Abstract:
A circuit for providing audio signals to a load such as a speaker is provided that uses the speaker or headphone amplifier structure as a current to voltage converter, thereby eliminating a separate current to voltage converter from the circuit. Such a design removes one of the elements that creates noise in the circuit architecture and improves the dynamic range for the audio signal. For example, the output of a digital to analog converter is a single ended output provided to the speaker or headphone amplifier. The digital to analog converter can include a series of current sources that are summed up to provide the single ended output. Where the current sources have positive and negative current source mismatch, a feedback mechanism is employed to correct for the mismatch and reduce introduction of harmonic noise into the signal through the digital to analog converter.

Description:
TECHNICAL FIELD 
     This invention relates generally to circuits for providing analog information to a load, and more specifically, for providing a low power and low noise circuit for providing audio input to a load. 
     BACKGROUND 
     Various circuits are known for providing information from a digital source to an analog output such as a speaker. For example, common consumer applications such as portable music or communication devices provide digital data for conversion to an analog output such as a speaker or headphones. A goal for such devices is to reduce the amount of power used in driving its playback circuit such that there can be an increase in the playback time for the device. For certain audio playback devices, the audio quality relies on the dynamic range at negative 60 decibels power output. Various elements of the playback circuits, however, can create noise that necessitates consuming more power to overcome the noise introduced by those circuit elements. 
       FIG. 1  depicts one such known architecture for a circuit that provides an audio signal to a headphone for playback for a listener. In this architecture, an over-sampled sigma delta modulator  105  receives an audio signal from an UP-sampler circuit  110 . The output of the over-sampled sigma delta modulator  105  is provided to a current steering digital to analog converter  115 . The digital to analog converter  115  provides two outputs that are received by a current to voltage converter  120 . The current to voltage converter includes multiple elements such as two resistors R 1  and R 2  and an amplifier A 1 . The output of this circuit is provided to a headphone amplifier  125 . The headphone amplifier  125  includes multiple resistors R 3 , R 4 , R 5 , and R 6  and a further amplifier. The output of the headphone amplifier  125  is provided to the headphone speakers that, in turn, produce the audible signal for the listener. In this circuit, the noise for the audio signal is created primarily by the current steering digital to analog converter  115 , the current to voltage converter  120 , and the headphone amplifier  125 . The noise provided by the circuit elements results in a minimum power needed to drive the audio signal to have a sufficient audio quality above the background noise created by the circuit elements. Moreover, the noise created by the circuit elements negatively affects the dynamic range of the audio signal thereby reducing the audio quality experienced by the listener. 
     SUMMARY 
     Generally speaking and pursuant to these various embodiments, a circuit for providing audio signals to a load such as a speaker is provided that uses the speaker or headphone amplifier structure as a current to voltage converter, thereby eliminating a separate current to voltage converter from the circuit. Such a design removes one of the elements that creates noise in the circuit architecture and improves the dynamic range for the audio signal. 
     In one such example, the output of a digital to analog converter of the audio signal may be a single ended output provided to the speaker or headphone amplifier. An example of such a digital to analog converter can include a series of current sources that are summed up to provide the single ended output to the speaker or headphone amplifier. In the case where the current sources have positive and negative current source mismatch, a feedback mechanism can be employed to correct for the mismatch and reduce introduction of harmonic noise into the signal through the digital to analog converter. So configured, noise introduced by a current to voltage converter in previous known circuits is eliminated. Implementation of new circuits such as those described herein improve the dynamic range of the audio signal and reduces noise, thereby improving audio quality and extending the battery life of devices implementing such a design. These and other benefits may become clear upon making a thorough review and study of the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above needs are at least partially met through the provision of the low noise and low power arrangement for playing audio signals described in the following detailed description, particularly when studied in conjunction with the following drawings wherein: 
         FIG. 1  comprises an example prior art circuit scheme as configured in accordance with various previously known circuit designs; 
         FIG. 2  comprises an example circuit scheme as configured in accordance with various embodiments of the invention; 
         FIG. 3  comprises a circuit diagram of an example digital to analog converter with an amplifier and feedback circuit as configured in accordance with various embodiments of the invention; 
         FIG. 4  comprises an example feedback circuit as configured in accordance with various embodiments of the invention; 
         FIG. 5  comprises a circuit diagram showing one cell of an example digital to analog converter connected to an amplifier into one representation of a feedback circuit as configured in accordance with various embodiments of the invention; 
         FIG. 6  comprises a representation of an example of cycling of current sources being connected to the feedback circuit with respect to the clock cycle of the circuit as configured in accordance with various embodiments of the invention; and 
         FIG. 7  comprises a flow diagram of an example method of operation of a circuit as configured in accordance with various embodiments of the invention. 
     
