Abstract:
A driver circuit includes a first driver amplifier that is configured to generate a first output in response to a first reference voltage input and a first audio input; a second driver amplifier that is configured to generate a second output in response to the first reference voltage and a second audio input; and a common mode (CM) amplifier, coupled to the first driver amplifier and the second driver amplifier. The CM amplifier is configured to generate an output in response to a second reference voltage input, the first reference voltage input being a divided version of the output. Gains of the first driver amplifier, second driver amplifier and the CM amplifier are equal. Noise at the output appears across a plurality of resistors coupled at the outputs of the first driver amplifier, second driver amplifier and the CM amplifier and cancels with respect to the output of the CM amplifier.

Description:
TECHNICAL FIELD 
     Embodiments of the disclosure relate to noise reduction in a stereo headset amplifier. 
     BACKGROUND 
     A stereo headset amplifier driving unit in audio applications is configured to drive an audio signal across a speaker coupled to the amplifier. The stereo headset amplifier driving unit includes a chip located in an audio system (e.g., mobile phone), and the speakers include a pair of earphones coupled to the audio system. A constituent driver amplifier of the driving unit is powered by a supply voltage, a fraction of which may bias the audio output of the driver amplifier. When the speaker is coupled to the headset amplifier driving unit, the DC bias across the speaker contributes to undesired power dissipation. Additionally, noise in the circuit will be audible across the speaker over the DC bias as a hum. 
     Schemes utilized in removing the DC bias in the audio output across the speaker may include a capacitive scheme configured to decouple the DC bias from the audio output, a common mode (CM) amplifier scheme having an extra CM amplifier to generate a CM voltage equal to the DC bias in the audio output, and a “negative voltage” scheme configured to enable the audio output to swing below a ground voltage. The “negative voltage” scheme involves generation of a negative supply voltage from the positive supply voltage through a charge pump. In the capacitive scheme and the CM amplifier scheme, the noise across the speaker coupled to the headset driving unit is dependent on a reference voltage input to the driver amplifier and/or the CM amplifier configuration. The “negative voltage” scheme solves the aforementioned problems associated with the capacitive scheme and the CM amplifier scheme, but causes increased power consumption from the positive supply voltage. Moreover, the generation of the negative supply voltage through the charge pump necessitates the use of an extra processing mask for creation of a deep N-well. Thus, there is an increased area/cost requirements associated with the “negative voltage” scheme. 
     SUMMARY 
     In one embodiment, a driver circuit includes a first driver amplifier that is configured to generate a first output in response to a first reference voltage input and a first audio input; a second driver amplifier that is configured to generate a second output in response to the first reference voltage and a second audio input; and a common mode (CM) amplifier, coupled to the first driver amplifier and the second driver amplifier. The CM amplifier is configured to generate an output in response to a second reference voltage input. The first reference voltage input is a divided version of the output. Also, gains of the first driver amplifier, second driver amplifier and the CM amplifier are equal. Noise at the CM amplifier output appears across a plurality of resistors coupled at the outputs of the first driver amplifier and second driver amplifier and cancels with respect to CM amplifier output. 
     In another embodiment, a method includes generating a CM voltage at an output of a CM amplifier based on a reference voltage input and deriving a reference voltage input to a first driver amplifier based on the CM voltage at the output of the CM amplifier. An audio input is provided to the first driver amplifier having an audio input. The method also includes replicating a noise at the output of the CM amplifier at an audio output of the first driver amplifier through a configuration of a feedback circuit associated with the CM amplifier and a feedback circuit associated with the first driver amplifier such that the CM amplifier and the first driver amplifier have same gain amplitude. 
