Abstract:
An audio amplifier such as for driving headphones. The amplifier includes multiple amplifier devices coupled in parallel. Both a bias generator and a volume control are responsive to a user setting. Under low output signal conditions, one or more of the amplifier devices are disabled in response to the user setting. Disabled amplifier devices do not consume output bias current. Thus the audio amplifier has reduced power consumption, and the system has longer battery life.

Description:
RELATED APPLICATION  
       [0001]     The present application claims benefit under 35 USC 119(e) of U.S. provisional Application No. 60/784,638, filed on Mar. 21, 2006, entitled “Adaptive Biasing Based on Volume Control Setting,” the content of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field of the Invention  
         [0003]     This invention relates generally to audio amplifiers, and more specifically to amplifier biasing and volume control.  
         [0004]     2. Background Art  
         [0005]     Amplifiers are typically biased at a minimum bias level, in order to ensure good fidelity, low distortion, and so forth. In general, the most difficult case in which to achieve good fidelity is at the amplifier&#39;s maximum output level. Therefore, amplifier systems have traditionally had their bias set at a level such that the fidelity and performance are adequate at large signal operation.  
         [0006]     At lower output level settings, this high bias level is typically far more than is necessary. This, unfortunately, results in an unnecessary and wasteful increase in power consumption when operating at small signal levels. This is especially unfortunate, given that most amplifiers are only rarely operated near their maximum output setting.  
         [0007]     In general, it is desirable to minimize the power consumption of an amplifier circuit, particularly in battery-powered systems. It is also desirable to minimize the heat generated by the amplifier, to reduce costs associated with heatsinks, fans, and power supply components.  
         [0008]     For a well-designed system which includes an output amplifier, the maximum output swing corresponds to the highest volume/level setting. Thus, the volume control setting and the output swing are related. In most systems, the volume control is implemented just before or as part of the power amplifier, or final stage in an amplifier chain, in order to maximize the dynamic range available to the preceding stages. In most systems, the maximum input level to the input of the volume control is known, and the peaks of the signal are generally made close to this limit, to maximize the signal-to-noise ratio of the preceding stages. Hence, it is possible to calculate the maximum possible output for each possible volume control setting.  
         [0009]      FIG. 1  illustrates an exemplary audio system  10  according to the prior art. The audio system includes a multi-stage amplifier which includes a first stage (Amp Stage  1 ) which receives the audio input signal (often referred to as the “Voice Signal” regardless of its content), one or more intermediate amplifier stages (Amp Stage  2 ), and a final power amplifier stage (Amp Stage  3 ). The power amplifier is coupled to drive a headphone or loudspeaker transducer (LS). The power amplifier (or other stages) may include a feedback loop. The power amplifier has a power reference input coupled to the output of a bias generator. It is this bias generator whose bias level is set at a predetermined level, typically set to optimize fidelity and performance at the maximum possible output level of the power amplifier. The gain of the power amplifier is set by a volume control mechanism. Typically, the volume control mechanism is dynamically adjustable according to a setting established by a user.  
         [0010]     What is needed is an improved audio amplifier system in which the gain established by the bias generator is dynamically adjustable, rather than being set at some predetermined point. Having a dynamically adjustable bias will enable the amplifier system to minimize power consumption and improve fidelity across a wide range of input signal levels and volume control settings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows an audio amplifier system according to the prior art.  
         [0012]      FIG. 2  shows an audio amplifier system according to one embodiment of this invention, in which the user-settable volume mechanism controls both (i) the bias generator to save power and (ii) the amplifier&#39;s feedback loop to adjust the amplifier gain.  
         [0013]      FIG. 3  shows an implementation of the audio amplifier system of  FIG. 2 .  
         [0014]      FIG. 4  shows an audio amplifier system according to another embodiment of this invention, in which the user-settable volume mechanism controls both (i) the bias generator to save power and (ii) the amplifier&#39;s effective multiplier number or “size” to adjust the amplifier gain.  
         [0015]      FIG. 5  shows an implementation of the audio amplifier system of  FIG. 4 .  
     
    
     DETAILED DESCRIPTION  
       [0016]     The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only.  
         [0017]      FIG. 2  illustrates one embodiment of an audio amplifier system  20  according to this invention. The system includes an amplifier, which may optionally be a multi-stage amplifier chain. The amplifier stage(s) may optionally be equipped with feedback loop(s). The feedback loop of at least one stage of the amplifier, preferably the final power amplifier stage, is coupled to receive a signal  22  or other indicator from the volume control mechanism. This signal dynamically adjusts the feedback loop, to set the gain of the amplifier. This signal also dynamically adjusts the bias setting of the bias generator, to set the bias provided to the amplifier stage(s).  
