Abstract:
Various embodiments of methods and apparatus for an amplifier with wide output voltage swing are disclosed. The amplifier may include multiple output stages, each associated with a distinct supply voltage. Each output stage may contribute current to the output of the amplifier over a range of amplifier output voltages and these ranges may overlap. Each output stage may contribute current until the amplifier output voltage reaches the supply voltage associated with that output stage. The amplifier output may be as great as the largest supply voltage minus a drop equal to Rdson for an output transistor multiplied by the output current. In a CMOS implementation, this voltage drop may be approximately 0.15V. When the amplifier output voltage is close to the supply voltage associated with an output stage, both that output stage and the output stage associated with the next highest supply voltage may contribute to the amplifier output.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to integrated circuits, and more particularly, to amplifiers implemented as integrated circuits.  
         [0003]     2. Description of the Related Art  
         [0004]     The basic function of an amplifier is to produce and output signal whose power is a multiple of the power of an input signal. In many applications it is desirable that the output waveform faithfully reproduce the shape of the input signal while magnifying its voltage and/or current in a linear fashion. Traditionally, amplifiers designed for these types of applications have been configured for class A operation.  
         [0005]     In an amplifier designed for class A operation, both output devices conduct continuously for the entire cycle of signal swing, or the bias current flows in the output devices at all times. The key ingredient of class A operation is that both devices are always on. There is no condition where one or the other is turned off. Because of this, class A amplifiers in reality are not complementary designs. They are single-ended designs with only one type polarity output devices. They may have “bottom side” transistors but these are operated as fixed current sources, not amplifying devices.  
         [0006]     Since a class A amplifier operates from only one power supply, the voltage level of the supply must be somewhat greater than the level of the peak output specified. Therefore, during times in which the input signal is very small, the difference between the amplitude of the output signal and the voltage of the power supply will be large. The amount of non-usable power to be dissipated in the output devices is the aforementioned voltage difference multiplied by the output current. Even in those instances where the output is at its maximum level, there will still be a non-negligible voltage drop across the output devices and corresponding level of non-usable power dissipated in the devices.  
         [0007]     Consequently class A is the most inefficient of all power amplifier designs, averaging only around 20% (meaning it consumes about 5 times as much power from the source as it delivers to the load!) Thus class A amplifiers are large, heavy and run very hot. All this is due to the amplifier constantly operating at full power. The positive effect of all this is that class A designs are inherently the most linear, with the least amount of distortion.  
         [0008]     In order to increase the efficiency of an amplifier while maintaining a high degree of linearity, a class G design may be employed. Class G operation involves changing the power supply voltage from a lower level to a higher level when larger output swings are required. There have been several ways to do this. The simplest involves a single class AB output stage that is connected to two power supply rails by a diode, or a transistor switch. The design is such that under most circumstances, the output stage is connected to the lower supply voltage, and automatically switches to the higher rails for large signal peaks. Another approach uses two class AB output stages, each connected to a different power supply voltage, with the magnitude of the input signal determining the signal path. Using two power supplies improves efficiency enough to allow significantly more power for a given size and weight.  
         [0009]     Typically, class G amplifier implementations employ current blocking diodes to prevent current from being driven into a lower voltage supply when the amplifier output exceeds the lower supply voltage. This effectively protects the lower voltage power supplies, but also limits the efficiency of their contribution to the amplifier output. The power dissipated in the diode will be the voltage drop across the diode times the output current. This power loss will occur any time a lower voltage supply is contributing to the output of the amplifier. In addition, each output stage normally includes a power device to control the flow of current to the load. This device dissipates power equal to the difference between the supply voltage and the amplifier output multiplied by the load current. Again, this power will be wasted any time the supply is contributing to the amplifier output.  
         [0010]     When a power supply is contributing maximum current to the amplifier output, the output device will typically be saturated and drop a few tenths of a volt. When added to the diode drop for a lower voltage supply, the total difference between the supply voltage and the amplifier output may be around one volt. While such a voltage drop and corresponding inefficiency may be acceptable in a relatively high voltage amplifier design where the output is several tens of volts, integrated circuit amplifiers for low-power applications are typically designed to operate with minimum supply voltages below two volts and such a drop in output stage voltage would limit the amplifier&#39;s maximum efficiency to less than fifty percent. Therefore, a more efficient design for a class G amplifier may be desirable.  