     In yet another embodiment, an audio system includes an audio source configured to generate an audio signal and a set of driver circuits including a first driver amplifier and a second driver amplifier. Each of the first driver amplifier and the second driver amplifier is configured to generate an audio output based on an audio input and includes a feedback circuit coupled between a terminal associated with the audio output and a terminal configured to receive the audio input. The audio system also includes a CM amplifier coupled to the first driver amplifier and the second driver amplifier and configured to generate a CM voltage at an output based on a reference voltage input. The CM amplifier includes a feedback circuit coupled between the output and an input terminal. A reference voltage input to the first driver amplifier and the second driver amplifier is derived based on the CM voltage through the input terminal of the CM amplifier. The feedback circuit of the CM amplifier, the feedback circuit of the first driver amplifier and the feedback circuit of the second driver amplifier are configured to enable the CM amplifier, the first driver amplifier and the second driver amplifier to have a same gain amplitude such that a noise at the output of the CM amplifier is replicated at each of the terminal of the first driver amplifier and the second driver amplifier associated with the audio output to render a differential noise across the terminal of the each of the first driver amplifier and the second driver amplifier associated with the audio output independent of the noise at the output of the CM amplifier. 
     Further, the audio system includes a first interface associated with the audio output of the first driver amplifier, a second interface associated with the output of the CM amplifier, a third interface associated with the audio output of the second driver amplifier, a first speaker and a second speaker. The noise at the output of the CM amplifier is differentially canceled across each of the first speaker and the second speaker when they are coupled between the first interface and the second interface and the third interface and the second interface respectively. 
     Other aspects and example embodiments are provided in the Drawings and the Detailed Description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE VIEWS OF DRAWINGS 
         FIG. 1  is a schematic view of a headset driving unit including a common mode (CM) amplifier coupled to a pair of speakers; 
         FIG. 2  is a schematic view of a headset driving unit, with capacitors configured to decouple a direct current (DC) bias from the outputs of driver amplifiers therein across a pair of speakers; 
         FIG. 3  is a schematic view of a headset driving unit coupled to a pair of speakers, with the common mode centered to a ground voltage; 
         FIG. 4  is a schematic view of a headset driving unit configured to cancel noise associated with a CM amplifier across speakers associated with a headset coupled, according to one or more embodiments; 
         FIG. 5  is a schematic view of an audio system including the headset driving unit of  FIG. 4 , with speakers; and 
         FIG. 6  is a process flow diagram detailing the operations involved in a method of realizing the headset driving unit of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Disclosed are a method, an apparatus and/or a system for noise reduction in a headset amplifier driver circuit. Although the present embodiments have been described with reference to specific example embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the various embodiments. 
       FIG. 1  illustrates a headset driving unit  100  including a common mode amplifier (CM amplifier)  114  coupled to a pair of speakers. The headset driving unit  100  includes a left amplifier  104  (L amplifier  104 ) associated with a left terminal  108  (L  108 ), a right amplifier  124  (R amplifier  124 ) associated with a right terminal  128  (R  128 ) and CM amplifier  114  associated with a common terminal  118  (COM  118 ). L amplifier  104 , R amplifier  124  and CM amplifier  114  are configured respectively to drive HSOL (Headset output left or simply output left)  106 , HSOR (Headset output right or simply output right)  126 , and HSOCM (headset common mode output)  116 , which are the outputs, at the aforementioned terminals. The inputs to L amplifier  104 , R amplifier  124  and CM amplifier  114  are L input  102 , R input  122  and CM input  112  respectively. L input  102  and R input  122  are outputs of a Digital-to-Analog Converter (DAC) configured to convert outputs of a digital audio source associated with the L channel and the R channel of a stereophonic audio system including headset driving unit  100  to an analog format suited to the requirements of L amplifier  104  and R amplifier  124 . 
     A chip associated with headset driving unit  100  including L amplifier  104  and R amplifier  124  utilizes a single supply voltage V DD+   140 , as illustrated in  FIG. 1 . HSOL  106  and HSOR  126  are biased at a voltage lower than V DD+   140 . CM amplifier  114  is configured to generate an output voltage, i.e., HSOCM  116 , equal to the bias voltage lower than V DD+   140  to which HSOL  106  and HSOR  126  are biased through an appropriate input voltage, CM input  112 . In an embodiment CM amplifier  114  is a part of the chip associated with headset driving unit  100  including L amplifier  104  and R amplifier  124 . The resistance associated with the L speaker is R L    110  and the resistance associated with the R speaker is R R    130 . While L amplifier  104  and R amplifier  124  are configured to drive HSOL  106  and HSOR  126  including audio signal components therein into R L    110  and R R    130  respectively, CM amplifier  114  is configured to enable removal of the direct current (DC) bias voltage (e.g., fraction of V DD+   140 ) across R L    110  and R R    130  in HSOL  106  and HSOR  126  through the generation of an output, HSOCM  116 , equal to the DC bias voltage. Thus, the DC voltage component of each of HSOL  106  and HSOR  126  is not dropped across the speakers associated with headset driving unit  100 . For example, if both HSOL  106  and HSOR  126  swing from 1+0.5 V to 1−0.5V (here, there is a 1V DC bias, along with the alternating current (AC) component), then CM amplifier  114  is configured to generate an HSOCM  116  voltage of 1V. Thus, the voltage across R L    110  and the voltage across R R    130  merely swings from +0.5V AC to −0.5 V AC. In one embodiment, under idle conditions where both HSOL  106  and HSOR  126  are 1V (i.e., no “audio output”), the voltages across R L    110  and R R    130  will be 0, leading to no wastage of power (due to zero current across R L    110  and R R    130 ) across R L    110  and R R    130 . It is noted that current is not drawn by R L    110  and R R    130  from L amplifier  104  and R amplifier  124  respectively. The CM voltage is also equal to a ground (GND) voltage. 
     Due to an extra amplifier, viz. CM amplifier  114 , in the circuit, the noise power in the circuit increases by a factor of two. Noise across each of R L    110  and R R    130  increases by √{square root over (2)}, as the noise power at COM  118  is uncorrelated to the noise power at L  108 /R  128 . Thus, in order to cut down noise power in the circuit, the area of L amplifier  104  and R amplifier  124  needs to be doubled, leading to stringent area budgets within the chip including headset driving unit  100  (i.e., including the amplifiers L amplifier  104 , R amplifier  124 , and CM amplifier  114 ). Additionally, the power consumed within the circuit including headset driving unit  100  and the speakers are increased due to the current associated with CM amplifier  114 . Headset driving unit  100  including L amplifier  104 , R amplifier  124  and CM amplifier  114  is a differential amplifier, where performance is impacted due to the area and power constraints. 
       FIG. 2  illustrates a headset driving unit  200 , with capacitors C C    250  configured to decouple the DC bias from HSOL  206  and HSOR  226  across R L    210  and R R    230 . HSOL  206 , HSOR  226 , L  208 , R  228 , L input  202 , R input  222 , R L    210 , R R    230 , L amplifier  204 , R amplifier  224  and V DD+   240  are analogous to HSOL  106 , HSOR  126 , L  108 , R  128 , L input  102 , R input  122 , R L    110 , R R    130 , L amplifier  104 , R amplifier  124  and V DD+   140  respectively. As capacitor blocks a DC signal and passes an AC signal, the voltage across R L    210  and R R    230  are AC voltages. As illustrated in  FIG. 2 , C C    250  is coupled between L  208  and R  228  (analogous to L  108  and R  128 ) of headset driving unit  200  and the corresponding R L    210  and R R    230 . 
     As C C    250  will suffice to decouple the DC bias across R L    210  and R R    230 , there is no requirement of a CM amplifier in the vein of CM amplifier  114 . Also, the reference voltage of the circuit is a ground voltage (e.g., GND  260 ) or any other voltage. The choice of capacitor C C    250  is made based on example Equation (1) as: 
                       C   c     =     1     2   ⁢   π   ⁢           ⁢     f   c     ⁢   Z         ,           (   1   )               
where f c  is the desired cut-off frequency and Z is the load-impedance associated with headset driving unit  200  including R L    210  and R R    230 . Assuming Z to be 16 ohms or 32 ohms, C C    250  has to be very large. For example, C C    250  varies between 22 μF and 220 μF, depending on the choice of parameters. Thus, the bulkiness of C C    250  necessitates increased board space associated with the circuit. Also, C C    250  is outside a chip including headset driving unit  200  (i.e., including L amplifier  204  and R amplifier  224 ). In addition to bulkiness of C C    250 , the cost associated therewith also adds to the cost of the circuit.
 
       FIG. 3  illustrates headset driving unit  300  coupled to a pair of speakers, with the CM centered to a ground voltage (GND  360 ). L input  302 , R input  322 , L  308 , R  328 , COM  318 , L amplifier  304 , R amplifier  324 , HSOL  306 , HSOR  326 , R L    310 , R R    330  and V DD+   340  are analogous to L input  102 , R input  122 , L  108 , R  128 , COM  118 , L amplifier  104 , R amplifier  104 , HSOL  106 , HSOR  126 , R L    110 , R R    130  and V DD+   140  respectively. In order to aid potential balance in the circuit, resistors R LCM    362  and R RCM    364  are coupled to L amplifier  304  and R amplifier  324  respectively through one terminal and to one another through the other terminal. The path coupling R LCM  (left common mode)  362  and R RCM  (right common mode)  364  are held at HSOCM  316 , which is equal to GND  360 . For example, the “audio output” of L amplifier  304  and R amplifier  324 , i.e., HSOL  306  and HSOR  326 , swings between 0+0.5V AC and 0−0.5V AC with no DC bias therein because the CM is centered to GND  360 . Thus, HSOL  306  and HSOR  326  swings below GND  360 . In order to drive HSOL  306 /HSOR  326  below GND  360 , the circuit requires a negative supply voltage (e.g., V DD−   360 . As seen in  FIG. 3 , L amplifier  304  and R amplifier  324  may utilize V DD−   360  as a supply voltage in addition to V DD+   340 ). Generating V DD−   360  requires the utilization of a negative charge pump, which includes one or more capacitors (e.g., two external capacitors) as storage elements in the aforementioned generation. Moreover, the charge pump draws a significant amount of current from V DD+   340  for the negative voltage generation, leading to increased power consumption. Most importantly, configuration of the negative charge pump necessitates the use of a separate extra processing mask for creation of a deep N-well. The extra processing mask adds an additional expense of 2-3% to the silicon (Si) floor plan associated with the chip design. 
       FIG. 4  illustrates a headset driving unit  400  configured to completely cancel noise across speakers associated with a headset, according to an embodiment. L amplifier  404  (first driver amplifier), R amplifier  424  (second driver amplifier), L input  402 , R input  422 , L  408 , R  428 , COM  418 , HSOL  406 , HSOR  426 , R L    410 , R R    430  and V DD+   440  are analogous to L amplifier  104 , R amplifier  124 , L input  102 , R input  122 , L  108 , R  128 , COM  118 , HSOL  106 , HSOR  126 , R L    110 , R R    130  and V DD+   140  respectively. In an embodiment, headset driving unit  400  does not include a negative supply voltage, akin to headset driving unit  300  of  FIG. 3 . 
     As illustrated in  FIG. 4 , each of L amplifier  404  and R amplifier  424  includes an input resistor R i    472  and a feedback resistor R f    474 . Therefore, the amplitude of the gain of each of L amplifier  404  and R amplifier  424  can be expressed in example Equation (1) as: 
                       G     L   ,   R       =       R   f       R   i         ,           (   1   )               
where G L  and G R  are the gain amplitudes associated with L amplifier  404  and R amplifier  424  respectively.
 
     Headset driving unit  400  includes a CM amplifier, CM amplifier  414  (common mode amplifier), which is operated using the same gain configuration as L amplifier  404  and R amplifier  424 . Therefore, CM amplifier  414 , for example, uses an appropriate configuration of an input resistor and a feedback resistor such that the gain amplitude associated with CM amplifier  414  is the same as the gain amplitude associated with L amplifier  404  and R amplifier  424 . As illustrated in  FIG. 4 , each of the input resistor and the feedback resistor associated with CM amplifier  414  are chosen as the same scalar multiple (e.g., p) of R i    472  and R f    474  (e.g., p·R i    482 , p·R f    484 ). Thus, the gain amplitude of CM amplifier  414  can be expressed in example Equation (2) as: 
                       G   CM     =         p   .     R   f         p   .     R   i         =       G     L   ,   R       =   G         ,           (   2   )               
where G CM  is the gain amplitude of CM amplifier  414 . G CM  and G L,R  are referred to as G for the sake of convenience due to the equality.
 
     Value of p is chosen to be as large as possible to reduce the feedback current through p·R f    484  associated with CM amplifier  414 . Thus, the input resistor and the feedback resistor associated with CM amplifier  414  is much larger than the corresponding R i    472  and R f    474  of L amplifier  404  and R amplifier  424 , the choice the Ri and Rf is constrained to a higher limit for noise reasons. As shown in  FIG. 4 , CM amplifier  414  is configured as a voltage divider. CM amplifier  414  includes an operational amplifier, with one of the input being a reference voltage, REF  492 . The other input terminal is coupled to a voltage divider including p·R i    482  and p·R f    484 . One terminal of p·R i    482  is coupled to a ground voltage, GND  460 , and the other terminal of p·R i    482  is coupled to the input terminal of CM amplifier  414  that is coupled to the voltage divider. Also, p·R f    484  constitutes the feedback path of CM amplifier  414 . As shown in  FIG. 4 , R f    474  and, consequently, p·R f    484  are varied to equally tune the gain amplitudes associated with the respective amplifiers. 
     In an embodiment, a same reference voltage, REFINT  494 , is generated for each of L amplifier  404  and R amplifier  424  from the virtual ground of the output of CM amplifier  414 , HSOCM  416 . Thus, REFINT (internal reference voltage of left and right amplifiers)  494  is derived from the terminal of p·R i    482  coupled to CM amplifier  414 . Ignoring factors other than G, the noise voltage associated with REFINT  494  and the noise voltage associated with HSOCM  416  is mathematically related, as expressed in example Equation (3) as: 
     
       
         
           
             
               
                 
                   
                     REFINT 
                     n 
                   
                   = 
                   
                     
                       HSOCM 
                       n 
                     
                     
                       ( 
                       
                         1 
                         + 
                         G 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     In Equation (3) and subsequent equations, the subscript n refers to noise. Thus, in Equation (3), REFINT n  refers to the noise voltage associated with REFINT  494  and HSOCM n  refers to the noise voltage associated with HSOCM  416 . L amplifier  404  and R amplifier  424  are configured to amplify REFINT n , as REFINT  494  is input to each of L amplifier  404  and R amplifier  424 . ignoring the noise of L amplifier  404 , R amplifier  424 , the noise due to R f    474  and factors other than G, the noise output voltage of L amplifier  404  and R amplifier  424 , viz. HSOL n  and HSOR n  respectively, are related to REFINT n  input in example Equation (4) as:
 
 HSO ( L,R ) n   =REFINT   n ·(1 +G )  (4)
 
     Therefore, HSOCM n  is equal to HSOL n  and HSOR n , as seen from Equations (3) and (4). Thus, the noise at COM  418  and at the noise at L  408 /R  428  is canceled when differentially measured across R L    410  and R R    430 . 
     Taking the noise of L amplifier  404 , R amplifier  424 , R f    474 , and p·R f    484  into account, the output noise power at COM  418  is expressed in example Equation (5) as:
 
 HSOCM   n   2   =REF   n   2   ·G   2   +CM   AMP     n     2 ·(1+ G ) 2 +4 kT·p·R   f ·(1+ G )  (5)
 
where HSOCM n   2  is the noise power at COM  418 , REF n   2  is the reference noise power, CM AMP     n     2  is the noise power due to CM amplifier  414 , k is the Boltzmann constant, and T is the temperature. The third contributor to the right side of Equation (5) is the noise power due to p·R f    484 .
 
     Output noise power at L  408 /R  428  is expressed in example Equation (6) as:
 
 HSO ( L,R ) n   2   =REFINT   n   2 ·(1+ G ) 2 +( L,R ) AMP     n     2 ·(1+ G ) 2 +4 kT·R   f ·(1+ G ),  (6)
 
where HSOL n   2  and HSOR n   2  are the noise power at L  408  and the noise power at R  428  respectively, REFINT n   2  is the noise power due to REFINT  494 , and L AMP     n     2  and R AMP     n     2  are the noise power due to L amplifier  404  and R amplifier  424  respectively. The third contributor to the right side of Equation (6) is the noise power due to R f    474 .
 
     Now, analogous to Equation (3), noise power due to REFINT  494  and the noise power at COM  418  is related, as expressed in Equation (7) as: 
     
       
         
           
             
               
                 
                   
                     REFINT 
                     n 
                     2 
                   
                   = 
                   
                     
                       HSOCM 
                       n 
                       2 
                     
                     
                       
                         ( 
                         
                           1 
                           + 
                           G 
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Through the substitution of Equation (7) in Equation (6), Equation (8) is obtained as:
 
 HSO ( L,R ) n   2   =HSOCM   n   2 +( L,R ) AMP     n     2 ·(1+ G ) 2 +4 kT·R   f ·(1+ G ),  (8)
 
     Thus, the noise power across each of R L    410  and R R    430  is expressed in example Equation (9) as:
 
 HSO ( L,R ) n   2   −HSOCM   n   2 =( L,R ) AMP     n     x ·(1+ G ) 2 +4 kT·R   f ·(1+ G )  (9)
 
     As seen in Equation (9), the noise power across each of R L    410  and R R    430  is independent of the noise power/contributions due to CM amplifier  414  (e.g., due to p·R f    484 ), and is dependent only on the noise contributions due to L amplifier  404  and R amplifier  424 . Even the noise contribution due to REF  492  does not affect the noise power across each of R L    410  and R R    430  as the appropriate REFINT  494  is input to each of L amplifier  404  and R amplifier  424 . Thus, the noise power due to CM amplifier  414  and any reference voltage/noise (e.g., REF  492 , REF n ) in the circuit is canceled across each of R L    410  and R R    430 . 
     In contrast, in  FIG. 1 , as the noise power at COM  118  and L  108 /R  128  are uncorrelated, the noise contribution due to an extra amplifier (e.g., CM amplifier  114 ) appears across R L    110  and R R    130 , in addition to the noise contribution due to L amplifier  104 /R amplifier  124 . In  FIG. 2 , each of L amplifier  204  and R amplifier  224  includes a reference voltage input (not shown), in addition to L input  202  and R input  204 . The aforementioned reference voltage is generated through another circuit, which contributes to noise power across R L    210  and R R    230 . Thus, headset driving unit  100  and headset driving unit  200  causes a noise power due to an extra amplifier (and resistors associated therewith) and/or a reference voltage source to be manifested across R L  ( 110 ,  210 ) and R R  ( 130 ,  230 ), in contrast to headset driving unit  400  of  FIG. 4 . 
     In an embodiment, headset driving unit  300  includes a reference voltage source. As headset driving unit  300  does not include an extra amplifier, the noise output across R L    310  and R R    330  only includes the contributions due to L amplifier  304  and R amplifier  324 . However, the generation of a negative supply voltage (e.g., V DD−   360 ) necessitates the utilization of a process mask to create a deep N-well, which increases chip area/costs associated therewith. Moreover, the current drawn from V DD+   340  is increased, as discussed above. Utilization of headset driving unit  400  avoids at least the aforementioned problems associated with headset driving unit  100 , headset driving unit  200 , and headset driving unit  300 . The driver amplifiers and the CM amplifiers discussed with regard to  FIGS. 1-4  are operational amplifiers. Again, as discussed above with reference to  FIG. 1 , CM amplifier  414  is configured to generate HSOCM  416 , which eliminates the DC bias level in HSOL  406  and HSOR  426  across R L    410  and R R    430 . Headset driving unit  400  also consumes lower power when compared to headset driving unit  100 , headset driving unit  200 , and headset driving unit  300 . Reduced area and reduced power consumption is accomplished through headset driving unit  400  without modifying the interfaces to the speakers associated with a headset. CM amplifier  414  is chosen to be as inexpensive as possible, in addition to the noisiness, as the noise power across R L    410  and R R    430  does not include the contributions associated with CM amplifier  414 . 
     In an embodiment, R i    472  and R f    474  is not increased by much due to the prospective increase in the noise contribution, but p·R i    482  and p·R f    484  are made as high as possible. The design focus is shifted solely to CM amplifier  414 . While headset driving unit  400  includes class AB amplifiers, the technique described herein applies to class G and class H amplifiers too, where the supply voltages are switched to a lower voltage level to save power. Further, HSOCM  416  (i.e., the CM voltage) is reduced through reducing the gain (G). When HSOCM  416  is reduced close to GND  460 , a class G/class H operation is performed seamlessly with headset driving unit  400 . Thus, the supply voltage (e.g., through Switched-Mode Power Supply (SMPS)) is reduced, leading to less power consumption from the power source. Also, as a user of an audio system including headset driving unit  400  may prefer to listen to audio with a reduced volume, the ability to freely move the CM voltage aids the utilization of a low supply voltage. 
     A high noise scenario involves small L amplifier  404  and R amplifier  424  and a small reference voltage (REF  492 ). The operation at low current discussed above allows for utilization of small capacitors for CM amplifier compensation. Therefore, the area savings (e.g., silicon area savings) associated with headset driving unit  400  is further increased. Headset driving unit  400  does not have a filtering requirement associated with REF  492 . Therefore, headset driving unit  400  leads to component savings. 
       FIG. 5  illustrates an audio system  500  including headset driving unit  400  with speakers coupled (shown as R L    410  and R R    430 ), according to one or more embodiments. Audio system  500  includes an audio source  502  coupled to an audio control unit  504 . For example, audio source  502  is a digital audio file stored in a memory of a data processing unit (e.g., a computing system, a mobile phone, an Apple® iPod™) or voice data associated with the data processing unit (e.g., an IP phone, a mobile phone). Audio source  502  is configured to output signals associated with both the L channel and the R channel. Audio control unit  504  may be configured to adjust parameters (e.g., volume adjustment) associated with the output of audio source  502 , and to transmit the adjusted output to an audio converter  506 . Audio converter  506  is a Digital-to-Analog Converter (DAC) configured to convert a digital signal to an analog format compatible with headset driving unit  400 . The L channel and R channel outputs associated with audio converter  506  are L input  402  and R input  422  respectively. Headset driving unit  400  has interfaces (e.g., ports) associated with L  408 , R  428 , and COM  418 . Loudspeakers/headphones/earphones/headsets are coupled to headset driving unit  400  by way of the aforementioned terminals. R L    410  and R R    430  are the resistances associated with the loudspeakers/headphones/earphones/headsets. 
     In an embodiment, a chip including headset driving unit  400  is provided in a mobile phone. The mobile phone includes appropriate circuitry (e.g., DAC) that is configured to detect the presence of headphones/headset/earphones. When the headphones/headsets/earphones are inserted, the circuitry detects the presence through an impedance measurement. Also, the chip associated with headset driving unit  400  can be in a sleep-mode which is activated following the detection of the presence of the headphones/headsets/earphones. 
     It is to be noted that the concepts discussed herein also applies to scenarios where outputs from a current DAC (IDAC) serves as L input  402  and R input  422 . In the aforementioned scenarios, resistors such as R i    472  and p·R i    482  are not required. Further, the gain (G) associated with CM amplifier  414 , L amplifier  404 , and R amplifier  424  is not a factor in the noise at the outputs. Therefore, the noise associated with REF  492  is low. It is noted that the headset driving unit  400  is called so for the sake of convenience. Examples of alternatives to headset driving unit  400  include headphone driving unit  400 , speaker driving unit  400  and earphone driving unit  400 . The concepts discussed herein are valid for all scenarios requiring audio output. The aforementioned scenarios utilize any form of electro-acoustic transducers (e.g., headsets, headphones, loudspeakers, earphones). 
       FIG. 6  illustrates a process flow diagram detailing the operations involved in a method of realizing headset driving unit  400 , according to an embodiment. Operation  602  generates a CM voltage (e.g., HSOCM  416 ) at an output of a CM amplifier (e.g., CM amplifier  414 ) based on a reference voltage input (e.g., REF  492 ). Operation  604  derives a reference voltage (e.g., REFINT  494 ) to a first driver amplifier (e.g., L amplifier  404 , R amplifier  424 ) based on the CM voltage (e.g., HSOCM  416 ) at the output of the CM amplifier (e.g., CM amplifier  414 ). An audio input is provided to the first driver amplifier (e.g., L input  402 , R input  422 ). Operation  606  replicates a noise at the output of the CM amplifier at an audio output (e.g., HSOL  406 /HSOR  426 ) of the first driver amplifier (e.g., L amplifier  404 , R amplifier  424 ) through an appropriate configuration of a feedback circuit associated with the CM amplifier and a feedback circuit associated with the first driver amplifier such that the CM amplifier and the first driver amplifier have a same gain amplitude associated therewith. It is noted that steps  602 - 606  is applicable in case of a second driver amplifier (having an audio input) associated with a second speaker in an audio system. 
     In addition, it will be appreciated that the various operations, processes, and methods disclosed herein may be embodied in a machine-readable medium or a machine accessible medium compatible with a data processing system, and may be performed in any order. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. 
     The forgoing description sets forth numerous specific details to convey a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without these specific details. Well-known features are sometimes not described in detail in order to avoid obscuring the invention. Other variations and embodiments are possible in light of above teachings, and it is thus intended that the scope of invention not be limited by this Detailed Description, but only by the following Claims.