasingly needed to efficiently drive speakers in mobile communication devices. However in a mobile environment, the amplifier powers up and powers down often. The accompanying pop and click noise therefore limits full adoption of class-D amplifiers in mobile devices. What is needed is a way to suppress pop and click noise in a Class-D amplifier while still preserving its otherwise advantageous characteristics, such as high efficiency and dynamic range. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The accompanying drawings, which are incorporated in and constitute part of the specification, illustrate embodiments of the invention and, together with the general description given above and the detailed descriptions of embodiments given below, serve to explain the principles of the present invention. In the drawings: 
           [0007]      FIG. 1  is a schematic diagram of a Class-D amplifier according to an embodiment of the present invention; 
           [0008]      FIG. 2  illustrates an exemplary modulated signal to illustrate the powering up and powering down of the Class-D amplifier according to an exemplary embodiment of the present invention; 
           [0009]      FIG. 3  illustrates an exemplary embodiment of a power driver that can be used as part of the Class-D amplifier according to an embodiment of the present invention; and 
           [0010]      FIG. 4  illustrates an exemplary embodiment of a charge pump and its corresponding output that can be used as part of the Class-D amplifier according to an embodiment of the present invention. 
       
    
    
       [0011]    Features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. 
       DETAILED DESCRIPTION 
       [0012]    The invention will be better understood from the following descriptions of various “embodiments” of the invention. Specific “embodiments” are implementations of the invention which provide views of the invention, but each embodiment does not itself represent the whole invention. In some cases individual elements from one particular embodiment may be substituted for different elements in another embodiment carrying out a similar or corresponding function. It is expected that those skilled in the art relating to this invention, and with access to the teachings provided herein, will recognize additional modifications, applications, and embodiments within the scope of the invention and additional fields in which the invention would be of significant utility. 
         [0013]    A Class-D amplifier is a specialized type of power amplifier that employs transistors that are configured and arranged in a push-pull configuration and driven to act as a switch. Typically, artifact noise, such as pop and click noise to provide an example, occurs during power up and power down of the Class-D amplifier. This artifact noise typically results from undesired transients in the Class-D amplifier which can produce audible pops and clicks when the Class-D amplifier is coupled to a speaker. Most often, the undesired transients can be generated when the Class-D amplifier changes its operating mode, such as power up/power down to provide an example. During these mode changes, abrupt stopping and starting of various components with the Class-D amplifier can lead to the undesired transients. For example, during normal operation, a loop filter, such as optional loop filter  108  as shown in  FIG. 1  to provide an example, can generate an unwanted DC offset within the Class-D amplifier. However, during power up of the Class-D amplifier, the loop filter suddenly generates this unwanted DC offset which causes a first undesired transient within the Class-D amplifier. Similarly, during power down of the Class-D amplifier, this unwanted DC offset is suddenly removed from the Class-D amplifier which causes a second undesired transient within the Class-D amplifier. The first and second undesired transient can produce audible pops and clicks when the Class-D amplifier is coupled to a speaker. Pops and can clicks may also be generated when PWM pulses suddenly start and/or stop. 
         [0014]    The present invention reduces undesired transients in a Class-D amplifier, especially during power up and/or power down, to substantially reduce or suppress artifact noise, such as the pop and click noise, within the Class-D amplifier while still preserving its otherwise advantageous characteristics, such as high efficiency and dynamic range. 
         [0015]      FIG. 1  is a schematic diagram of a Class-D amplifier according to an embodiment of the present invention. A Class-D amplifier  100  amplifies an input signal  150  to provide a primary output signal  152  for presentation to a speaker  102 . However, the speaker  102  as shown in  FIG. 1  is for illustrative purposes only, those skilled in the relevant art(s) will recognize that the Class-D amplifier  100  may be coupled to other devices without departing from the spirit and scope of the present invention. Instead of presenting undesired transients to the speaker  102  during power up and/or power down which can cause audible pops and clicks as discussed above, the Class-D amplifier  100  diverts these undesired transients from being presented to the speaker  102 . As shown in  FIG. 1 , the Class-D amplifier  100  includes a primary feedback loop  104  and an auxiliary feedback loop  106 . The primary feedback loop  104  operates in conjunction with the auxiliary feedback loop  106  to divert undesired transients from being presented to the speaker  102  during power up and/or power down. 
