Abstract:
An apparatus includes first and second amplifiers, each having an input coupled to an audio signal input and an output coupled to an audio signal output. The second amplifier also has a feedback path from its output to its input. A first switch can be operated to selectively couple and uncouple the audio signal input and the input to the first amplifier to and from a first node. A second switch can be operated to selectively couple and uncouple the input of the second amplifier to and from a second node. A third switch can be operated to selectively couple and uncouple the first node to and from ground.

Description:
TECHNICAL FIELD 
     This disclosure is generally directed to amplifier circuits. More specifically, this disclosure is directed to a method and system for glitch suppression in dual-amplifier circuit. 
     BACKGROUND 
     There is continual market pressure to design portable devices to be smaller, smarter and more complex. In some portable devices, an audio device with multiple input signal sources that drives a speaker for both earpiece and hands free operation may be integrated into a single, mixed-signal sub-system. Such integration provides a better form factor and fewer external components. 
     Power consumption is one major issue faced by manufacturers of portable devices. This is one reason that manufacturers are using highly efficient class D power amplifiers in more and more handheld and other portable devices. However, a class D power amplifier may have a higher noise level than a class AB power amplifier, which can make the class D amplifier less suitable for earpiece or headphone applications. 
     One way to reduce printed circuit board space and component costs is to use the same speaker for both hands free and earpiece operation. One way to achieve this is to couple the single speaker to both the output of a class AB power amplifier (for earpiece operation) and the output of a class D power amplifier (for hands free operation). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure and its features, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a system that includes a speaker and an amplifier circuit; 
         FIG. 2  illustrates an output waveform of a class D amplifier driving an inductive load; 
         FIG. 3  illustrates glitches conducted to an input of the amplifier circuit of  FIG. 1 ; 
         FIG. 4  illustrates a system that includes a speaker and an amplifier circuit in accordance with this disclosure; 
         FIG. 5  illustrates an audio device employing an amplifier circuit in accordance with this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 through 5 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any type of suitably arranged device or system. 
       FIG. 1  illustrates a system  100  that includes a speaker  112  and an amplifier circuit  102 . The amplifier circuit  102  includes an audio signal input  104  and an audio signal output  106 , which is coupled to the speaker  112 . The amplifier circuit  102  further includes an audio signal output  108  and an audio signal output  110 . 
     The amplifier circuit  102  includes an amplifier  114 , which may be a class D amplifier, and a second amplifier  116 , which may be a class AB amplifier. The amplifier circuit  102  further includes amplifier  118  and amplifier  120 . Conductors  130  couple the audio signal input  104  in parallel to inputs of the amplifiers  114 ,  116 ,  118 , and  120 . In this example, the audio signal input  104  is a balanced input, although it will be understood that a single-ended audio signal input could also be used. 
     The amplifiers  114  and  116  provide balanced outputs and are coupled in parallel to the audio signal output  106  and, hence, to the speaker  112 . Each of the amplifiers  118  and  120  provides a single-ended output, and the amplifiers  118  and  120  are respectively coupled to the audio signal output  108  and the audio signal output  110 . It will be understood that any combination of balanced and single-ended audio signal outputs could be used. 
     Feedback circuits  126   a  and  126   b  coupled each side of the balanced audio signal output  106  to the balanced input of the amplifier  116 . As shown in  FIG. 1 , the feedback circuits  126   a  and  126   b  are R-C circuits, however it will be understood that other feedback circuits could be used. A switch  122   a  and a resistor  128   a  couple one side of the balanced input of the amplifier  116  to one side of the balanced audio signal input  104 . Similarly, a switch  122   b  and a resistor  128   b  couple the other side of the balanced input of the amplifier  116  to the other side of the balanced audio signal input  104 . In the amplifier circuit  102 , the switches  122   a  and  122   b  are implemented using back-to-back p-channel and n-channel devices, although other switches could be used. 
     In some applications, the system  100  is used in a handheld device where a user, at certain times, holds the speaker  112  to her ear for use as an earpiece and, at other times, holds the speaker away from her ear, for hands-free or walkie-talkie operation. In the first instance, the amplifier  116  may be used to power the speaker  112 , to reduce power consumption by the system  100  and noise. In the second instance, the amplifier  114  may be used to produce higher sound volumes from the speaker  112 . 
     In the first instance, a control input  124  to the amplifier  114  may be used to inactivate the amplifier  114 , a control input  125  may be used to activate the amplifier  116 , and control inputs  123  may be used to configure the switches  122   a  and  122   b  to couple the audio signal input  104  to the input of the amplifier  116 . In this configuration, the amplifier  114  is inactive and the amplifier  116  is active and operating to amplify a signal from the audio signal input  104  and supply the amplified signal to the speaker  112 . 
     In the second instance, the control signal  124  may be used to activate the amplifier  114 , the control input  125  may be used to inactivate the amplifier  116 , and the control inputs  123  may be used to configure the switches  122   a  and  122   b  to decouple the audio signal input  104  from the input of the amplifier  116 . In this configuration, the amplifier  116  is inactive and has no input to amplify while the amplifier  114  is active and operating to amplify the signal from the audio signal input  104  and supply the amplified signal to the speaker  112 . 
