Abstract:
Presently many audio chips suffer from pop issues, which is especially serious for single ended audio drivers. An audio pop is a disturbance in the output caused by a sudden transition of chip power, particularly when a chip is powered on or powered off. Furthermore, compensation networks included in the amplifiers on audio chips for stability offer a significant path for transmitting power disturbances to the output. Hence, circuitry is developed to suppress pops in the output stages of an amplifier.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. Ser. No. 12/488,438, filed Jun. 19, 2009 entitled “ANTI-POP CIRCUIT” which is hereby incorporated by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to audio amplifiers and drivers and specifically with the mitigation of audio pops. 
     2. Related Art 
     A significant issue with audio drivers in present technology arises from audio “pops.” Term “pop” in the audio field is an output disturbance caused by a sudden transition of a chip&#39;s power. In particular, the problem is especially pronounced when power is initially turned on (or equivalently off). Pops during when a chip powers up because the various components receive power and may cause voltage to be seen at the output even though there is no signal. Similarly, a pop can occur when the power is switched off because energy may be stored in the circuitry and the power may inadvertently be dissipated through the output even though no input signal is received. Furthermore, because of capacitive and inductive effects, the voltage can even spike when the power to a chip is switched on or off. 
     The pop issue manifests itself frequently in the driver or power portion of an audio circuit. This is commonly implemented with some form of amplifier. Two stage amplifiers are commonly used in audio applications. In particular the first stage is referred to as the amplifier stage and the second stage referred to as the output stage. Generally speaking, the amplifier stage supplies the gain and the output stage provides high current driving capability, low impedance. In the case of amplifier with differential inputs, the amplifier stage can supply either a single output representative of the difference between the input signals or can provide a differential output. 
       FIG. 1A  illustrates a conventional design for a two stage amplifier. In this example, the circuit is operational amplifier  100  with amplifier stage  110  and output stage  160 . Amplifier stage  110  comprises field effect transistors (FETs)  112 ,  114 ,  116 ,  118 , and  120 . A differential input V IN+  and V IN−  are received by FET  116  and FET  118 . An output based on the difference between V IN+  and V IN−  is supplied to the output stage at node A. Output stage  160  comprises FET  162  which receives the output at node A and FET  164 . The output stage produces an amplified output signal V OUT  which is based on the difference between V IN+  and V IN− . The primary purpose of the output stage is not to provide gain but to maintain the output regardless of the current drawn through it. However, in some implementations the output stage may supply some amount of gain. 
     One of ordinary skill in the art would recognize there are countless designs for the amplifier stage and output stage. The design shown in  FIG. 1A  is a representative design. In order to simplify the remaining disclosure, where appropriate, the amplifier stage and/or output stage are represented by a symbol. 
       FIG. 1B  illustrates a design for a general design for a two-stage amplifier. Amplifier  100  comprises amplifier stage  110  which derives a signal from V IN+  and V IN−  and provides an output a node A. It also comprises output stage  160  which takes the output signal at node A and maintains output signal V OUT  regardless of the current drawn by the attached load. 
     The difficulty with two-stage amplifiers is that generally they are inherently unstable. In order to address this issue, many compensation circuits exist. One of the most basic is to add an resistor and capacitor in feedback from the output to the input of the output stage. 
       FIG. 2  illustrates a two-stage amplifier with a basic compensation network. In amplifier  200 , capacitor  202  and resistor  204  are added to output stage  220  in feedback from the output to input at node A. The original circuitry described as output stage  160  in amplifier  100  are now referred to as the core output stage to avoid confusion. This feedback from the output to node A provides stability to the two-stage amplifier. However, it provides another source of pop. For example, when amplifier stage  110  is powered up the voltage at node A may be spike. Through capacitor  202  and resistor  204 , that spike can be transmitted to the output causing the pop. 