         [0016]    The primary feedback loop  104  includes an optional loop filter  108 , a pulse width modulation (PWM) generator  110 , a reference generator  112 , and a power driver  114 . The optional loop filter  108  receives the input signal  150  via an input resistance R IN . Often, the input signal  150  represents an audio signal having a frequency range from approximately 20 Hz to approximately 20 kHz; however, those skilled in the relevant art(s) will recognize that other types of signals are possible for the input signal  150  without departing from the spirit and scope of the present invention. Typically, the optional loop filter  108  is used for maintaining loop dynamics, also referred to as stability, for the optional loop filter  108 . The optional loop filter  108  can additionally reduce unwanted noise within the primary feedback loop  104  to provide a filtered signal  154 . 
         [0017]    The PWM generator  110  pulse width modulates the input signal  150  or, optionally, the filtered signal  154 , in accordance with a reference signal  156 . Typically, the PWM generator  110  provides a modulated signal  158  at a first logic level, such as a logic one to provide an example, when the input signal  150  or, optionally, the filtered signal  154 , is greater than or equal to the reference signal  156 . Similarly, the PWM generator  110  provides the modulated signal  158  at a second logic level, such as a logic zero to provide an example, when the input signal  150  or, optionally, the filtered signal  154 , is less than the reference signal  156 . 
         [0018]    The power driver  114  amplifies the modulated signal  158  in accordance with a programmable gain to provide the primary output signal  152 . As to be discussed below, the programmable gain may be smoothly ramped up from a minimum programmable gain to a maximum programmable gain and/or smoothly ramped down from the maximum programmable gain to the minimum programmable gain. Typically, the primary output signal  152  is provided to the speaker  102 . The speaker  102  may be characterized as having an inductive impedance and some parasitic capacitance which together operate to filter the primary output signal  152  to be an amplified representation of the input signal  150 . The primary feedback loop  104  additionally includes a resistor R FDBK  that, in conjunction with the input resistance R IN  and/or the programmable gain, determines a gain of the primary feedback loop  104 . 
         [0019]    The reference generator  112  provides the reference signal  156  to the PWM generator  110 . In an exemplary embodiment, the reference generator  112  provides a ramp signal as the reference signal  156 . However, those skilled in the relevant art(s) will recognize that the reference generator  112  may provide other types of signals to the PWM generator  110  without departing from the spirit and scope of present invention. Typically, the reference signal  156 , as well as these other types of signals, are characterized as having a frequency that is greater, such as twice the Nyquist rate to provide an example, than a frequency of the input signal  150 . 
         [0020]    The auxiliary feedback loop  106  includes the optional loop filter  108 , the PWM generator  110 , the reference generator  112 , and an auxiliary driver  116 . The optional loop filter  108  and the PWM generator  110  operate upon the input signal  150  to provide the modulated signal  158  in a substantially similar manner as described above. The auxiliary driver  116  amplifies the modulated signal  158  in accordance with a programmable gain to provide an auxiliary output signal  158 . As to be discussed below, the programmable gain may be smoothly ramped up from a minimum programmable gain to a maximum programmable gain and/or smoothly ramped down from the maximum programmable gain to the minimum programmable gain. In an exemplary embodiment, the programmable gain of the auxiliary driver  116  is inversely related to the programmable gain of the power driver  114 . In this exemplary embodiment, the programmable gain of the auxiliary driver  116  smoothly ramps up as the programmable gain of the power driver  114  smoothly ramps down and/or the programmable gain of the auxiliary driver  116  smoothly ramps down as the programmable gain of the power driver  114  smoothly ramps up. The auxiliary feedback loop  106  additionally includes a resistor R AUX  that, in conjunction with the input resistance R IN  and the programmable gain, determines a gain of the auxiliary feedback loop  106 . Typically, a maximum programmable gain of the auxiliary driver  116  is a small fraction of a maximum programmable gain of the power driver  114 . 