     Where the amplifier  114  is a class D amplifier, its output switches between ground and V DD  (a supply voltage) with a duty cycle that is determined by the signal from the audio signal input  104 . In some class D amplifier circuits, a low-pass filter circuit operates on the output of the amplifier to filter out the amplifier switching frequency, leaving primarily an audio frequency amplified signal. In the amplifier circuit  102 , a separate filter circuit is not used, and the output of the amplifier  114  is coupled directly to the speaker  112 . The resistive and inductive loading properties of the speaker  112  operate to filter out the amplifier switching frequency. 
       FIG. 2  illustrates an output voltage waveform  200  of the amplifier  114  on the audio signal output  106 , driving the speaker  112 . As described above, the output of the amplifier  114  switches between ground  202  and V DD    204 . Because of the rapid transition between ground  202  and V DD    204 , inductive kick back from the speaker  112  may cause the voltage on the audio signal output  106  to rise above V DD    204  as shown at portions  206 ,  210  and  214  of the waveform  200 . Similarly, inductive kick back from the speaker  112  may cause the voltage on the audio signal output  106  to fall below ground  202  as shown at portions  208  and  212  of the waveform  200 . 
     Because the output of the amplifier  116  is coupled in parallel with the output of the amplifier  114  to the audio signal output  106 , these voltages above V DD    204  and below ground  202  will also be present at the output of the amplifier  116 . The feedback circuits  126   a  and  126   b  may conduct these voltages above VDD  204  and below ground  202  to the input of the amplifier  116  and to the switches  122   a  and  122   b . Even though the switches  122   a  and  122   b  are configured to decouple the input of the amplifier  116  from the audio signal input  104  when the amplifier  114  is operating, voltages above V DD    204  and below ground  202  may be conducted through the switches  122   a  and  122   b  and the resistors  128   a  and  128   b  to the audio signal input  104 . 
       FIG. 3  illustrates a waveform  300  that may appear on one or both of the conductors  130  as a result of the conduction voltages above V DD    204  and below ground  202  through the switches  122   a  and  122   b  and the resistors  128   a  and  128   b  to the audio signal input  104 . Voltage spikes (or ‘glitches’) may occur in portions  306 ,  308 ,  310 ,  312  and  314  of the waveform  300 , corresponding to the portions  206 ,  208 ,  210 ,  212  and  214  of the waveform  200  where the voltage of the waveform  200  rises above V DD    204  or falls below ground  202 . Because the conductors  130  couple the audio signal input  104  to the inputs of the amplifier  114 , the amplifier  118  and the amplifier  120 , each of those amplifiers will receive and amplify a signal that includes both the program signal at the audio signal input  104  and the noise signal of waveform  300 . As such, the noise glitches of the waveform  300  will be reproduced in the amplified outputs of the amplifier  114 , the amplifier  118  and the amplifier  120 . This will cause undesirable noise at the outputs  106 ,  108  and  110 . 
       FIG. 4  illustrates a system  400  that includes a speaker  412  and an amplifier circuit  402  in accordance with this disclosure. The amplifier circuit  402  includes an audio signal input  404  and an audio signal output  406 , which is coupled to the speaker  412 . The amplifier circuit  402  further includes an audio signal output  408  and an audio signal output  410 . 
     The amplifier circuit  402  includes an amplifier  414 , which may be a class D amplifier, and a second amplifier  416 , which may be a class AB amplifier. The amplifier circuit  402  further includes amplifier  418  and amplifier  420 . Conductors  430  couple the audio signal input  404  in parallel to the inputs of the amplifiers  414 ,  416 ,  418 , and  420 . The audio signal input  404  is a balanced input, although it will be understood that a single-ended audio signal input could also be used. 
     The amplifiers  414  and  416  provide balanced outputs and are coupled in parallel to the audio signal output  406  and, hence, to the speaker  412 . Each of the amplifiers  418  and  420  provides a single-ended output, and the amplifiers  418  and  420  are respectively coupled to the audio signal output  408  and the audio signal output  410 . It will be understood that any combination of balanced and single-ended audio signal outputs could be used. 
     Furthermore, while the outputs of the amplifiers  414  and  416  are shown coupled to each other within the amplifier circuit  402 , it will be understood that the outputs of the amplifiers  414  and  416  could be coupled to each other externally to the amplifier circuit  402 . 
     Feedback circuits  426   a  and  426   b  couple each side of the balanced audio signal output  406  to the balanced input of the amplifier  416 . As shown in  FIG. 4 , the feedback circuits  426   a  and  426   b  are R-C circuits, however other feedback circuits could be used. A switch  422   a  couples one side of the balanced input of the amplifier  416  to a node  436   a . Similarly, a switch  422   b  couples the other side of the balanced input of the amplifier  416  to a node  436   b.    