     Previous solutions have been applied to audio systems having additional circuitry. Specifically,  FIG. 3  illustrates an audio system which comprises in addition to amplifier  200 , a sound output apparatus comprising low pass filter  302  and output circuit  304  is inserted between V OUT  and the load such as a speaker. In many applications low pass filters are used prior to attaching an audio system to a load. In addition output circuit  304  comprises an electrostatic protection circuit which is used to shunt harmful external static electricity away from the remainder of the audio system. 
     In such an implementation, previous solutions have added anti-pop circuit  310  into the sound output apparatus. Anti-pop circuit  310  comprises shunting capacitor  314  and switch  312 . Control circuit  316  closes switch  312  before power is switched on and switched off. When switch  312  is closed, the output of low pass filter  302  is shunted to ground through shunting capacitor  314 , thus draining any voltage spikes to ground before they can manifest themselves as a peak. 
     The primary drawback to this type of solution is that it requires a sound output apparatus to be placed external to the amplifier. In modern audio systems, there is a desired to eliminate the sound output apparatus. In particular, because the low pass filter is placed near the output, the low pass filter must be designed to accommodate high power. As a result, the low pass filter is bulky, expensive and consumes a lot of power. The elimination of the low pass filter and/or output circuit can reduce power consumption and expense, but it also eliminates the opportunity to deploy an anti-pop circuit such as anti-pop circuit  310 . 
     Thus there is a need in the industry for an inexpensive, compact solution that reduces or eliminates the audio pop in an audio amplifier without the need for expensive additional circuitry. 
     SUMMARY OF INVENTION 
     A circuit and method for use in a reduced pop amplifier is disclosed comprising an amplifier stage and an output stage. The output stage comprises a core output stage and a compensation network coupled to the input and output of the core output stage to provide amplifier stability. The compensation network comprises a capacitor and two switches with the first switch breaking the connectivity between the input and output of the core amplifier stage and a second switch which shunts the capacitor to ground when the control signal is low. Typically, the compensation network also comprises a resistor. Optionally, the output stage comprises a third switch which pulls the output of the core amplifier stage to ground. 
     In a specific implementation, the core output stage is a push-pull output stage receiving two bias signals which can be a class AB control signal. Two compensation networks are used to feedback the output of the push-pull output stage to each of the two bias inputs. In addition to switches to break the connectivity between each bias input and the output and the switch to pull the output to ground. 
     The control signal used to control the switches is low when the amplifier is powered down and remains low until after the amplifier is powered up and remains high until before the amplifier is powered down. 
     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 
       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. 
         FIG. 1A  illustrates a conventional design for a two stage amplifier; 
         FIG. 1B  illustrates a design for a general design for a two-stage amplifier; 
         FIG. 2  illustrates a two-stage amplifier with a basic compensation network; 
         FIG. 3  illustrates an audio system which comprises in addition to an amplifier, a sound output apparatus; 
         FIG. 4  illustrates an amplifier equipped with an anti-pop circuit which can be implemented with simple switches; 
         FIG. 5  illustrates an amplifier equipped with an improved anti-pop circuit; 
         FIG. 6  illustrates an amplifier equipped with an alternative anti-pop circuit. 