    
    
     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein. 
     DETAILED DESCRIPTION 
     Referring now to the drawings, and in particular, to  FIG. 2 , an illustrative circuit architecture that is compatible with many of these teachings will now be presented. The example apparatus  200  of  FIG. 2  includes a modulator  205  configured to receive an input signal and to output a digital signal that is a modulated form of the input signal. A digital to analog converter  215  is configured to receive the digital signal, by one example audio data, and provide a single ended analog output, representing in this example audio signaling, at output  217 . Other types of digital data can be so processed. The described digital to analog converter  215  in one example comprises a Class B style single ended output digital to analog converter. The apparatus  200  further includes a current to voltage converter  220 , which may be a speaker or headphone amplifier, including a resistor  225  and an amplifier  230 . The amplifier  230  in this example includes at least a first input  233  configured to receive the single ended analog output from output  217  and a second input  237  connected to a common mode voltage (Vcm). The resistor  225  is connected between the first input  233  of the amplifier  230  and an output  239  of the amplifier  230 . The output  239  is configured to connect to a load, for example, a speaker or headphone. So configured, the apparatus  200  can provide an audio signal through a modulator and digital to analog converter to a load such as the headphone speaker without including a separate current to voltage converter, instead using the headphone amplifier as the current to voltage converter. 
     With reference to  FIG. 3 , an example approach to the digital to analog converter  215  will be described. In this example, the digital to analog converter  215  includes a plurality of positive current sources  320  and a plurality of negative current sources  330 . The individual current sources  320  and  330  of this example feed into one of three paths through switches  340 . The three paths include the single ended analog output  217 , the common mode voltage (Vcm), and a path  350  to a feedback circuit  360 . In this example, the digital to analog converter  215  is a “2N−1” level digital to analog converter implemented using current sources in sync. In this example, the modulator  205  is a delta sigma modulator that also provides a “2N−1” level output. In other words, the outputs from the modulator  205  are N−1, N−2, . . . , 1, 0, −1, −2, . . . , −N+2, and −N+1. The output of the digital to analog converter  215  is single ended so that it can be fed directly to the headphone amplifier  220 . More specifically, in the example of  FIG. 3 , the digital to analog converter has N positive current sources (Ip 1  through Ipn) and N negative current sources (Im 1  through Imn). The current sources are individually connected to the amplifier  230  depending on the output of the modulator  205 . Table 1 below summarizes the connection of the current sources  320  and  330  to the amplifier  230  in the example of  FIG. 3 . 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Number of Positive  
                 Number of Negative  
               
               
                   
                   
                 Current Sources 
                 Current Sources  
               
               
                 Serial 
                 Modulator 
                 Connected to 
                 Connected to 
               
               
                 No. 
                 Output 
                 the Amplifier A2 
                 the Amplifier A2 
               
               
                   
               
             
             
               
                 1 
                 N − 1 
                 N − 1 
                 0 
               
               
                 2 
                 N − 2 
                 N − 2 
                 0 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 N − 1 
                 1 
                 1 
                 0 
               
               
                 N 
                 0 
                 0 
                 0 
               
               
                 N + 1 
                 −1 
                 0 
                 1 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 2N − 2 
                 −N + 2 
                 0 
                 N − 2 
               
               
                 2N − 1 
                 −N + 1 
                 0 
                 N − 1 
               
               
                   
               
             
          
         
       
     
     The table shows the number of and type of current sources connecting to the amplifier  230  as decided by the modulator output. In other words, the digital to analog converter  215  is configured to connect the individual current sources  320  and  330  to the amplifier  230  based upon the single ended digital output from the modulator  205 . Per Table 1 above, zero current sources connect to the amplifier when the modulator  205  output is 0, and the number of sources connected to the amplifier  230  increases when signal strength increases. The type of current source connected to the amplifier  230 , positive or negative, is determined in response to the signal polarity. Such an arrangement is common with Class B type digital to analog converters. In one approach, the digital to analog converter  215  comprises an analog finite impulse response (FIR) filter including a plurality of cells  370 . Individual cells  370  each have a plurality of positive current sources  320  and a plurality of negative current sources  330 . Individual ones of current sources  320  and  330  feed into one of three paths through the switches  340  as described above. In the example of  FIG. 3 , an individual cell  370  is illustrated where multiple individual cells are contemplated to be included in the circuit structure with their combined output being provided to the single ended output  217  to the amplifier  230 . When not connected to the amplifier  230 , individual current sources  320  or  330  are connected to either the common mode voltage Vcm or to the feedback circuit  360 . 
     An example feedback circuit  360  will be described with reference to  FIG. 4  and  FIG. 5 . The feedback circuit  360  includes an integrator circuit  410  including a summing amplifier  413  and an integrating capacitor  417  connected together receive an error signal. The error signal includes current from individual ones of the positive current sources  320  and negative current sources  330  connected to the path  350  to the feedback circuit  360 . The feedback circuit  360  of this example further includes switches  420  with gates  430  connected to a node  440  electrically connecting an output  415  of the summing amplifier  413  and a compensation capacitor  450 . The compensation capacitor  450  is connected between the output  415  of the summing amplifier  413  and a node  460  between the integrating capacitor  417  and a resistor  470 . The switches  420  of the feedback circuit  360  individually correspond to the individual cells  370  of the digital to analog converter  215 , and the switches  420  are individually configured to route current to the individual cells  370  to control positive and negative currents to effect current value matching for the individual cells  370 . 
     In the example where the digital to analog converter  215  implements an analog FIR filter by having multiple cells  370 , each of which having multiple positive current sources  320  and negative current sources  330 . This implementation of the filter allows the digital to analog converter  215  to reduce out of band noise for the analog output signal. The positive current sources  320  and negative current sources  330 , however, can have the mismatch between them. The current mismatch can cause harmonics to appear in the output of the digital analog converter thereby degrading the audio quality of the ultimate output of the circuit. For example, positive current sources  320  may be implemented using PMOS transistors whereas negative current sources  330  can be implemented using the NMOS transistors. Thus, because the positive current sources  320  and negative current sources  330  are using different types of devices, a mismatch in the individual current output of the individual sources can occur. 
     Implementation of the example feedback circuit of  FIG. 4 , however, can correct this current mismatch. In one approach, the positive and negative current sources that are connected to the feedback circuit  360  are rotated over time. The feedback structure integrates this mismatch signal that is manifested in an error signal provided on the path  350 . In response, the switches  420  provide a control circuit signal in the form of a current that is routed back to individual cells  370  of the digital to analog converter  215 . An example of such a structure is shown in  FIG. 5 . 
     In  FIG. 5 , the output from an individual switch  420  of the feedback circuit  360  is connected via an electronic path  575  back to the individual cell  370  of the digital to analog converter  215 . This path  575  provides the feedback signal to the controls for the individual current sources  320  and  330 , thereby effecting a correction of the mismatch between the positive current sources  320  and the negative current sources  330 . A control for matching the positive and negative sources can be done in one of three ways. First, the control can be done such that there is a control of all the positive current sources  320  provided together to a given cell  370 , which is known in the art as gang control of the current sources  320 . Second, control of all the negative sources  330  of a given cell  370  can be ganged together. Third, there can be individual control of both types of current sources, the positive current sources  320  and the negative current sources  330 . The example of  FIG. 5  illustrates gang control of the positive current sources  320  through connection of the feedback path  575  to a link  580  to the controls for the positive current sources  320  of cell  370 . The current provided from the feedback circuit  360 , therefore, is used to control the positive current sources  320  to help correct the current mismatch with the negative current sources  330 . Implementation of control of the negative current sources  330  or of simultaneous feedback control of both types of current sources can be implemented by one of skill in the art using similar approaches. 
     An example approach to connecting positive and negative current sources from the digital to analog converter  215  to the feedback circuit  360  will be discussed with respect to  FIG. 6 . In this approach, individual cells  370  are configured to rotate connection of individual positive current sources  320  together with individual negative current sources  330  to the path  350  to the feedback circuit  360 . For example, current sources Ip(n−1) and Im(n−1) are connected to the path  350  to the feedback circuit  360  for one clock cycle  610  of the circuit. This connection is effected through the switches  340 , which are controlled by a separate controller (not shown) controlling the operation of the circuit. The effect of the connection is to have a positive current source&#39;s signals and its corresponding negative current source&#39;s signals add together on the path  350  to the integrator circuit  410 , such that the mismatching current between the individual positive current source and the corresponding individual negative current source is detected for this single clock cycle  610 . On the next clock cycle  620 , a second pair of a positive current source Ipn and a corresponding negative current source Imn is connected to feed into the path  350  to the feedback circuit  360 . Again, this connection of a pair of current sources, one positive current source Ipn and one negative current source Imn, is connected to the path  350  to the feedback circuit  360  for one clock cycle  620 . At the start of the next clock cycle  630 , another different pair of current sources, one positive Ip 1  and one negative Im 1 , is connected to the path  350  to the feedback circuit  360 . Although the integrating of the error signals is described as happening over a single clock cycle for the circuit, other time periods or methods can used for sensing the error signal and providing the feedback control to the individual cells. 
     So configured, the chosen positive and negative current sources are rotated every cycle such that the integrator circuit sees the average error over time. Based on this average error, a feedback correction current is provided from the individual switch  420  that corresponds to the cell  370  corresponding to the sensed current sources along the feedback path  575  as described above. So configured, the digital to analog converter  215  can be controlled to manage errors introduced by mismatches of the positive current sources  320  and the negative current sources  330  and still provide a low noise and low power single ended output through output  217  to the amplifier  230 . 
     With reference to  FIG. 7 , an example method of operation of the circuit such as that described above will be described. The method includes receiving  710  the modulator output signal at a digital to analog converter. In one example, the receiving includes receiving the modulator output signal at a current steering digital to analog converter. The method further includes controlling  720  a single ended output from the digital to analog converter based on the modulator output signal. The controlling  720  includes controlling connection of current sources of the digital to analog converter to the single ended output based on the modulator output signal received at the digital to analog converter. In one example, this controlling includes connecting an increasing number of current sources to the single ended output in response to receiving increasing signal strength of the modulator output signal. The type of current source connected to the single ended output is determined in response to the polarity of the modulator output signal. Such an approach can be effected through the use of a Class B digital to analog converter. In yet another example, the method may further include implementing an analog finite impulse response filter in the digital to analog converter. 
     Referring again to  FIG. 7 , the illustrated method includes matching  730  positive and negative current values for the digital to analog converter through a feedback circuit electrically connected to the digital to analog converter. The method further includes at  740  sending an output signal from the single ended output to a headphone amplifier connected to receive the output signal and a common mode voltage. The matching positive and negative current values through the feedback circuit may in one approach include integrating current sources from the digital to analog converter and providing a current control signal to individual cells of the digital to analog converter. So configured, a circuit implementing this method can provide a single ended output to an amplifier to eliminate the noise introduced by a separate current voltage converter that is typically implemented in audio circuits of this kind. Accordingly, noise is reduced, and an improved dynamic range can be realized in the audio output through the method executed by such a circuit. 
