Abstract:
An audio amplifier circuit is described which comprises an operational amplifier. The operational amplifier receives an audio input signal and provides an output suitable for connecting a headphone, or a loudspeaker. A step-up converter is provided which supplies the operational amplifier. The audio amplifier is configured to operate in one of multiple operating modes, each of which uses a distinct supply voltage Vcc of the operational amplifier in the audio amplifier. Comparators are used to compare the output voltage of the operational amplifier with a first reference voltage to raise the supply voltage of the operational amplifier, if clipping is imminent. A second comparator is used to compare the output voltage of the operational amplifier with a second reference voltage, indicating that the supply voltage of the operational amplifier can be lowered without the risk of clipping.

Description:
TECHNICAL FIELD 
       [0001]    The present invention generally relates to amplifier circuits, and more particularly, to high efficiency amplifier circuits for use in battery powered applications. 
       BACKGROUND OF THE INVENTION 
       [0002]    Portable audio devices, such as for example MP3 players, radios, or external headphone amplifiers, are typically powered by a battery. The battery&#39;s voltage limits the maximum amplitude of the amplifier&#39;s output, and thereby the volume of an attached speaker or headphone. The amplifier&#39;s output voltage is inherently limited to stay below the supply voltage of the amplifier&#39;s end stage. Audio clipping, a distortion of the audio signal, occurs, if the amplifier&#39;s end stage becomes saturated. 
         [0003]    Class G audio amplifier circuits are known to conserve power and avoid audio clipping by adjusting the voltage that is supplied to an amplifier&#39;s end stage. An exemplary class G headphone amplifier circuit is disclosed in U.S. patent application Ser. No. 12/255,537, which is hereby incorporated by reference. The known audio amplifier circuit comprises a voltage generator that generates pairs of differential output voltage at variable amplitudes which are supplied to a headphone amplifier. The voltage generator is controlled by an automatic signal level detector, which analyzes the amplifier&#39;s input signal. 
         [0004]    Similarly, U.S. patent application Ser. No. 12/434,424 discloses amplifier circuits and methods of operation thereof. The &#39;424 application discloses a variable voltage power supply, which may be designed to output a plurality of discrete output voltages. The variable voltage power supplies are controlled by the input signal to the amplifier. U.S. patent application Ser. No. 12/434,424 is hereby incorporated by reference. 
         [0005]    U.S. Pat. No. 5,834,977 discloses an amplifying circuit comprising a step-up converter, which increases the supply voltage to an amplifying circuit from a battery voltage of 12 volts to an increased voltage of 18 volts based on a comparator which compares the output voltage Va of the amplifier with a fixed reference voltage of 10 volts. 
       SUMMARY OF THE INVENTION 
       [0006]    A battery-powered audio amplifier circuit is provided which comprises an operational amplifier. The operational amplifier is configured to receive an audio input signal, for example from an MP3 player, a radio, a cellular telephone, or any other source of electrical audio signals. The operational amplifier provides an audio output signal to a headphone, a loudspeaker, or any other audio device for converting electric energy into sound. The operational amplifier is operatively connected to a step-up converter, which provides a variable supply voltage Vcc to the operational amplifier. The step-up converter is powered by a battery having a voltage Vbat. The step-up converter is controlled by a microcontroller and configured to raise the operational amplifier variable supply voltage Vcc above the battery voltage Vbat. The audio amplifier circuit operates in one of multiple operating modes. Each of the multiple operating modes is characterized in that a distinct supply voltage Vcc is provided to the operational amplifier. 
         [0007]    In one particular operating mode the step-up converter may be disabled, and battery voltage Vbatt may be applied directly to the operational amplifier. This particular operating mode with disabled step-up converter is preferably selected, if the output signal amplitude is low. If the output signal amplitude is high, the variable supply voltage Vcc to the operational amplifier may be raised in several steps from a minimum voltage to a maximum voltage. In a particular example the step-up converter may be configured to raise the variable supply voltage Vcc to the operational amplifier in 3 steps of 2.3V each from a minimum voltage of 5V to a maximum voltage of 11.9V. 