         FIG. 7  illustrates an amplifier comprising an anti-pop circuit with different switch locations; 
         FIG. 8  illustrates an amplifier with an anti-pop circuit using the principles illustrated in the anti-pop circuits described for amplifiers  400  and  500   
         FIG. 9A  illustrates a two-stage differential amplifier comprising a differential amplifier stage and differential output stage; 
         FIG. 9B  shows a differential amplifier with analogous anti-pop circuitry to the single ended amplifiers described above; 
         FIG. 10A  illustrates the preliminary stages of an amplifier using a push-pull output stage; 
         FIG. 10B  illustrates a circuit diagram for an exemplary bias circuit; 
         FIG. 10C  illustrates an amplifier with a push-pull output stage; 
         FIG. 10D  illustrates an amplifier with a push-pull output stage and analogous anti-pop circuitry to the amplifiers described above; 
         FIG. 11  illustrates an example of an amplifier with push-pull output stage; 
         FIG. 12  shows the timing of the control signal. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  illustrates an amplifier equipped with an anti-pop circuit which can be implemented with simple switches. Amplifier  400  is similar to amplifier  200 . Amplifier  400  comprises amplifier stage  110  and output stage  420 . Like output stage  220  of amplifier  200 , output stage  420  comprises core output stage  160  and a compensation network comprising capacitor  202  and resistor  204 . The described components function essentially the same as that described for amplifier  200 . However, output stage  420  further comprises switch  402 . When closed switch  402 , it drags the output voltage V OUT  to V SS  which is shown as ground in  FIG. 4 . It should be noted that often V SS  is fixed to ground. However, for the purposes of this disclosure ground and V SS  are used interchangeably and should be construed to be the low power rail and not necessarily a zero voltage. 
     Switch  402  is controlled by a control signal. Therefore the switch initially is closed when the control signal is low but the switch is opened when the control signal is high. The control signal should be activated prior to power supply V DD  ramping up to avoid an output pop. As V DD  increases switch  402  is eventually closed, but during the initial ramp up period, switch  402  may remain open thus permitting some pop to be manifested at the output. In order to maintain generality, V DD  is often referred to as the high power voltage or high power rail and V SS  is often referred to as the low power voltage, low power rail or ground. It should be noted that notationally, the switches described in each of these diagrams is con rolled by an individual input (not to be confused with a control signal given to the amplifier (ctrl) as described above). For the sake of notation, these switches are open when the input is low and closed when the input is high. For that reason switch  402  is shown to be controlled by the logical complement of ctrl that is  ctrl . 
     However, only switch  402  is not enough for the pop control because, even though output V OUT , is grounded during the power up (or power down) periods, the voltage built up at node A can still tend to drive the V OUT  up through the compensation network, so even though ideally, switch  402  pulls the output voltage to ground, the voltage a node A can still cause a pop at the output, albeit a suppressed pop. 
       FIG. 5  illustrates an amplifier equipped with an improved anti-pop circuit. Amplifier  500  is similar to amplifier  200 . Amplifier  500  comprises amplifier stage  110  and output stage  520 . Like output stage  220  of amplifier  200 , output stage  520  comprises core output stage  160  and a compensation network comprising capacitor  202  and resistor  204 . The described components function essentially the same as that described for amplifier  200 . However, output stage  520  further comprises switch  502  and switch  504 . Switch  502  is closed when the control signal is high and switch  504  is opened when the control signal is high. When the control signal is high, the circuit behaves essentially the same as amplifier  200 . Compensation capacitor  202  and compensation resistor  204  feed back V OUT  to node A to provide stability to amplifier  500 . However, when the control signal is low such as prior to power up, node A is shunted through capacitor  202  to V SS . Furthermore, with switch  502  open, the path from node A to V OUT  through the compensation network is broken. As a result, node A does not influence V OUT  until the circuit is powered up, thus, mitigating any pop at the output. 
     Ideally, the control signal is low during any power transition, i.e., power up or power down. It is also important to note that switch  504  also prevents capacitor  202  from floating. If capacitor  202  was allowed to remain floating, the absolute voltage of each electrode of the capacitor will change due to the changes in the amplifier stage, even though the charge in the capacitor and therefore the voltage across the electrodes of the capacitor will remain unchanged. At the same time. V OUT  should stay at V SS . Thus, when switch  502  is closed, the voltage difference between node B of and V OUT  will cause a pop at V OUT . 