     A switch  432   a  and a resistor  128   a  couple the node  436   a  to one side of the balanced audio signal input  104 . Similarly, a switch  432   b  and a resistor  428   b  couple the node  436   b  to the other side of the balanced audio signal input  104 . A switch  434  couples both nodes  436   a  and  436   b  to ground. In the amplifier circuit  403 , the switch  434  is implemented using two n-channel devices coupled in series with their common node coupled to ground and their independent nodes coupled to nodes  436   a  and  436   b , respectively. In the amplifier circuit  402 , each of the switches  422   a ,  422   b ,  432   a  and  432   b  is implemented using back-to-back p-channel and n-channel devices, although other switches could be used. It will be understood that the control signal  423  may have an opposite polarity at the control terminal of a p-channel device than at the control terminal of an n-channel device. 
     As with the system  100 , shown in  FIG. 1 , in some applications, the system  400  is used in a handheld device where a user, at certain times, holds the speaker  412  to her ear for use as an earpiece and, at other times, holds the speaker away from her ear, for hands-free or walkie-talkie operation. In the first instance, the amplifier  416  may be used to power the speaker  412 , to reduce power consumption by the system  400  and noise. In the second instance, the amplifier  414  may be used to produce higher sound volumes from the speaker  412 . 
     In the first instance, a control input  424  to the amplifier  414  may be used to inactivate the amplifier  414 , a control input  425  may be used to activate the amplifier  416 , and control inputs  423  may be used configure the switches  422   a ,  422   b ,  432   a  and  432   b  to couple the audio signal input  104  to the input of the amplifier  416 . Additionally, a control input  435  may be used to uncouple the nodes  436   a  and  436   b  from ground. In this configuration, the amplifier  414  is inactive and the amplifier  416  is active and operating to amplify a signal from the audio signal input  404  and supply the amplified signal to the speaker  412 . 
     In the second instance, the control signal  424  may be used to activate the amplifier  414 , the control signal  425  may be used to inactivate the amplifier  416 , and the control inputs  423  may be used to configure the switches  422   a  and  422   b  to decouple the input of the amplifier  416  from the nodes  436   a  and  436   b . The control inputs  423  may further be used to configure the switches  432   a  and  432   b  to decouple the audio signal input  404  from the nodes  436   a  and  436   b . Additionally, the control input  435  may be used to couple the nodes  436   a  and  436   b  to ground. In this configuration, the amplifier  416  is inactive and has no input to amplify, while the amplifier  414  is active and operating to amplify the signal from the audio signal input  404  and supply the amplified signal to the speaker  412 . 
     As described for the amplifier circuit  102  shown in  FIG. 1 , voltages above V DD  and below ground at the audio signal output  406  may be conducted by the feedback circuits  426   a  and  426   b  to the input of the amplifier  416  and through the switches  422   a  and  422   b . However, in the amplifier circuit  402  such voltages are further isolated from the audio signal input  404  by the switches  432   a  and  432   b , as well as being conducted to ground by the switch  434 . In this way, the noise glitches of the waveform  300  are greatly reduced or eliminated and, therefore, are greatly reduced or eliminated in the amplified outputs of the amplifier  414 , the amplifier  418  and the amplifier  420 . 
     Although  FIG. 4  illustrates one example of a system  400  that includes a speaker  412  and an amplifier circuit  402 , various changes may be made to  FIG. 4 . For example, the system  400  could include any suitable number of amplifiers, such as only two amplifiers. Also, the system  400  could use any suitable type(s) of amplifiers. 
       FIG. 5  illustrates an audio device  500  employing an amplifier circuit  502  in accordance with this disclosure. The audio device  500  may be a wireline telephone or may be a cellphone, personal digital assistant, or other wireless communication device. The audio device may also be a portable music or gaming system. 
     The audio device  500  includes an audio source  504 , which may be recorded audio or audio decoded from a wireless or wireline communication system. An audio output of the audio source  504  is coupled to an audio signal input of the amplifier circuit  502 . A first audio signal output of the amplifier circuit  502  is coupled to a speaker  506 . A second audio signal output of the amplifier circuit  502  is coupled to a headphone jack  508 . In this example, the amplifier circuit  502  could represent the amplifier circuit  402  of  FIG. 4 , which uses different amplifiers to provide different levels of amplification for signals sent to the speaker  506  and the headphone jack  508 . 
     Although  FIG. 5  illustrates one example of an audio device  500  employing an amplifier circuit  502 , various changes may be made to  FIG. 5 . For example, amplified signals could be provided to any other suitable destination(s). 
     It may be advantageous to set forth definitions of certain words and phrases that have been used within this patent document. The term “couple” and its derivatives refer to any direct or indirect communication between two or more components, whether or not those components are in physical contact with one another. The terms “transmit,” “receive,” and “communicate,” as well as derivatives thereof, encompass both direct and indirect communication. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The term “each” means every one of at least a subset of the identified items. The phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. 
     While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.