Abstract:
According to the invention, an audio amplifier system for use with a single-ended portable power supply that is referenced to ground, such as a small battery, has a single-channel class G amplifier section, a multiple voltage output charge pump subsystem for supplying complementary pairs of power supply voltages at selected ratiometric levels to an amplifier section, a set of switches on the power supply rails and a power-measuring comparator for selecting which complementary pair of power supply voltages is provided to the amplifier section.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application claims benefit under 35 USC 119(e) of U.S. provisional Application No. 60/982,358, filed on Oct. 24, 2007, entitled “High Efficiency Audio Amplifier,” the content of which is incorporated herein by reference in its entirety. 
    
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     NOT APPLICABLE 
     REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK 
     NOT APPLICABLE 
     BACKGROUND OF THE INVENTION 
     This invention relates to audio amplifiers, particularly audio amplifiers where efficiency is important, such as battery-operated portable applications, for example portable amplifiers connected to headphones. 
     Minimizing unnecessary amplifier power dissipation is important in portable applications, such as headphones with a battery operated amplifier, such as a portable music player or telephones. In portable music players, such as MP3 players, designs are leading to decreasing MP3 encoding power dissipation as small feature size processes are used for the digital sections. This leaves the headphone as a major power dissipation contributor. In cellular telephones, especially where MP3 functions are integrated, the percentage of the power dissipated compared to the power dissipated in the transmitter and receiver is small. However, the headphones are used for a much longer period of time, making accumulated battery drain important. 
     Often very large AC-coupling capacitors are used to allow ground-referenced headphone return paths, i.e., headphone connections, where one of the terminals is grounded. It is desirable to remove these large capacitors and to achieve high power efficiency. 
     In the past, audio amplifiers have employed bipolar power supplies for AB amplifiers with a reference voltage, typically ground reference, between high and low (positive and negative) voltage rails. However, in relatively low-cost, low-power applications, demands have made it far more costly to provide a negative power supply voltage than to add large AC-coupling capacitors. 
     Some types of audio amplifiers with bipolar power supplies employ an integrated negative-supply generating charge pump in order to make it more cost effective to include the negative power rail, thereby making ground-referenced headphones easier for customers to use. Known art uses a charge-pump to generate a negative rail. 
     Reference is made to U.S. Pat. Nos. 7,061,327, 7,061,328, and 7,183,857 for background. Referring to  FIG. 1 , the techniques described therein use inefficient class AB amplifiers  12 ,  14  that employ two fixed voltage rails  16 ,  18  at +VCC and −VCC) with a battery  20  (at +VCC=1.8 VDC from ground) and a charge pump  22  and therefore suffer from high power dissipation at the loads, which are stereo earphone speakers  24 ,  26  referenced to ground. (The power dissipation in a class AB amplifier with a sinusoidal signal is minimal at the zero crossing and at the peak but maximum at points in between.) An alternative scheme described therein uses a single fixed voltage rail, which is likewise inefficient and requires a coupling capacitor to couple a signal to a ground-referenced load. 
     Class G amplifiers are known in the art. Referring to  FIG. 2 , a conventional class G amplifier  30  employs two parallel class AB amplifiers  32 ,  34  operating with complementary fixed voltage rail pairs  36 ,  38  and  40 ,  42  with power supplies at different maximum voltages (e.g. 3.0 VDC and 2.25 VDC), where connection to the rails are alternately switched by an equivalent ganged switch  44  (such as transistor switches) during each power cycle so that each amplifier  32 ,  34  operates only during a segment of different parts of the power cycle. The proper selection of the cycle parts improves efficiency to the output load, a loudspeaker  46 . To accommodate the d.c. voltage shift of the a.c. signal, a coupling capacitor  48  is needed. 
     It is important to consider how battery voltage maps into power supply requirements. For example, where a Li-Ion battery is used, the output voltage is a nominal 3.6V. However, producing +/−3.6V and the associated rails can be costly due to the high 7.2V requirements. Efficiency degrades when many rails are employed. Supplying the input voltage from the output of a +DC, −DC power source is possible but at added cost. 
     More efficient multiple-rail power supplies are needed. 
     SUMMARY OF THE INVENTION 
     According to the invention, an audio amplifier system for use with a single-ended portable power supply that is referenced to ground, such as a small battery, has a single-channel class G amplifier section, a multiple voltage output charge pump subsystem for supplying complementary pairs of power supply voltages at selected ratiometric levels to an amplifier section, a set of switches on the power supply rails and a power-measuring comparator for selecting which complementary pair of power supply voltages is provided to the amplifier section. 