     Alternatively,  FIG. 6  illustrates an amplifier equipped with an anti-pop circuit. Instead of modifying the compensation network as in the manner shown for amplifier  500 . Output stage  620  of amplifier  600  comprises switch  602  which when opened breaks the compensation network between resistor  204  and the output of the amplifier rather than between resistor  204  and capacitor  202  as in amplifier  500 . When the control signal is high, amplifier  600  operates normally like that of amplifier  200 . When the control signal is low, switch  604  shunts capacitor  202  to ground through resistor  204  and switch  602  disconnects node A from V OUT . 
     It should be noted principles of modifying a compensation network to disconnect node A from V OUT , while simultaneously draining any residual charges in the compensation network can be applied to other compensation networks. Furthermore, the placement of the various switches can be varied with the same result. For example,  FIG. 7  shows an amplifier comprising an anti-pop circuit where the switch  702  functions similarly to switch  502  of amplifier  500 , but located in a different location in the path between node A and V OUT . A compensation network with the capacitor and resistor transposed from that shown for amplifiers  200 ,  400 ,  500 ,  600 , and  700  introduces countless more combinations of switch positions. No doubt the various combinations of switch locations and compensation network elements would be apparent to one of ordinary skill in the art. 
       FIG. 8  illustrates an amplifier with an anti-pop circuit using the principles illustrated in the anti-pop circuits described for amplifiers  400  and  500 . Again, amplifier  800  is similar to that described in the previous figures. Similar to output stages of previously described amplifiers, output stage  820  incorporates switch  402  to drag down V OUT  as well as switch  502  to break the path between node A and V OUT . In Addition, switch  504  shunts node A to V SS  through capacitor  202 . Similar to that described for amplifier  400 , switch  402  drags down V OUT  to V SS  when the control signal is low. Therefore, prior to power up, switch  402  is closed. Switch  502  and  504  behave in essentially the same manner as described for amplifier  500 . Therefore, when the control signal is high, the amplifier behaves essentially like amplifier  400 . However, when the control signal is low, such as prior to power up or just after power down, V OUT  is dragged to ground, Node A is shunted to V SS  and the connection between capacitor  202  and resistor  204  is broken. 
     For simplicity, the earlier examples have used a single ended amplifier stage in a single ended amplifier.  FIG. 9A  illustrates a two-stage differential amplifier comprising differential amplifier stage  910  and differential output stage  920  having core output stage  960 . Differential amplifier stage  910  takes differential inputs V IN+  and V IN−  and provides outputs to differential output stage  920  at nodes A +  and A − . Differential output stage  920  has two output V OUT+  and V OUT− . To supply stability to a compensation network with a feedback path from V OUT+  to node A −  and a compensation network with a feedback path from V OUT−  to node A +  are added to differential output stage  920 . In a typical implementation, the differential stage is inverting hence, the voltage V OUT+  is fed back in a compensation network to node A −  and node A + . In the example of  FIG. 9A , the compensation networks can be as simple as comprising a capacitor and a resistor. Output stage  920  of amplifier  910  comprises a compensation network with resistor  902  and capacitor  904  which provides a path between V OUT+  and node A −  and a compensation network with resistor  906  and capacitor  908  which provides a path between V OUT−  and node A + . The paths during power up and power down unfortunately provide a path for a spike to traverse from differential amplifier stage  910  to output V OUT+  and/or output V OUT− . 
       FIG. 9B  shows a differential amplifier with analogous anti-pop circuitry to the single ended amplifiers described above. Amplifier  950  comprises output stage  970  which is similar to output stage  920 , but includes switches for breaking the path from output to the input node via the compensation network. Furthermore, it comprises a switch for shunting the capacitor in the compensation network to V SS . More specifically switch  912  is open during power up or power down and breaks the path between V OUT+  and node A −  and switch  914  is closed during power up or power down and shunts capacitor  902  to V SS . Similarly, switch  916  is opened during power up or power down and breaks the path V OUT−  and node A +  and switch  918  is closed during power up or power down and shunts capacitor  906 . During power up or power down the control signal supplied to the switches is low, otherwise it is high. When the control signal is high amplifier  950  behaves like amplifier  900 . 
     While not shown, one of ordinary skill in the art could vary the switch placement and the type of compensation network. Furthermore, switches can be placed at each of the differential outputs to pull down V OUT+  and V OUT−  to V SS . 
     Another common amplifier implementation is a push-pull output stage. In a typical push-pull output stage, two complementary transistors are placed in series such as shown in  FIG. 11  with FET  1102  and FET  1104 . The output is tapped between the two transistors. Often, the complementary transistors are an n-channel FET (NFET) and a p-channel FET (PFET), other configurations include a npn bipolar transistor and a pnp bipolar transistor. Quite often the inputs to the transistors (such as the gate on FET) require different biasing. Because the inputs to the transistors often require different bias voltages. A bias circuit is often used between the amplifier stage and the output stage. The output of the bias circuit generates two voltages one for each transistor in a push-pull output stage. 
       FIG. 10A  illustrates the preliminary stages of an amplifier. Preliminary stages  1020  comprises amplifier stage  1010  which behaves similarly to the amplifier stage  110  described above. Amplifier stage  1010  receives differential inputs with voltages V IN+  and V IN−  and produces an output which is the amplified difference between V IN+  and V IN− . The output having a voltage of V A  is separately biased for use by a push-pull output stage, by bias circuit  1012  such as class AB bias control. The outputs of bias circuit  1012  have voltages equal to the input of V A  with a fixed bias. Specifically, V Ap= V A +V bias1  and V An =V A −V bias2 . 
       FIG. 10B  illustrates a circuit diagram for an exemplary bias circuit. The input voltage has a fixed bias added and subtracted with voltage source  1014  and  1016 . The voltage sources maintain a fixed voltage between its two terminals. Thus if the potential across voltage source  1014  is V bias1  then V Ap= V A +V bias1  and if the potential across voltage source  1016  is V bias2  then V An =V A −V bias2 . One of ordinary skill in the art should recognize that even though voltage sources  1014  and  1016  are symbolically represented by a battery any voltage source circuit can be used. 
       FIG. 10C  illustrates an amplifier with a push-pull output stage. Amplifier  1000  comprises preliminary stages  1020 . Preliminary stages  1020  receives differential input V IN+  and V IN−  and produces an output which is the amplified difference between V IN+  and V IN− , but the output is presented with a bias. At node A p , the output is appropriately biased to control a PFET in push-pull output stage  1060  and at node A n , the output is appropriately biased to control a NFET in push-pull output stage  1060 . The signals at nodes A p  and A n  are referred to as p_cntl and n_cntl, respectively. In order to stabilize amplifier  1000 , output stage  1030  further comprises compensation network comprising capacitor  1032  and  1034  which provides a feedback path from V OUT  to node A p  and a compensation network comprising capacitor  1036  and resistor  1038  which provide a feedback path from V OUT  to node A n . Once again, the feedback paths introduced by the compensation networks provide paths for a pop to travel from preliminary stages  1020  to the output V OUT . 
       FIG. 10D  shows an amplifier with a push-pull output stage and analogous anti-pop circuitry to the amplifiers described above. Amplifier  1050  comprises output stage  1070  which is similar to output stage  1030 , but includes switches for breaking the path from the output to each input node via the compensation network. Furthermore, it comprises a switch for shunting the capacitor in the compensation network to V SS . More specifically switch  1042  is open during power up or power down and breaks the path between V OUT  and node A p  and switch  1044  is closed during power up or power down and shunts capacitor  1032  to V SS . Similarly, switch  1046  is open during power up or power down and breaks the path V OUT  and node A n  and switch  1048  is closed during power up or power down and shunts capacitor  1036 . During power up or power down the control signal supplied to the switches is low, otherwise it is high. When the control signal is high amplifier  1050  behaves like amplifier  1000 . 