       SUMMARY  
       [0011]     Various embodiments of methods and apparatus for an amplifier with wide output voltage swing are disclosed. The amplifier may include multiple output stages, each associated with a distinct supply voltage. Each output stage may contribute current to the output of the amplifier over a range of amplifier output voltages and these ranges may overlap. Each output stage may contribute current until the amplifier output voltage reaches the supply voltage associated with that output stage. The amplifier output may be as great as the largest supply voltage minus a drop equal to Rdson for an output transistor multiplied by the output current. In a CMOS implementation, this voltage drop may be approximately 0.15V. When the amplifier output voltage is close to the supply voltage associated with an output stage, both that output stage and the output stage associated with the next highest supply voltage may contribute to the amplifier output.  
         [0012]     As the amplifier input voltage increases from zero, the output stage associated with the lowest supply voltage may supply all of the amplifier output current until the amplifier output voltage reaches a level that is delta V below the lowest supply voltage, where delta V may be a few tenths of a volt and may be set by ratio of channel geometries of transistors in the output stage controller. At this point the output stage associated with the next highest supply voltage may begin to contribute current to the output of the amplifier. Both output stages may continue to contribute current to the amplifier output until the amplifier output voltage reaches the level of the lowest supply voltage. At this point the output stage controller may assert a signal that causes the output of the output stage associated with the lowest supply voltage to be coupled to the input for that stage through an analog switch, thus isolating the lowest supply voltage from the amplifier output when the amplifier output voltage is above the lowest supply voltage.  
         [0013]     When the amplifier output voltage is above the lowest supply voltage the output stage associated with the next highest supply voltage may contribute all the current to the amplifier output. When the amplifier output rises to within delta V of the supply voltage for this output stage the output stage associated with the next highest supply voltage may begin to contribute current to the amplifier output. The transition regions in which two output stages whose supply voltages are adjacent share the current load of the amplifier may begin at delta V below each supply voltage. At this voltage, the output stage associated with the supply voltage may be supplying the preponderance of the amplifier output current, while the output stage associated with the next highest supply voltage may supply only negligible current. As the output voltage of the amplifier increases through delta V, the brunt of the amplifier output current may switch sources, so that by the time the amplifier output voltage reaches the supply voltage, the output stage associated with the next highest supply voltage may bear the complete burden. This transition mechanism may serve to reduce noise associated with switching the current supply between output stages.  
         [0014]     The transitioning of amplifier output load between output stages may be repeated for each pair of output stages included in the amplifier. A larger number of output stages may correspond to a greater amplifier efficiency. In one embodiment, an amplifier may be implemented as an IC using CMOS technology and delta V may be designed to be approximately 0.3 V. This IC may be used to drive a cooling fan in a personal computer environment and may use supply voltages of 2.5 V, 3.3 V, and 5 V that are common in this type of system. Each output stage of the amplifier may provide a voltage gain greater than unity.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     Other aspects of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which:  
         [0016]      FIG. 1  shows a diagram of a class G amplifier, according to one embodiment.  
         [0017]      FIG. 1A  shows a schematic diagram of one embodiment of a output stage controller included in a class G amplifier.  
         [0018]      FIG. 2  is a flowchart of a method for operating a class G amplifier with three output stages, according to one embodiment.  
         [0019]      FIG. 3  illustrates a mechanism for protecting the power supply of an output stage of a class G amplifier from reverse current flow, according to one embodiment.  
         [0020]      FIG. 4  is a flowchart of a method for operating an output stage of a class G amplifier, according to one embodiment.  
         [0021]      FIG. 5  illustrates circuitry for limiting the total output current of an amplifier as a function of the amplifier output voltage, according to one embodiment.  
         [0022]      FIG. 6  is a flowchart of a method for providing output current limiting in a class G amplifier, according to one embodiment. 
     
    
       [0023]     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and description thereto are not intended to limit the invention to the particular form disclosed, but, on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling with the spirit and scope of the present invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF EMBODIMENTS  
       [0024]      FIG. 1  shows a diagram of a class G amplifier, according to one embodiment. Voltage-controlled current sink  100  may draw current from the control sections of the output stages  190  in proportion to the amplifier input voltage  180 . For example, output stage  190 A may be associated with the lowest power supply voltage  120  of any output stage. When current sink  100  begins to draw current in response to input voltage  180 , switches  105  and  110  associated with the control sections of output stages  190 B and  190 C respectively may be open. In this instance, all current drawn by current sink  100  may come from the control section of output stage  190 A through resistor  115 .  
         [0025]     Current-controlled current source  125  may source current from the power supply of output stage  190 A in proportion to the current through resistor  115 . Switch  130  may be closed for amplifier output voltage  175  in the range of zero to the voltage  120  of the power supply associated with output stage  190 A. Current source  125  may supply current through switch  130  to produce amplifier output voltage  175 . For values of amplifier output voltage  175  below supply voltage  120  minus a small voltage increment, delta V, output stage  190 A may be the only contributor to the current through amplifier load  195 .  