         [0021]    A controller module  118  controls overall operation of the Class-D amplifier  100 . The controller module  118  provides a gain control  160  to control the programmable gains of the power driver  114  and the auxiliary driver  116 . At the beginning of power up of the Class-D amplifier  100 , the programmable gain of the power driver  114  is minimized and the programmable gain of the auxiliary driver  116  is maximized. This causes the input signal  150  to be entirely operated upon by the auxiliary feedback loop  106 . As a result, any undesired transients that may result from powering up the Class-D amplifier  100  are diverted away from the power driver  114  and passed onto the auxiliary driver  116 . The programmable gain of the power driver  114  is gradually increased from its minimum value to its maximum value while the programmable gain of auxiliary driver  116  is gradually decreased from its maximum value to its minimum value. In an exemplary embodiment, the auxiliary driver  116  may be characterized as being an open circuit when the programmable gain is at its minimum value. Once the primary feedback loop  104  and/or the auxiliary feedback loop  106  have settled, namely are free from undesirable transients, the input signal  150  is effectively handed off from the auxiliary feedback loop  106  to the primary feedback loop  104 . Typically, the handoff occurs once common components between the primary feedback loop  104  and the auxiliary feedback loop  106 , such as the optional loop filter  108  and the PWM generator  110  to provide some examples, have settled. 
         [0022]    During normal operation, the programmable gain of the power driver  114  is at its maximum value while the programmable gain of auxiliary driver  116  is at its minimum value. This causes the input signal  150  to be entirely operated upon by the primary feedback loop  104 . 
         [0023]    At the beginning of power down of the Class-D amplifier  100 , the programmable gain of the power driver  114  is maximized and the programmable gain of the auxiliary driver  116  is minimized. The programmable gain of the power driver  114  is gradually decreased from its maximum value to its minimum value while the programmable gain of auxiliary driver  116  is gradually increased from its minimum value to its maximum value. In an exemplary embodiment, the power driver  114  may be characterized as being an open circuit when the programmable gain is at its minimum value. Once the primary feedback loop  104  and/or the auxiliary feedback loop  106  have settled, namely are free from undesirable transients, the input signal  150  is effectively handed off from the primary feedback loop  104  to the auxiliary feedback loop  106 . Typically, the handoff occurs once common components between the primary feedback loop  104  and the auxiliary feedback loop  106 , such as the optional loop filter  108  and the PWM generator  110  to provide some examples, have settled. As a result, any undesired transients that may result from powering down the Class-D amplifier  100  are diverted away from the power driver  114  and passed onto the auxiliary driver  116 . The input signal  150  is effectively smoothly handed-off from the primary feedback loop  104  to the auxiliary feedback loop  106  by this ramping of their respective programmable gains. 
         [0024]      FIG. 2  illustrates an exemplary modulated signal to illustrate the powering up and powering down of the Class-D amplifier according to an exemplary embodiment of the present invention. As discussed above, a Class-D amplifier, such as the Class-D amplifier  100  to provide an example, can operate in a power up mode of operation  250 , a normal mode of operation  252 , and a power down mode of operation  254 . In the power up mode of operation  252 , a programmable gain of a primary feedback loop, such as the primary feedback loop  104  to provide an example is at its minimum value and a programmable gain of an auxiliary feedback loop, such as the auxiliary feedback loop  106  to provide an example is at its maximum value. The programmable gain of the primary feedback loop is gradually increased from its minimum value to its maximum value while the programmable gain of the auxiliary feedback loop is gradually decreased from its maximum value to its minimum value. 