         [0018]     The bias generator (or, alternatively, the volume control mechanism) monitors and derives a control value from the volume control setting. The bias generator modifies the quiescent bias currents provided to the amplifier, to satisfy only the maximum signal level possible at the current volume control setting. In this manner, the bias current and hence the power consumption of the system can be reduced as the volume setting is reduced. Audio systems are rarely used at their maximum volume setting, and using this technique will reduce power consumption during most operating conditions.  
         [0019]     The amplifier system is coupled to be powered by a power source which, in many embodiments, may be a battery. The output of the amplifier system is coupled to drive a transducer such as a headphone or loudspeaker.  
         [0020]      FIG. 3  illustrates one embodiment of an audio amplifier system  30  similar to that of  FIG. 2 . The amplifier system includes an output amplifier  32  with a combined feedback loop and volume control mechanism  34 , implemented as a tap selector and a resistor chain  36  between the output node and the input node. The volume is set by selecting a suitable point (tap) along the resistor chain for the feedback point of the amplifier. Selecting a tap closest to the output will provide the minimum gain and thus the lowest volume setting. Selecting a tap closest to the input will provide the maximum gain and thus the highest volume setting. The tap selector is typically implemented as an array of switches (not shown) controlled by a digital word (Volume Setting). The same digital Volume Setting word can be used to control the bias of the amplifier, as previously described, via a connection  38  to the bias generator.  
         [0021]      FIG. 4  illustrates an audio amplifier system  40  according to another embodiment of this invention. In this embodiment, the user-settable volume control mechanism is coupled to adjust the bias produced by the bias generator, and to directly adjust the amplifier rather than a feedback loop of the amplifier. The volume control mechanism controls the gain of the amplifier by altering the effective multiplier number or “size” of the amplifier.  
         [0022]      FIG. 5  illustrates one embodiment of an audio amplifier circuit  70  similar to that of  FIG. 4 . The circuit includes an input stage (IS) which receives a signal from an input terminal, and provides gain and complementary output signals to a bias and driver top side circuit (BDTS) and to a bias and driver bottom side circuit (BDBS). The BDTS receives the output from the input stage and provides an output signal that is capable of driving and suitably biasing the relatively large output devices connected to the Vdd rail. The BDBS accepts another output from the input stage and provides an output signal that is capable of driving and suitably biasing the relatively large output devices connected to the Vss rail.  
         [0023]     One suitable way to implement the output stage of the amplifier is to configure the output stage as multiple parallel devices which can be independently or sequentially powered down as the volume setting is reduced. In this manner, the output bias current, which flows between output devices  76  and  90 , and between output devices  80  and  94 , and between output devices  84  and  98 , etc., can be reduced as the output swing requirements are reduced. Since the output bias current is generally a dominant component of the overall power consumption, large power savings can be made.  
         [0024]     The circuit includes a plurality of parallel output stages  76 ,  90 ,  80 ,  94 ,  84 ,  98 ,  88 , and  102 , coupled in parallel to drive the output node Vout. The number, size, and/or power handling capacities of the various output stages can be selected according to the needs of the application at hand, and particularly in view of an anticipated volume setting usage model. In the example shown, the first output stage comprising devices  76  and  90  has a multiplier (M) value of N and is always active and directly coupled to the output. N can be any value suitable to create a minimum bias current that performs correctly for the minimum signal level with all other amplifier devices switched off. In some applications, N will be greater than 1. The values N, 1, 2, 4, etc. are for illustration only; in practice, the values do not need to be integer multiples.  
         [0025]     A second output stage comprising devices  80  and  94  has a multiplier value of 1 and is coupled to be enabled and disabled by switches  78  and  92 . A third output stage comprising devices  84  and  98  has a multiplier value of 2 and is coupled to be enabled and disabled by switches  82  and  96 . A fourth output stage comprising devices  88  and  102  has a multiplier value of 4 and is coupled to be enabled and disabled by switches  86  and  100 .  
         [0026]     The switches are controlled by a binary weighted control word which may, in some embodiments, also be used to set a digitally controlled volume control (not shown). The binary weighted control word may be derived from the same digital signal that is used as an input to the digital volume control (not shown).  
         [0027]     Amplifier output stages having a larger multiplier number (M) are able to drive larger output voltage swings into the loudspeaker load, but the output bias currents also increase by a factor of the multiplier M. By allowing the effective aggregate size of the amplifier output stage, and hence the overall output stage biasing current, to be adjusted by the volume control, the amplifier can be better optimized to match the output signal swing conditions while saving power.  
       CONCLUSION  
       [0028]     When one component is said to be “adjacent” another component, it should not be interpreted to mean that there is absolutely nothing between the two components, only that they are in the order indicated.  
         [0029]     The various features illustrated in the figures may be combined in many ways, and should not be interpreted as though limited to the specific embodiments in which they were explained and shown.  
         [0030]     Those skilled in the art, having the benefit of this disclosure, will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.