         [0008]    A first comparator may be used to compare the amplitude of the output signal of the operational amplifier with a first reference voltage Vref 1  to detect the risk of clipping. The comparator may be operatively connected to the microcontroller, which controls the step-up converter. If the output signal falls below the first reference voltage Vref 1  the microcontroller causes the step-up converter to increase its output voltage Vcc and thereby the supply voltage of the operational amplifier by one step. 
         [0009]    A second comparator may be used to compare the amplitude of the output signal of the operational amplifier with a second reference voltage Vref 2  to detect, if the supply voltage of the operational amplifier can be lowered by one step without risking clipping. The second comparator may be operatively connected to the microcontroller. The microcontroller may be configured to lower the output voltage Vcc of the step-up converter by one step, if the output signal has not fallen below Vref 2  for a predetermined amount of time. 
         [0010]    The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a circuit diagram of an exemplary class G audio amplifier circuit. 
           [0012]      FIG. 2  is an illustration of various output signals of a class G audio amplifier circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring to  FIG. 1 , a circuit diagram of an exemplary class G audio amplifier circuit  1  in which the principles of the present invention may be advantageously practiced is illustrated generally. The exemplary audio amplifier circuit  1  comprises an operational amplifier  100 , which is configured to amplify an audio signal received from input connector  102 , and provide an amplifier audio output signal to output connector  104 . Connectors  102  and  104  may be standard 2.5 mm, 3.5 mm, or 6.4 mm stereo jacks. Input connector  102  may be used to connect an audio source, e.g. a portable MP3 player, a cellular telephone, a radio or the like. Output connector  104  may be used to connect a headphone, a speaker or the like. 
         [0014]    The amplitude of the audio output signal at output connector  104  and thereby the volume of an attached speaker or headphone can be adjusted through potentiometer  106 . Potentiometer  106  acts as a voltage divider for the audio input signal provided to input connector  102 . The potentiometer output voltage is provided to the operational amplifier  100  through a decoupling capacitor  108 , which eliminates any DC bias of the audio input signal. A voltage divider formed by resistors  120  and  122  is used to bias operational amplifier  100 . The tap point on the voltage divider is bypassed for AC signals by capacitor  118 , restoring some AC power supply rejection. Resistor  116  provides a DC return path for the VCC/2 reference voltage and also sets the circuit&#39;s (AC) input impedance. Resistors  110  and  112  and capacitor  114  determine the circuit&#39;s gain and bandwidth as described in AN-581 Application Note by Analog Devices, available online at http://www.analog.com, which is hereby incorporated by reference. 
         [0015]    Operational amplifier  100  may e.g. be a Rail-to-Rail, High Output Current Amplifier AD8397 manufactured by Analog Devices. Output  101  of operational amplifier  100  is at a potential of half of its supply voltage Vcc. Theoretically, the maximum amplitude of output  101  could hence be Vcc. In practice, however, the maximum amplitude of an AD8397 operational amplifier has to stay within 0.5V of supply rails. The maximum amplitude is hence Vcc-1V. If the operational amplifier AD8397 were powered directly by a battery having a battery voltage Vbat of 4.2V, the maximum amplitude of the output would hence be only 3.2V. 
         [0016]    To achieve higher output voltages operational amplifier  100  is powered by a variable supply voltage Vcc, which is generated in step-up converter  134 . Vcc may be higher than the battery voltage Vbatt powering audio amplifier circuit  1 . Step-up converter  134  is configured to raise battery voltage Vbat to operational amplifier variable supply voltage Vcc at its output  158 . Vcc is selected to be sufficiently high in order to avoid clipping of the audio output signal at output  101  of operational amplifier  100 . Variable supply voltage Vcc is selected from a set of predetermined voltages Vcc 1  . . . . Vccx. 