         [0025]    From the discussion above, the primary feedback loop amplifies an input signal, such as the input signal  150  to provide an example, using from a modulated signal  200 , such as the modulated signal  158  to provide an example. As shown in  FIG. 2 , the auxiliary feedback loop draws power from the modulated signal  250  during the power up mode of operation  250 . This power draw is the largest when the programmable gain of the auxiliary feedback loop is at its maximum value and gradually decreases as the programmable gain of the auxiliary feedback loop is gradually decreased to its minimum value. As a result, the modulated signal  250  itself gradually increases from its minimum value corresponding to the auxiliary feedback loop being at its maximum value to its maximum value corresponding to the auxiliary feedback loop being at its minimum value. This gradual increasing of the modulated signal  250  leads to a gradual increase in an output signal, such as the primary output signal  152  to provide an example, of the primary feedback loop. 
         [0026]    When the programmable gain of the primary feedback loop is at its maximum value and/or the programmable gain of the auxiliary feedback loop is at its minimum value, the Class-D amplifier may be characterized as being in the normal mode of operation  252 . 
         [0027]    In the power down mode of operation  254 , the programmable gain of the primary feedback loop is at its maximum value and the programmable gain of the auxiliary feedback loop is at its minimum value. The programmable gain of the primary feedback loop is gradually decreased from its maximum value to its minimum value while the programmable gain of the auxiliary feedback loop is gradually increased from its minimum value to its maximum value. 
         [0028]    As additionally shown in  FIG. 2 , the auxiliary feedback loop begins to draw power from the modulated signal  250  during the power down mode of operation  252 . This power draw is the largest when the programmable gain of the auxiliary feedback loop is at its maximum value and gradually decreases as the programmable gain of the auxiliary feedback loop is gradually decreased to its minimum value. As a result, the modulated signal  250  itself gradually decreases from its maximum value corresponding to the auxiliary feedback loop being at its minimum value to its minimum value corresponding to the auxiliary feedback loop being at its maximum value. This gradual decreasing of the modulated signal  250  leads to a gradual decrease in the output signal of the primary feedback loop. 
         [0029]      FIG. 3  illustrates an exemplary embodiment of a power driver that can be used as part of the Class-D amplifier according to an embodiment of the present invention. A power driver  300  amplifies an input signal  350  to provide an output signal  352 . The power driver  300  may represent an exemplary embodiment of the power driver  114 . As such, the input signal  350  and the output signal  352  may represent exemplary embodiments of the modulated signal  158  and the primary output signal  152 , respectively. The power driver  300  includes pre-driver stages  302 . 1  through  302 . n , a charge pump  304 , and an output stage  306 . 
         [0030]    The pre-driver stages  302 . 1  through  302 . n  amplify their respective input signals to provide respective output signals  354 . 1  through  354 . n  and output signals  356 . 1  through  356 . n . Those skilled in the relevant art(s) will recognize that the pre-driver stages  302 . 1  through  302 . n  may include a single pre-driver stage or multiple pre-driver stages without departing from the spirit and scope of the present invention. A first pre-driver stage from among the pre-driver stages  302 . 1  through  302 . n  amplifies the input signal  350  to provide output signals  354 . 1  and  356 . 1 . A next pre-driver stage from among the pre-driver stages  302 . 1  through  302 . n  amplifies output signals from a previous pre-driver stage from among the pre-driver stages  302 . 1  through  302 . n.    
         [0031]    In an exemplary embodiment, the pre-driver stages  302 . 1  through  302 . n  are implemented in a substantially similar manner; therefore, only the pre-driver stage  302 . n  is to be discussed in further detail. The pre-driver stage  302 . n  includes a first stage  308 . 1  and a second stage  308 . 2  to amplify the output signals  354 .( n− 1) and  356 .( n− 1), respectively, to provide the output signals  354 . n  and  356 . n , respectively. The first stage  308 . 1  is substantially similar to the second stage  308 . 2 ; therefore, only the first stage  308 . 1  is to be discussed in further detail. The first stage  308 . 1  includes an amplifier  310 , such as a CMOS inverting amplifier to provide an example though any suitable amplifier may be used that will be apparent to those skilled in the relevant art(s) without departing from the spirit and scope of the present invention, and a switching transistor  312 . The amplifier  310  amplifies the output signal  354 .( n− 1) in response to a state of the switching transistor  312 . The switching transistor  312  can be in a conducting or “on” state or in a non-conducting or “off” state depending upon a corresponding amplifier control signal from among amplifier control signals  358 . 1  through  358 . n . For example, the amplifier  310  amplifies the output signal  354 .( n− 1) when the switching transistor  312  is in the conducting or “on” state and does not amplify the output signal  354 .( n− 1) when the switching transistor  312  is in the non-conducting or “off” state. 