     Additional switches can be added to push-pull output stage  1060 .  FIG. 11  illustrates in greater detail an example of an amplifier with push-pull output stage. Amplifier  1100  comprises amplifier stage  1020  which is similar to that described for amplifiers  1000  and  1050 . Furthermore, amplifier  1100  comprises output stage  1120  which comprises a push-pull output stage comprising PFET  1102  and NFET  1104 . As can be seen, node A p  is the input that provides PFET  1102  with the p_cntl signal and node A n  is the input that provides NFET  1104  with the n_cntl signal. In principle, the p_cntl signal and n_cntl signal represent the same input but are biased differently. Though shown specifically as a generic FET, PFET  1102  is often a p-channel metal-oxide-semiconductor FET (MOSFET) in enhancement mode. Likewise, NFET  1104  is often an n-channel MOSFET in enhancement mode. In addition to switches  1042  and  1046  breaking the path provided by compensation networks from V OUT  to the respective nodes A p  and A n  and in addition to switches  1044  and  1046  which shunt capacitors  1032  and  1036  to V SS  as described for amplifier  1050 . Switch  1106  which is closed during power up and power down pulls V OUT  to V SS  output stage having anti-pop circuitry added. Switch  1106  operates similarly to switch  402  described for amplifier  800 . 
     In addition, output stage  1120  further comprises switch  1108  which drags the voltage at node A p  to V DD , that is p_cntl is V DD  when switch  1108  is closed. During power up and power down, switch  1108  is closed, by forcing p_cntl to be V DD , PFET  1102  as a gate-to-drain voltage of zero effectively shutting PFET  1102 . Essentially, this insures that no current is flowing through PFET  1102 . This also has the effect of charging capacitor  1042  so that even after the control signal goes high and switches  1108  and  1044  open, p_cntl begins initially at V DD  therefore PFET  1102  begins with no current flowing through it, thus preventing a pop from manifesting after the control signal causes switch  1108  and  1044  to open and switch  1042  to close. 
     When the control signal is high, switches  1044 ,  1048 ,  1106  and  1108  are open and switches  1042  and  1046  are close. Hence output stage  1120 , functions as a compensated push-pull output stage. 
     There are several methods to implement a control signal. As mentioned before, the ideal control signal should be low during power up and power down. For example, the control signal could be latched to V DD  as soon as V DD  reaches a predetermined level, the control signal goes high and as soon as V DD  drops below a predetermined level the control signal goes low. However, this simple approach leaves the possibility of an audio pop. 
       FIG. 12  shows the timing of an alternative control signal. At time  1202 , the power supply voltage V DD  begins to amp up. Prior to this time the control signal is low and remains low. At time  1204 , V DD  reaches normal operating level, but the control signal still remains low. Up to this time, V OUT  is forced to V SS . A short time later at time  1208 , control signal goes high and the amplifier begins to operate normally. Because the amplifier is allowed to completely powered up before activating the control signal any audio pop is completely suppressed. In the power down sequence, at time  1212 , the control signal goes low, however, the power supply voltage V DD  remains at normal operating levels. At this point, the amplifier is essentially deactivated and is forced V OUT  is forced to V SS . A short time later at time  1216 , V DD  begins to ramp down. At time  1218 , V DD  has completely powered down. 
     Such timing can be implemented without the need of a second voltage supply. This control signal is a non-overlapping version of power supply signal, V DD . For example, a control signal latched to V DD  by way of a delay circuit can delay the control signal going high until a small time interval after V DD  has reached normal operating voltage. In many applications, such as this example, the circuitry is controlled by a digital control. As an example a power down bar (pdb) signal used to indicate whether the amplifier block is powered up or down. For the power up sequence, the pdb signal goes high at time  1206  shortly there after the control signal goes high. During power down the digital circuitry begins to power down the block. First the control signal goes down at  1212 , then the pdb signal goes down at  1214  and finally the power signal begins to ramp down at  1216 . 
     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.