         [0017]    Audio output voltage  101  of operational amplifier  100  is compared with a first reference voltage Vref 1  in a first comparator  130 . The comparison, which is reflected in the output  131  of first comparator  130 , is used to determine the level of operational amplifier supply voltage Vcc that is required to avoid clipping. The audio output signal at output  101  swings around Vcc/2. Vref 1  is selected to be below Vcc/2, but above a minimum voltage at which clipping would occur. The minimum voltage at which clipping may occur in an AD8397 operational amplifier is 0.5V. Vref 1  may therefore be selected as 0.53V. If the voltage at output  101  falls below Vref 1  operational amplifier  100  is close to its limits, and its output is at risk of getting distorted. To avoid such distortion Vcc is increased, if the voltage at operational amplifier output  101  falls below Vref 1 . Vcc may be raised in several discrete voltage steps Vstep as long as first comparator  130  through its output  131  indicates a voltage at output  101  below Vref 1 , or until a maximum voltage Vccmax is reached. Output  131  of first comparator  130  provides a step-up signal to a microcontroller  166  input IN 1 . The microcontroller  166  controls step-up converter  134 , which may for example be a 1.2 MHz DC-to-DC Step-Up Switching Converter ADP1610 manufactured by Analog Devices. Step-up converter  134  is powered by a battery through battery input  136 . Inductor  142  stores energy during the on-time of step-up converter  134 , and transfers that energy to the output  158  through the output rectifier  144  during the off-time. Resistor  145  and capacitor  147  smoothen the output of step-up converter  134 . 
         [0018]    While a high supply voltage Vcc of operational amplifier  100  is beneficial to eliminate clipping and distortion of the audio signal at operational amplifier output  101 , it comes at a cost of increased losses in step-up converter  134  and correspondingly reduced battery life. Beneficially, Vcc is hence only raised as much as necessary to avoid distortion of the output  101  of operational amplifier  100 . If the signal at output  101  of operational amplifier  100  is small, Vcc may be lowered. A second comparator  132  compares the voltage of output  101  with a second reference voltage Vref 2 . Vref 2  is selected to indicate, if the voltage of output  101  would be clipped, if Vcc were to be lowered by Vstep. Vref 2  is hence selected as Vref 2 =Vstep/2+Vref 1 . In one exemplary embodiment Vstep may be 2.3V and Vref 1  may be 0.53V. Hence, Vref 2  in this example will be 2.3V/2+0.53V=1.68V. 
         [0019]    Output  133  of second comparator  132  may be used to assess, if Vcc can be lowered by one step Vstep without risking distortion of the audio signal at operation amplifier output  101 . Vcc may be lowered by one step Vstep, if the voltage at output  101  of operational amplifier  100  has not fallen below Vref 2  for a predetermined amount of time, e.g. for 20 sec. Output  133  of comparator  132  may be operatively connected to an input IN 2  of the microcontroller  166 . The microcontroller is also operatively connected to step-up converter  134 , and more particularly controls switches  138 ,  152 ,  154  and  156  of step-up converter  134 . Operational amplifier  100 , step-up converter  134 , first comparator  131  and second comparator  132  are operatively connected to a common ground. 
         [0020]    An exemplary use of amplifier circuit  1  is further explained with respect to operating modes as illustrated in  FIG. 2 . In a first operating mode as illustrated in graph  201  Vcc 1  is equal to Vbat, which is e.g. 4.2V. In this operating mode step-up converter  134  is disabled. Disabling of step-up converter  134  is achieved by opening switch  138 , so that the SD input of step-up converter  134  is low. Switch  138  is controlled by the microcontroller  166 . In this first operating mode bypass diode  140  is conductive, so that Vcc is approximately Vbat. More precisely, Vcc equals Vbat minus the voltage drop across diode  140 . As indicated in graph  201  at a time t 1  the output signal falls below Vref 1  (0.53V), causing the output  131  of first comparator  130  to turn low. The low-signal of output  131  is read by the microcontroller  166 , which responsive to the low-signal of output  131  closes switch  138 . As shown in graph  202  this activates step-up converter  134 , which now produces a constant voltage Vcc 2  of 5V. 
         [0021]    In the second operating mode as illustrated in graph  202  step-up converter  134  is active, producing a voltage Vcc 2  at its output  158  of 5V. At time t 2  the voltage at output  101  of operational amplifier  100  falls once again below Vref 1 , i.e. below 0.53V, which indicates the operational amplifier is close to its limits, and the output may get distorted when the signal is further increased. Output  131  of first comparator  130  turns low again at time t 2 , which is detected by the microcontroller. The microcontroller now closes switch  152 , which is operatively connected to the feedback pin FB of step-up converter  134 . The feedback pin FB of step-up converter  134  is configured such, that its voltage has to be constant at 1.23V. The voltage at feedback pin FB of step-up converter  134  is a function of the output voltage Vcc at output  158 , which is divided by a voltage divider formed by resistors  146 ,  148 ,  160 ,  162 , and  164 . By closing switch  152  resistor  160  is now in parallel to resistor  148 , thus lowering the total resistance below that of resistor  148  alone. Consequently, to reach the same voltage of 1.23 V at feedback pin FB, Vcc has to go up by Vstep. The values of resistors  146 ,  148 ,  160 ,  162 , and  164  are selected such, that Vstep is 2.3V. By closing switch  152  the amplifier enters a third operating mode illustrated in graph  203 . 
