Abstract:
Direct current (DC) offset in and audio driver can cause a constant drain on power even when there is no sound. Furthermore it can cause an audible pop when the audio driver is enabled. A scaled replica output stage can be employed to perform DC offset cancellation offline during a sampling phase. Once DC offset cancellation is achieved, the audio driver uses a full scale output stage during the operation phase.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to audio drivers and specifically with DC offset cancellation. 
         [0003]    2. Related Art 
         [0004]      FIG. 1  shows a conventional digital audio driver. Driver  100  comprises a digital to analog converter (DAC)  102 , amplifier stage  104  and an output stage. 106 . DAC  102  converts a digital audio signal into an analog audio signal. Amplifier stage  104  amplifies the analog audio signal. The primary purpose of output stage  106  is to maintain the output regardless of the current drawn through it, but in some implementations the output stage may supply some amount of gain. 
         [0005]    In most analog circuits, such as the audio driver, a DC offset is present.  FIG. 2A  schematically shows the DC offset as fixed voltage  202  in series with an ideal audio driver component  204 , which provides audio output  206 , referenced as V OUT . This output is used to drive load  208  represented here by headphones. 
         [0006]    As shown in  FIG. 2B , issues with a DC offset in an audio driver are often resolved where the load  208  is AC-coupled, such as with external capacitor  210  which is inserted between the output of the audio driver and load  208 . However, modern integrated audio drivers consolidate components onto a single integrated circuit eliminating costlier components such as a large capacitor. Larger capacitors provide better isolation while minimizing the affect on the frequency response of the audio driver. If a smaller capacitor were used, there would be more attenuation in the low frequencies resulting in perceived diminished “bass” by a listener. 
         [0007]    Integrated audio drivers that are directly coupled to the load introduce problems due to the presence of DC offset at the driver output. The DC offset will cause current to flow into the load even when no sound is present increasing the average power consumption. This is particularly a problem when driving low-impedance headphones. Another problem is that when the driver is enabled, the output voltage will abruptly change, causing an audible “pop” sound. 
         [0008]      FIG. 3  illustrates the pop problem in direct coupled audio drivers. Graph  302  is representative of the enable signal received by the audio driver. Graph  304  is representative of the output voltage of the audio driver around the time the audio driver is enabled. At time  306 , the output voltage jumps from 0V to V OS , resulting in an audible pop even for offset voltages as low as 1 mV. 
         [0009]    Traditional techniques for removing DC offsets from amplifiers fall into two categories, auto-zeroing and chopper stabilization.  FIG. 4  illustrates an analog method of auto-zeroing applied to an amplifier. When using auto-zeroing the amplifier has a sampling phase where the DC offset is removed and an operation phase where the amplifier amplifies an input signal. In the example shown in  FIG. 4 , switch  410  is connected to ground, or in the case of a differential amplifier, the positive and negative inputs of the differential amplifier are connected to each other (i.e., a zero input in either case). Offset voltage  402  and amplifier  404  produce an output which should be representative of the DC offset because the input is effectively zero due to switch  410 . The output is sampled by sample/hold  408 . Optionally, buffer  406  is used to supply the voltage to sample/hold  408 . Buffer  406  may be an amplifier with or without gain. The purpose is to sample the voltage at the output without supplying a significant load which could affect the output voltage. Sample/hold  408  feeds back the voltage to amplifier  404 , which is adjusted to zero out the offset. After the sampling phase is completed, the adjustment to the amplifier is fixed and switch  410  connects the amplifier back to the input and the amplifier functions now in the operational stage. 
         [0010]      FIG. 5  illustrates a digital implementation of auto-zeroing applied to an amplifier. Switch  512  operates similarly to switch  410  in  FIG. 4 , controlling the input to the amplifier when in the sampling phase and the operational phase. The remaining components are similar to their counter parts in  FIG. 4 . The key difference is successive approximation register (SAR)  508  is used to determine the voltage after a number of clock cycles. The digital representation of the voltage is fed to DAC  510  which supplies the voltage to the amplifier so that the voltage can be adjusted to a zero offset. Once the offset voltage is determined, the SAR output can be fixed and the amplifier transitions to the operational stage and functions with zero offset. 
         [0011]    The chopper stabilization approach applies a modulation to the input signal and a corresponding demodulation to the output signal. Since the DC offset only encounters the demodulation, it is effectively modulated to a higher frequency. More specifically,  FIG. 6  illustrates a basic chopper stabilized amplifier. Again the amplifier is shown as ideal amplifier  604  with fixed voltage offset  602 . The input signal is modulated with mixer  606  where carrier signal  612  is the desired frequency that the DC offset is displaced to. The output signal is demodulated with mixer  610  with carrier signal  614 . Typically the carrier signals are square waves at a given frequency. Because the input is modulated and then demodulated, its frequency profile does not change. However, the DC offset is effectively modulated by mixer  610  to the frequency of  614 . In this fashion, the DC offset is removed from the amplifier. 
         [0012]    A drawback of the auto zeroing approach is that in an audio driver the load may be connected to the driver before it is enabled. When the amplifier is first enabled, the offset would be present because the auto-zeroing has not been applied to remove the DC offset, and a pop would still be heard. One attempt to remedy this is to use two amplifiers and “ping-pong” between them. That is while one amplifier is in the sampling phase, the other amplifier is in the operational phase and only the one in the operational phase is connected to the output. This can prove costly to implement because it doubles the hardware used in the amplifier stage and, furthermore, it may not solve the problem because neither amplifier can be auto-zeroed until they are enabled, so there may still be an initial pop. 
         [0013]    A chopper stabilized amplifier would have no DC offsets from the time it is enabled because there is no sampling phase like the auto-zeroing technique. The drawback of the chopper stabilization is that it is a more complicated solution. Generally the chopper stabilization is used only to remove the offset from an amplifier leaving potential DC offsets from other components in the audio driver. Even though the DC offset is modulated, it still is present except as a higher frequency signal. Unless this signal is filtered out, it can still cause current to flow to the load. 
         [0014]      FIG. 7  illustrates another approach to isolating an amplifier until the DC offset can be removed by auto-zeroing. For the sake of example, the auto-zeroing technique of  FIG. 5  is used. During the sampling phase, switch  702  disconnects the audio driver from load  704 . After the sampling phase is completed switch  702  is closed and connects the audio driver to load  704 . The switch  702  is required to switch a signal after it has been amplified and can carry substantial power. Some drawbacks of using such a switch are that the switch can be very expensive or/and degrade the driver performance. 
         [0015]    There is a need in the industry to eliminate DC offset from an audio driver which is not expensive and does not degrade the driver performance. 
       SUMMARY OF INVENTION 
       [0016]    An audio driver equipped with DC offset cancellation comprising an input stage that receives and amplifies an audio sound signal, an output stage for driving a load, a replica output stage coupled to the input stage, that connects the output stage to the input stage and an auto-zeroing circuit coupled to the output of the replica stage which adjusts the input stage to cancel any DC offset detected at the output of the replica output stage. The output stage can comprise an NFET and a PFET. The audio driver can further comprise a switch which connects the gate of the NFET to the high supply voltage and another switch which connects the gate of the PFET to the low supply voltage to put the output stage into a high-impedance mode during the sampling. Typically the replica output stage is substantially smaller than the output stage. 
         [0017]    The auto-zeroing circuit comprises a comparator and successive approximation logic, which is frozen after the sampling phase is completed. The auto-zeroing circuit can comprise a low-pass filter. 
         [0018]    This auto-zeroing configuration can be applied to output stages that are single ended or differential. It can also be applied in a class AB amplifier where the output stage is in a push-pull configuration and receives two inputs. 
         [0019]    Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1  shows a conventional digital audio driver 
           [0021]      FIG. 2A  schematically shows the DC offset as a fixed voltage in series with an ideal audio driver component, which provides audio output; 
           [0022]      FIG. 2B  shows the DC offset in an audio driver as resolved by AC coupling the load with an external capacitor inserted between the output of the audio driver and the load; 
           [0023]      FIG. 3  illustrates the pop problem in direct coupled audio drivers; 
           [0024]      FIG. 4  illustrates an analog method of auto-zeroing applied to an amplifier; 
           [0025]      FIG. 5  illustrates a digital implementation of auto-zeroing applied to an amplifier; 
           [0026]      FIG. 6  illustrates a basic chopper stabilized amplifier; 
           [0027]      FIG. 7  illustrates another approach to isolating an amplifier until the DC offset can be removed by auto-zeroing; 
           [0028]      FIG. 8  shows an embodiment of an auto-zeroing audio driver in accordance with the present invention; 
           [0029]      FIG. 9  shows the amplifier and output stages of an audio driver; 
           [0030]      FIG. 10  illustrates an audio driver where the pop from an initial DC offset is removed; 
           [0031]      FIG. 11  illustrates an alternate embodiment of an audio driver where the audible pop resulting from an initial DC offset is removed; 
           [0032]      FIG. 12  illustrates another implementation of an audio driver; 
           [0033]      FIG. 13  illustrates another embodiment of an audio driver employing the auto-zeroing approach of  FIG. 8 ; 
           [0034]      FIG. 14  illustrates an embodiment of an audio driver where the output stage receives only one input from the amplifier; 
           [0035]      FIG. 15  shows an embodiment of an output stage and replica output stage that can be used in audio driver as output stages, respectively; and 
           [0036]      FIG. 16  is an example of an audio driver with a fully differential output stage. 
       
    
    
       [0037]    Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
       DETAILED DESCRIPTION 
       [0038]    A detailed description of embodiments of the present invention is presented below. While the disclosure will be described in connection with these drawings, there is no intent to limit it to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications and equivalents included within the spirit and scope of the disclosure. 
         [0039]      FIG. 8  shows an embodiment of an auto-zeroing audio driver in accordance with the present invention. The auto-zeroing audio driver comprises DAC  102 , amplifier stage  104  and output stage  106 . In addition, an auto-zeroing circuit comprises comparator  806 , optional low-pass filter (not shown) and SAR logic  804 . Additionally, it can comprise adder  802 . 
         [0040]    During the sampling phase, the input to the DAC  102  is fixed at zero. In one embodiment, unlike the auto-zeroing of an amplifier, the input can be under software control so that a physical switch is not needed. Comparator  806  receives a signal at the output of the audio driver, but optionally could receive the signal at the output of amplifier stage  104  if the auto-zeroing including the output stage is not desired. Comparator  806  compares the output voltage to ground. Optionally, the output can be low-pass filtered to isolate the DC offset if transient sources of voltage are present such as noise. Based on the sign of comparator  806 , SAR logic  804  increments or decrements its internal register and adds the result to the input of DAC  102 . Specifically, if comparator  806  determines that the current output voltage is greater than ground, SAR logic  804  decrements its internal register and if comparator  806  determines that the current output voltage is less than ground, SAR logic  804  increments its internal register. 
         [0041]    The result in the SAR logic&#39;s register is added to DAC  102  by the use of adder  802 . In one embodiment, adder  802  is not included and the result of SAR logic  806  can be fed back to controlling software or firmware which adds it to the value sent to the DAC. 
         [0042]    When the result from SAR logic  804  is added to the input of DAC  102 , the output may change. Upon each iteration, the amount SAR logic  804  increments or decrements its register decreases. Eventually, the register in SAR logic  804  will converge to a value that causes the DC offset to be arbitrarily small. In one embodiment, SAR logic  804  begins with the largest increment first and uses an increment that is half as big as the previous iteration until the increment size reaches the minimum resolution of the register. 
         [0043]    When convergence is reached, the output from SAR logic  804  is frozen, such that the value added to the input to DAC  102  is fixed and the audio driver can operate without a DC offset. This configuration has advantages over prior solutions because the DC offset contribution of certain components in the analog portion of an audio driver is removed. 
         [0044]    Embodiments addressing the problem of the audible pop which occurs at the start of the sampling phase will now be described. Referring to  FIG. 9 , the amplifier and output stages of an audio driver are shown. Audio driver  900  comprises DAC  902 , amplifier  904  and output stage  920 . It comprises feedback network  906  in a negative feedback configuration which is used to provide stability to amplifier structure. In this configuration, amplifier  904  provides a dual input to output stage  920 . Depending on the application, amplifier  904  drives output stage  920  which is shown in a push-pull configuration where the two inputs to the output stage differ only by a bias voltage as is often the case in a class AB amplifier structure. The audio driver can drive load  912  depicted here as a headphone. 
         [0045]    The output stage comprises p-channel field effect transistor (PFET)  908  and n-channel field effect transistor (NFET)  910 , where the output is tapped between the drain of PFET  908  and the drain of the NFET  910 . The source of PFET  908  is coupled to the high supply voltage rail and the source of NFET  910  is coupled to the low supply rail. In a typical application, the low supply voltage rail may actually be a negative voltage relative to ground and often equal in magnitude to the high supply voltage. In other applications it may be tied to the ground potential. 
         [0046]      FIG. 10  illustrates an audio driver where the pop from an initial DC offset is removed. Audio driver  1000  comprises DAC  902 , amplifier  904 , output stage  920 , and feedback network  906 . The audio driver  1000  further comprises duplicate output stage  1010  which comprises PFET  1002  and NFET  1004 . Auto-zeroing circuit  1006  is connected to the output of duplicate output stage  1010 . In one embodiment, auto-zeroing circuit  1006  uses one of the circuit techniques shown in  FIG. 4 ,  5  or  8 . 
         [0047]    During sampling, output stage  920  is disconnected from amplifier  904 , the output of output stage  920  is grounded and the output stage should exhibit high impedance so that it doesn&#39;t draw significant power during the sampling phase. Switches  1012  and  1014  are opened during the sampling phase and closed during the operational phase. During the sampling phase, switches  1012  and  1014  disconnect output stage  920  from amplifier  904  and connect output stage  920  to amplifier  904  during the operational phase. Switches  1016  and  1018  are closed during the sampling phase and opened during the operational phase. During the sampling phase, switch  1016  connects the gate of PFET  908  to the high supply voltage rail placing PFET  908  into a high impedance state and switch  1018  connects the gate of  910  to the low supply voltage rail placing NFET  910  into a high impedance state to insure little current flows through PFET  908  and NFET  910 . Switch  1020  is closed during the sampling phase and opened during the operational phase. During the sampling phase this insures that the output is grounded. After the DC offset is corrected during the sampling phase, the output from the output stage  920  will initially be zero, so when output stage  920  is connected to amplifier  904 , there is no voltage change during the transition and hence no pop is heard. 
         [0048]    In one embodiment, output stage  1010  is a “scale model” of output stage  920 , using smaller field effect transistor (FETs) with the same electrical characteristics. Smaller FETs do not occupy as much space on an integrated circuit and hence are less costly than a full scale output stage. Because the FETs are smaller they cannot handle the same level as current as output state  920 , but that is not necessary since duplicate output stage  1010  drive the comparator, not the load. Any difference between the DC offset seen at the output of output stage  920  and duplicate output stage  1010  in this embodiment is negligible in light of the high comparative gain of amplifier  904 . 
         [0049]    During the sampling phase, output stage  920  is disconnected from the amplifier stage, set to high impedance so the output stage and load do not draw a significant current and grounded at the output. Meanwhile, the auto-zeroing circuit is adjusting amplifier  904  to cancel any DC offset seen at the output of duplicate output stage  1010 . The response of the audio driver should be substantially the same as when output stage  920  is connected, so there should be no significant difference in DC offset seen at the output of output stage  920  and duplicate output stage  1010 . After the DC offset is zeroed out, the audio driver can be placed in the operational phase. During this transition, the output stage  920  is connected to amplifier  904  by closing switches  1002  and  1004 , disconnected from its high impedance state by opening switches  1016  and  1018  and disconnected from ground via switch  1020 . Optionally, the duplicate output stage  1010  can be disconnected. However, because duplicate output stage is identical and scaled down, leaving duplicate output stage connected does no harm. 
         [0050]      FIG. 11  illustrates an alternate embodiment of an audio driver where the audible pop resulting from an initial DC offset is removed. In this embodiment audio driver  1100  differs from audio driver  1000  in that the DC offset seen at duplicate output stage  1010  through auto-zeroing circuit  1102  is fed back and added to the input of DAC  902 . All other components function essentially the same way as described for audio driver  1000 . In this way not only is the DC offset introduced by the amplifier cancelled, but any DC offset introduced by the DAC is also cancelled. 
         [0051]      FIG. 12  illustrates another implementation of audio driver  1100 . In this implementation audio driver  1100  employs the auto-zeroing circuitry described in  FIG. 8 . The auto-zeroing circuit comprises comparator  1202 , digital low-pass filter  1204  and SAR logic  1206 . An embodiment of the operation of these circuit elements is described above with reference to  FIG. 8 . 
         [0052]      FIG. 13  illustrates another embodiment of an audio driver employing the auto-zeroing approach of  FIG. 8 . In this embodiment, output stages  920  and  1010  are audio output stages and are not limited to the output stages shown in the previous figures. For example, the output stages could comprise bipolar transistors rather than FETs or could comprise an alternate topology. For clarity, the ability to switch output stage  920  into a high impedance mode is abstracted into output stage  920  and controlled by input  1302  which controls whether the audio driver is in the sampling phase or the operational phase. If it is in the sampling phase, it puts output stage into high impedance mode and takes it out of high impedance mode when the driver is in the operational phase. 