     A more specific example of a circuit embodying the teachings as described herein will be described with reference to  FIGS. 2 and 5 . In this example, a sigma delta modulator  205  is configured to receive an UP-sampled audio signal from an UP-sampler circuit  210  and to output a modulated digital signal. A current steering digital to analog converter  215  is configured to receive the modulated digital signal from the sigma delta modulator  205  and provide a single-ended analog output  217 . The current steering digital to analog converter  215  includes at least a plurality of cells  370  configured to allow the current steering digital to analog converter  215  to operate as an analog finite impulse response filter. Individual cells  370  of the plurality of cells  370  include at least a series of positive current sources  320  and a series of negative current sources  330 . The current steering digital to analog converter  215  also includes switches  340  configured to connect the positive current sources  320  and the negative current sources  330  individually to one of the group including single-ended analog output  217 , a common mode voltage (Vcm), and a path  350  to a feedback circuit  360 . The switches  340  are configured to connect a positive current source  320  and a corresponding negative current source  330  for a given cell  370  to the path  350  to the feedback circuit  360  for a clock cycle for the apparatus  200  and to connect a different positive current source and a corresponding negative current source for the given cell  370  to the path  350  to the feedback circuit  360  for a next cycle for the apparatus  200 . The connection of current sources to the path to the feedback circuit for a single cycle is further illustrated in the example of  FIG. 6  and described above. 
     The feedback circuit  360  of this example includes at least an integrator circuit  410  including a summing amplifier  430  and an integrating capacitor  417  connected to together to receive an error signal including current from individual ones of the positive current sources  320  and the negative current sources  330  connected to the path  350  to the feedback  360 . The feedback circuit  360  further includes feedback switches  420  with gates  430  connected to a node  440  connecting an output  415  of the summing amplifier  413  and a compensation capacitor  450 . The compensation capacitor  450  is connected between the output  415  of the summing amplifier  413  and a node  460  between the integrating capacitor  417  and a resistor  470 . In one example, the resistor  470  has a value of 100 kilo ohms, the integrating capacitor  417  has a capacitance of 10 pico farads, and the compensation capacitor has a capacitance of 1 pico farad, although other values of course may be used in other applications. The switches  420  individually correspond to the individual cells  370  and are individually configured to route current to the individual cells  370  to control positive and negative currents to effect current value matching for the individual cells  370 . 
     The single-ended analog output  217  is configured to feed to a headphone amplifier  220 , which is configured to receive the single-ended analog output  217  and the common mode voltage (Vcm). In this example, the headphone amplifier includes the resistor  225  and an amplifier  230 . The amplifier  230  includes at least a first input  233  configured to receive a single-ended amplifier output  217  and a second input  237  connected to the common mode voltage (Vcm). The resistor  225  is connected between the first input  233  of the amplifier  230  and an output  239  of the amplifier  230 , which is configured to connect to a load. The load typically is a speaker or a headphone speaker. In this example, the headphone amplifier  220  is configured to act as a current voltage converter between the current steering digital to analog converter  215  and the load. 
     So configured, the digital to analog converter is designed to reduce noise in the audio signal provided to a speaker. Moreover, in various examples, a differential signal chain implemented in the digital to analog converter provides an output directly to the headphone amplifier, which itself acts as a current to voltage converter. Accordingly, a separate current to voltage converter circuit is not needed, thereby eliminating a potential source of noise in the system. The dynamic range of such an arrangement increases the volume in the digital and reduces the gain in the analog for lower input signals. This is done dynamically but does not provide any dynamic change to the signal and therefore introduces limited audio artifacts for the listener. Accordingly, the circuit arrangement described above can provide a low power, high dynamic range digital to analog conversion, for example, in a headphone application, which is common for portable consumer music devices such as MP3 players and the like. 
     Those skilled in the art will recognize that a wide variety of modifications, alterations and combinations can be made with respect to the above described embodiments without departing from the scope of the invention. Such modifications, alterations and combinations to be viewed as being within the ambient concept.