         [0022]    In the third operating mode as illustrated in graph  203  step-up converter  134  produces a constant output voltage Vcc 3  of 7.3V. At time t 3  the voltage at output  101  of operational amplifier  100  once again falls below Vref 1 . This causes the microcontroller  166  to further raise Vcc by closing switch  154 . The resulting fourth operating mode is illustrated in graph  204 . 
         [0023]    In the fourth operating mode as illustrated in graph  204  step-up converter  134  produces a constant output voltage Vcc 4  of 9.6V. At time t 4  the voltage at output  101  of operational amplifier  100  once again falls below Vref 1 . This causes the microcontroller  166  to further raise Vcc by closing the last remaining switch  156 . The resulting fifth operating mode is illustrated in graph  205 . 
         [0024]    In the fifth operating mode as illustrated in graph  205  the operational amplifier  100  is powered by the maximum supply voltage Vcc 5  of 11.9V. Vcc cannot be raised any further. Vcc 5  equals Vccmax of 11.9V and has been selected to not exceed the operating limits of the step-up converter  134 , the operational amplifier  100 , the comparators  130 / 132 , the capacitors and the resistors in the circuit and the maximal acceptable battery uptake current. 
         [0025]    As illustrated in graph  206  the voltage at output  101  of operational amplifier  100  is above Vref 2 , which in this fifth operating mode is 1.68V. Output  133  of second comparator  132  is high. This indicates that Vcc 5  could be lowered by Vstep (2.3V) without causing the output signal to be clipped. The microcontroller  166  is operatively connected to output  133  and configured to read output  133 . If output  133  has been high for a predetermined amount of time, e.g. 20 seconds, the microcontroller  166  opens switch  156 , thus lowering Vcc from its level Vcc 5  of 11.9V to Vcc 4  of 9.6V. Comparator  132  indicates, if sufficient headroom is available to lower Vcc by one step Vstep, and still keep the signal above Vref 1 =0.53V. Hence Vref 2 =Vstep/2+Vref 1 =1.68V. 
         [0026]    Graph  207  illustrates the amplifier circuit  1  in the fourth operating mode with Vcc 4 =9.6V. The output signal is shown not to fall below Vref 2  of 1.68V for a predetermined amount of time. Consequently, microcontroller  166  opens switch  154  to lower the supply voltage of operational amplifier  100  to Vcc 3  of 7.3V as shown in graph  208 . 
         [0027]    Graph  208  illustrates the amplifier in the third operating mode with Vcc 3 =7.3V. The output signal is shown not to fall below Vref 2  of 1.68V for a predetermined amount of time. Consequently, microcontroller  166  opens switch  152  to lower the supply voltage of operational amplifier  100  to Vcc 2  of 5V as shown in graph  209 . 
         [0028]    Graph  209  illustrates the amplifier in the second operating mode with step-up converter  134  producing supply voltage for operational amplifier  100  of Vcc 2 =5V. In this second operating mode Vref 2  is 0.93V and not 1.68V as in the previous steps. This is, because Vcc 2  (5V) is only 0.8V above Vcc 1  (4.2V). Again, the microcontroller  166  determines if the voltage of output signal  101  stays above Vref 2  for a predetermined amount of time. If it does, the microcontroller  166  opens switch  138  to disable step-up converter  134 . This causes the supply voltage of operational amplifier  100  to drop to Vcc 1 =4.2V, as illustrated in graph  210 . 
         [0029]    While the present invention has been described with reference to exemplary embodiments, it will be readily apparent to those skilled in the art that the invention is not limited to the disclosed or illustrated embodiments but, on the contrary, is intended to cover numerous other modifications, substitutions, variations and broad equivalent arrangements that are included within the spirit and scope of the following claims.