         [0032]    The charge pump  304  provides the amplifier control signals  358 . 1  through  358 . n  to the pre-driver stages  302 . 1  through  302 . n  in response to a gain control  360 , such as the gain control  160  to provide an example. The amplifier control signals  358 . 1  through  358 . n  gradually increase and/or decreases programmable gains of the pre-driver stages  302 . 1  through  302 . n . For example, the amplifier control signals  358 . 1  through  358 . n  gradually transition switching transistors of the pre-driver stages  302 . 1  through  302 . n  to transition from the “on” state or to the “off” state and/or from the “off” state to the “on” state. 
         [0033]    The output stage  306  includes a p-type switching transistor  314  and an n-type switching transistor  316 . The p-type switching transistor  314  and the n-type switching transistor  316  typically represent complementary transistors whereby only the p-type switching transistor  314  and the n-type switching transistor  316  is conducting at a given instance in time. For example, when the output signal  354 . n  causes the p-type switching transistor  314  to conduct, the output signal  352  is coupled to a first logic level that can be represented by a first potential V DD . In this example, the n-type switching transistor  316  is not conducting when the p-type switching transistor  314  is conducting. As another example, when the output signal  356 . n  causes the n-type switching transistor  316  to conduct, the output signal  352  is coupled to a second logic level that can be represented by a second potential, such as a ground potential to provide an example. In this other example, the p-type switching transistor  314  is not conducting when the n-type switching transistor  316  is conducting. Typically, the output stage  306  may be characterized as being a half H-bridge stage that is coupled to a first coupling of a speaker, such as the speaker  102  to provide an example. Although not shown in  FIG. 3 , another half H-bridge stage that is substantially similar to the output stage  306  may be included within the power driver  300  to couple to a second coupling of the speaker. 
         [0034]      FIG. 4  illustrates an exemplary embodiment of a charge pump and its corresponding output that can be used as part of the Class-D amplifier according to an embodiment of the present invention. A charge pump  400  provides a control signal V OUTN  which can gradually increase and/or decreases programmable gains of programmable amplifiers, such as the pre-driver stages  302 . 1  through  302 . n  to provide an example. The charge pump  400  may represent an exemplary embodiment of the charge pump  304 . 
         [0035]    The charge pump  400  includes switches  402  through  408  and capacitors  410  through  414 . The switches  402  through  404  charge and/or discharge the capacitors  410  through  414  in response to a clock signal to provide an. The clock signal may represent an exemplary embodiment of the gain control  360 . 
         [0036]    The clock signal includes clock signals φ, φb, φ 1 , and φ 1   b . A pumping up mode of operation for the charge pump occurs when the clock signals φ, φb, φ 1 , and φ 1   b  cause the switches  402  through  404  to charge the capacitors  410  through  414 . The charging of the capacitors in this manner produces a gradual increase in the output V OUTN  from its minimum value to its maximum value. A pumping down mode of operation occurs when the clock signals φ, φb, φ 1 , and φ 1   b  cause the switches  402  through  404  to discharge the capacitors  410  through  414 . The charging of the capacitors in this manner produces a gradual decrease in the output V OUTN  from its maximum value to its minimum value. 
         [0037]    For example, the amplifier control signals  358 . 1  through  358 . n  gradually transition switching transistors of the pre-driver stages  302 . 1  through  302 . n  to transition from the “on” state or to the “off” state and/or from the “off” state to the “on” state. 
       CONCLUSION 
       [0038]    The invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. 
         [0039]    The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. 
         [0040]    Various embodiments of the present invention have been described above. It should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made from those specifically described without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments. 
         [0041]    The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. 
         [0042]    The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments.