Abstract:
An apparatus for providing an audio signal to drive a speaker system includes first and second audio channels. The first audio channel has a first class-D amplifier for receiving an input signal, and a first reconstruction filter for receiving an output from the first class-D amplifier and reconstructing therefrom an output audio signal for driving the speaker system. The second audio channel has a second class-D amplifier for receiving an input audio signal, and a second reconstruction filter for receiving an output from the second class-D amplifier and reconstructing therefrom an output audio signal for driving the speaker system. The first and second reconstruction filters have corresponding first and second planar inductors, with the second planer inductor being magnetically coupled to the first planar inductor.

Description:
FIELD OF DISCLOSURE 
     This invention relates to audio amplifiers, and in particular, to class D amplifiers. 
     BACKGROUND 
     Because of their high efficiency, class D amplifiers are particularly useful for portable and compact audio applications, such as in automotive applications. 
     A typical class D amplifier system  10 , shown in  FIG. 1 , features a class-D audio amplifier  12  that transforms an incoming audio signal  14 , which can be analog or digital, into an amplified pulse-coded representation of that incoming signal, hereafter referred to as the amplifier output  16 . A reconstruction filter  22  converts this amplifier output  16  into a corresponding analog audio signal  20 , which can then be used to drive a speaker  18 . This process is carried out by a reconstruction filter  22 . An important component of this reconstruction filter  22  is an inductor. 
     Inexpensive and coupled or uncoupled compact planar inductors  24 , such as that shown in  FIG. 2  can be used in reconstruction filters  22  of class-D amplifier systems  10 . A planar inductor  24  typically includes a printed circuit board  26  having concentric traces  28  that function as the windings of the inductor. First and second low-reluctance structures  30 ,  32  resting on opposing sides of the printed circuit boards function as the core of the inductor  24 . Typical low-reluctance structures  30 ,  32  often comprise but are not limited to a ferromagnetic material, such as powdered iron, or ferrite. In an effort to linearize the change in inductance as a function of frequency, such structures often incorporate air gaps. These air gaps can result in cross-talk caused by magnetic linkage between nearby inductors. 
     SUMMARY 
     The invention is based on the recognition that cross-talk between nearby planar inductors can be significantly mitigated by proper feedback. As a result, the invention makes it possible to construct amplifiers having small footprints with inexpensive planar inductors. 
     In one aspect, the invention features an apparatus for providing an audio signal to drive a speaker system. Such an apparatus includes first and second audio channels. The first audio channel has a first class-D amplifier for receiving an input signal, and a first reconstruction filter for receiving an output from the first class-D amplifier and reconstructing therefrom an output audio signal for driving the speaker system. The second audio channel has a second class-D amplifier for receiving an input audio signal, and a second reconstruction filter for receiving an output from the second class-D amplifier and reconstructing therefrom an output audio signal for driving the speaker system. The first and second reconstruction filters have corresponding first and second planar inductors, with the second planer inductor being magnetically coupled to the first planar inductor. 
     Some embodiments of the apparatus also include a first feedback loop for providing a signal from an output of the first reconstruction filter back to the first class-D amplifier. Among these embodiments are those that also include a second feedback loop for providing a signal from an output of the second reconstruction filter back to the second class-D amplifier. 
     Yet other embodiments further include means for reducing distortion due to magnetic coupling between the first and second planar inductors. Among these are those in which the means for reducing distortion includes means for providing feedback from the first reconstruction filter to the first class-D amplifier. These latter embodiments also include those in which the means for reducing distortion further includes means for providing feedback from the second reconstruction filter to the second class-D amplifier. 
     Also included among the embodiments of the apparatus are those in which the first class-D amplifier includes a first output for providing a first signal, a second output for providing a second signal modulated out-of-phase with the first signal, a first feedback input, and a second feedback input. In these embodiments, the reconstruction filter includes first and second LC circuits for receiving the first and second signals respectively. These embodiments also include first and second feedback loops. The first feedback loop connects an output of the first LC circuit to the first feedback input. The second feedback loop connects an output of the second LC circuit to the second feedback input. 
     Yet other embodiments of the apparatus include first and second printed circuit boards. Class-D amplifiers are mounted on the first printed circuit board, and planar inductors are mounted on the second printed circuit board. These printed circuit boards are oriented perpendicular to each other. 
     In some embodiments, the first and second audio channels are configured such that cross-talk between the first and second channels is at least 54 dB below a signal level of the first audio channel at 10 kHz. 
     Among the embodiments of the apparatus are those in which the central axes of each of the first and second inductors are separated by no more than one body length. 
     In some embodiments, wherein the first and second planar inductors are mounted on a printed circuit board having conductive traces between the first and second planar inductors. In these embodiments, the first planar inductor has a first edge and the second planar inductor has a second edge facing the first edge and separated from it by no more than a width of the conductive trace. 
     In some embodiments, the first class-D amplifier is configured to operate in BD mode. Among these are those in which the first audio channel is configured to operate in differential mode at audio frequencies and in common mode at frequencies higher than the audio frequencies. 