         [0053]    The example shown is for a push-pull output stage, and the amplifier supplies two outputs to the output stage which has a single output. The use of a replica output stage can be applied to many other circumstances such as a single input output stage or a fully differential output stage. A replica of the output stage is added to the audio driver and during the sampling phase, the output stage is disconnected, placed into a high impedance stage and the output is zeroed. Generically, zeroed is meant to imply a zero output value, so in the case of a single ended driver, zeroed means grounded, but in the case of a differential driver, zeroed means that the difference between the two outputs is zero. In either case, zero includes a range of error within which the audible artifacts associated with the DC offset are suitably removed. This range of permissible error is often defined by requirements. For example some requirements defined this “inaudible” threshold has 65 dB below the maximum voltage swing, that is, the voltage of the high power rail minus the voltage of the low power rail. Often this translates to ½ to 1 millivolt. 
         [0054]      FIG. 14  illustrates an embodiment of an audio driver where the output stage receives only one input from the amplifier. Audio driver  1400  comprises DAC  1402 , amplifier  1404 , and feedback network  1406  which function essentially the same as DAC  902 , amplifier  904  and feedback network  906  except that amplifier  1404  provides a single output to the output stage. Audio driver  1400  comprises output stage  1414  which can optionally be put into a high impedance mode shown here as controlled by input  1410 . Switch  1412  disconnects output stage  1414  during the sampling phase and switch  1416  grounds the output during the sampling phase. Because there is only one input to output stage  1414  only one switch is needed to disconnect it from amplifier  1404 . The auto-zeroing circuitry in audio driver  1400  comprises comparator  1202 , digital low-pass filter  1204  and SAR logic  1206 . Though shown with this specific auto-zeroing circuitry, other auto-zeroing techniques and configurations such as shown in  FIGS. 4 ,  5 ,  10  and  11  could be used. 
         [0055]      FIG. 15  shows an embodiment of an output stage  1500  and replica output stage  1550  that can be used in audio driver  1400  as output stages  1414  and  1408 , respectively. Output stage  1500  comprises PFET  1502  and NFET  1504 , where the output is tapped between the drain of PFET  1502  and the drain of the NFET  1504 . The source of PFET  1502  is coupled to the high supply voltage rail and the source of NFET  1504  is coupled to the low supply rail. Unlike output stage  920 , the input to the output stage is coupled to the gate of PFET  1502  and the gate of NFET  1502  is coupled to a bias voltage. Switch  1506  is added to the conventional output stage and used to place output stage  1500  and more specifically PFET  1502  into a high impedance state. Switch  1506  can be controlled by signal  1410 . Replica output stage  1550  comprises PFET  1552  and NFET  1554  connected in essentially the same configuration as output stage  1500 . However, PFET  1552  and NFET  1554  can be a smaller version of PFET  1502  and NFET  1504 . This type of output stage may be used in a class A amplifier. 
         [0056]      FIG. 16  is an example of an audio driver with a fully differential output stage. Audio driver  1600  comprises DAC  1602 , amplifier  1604 , feedback network  1606  and feedback network  1608 . Amplifier  1604  differs from the previous amplifiers described in that outputs constitute a differential signal. Audio driver  1600  also comprises output stage  1620  which is a fully differential output stage. Because of the differential outputs, two feedback networks  1606  and  1608  are included to provide stability to the driver. To enable the auto-zeroing, replica output stage  1610  is added which functions similar to output stage  1620  and may be a smaller version with smaller transistors. Each differential output of the replica output stage is supplied to comparator  1632  which differs from comparator  1202  in previous figures because it compares the positive and negative differential outputs rather than comparing a single output to ground. When no signal is supplied, the positive and negative differential outputs should be the same. Digital low-pass filter  1204  and SAR logic  1206  may function as described in earlier embodiments. Audio driver  1600  further comprises switches  1612  and  1614  which are open during the sampling phase and closed during the operational phase and serve to disconnect and connect output stage  1620  to amplifier  1604 . Switch  1632  is used to tie the positive and negative differential outputs during the sampling phase thus “zeroing” the output during the sampling phase. Differential output stage  1620  drives differential load  1640  (shown as a speaker here) during the operational phase. 
         [0057]    The DC offset cancellation described can be applied to any multi-stage audio driver or multi-stage amplifier applications, having a separate output stage. The offset is performed off-line before the output stage is enabled eliminating any transitions during startup. By feeding back the auto-zeroing value into the digital domain, no additional analog components are required for the correction eliminating a potential source of distortion. Furthermore, any DC offset contributed by any component in the analog portion of the driver including the DAC can be removed. All added components are comparatively small to the ordinary components in an audio driver, and thus have negligible impact on the die size of the audio driver system. 
         [0058]    It should be emphasized that the above-described embodiments are merely examples of possible implementations. Many variations and modifications may be made to the above-described embodiments without departing from the principles of the present disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.