     The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a prior art amplifier system. 
         FIG. 2  is a block diagram of a prior art class G amplifier. 
         FIG. 3  is a schematic diagram of a first embodiment of the invention. 
         FIG. 4  is a schematic diagram of a charge pump subsystem employed in connection with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention involves ground-reference class G amplifier solutions powered by a multiple-rail charge pump subsystem as shown in  FIG. 3  and  FIG. 4 . These configurations yield the best results of both high power efficiency and no requirement for output capacitors. In the example of the circuit of  FIG. 3 , a class G amplifier system  100  operates with a battery power source  102  at a voltage VDD of 1.8V. There is an optional voltage regulator  104  to assure constancy. This is primarily employed where the source voltage is subject to unexpected variations or where there is sufficient headroom in the voltage source to permit a voltage drop in the regulator circuit. In direct connection, the battery  102  supplies the voltage to the VDD voltage rail  106  for an outer rail or power supply terminal, and ratio metrically related voltages are applied at inner rails VDD 1 , VSS 1  and outer rail VSS, which are power supply terminals at voltages VDD 1 , −VSS 1  and −VSS, respectively, that are in pairs of complementary voltages around ground (zero volts). Where VDD is at 1.8V, VSS is at −1.8V. Where VDD 1  is set at 0.6V, VSS 1  is at −0.6V. These voltages are produced by a charge pump subsystem consisting of charge pump  1   114  and charge pump  2   116  connected to the other rails  108 ,  110  and  112 , as hereinafter explained. The battery and the output load, namely a loudspeaker or earphone  118 , are both referenced to ground  120 , as is the second charge pump  2   116 . Significantly, there is no output capacitor to add inefficiencies and drain the battery. The charge pump subsystem may supply power to a plurality of amplifiers, as in a stereo system. Only one amplifier is shown in  FIG. 3  for simplicity. It is also to be understood that VDD may be any voltage, not just 1.8V, although the VDD 1  rail is selected to be at a ratiometric level in relation thereto, such as 1.2V or 0.6V, in accordance with charge pump design. 
     The class G amplifier  100  is coupled to receive and amplify a differential audio signal of VINP and VINN at differential input terminals  122 ,  124  of a preamplifier  126 , which in turn generates differential current that is output through output terminals  128 ,  130 . Two rail selector switches SWP  132  and SWN  134  direct whether the preamplifier  126  is connected through inner current mirrors  136 ,  138  to the inner rail pair  108 ,  110  (switch position S 0 ) or through the outer current mirrors  140 ,  142  to the outer rail pair  106 ,  112  (switch position S 1 ). The efficiency of the amplifier  100  is determined by the switching thresholds, as hereinafter explained. 
     The current mirrors  136 ,  138 ,  140 ,  142  are respectively coupled at their source terminals to the voltage rails VDD 1 , VSS 1 , VSS and VDD, whereas the drain terminals of each of their output stages M are all connected to the output node  144  that directly drives the load  118 , as intended. The reference M indicates that there is a substantial amplification in comparison to the input stages of the current mirrors. The drain terminals of the input stages “1” of each of the current mirrors  136 ,  138 ,  140 ,  142  are respectively switched to current sources  128 ,  130 , alternately activating inner current mirrors  136 ,  138  and outer current mirrors  140 ,  142  supplied respectively by the inner rails  108 ,  110  and the outer rails  106 ,  112 . 
     The switches SWP  132  and SWN  134  are controlled by comparators  146 ,  148 , which operate independently of one another. One input terminal of each comparator  146 ,  148  is coupled to the output node  144  to monitor instantaneous voltage at the output, one comparator  146  sensing for positive deviations from neutral reference and the other comparator  148  sensing for negative deviations. (In an alternative embodiment, the terminal of the comparators  146 ,  148  can be coupled to monitor, respectively input terminals  142  and/or  144  with appropriate modification of threshold requirements.) A positive voltage threshold element Vtp  150  is coupled between the inner rail VDD 1   108  and the other input to the comparator  146 . A negative voltage threshold element Vtn  152  is coupled between the inner rail VSS 1   110  and the other input to the comparator  148 . The respective threshold levels are selected to be approximately 150 mV. In operation, a positive voltage deviation (above ground reference) creating less than 150 mV between the voltage at output terminal  144  and inner rail VDD 1   108  causes the comparator  146  to switch the switch  132  from position S 0  to S 1 , that is from inner rail  108  to outer rail  106 , permitting greater output voltage amplitude than is permitted by the inner rail  108 . The reverse is also true to cause the switch  132  to change back from position S 1  to S 0 , with some hysteresis. 