     In another aspect, the invention features another apparatus for amplifying an audio signal. This apparatus includes first and second audio channels. The first audio channel has a first class-D amplifier for receiving an input signal, and a first reconstruction filter for receiving an output from the first class-D amplifier and reconstructing therefrom an output audio signal for driving the speaker system. The second audio channel has a second class-D amplifier for receiving an input audio signal, and a second reconstruction filter for receiving an output from the second class-D amplifier and reconstructing therefrom an output audio signal for driving the speaker system. The first and second reconstruction filters have corresponding first and second planar inductors. These planar inductors are magnetically coupled to each other. The apparatus further includes means for reducing signal distortion arising from this magnetic coupling between the first and second planar inductors. 
     Among the many embodiments of the foregoing apparatus are those in which the means for reducing signal distortion includes means for providing, to the first class-D amplifier, a feedback signal from the first reconstruction filter. 
     In yet another aspect, the invention features an apparatus for providing an audio signal to drive a speaker system. Such an apparatus includes a circuit having a planar inductor, as well as a first audio channel. The audio channel has a first class-D amplifier for receiving an input signal, and a first reconstruction filter for receiving an output from the first class-D amplifier and reconstructing therefrom an output audio signal for driving the speaker system. This first reconstruction filter includes a first planar inductor that is magnetically coupled to the planar inductor associated with the circuit. 
     There are numerous embodiments of the foregoing apparatus, some of which are listed below. 
     Among the many embodiments are those that include means for reducing signal distortion arising from the magnetic coupling between the first and second planar inductors. 
     In additional embodiments, the circuit having a first planar inductor includes a power supply for the first audio channel. 
     Other embodiments include a first feedback loop for providing a signal from an output of the first reconstruction filter back to the first class-D amplifier. 
     Also among the many embodiments of such an apparatus are those in which the first class-D amplifier includes a first output for providing a first signal, a second output for providing a second signal modulated out-of-phase with the first signal, a first feedback input, and a second feedback input. In such embodiments, the reconstruction filter includes first and second LC circuits for receiving the respective first and second signals, a first feedback loop connecting an output of the first LC circuit to the first feedback input; and a second feedback loop connecting an output of the second LC circuit to the second feedback input. 
     These and other features of the invention will be apparent from the following detailed description and the accompanying figures, in which: 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a typical class-D audio amplifier system having an amplifier that both amplifies and modulates an input signal, and a reconstruction filter that a signal that can drive a speaker; 
         FIG. 2  shows a planar inductor for use in the reconstruction filter shown in  FIG. 1 ; 
         FIG. 3  shows a circuit with feedback loops to reduce coupling between planar inductors in neighboring audio channels; 
         FIG. 4  shows one exemplary embodiment in which the planar inductors, rather than being on a main board as they are in other embodiments, are instead placed on a separate daughter board to further reduce amplifier footprint; and 
         FIG. 5  shows a circuit with feed back loops to reduce coupling with planar inductors in a neighboring circuit. 
     
    
    
     DETAILED DESCRIPTION 
     A difficulty that arises with planar inductors that incorporate an air gap is the significant magnetic flux leakage that results from that air gap. As a result of this magnetic flux leakage, neighboring planar inductors can function essentially as a transformer. The magnetic coupling between the two inductors leads to cross-talk. In applications where such magnetic coupling is unwanted, one typically spaces planar inductors far from each other. Although this generous spacing between planar inductors does indeed reduce magnetic coupling between them, it also results in circuits having unacceptably large footprints. 
     In an audio amplifier with two or more independent channels, it is desirable to maintain cross-talk between channels at least 54 dB below the signal of interest in any one channel. When using planar inductors in the reconstruction filters of such amplifiers, it has been found desirable to space the planar inductors by at least three times the body length (i.e. the length of the low-reluctance structures  30 ,  32 ) of a planar inductor to weaken the magnetic coupling between adjacent inductors enough to reach this cross-talk level. This spacing results in unreasonably large audio amplifiers that are difficult to mount within the confined space of a typical motor vehicle. For this reason, class D audio amplifiers that rely on planar inductors in their reconstruction filters are typically restricted to being single channel amplifiers, such as bass amplifiers. 
       FIG. 3  shows a multi-channel class D audio amplifier system having feedback loops configured to effectively reduce the effect of magnetic coupling between planar inductors in adjacent audio channels  36 ,  38 . The presence of such feedback loops enables channels to be mounted closer to each other without introducing unacceptable cross-talk. This, in turn, enables one to construct compact multi-channel class D audio amplifier systems that can easily be mounted within the confined spaces of a motor vehicle. 
     Expressed differently, for any level of acceptable cross-talk, there exists a distance below which cross-talk becomes unacceptable. In the configuration shown in  FIG. 3 , this distance is smaller than it would have been in the absence of the illustrated feedback loops. 
     The illustrated system includes only first and second channels  36 ,  38  adjacent to each other. However, the principles described herein are applicable to any number of channels. 
     As is apparent from  FIG. 3 , the first and second channels  36 ,  38  have similar structures. Accordingly, only the first channel  36  is described in detail, with the understanding that other channels have similar construction. 
     The first channel  36  features a class-D audio amplifier  40  in series with a reconstruction filter  42 . The class-D audio amplifier  40  accepts an audio signal  44 , which can be digital or analog, and modulates the audio signal  44  to output first and second amplified pulse-coded versions of the audio signal, hereafter referred to the “amplifier output signals”  46 ,  48 . The first and second class-D audio amplifier output signals  46 ,  48  are preferably modulated in-phase (class “BD” modulation) to reduce electromagnetic interference, to reduce pop noise during amplifier start-up and shut-down, and to cause the reconstruction filter  42  to work properly with center-pole gapped coupled planar inductor structures. 
     The amplifier output signals  46 ,  48  are provided to the reconstruction filter  42  (e.g., a low-pass filter). 
     At a minimum, the reconstruction filter  42  includes a first planar inductor  50  and a first capacitor  52  connected to form a first LC circuit for low-pass filtering the first amplifier output signal  46  and a second planar inductor  54  and a second capacitor  56  connected to form a second LC circuit for filtering the second amplifier output signal  48 . This reconstruction filter  42  thus removes the high (i.e. RF) frequencies associated with the pulses in the amplifier output signals  46 ,  48 , leaving behind a baseband amplified audio signal  58  that can then be used to drive a speaker  60 . 
     Typical values of differential inductance of the first and second planar inductors  50 ,  54  are on the order of 10 micro-henries. In one embodiment, the inductance is 10.7 micro-henries measured at 10 kHz with 1 Vrms. Typical capacitance values for the first and second capacitors  52 ,  56  are on the order of 1 microfarad with a saturation current of 8 amps. 
     In some embodiments, the planar inductor is one of size EFR28 that uses 3C92 ferrite material, with a total air gap of 250 micrometers. 
     Each channel  36 ,  38  also includes first and second feedback loops  62 ,  64 . The first feedback loop  62  provides a first signal to a first feedback input  66  in the class-D audio amplifier  40 , while the second feedback loop  64  provides a second signal a second feedback input  68  in the class-D audio amplifier  40 . The first and second signals are positive and negative outputs that are out of phase relative to each other. 
     The first and second feedback loops  62 ,  64  operate to correct errors arising from non-linearity at the audio output  58 . As a result, the feedback loops  62 ,  64  can be used to correct for both total harmonic distortion, and distortion that arises from magnetic coupling of planar inductors in an adjacent channel  38 . This, in turn, allows adjacent channels  36 ,  38  to be positioned closer to each other, resulting in an amplifier system  42  arranged in a physically smaller layout and low cost. 
     The circuit configuration shown in  FIG. 3  enables the first audio channel  36  to function as a differential filter at audio range frequencies and as a common mode filter at RF frequencies. This arises in part because the planar inductor functions as a common mode filter that tends to block higher frequencies. As a result, these high frequency components tend to be attenuated more than they would be in more conventional approaches. 
     The configuration shown in  FIG. 3  allows separate channels  36 ,  38  to share the same printed circuit board. However, the configuration shown in  FIG. 3  also allows pairs of planar inductors  50 ,  54  from separate channels  36 ,  38  to be placed on a separate daughter board  70 , as shown in  FIG. 4 , which can then be mounted perpendicular to the board  72  containing the class-D audio amplifiers  66  for each channel  36 ,  38 . This further reduces the physical footprint of the overall amplifier system  42 . 
     As shown in  FIG. 4 , each planar inductor  50 ,  54  has a central body axis  74 , first and second edges  76 ,  78  that are parallel to this body axis  74 , and third and fourth edges  80 ,  82  that are perpendicular to this body axis  74  and also to the first and second edges  76 ,  78 . The third and fourth edges  80 ,  82  are separated by a body length. The planar inductors  50 ,  54  are separated from each other by a dimension that is less than this body length. Despite this small separation, the presence of the feedback loops  62 ,  64  shown in  FIG. 3  prevents the magnetic coupling between planar inductors  50 ,  54  in  FIG. 4  from exceeding 54 dB below the signal level of any audio channel  36 ,  38 . 
     As is also apparent from  FIG. 4 , each pair of planar inductors  50 ,  54  is separated by no more than a width of a conductive trace  84  on the daughter board  74 . Despite this small separation, the presence of the feedback loops  62 ,  64  shown in  FIG. 3  prevents the magnetic coupling between planar inductors  50 ,  54  in  FIG. 4  from exceeding 54 dB below the signal level of any audio channel  36 ,  38 . 
     The possibility of magnetic coupling, and its accompanying audio frequency distortion, arises when an audio channel having a planar inductor is mounted near any circuit that also includes planar inductors. Thus, the feedback mechanism described herein is applicable to cases in which the first channel  36  is mounted near another circuit that may not necessarily be another audio channel, but that may nevertheless include one or more planar inductors close enough to the audio channel to cause audible distortion. For example, as shown in  FIG. 5 , a circuit  38  mounted near the first channel  36  could be a power supply having one or more planar inductors that are close enough to magnetically couple to a planar inductor in the first channel  36 .