     Similarly, but independently, in operation, a negative voltage deviation (below ground reference) creating less than 150 mV between the voltage at output terminal  144  and inner rail VSS 1   110  causes the comparator  148  to switch the switch  134  from position S 0  to S 1 , that is from inner rail  110  to outer rail  112 , permitting greater output voltage amplitude than is permitted by the inner rail  110 . Thus efficiency is achieved for both low amplitude deviations and high amplitude deviations, depending upon instantaneous output voltage at the output node  144 . The reverse is also true to cause the switch  132  to change back from position S 1  to S 0 , with some hysteresis 
     A suitable charge pump subsystem suitable for the circuitry of  FIG. 3  is shown in  FIG. 4 . A charge pump works by charging a floating switched capacitor for a switching period, then connecting it in series with another voltage reference, such as another capacitor, or by reversing terminals, for a power supplying cycle.  FIG. 4  illustrates charge pump  1   114  built on capacitor CF 1   160  connected to charge pump  2   116  built on capacitor CF 162 , all connected to voltage rails VDD  106 , VDD 1   108 , GND  120 , VSS 1   110 , VSS  112 , where battery  102  is connected between VDD  106  and GND  120 . the respective voltages produced from a 1.8V source are 1.8V, 0.6V, 0V, −0.6V and −1.8V. This is accomplished with charge pump  1   114  operative in three states and charge pump  2   116  operative in two states. This means that the charge pumps dwell in each state either one-third or one-half of each of their respective charge cycles. Charge pump  1   114  is provisioned with first and second ganged three-state switches  164 ,  166  that dwell in each of its states one-third of the time of a cycle. Switch  1   164  has terminal S 2  connected to VDD, terminal S 2  connected to VDD 1  and terminal S 0  connected to VSS 1 . Ganged switch  2   166  has its terminal S 2  connected to VDD 1  and its terminal S 1  connected to terminal VSS 1  and its terminal S 0  connected to terminal VSS. In state S 0 , capacitor  160  is coupled across VSS and VSS 1 ; in state S 1 , capacitor  160  is coupled across VDD 1  and VSS 1  and in state S 2 , capacitor  160  is coupled across VDD and VDD 1 . 
     Charge pump  2   116  is provisioned with third and fourth ganged two-state switches  168 ,  170  that dwell in each of its states one-half of the time of a cycle. Switch  168  in switch position S 0  is connected to ground  120  and in switch position S 1  is connected to VDD rail  106 . Switch  170  in switch position S 0  is connected to VSS rail  112  and in switch position S 1  is connected to Ground. Thus switching between states S 0  and S 1  causes capacitor  162  to have its terminals alternately connected between ground and the rail VDD with the constant power supply voltage at VDD and the rail VSS that carries the inverse of the power supply voltage. The periods of dwell in each state is equal, so that charge pump  2   116  is reversing the sense of charged capacitor  162  each period of the charging or discharging of capacitor  160  of charge pump  1 . Therefore, as configured, charge pump  1  produces a differential voltage of 1.2V across capacitor  160 . This differential voltage is applied with proper polarity and voltage differential across voltage rails where that desired differential is maintained to yield the voltages at DC potential of 1.8V, 0.6V, 0V, −0.6V and −1.8V with sufficient charge to supply the amplifier circuit of  FIG. 3 . 
     Ratiometric spacings of 33% have been shown in the embodiment of  FIG. 3 . For example 33% spacings (VDD/3) are arranged to give 1.8V, 0.6V, −0.6V and −1.8V. Other ratiometric spacings are possible, such as 25%. 
     Other rail combinations are possible when a lithium-ion supply is considered. The Li-Ion supply is typically at 3.6V. A 3-state charge pump can produce +3.6V, 1.8V, 0V and also −1.8V. In such a case, the amplifier can be class AB running of +/−1.8V modified to a class G configuration. Alternatively, a 6-state charge pump supplied by a +3.6V supply can produce power outputs of +3.6V, 2.7V, 1.8V, 0.9V, 0V, −0.9V and −1.8V. Other combinations exist having different ratiometric relationships. 
     It is important to consider the actual signal to be amplified when selecting the rail options. The average power dissipation is related to the probability density function of the signal and the volume setting. Music and voice dictate different choices for selecting rail options. 
     The invention has been explained with reference to various embodiments. Other embodiments will be evident to those of skill in the art. It is therefore not intended that this invention be limited, except as indicated by the appended claims.