         [0026]     As amplifier input voltage  180  increases, current sink  100  may draw more and more current through resistor  115 . Increasing current through resistor  115  may cause a proportional increase in current from source  125  through load  195 , and a corresponding rise in amplifier output voltage  175 . Output stage  190 A may continue to source current to load  195  until the amplifier output voltage  175  reaches supply voltage  120 . At this point the efficiency of supplying power to the load  195  is quite high. For example, if supply voltage  120  were 3.3V, the amplifier might be supplying current to load  195  with an efficiency of greater than 95%.  
         [0027]     In a typical class G amplifier, when the output voltage increases to a level exceeding the capabilities of a lower voltage supply, the lower supply is disconnected from the amplifier output while the next higher voltage supply is switched in to furnish power to the load. This switching of the load between power supplies may result in glitches or dropouts in the amplifier output voltage. If the lower voltage supply switches off before the higher voltage supply comes on, there may be a momentary drop in the amplifier&#39;s output. Conversely if the lower voltage supply is not completely out of the circuit before the higher voltage supply kicks in, a voltage spike may occur at the output. Both of these conditions may result in the generation of electromagnetic noise, which could be detrimental to the function of circuitry in the proximity of the amplifier.  
         [0028]     In order to insure smooth transitions of load current between power supplies and minimize generated noise, the amplifier of  FIG. 1  establishes a transition zone for each pair of output stages  190 . The transition zone may begin when the amplifier output voltage  175  increases to a level that is some small voltage, delta V, below the level of the lower supply voltage. In the case of a transition between output stages  190 A and  190 B, switch  105  may close when amplifier output voltage  175  reaches a level that is equal to supply voltage  120  minus delta V. When switch  105  closes, the control section of output stage  190 B may start contributing current from power supply  140  through resistor  135  to current sink  100 . Switch  150  may be closed, and the current through resistor  135  may cause current-controlled current source  145  to source a proportional current to load  195 . At this point the current corresponding to the amplifier input voltage  180  is shared between resistors  115  and  135 , and the load current is shared between current sources  125  and  145 .  
         [0029]     As amplifier output  175  increases from supply voltage  120  minus delta V, the proportion of the current through sink  100  shifts from resistor  115  to resistor  135 . Likewise, the load current source may shift from  125  to  145  such that when the amplifier output reaches the lower supply voltage  120 , output stage  190 B is supplying the total output current for load  195 . At this point, a very small amount of current may continue to flow through resistor  115 , but it is insignificant relative to the current through resistor  135 . This transitional sharing of the load current may insure that the amplifier output will be monotonic and linear with respect to an increasing input signal.  
         [0030]     Once the amplifier output  175  reaches supply voltage  120 , switch  130  may opened to prevent current from flowing through current source  125  in the reverse direction and damaging power supply  120 . This protective feature will be described in greater detail below. As the input voltage increases, current sink  100  draws more current through resistor  135 , which causes proportionally more current to be sourced from  145  through load  195  with a proportional increase in amplifier output  175 . A second transition region occurs between output stages  190 B and  190 C beginning when the amplifier output increases to supply voltage  140  minus delta V. Note that the value for the voltage difference, delta V, for the transition between output stages  190 B and  190 C may be the same as or different from the voltage difference for the transition between output stages  190 A and  190 B. Delta V may be set by the width to length parameters of transistors included in the output stage controller  107 . Delta V may be set within a range of values from 0V to 0.8V or potentially greater depending upon the specific application. In one embodiment, the value chosen for delta V may be 0.3V. Smaller values for delta V may increase the overall efficiency of the amplifier by allowing each output stage to solely contribute current to the amplifier output until the output is closer to the supply voltage of the output stage.  
         [0031]     As previously described with regard to the transition between output stages  190 A and  190 B, the preponderance of current through sink  100  switches from resistor  135  to resistor  155  and as a result, the load current sourced from  145  decreases while that from  165  increases such that when the amplifier output reaches the level of supply voltage  140 , the entire output current is produced by output stage  190 C. Further increases in input voltage  180  result in increased current through resistor  155  with a corresponding increase in current sourced from  165  through load  195 . This causes a proportionate rise in amplifier output  175  until the maximum level of supply voltage  160  minus the amplifier output current times Rdson of the output transistor is reached.  
         [0032]     Note that the drain-to-source on resistance for output transistor implementations using current CMOS technology may be inversely proportional to the width of the channel. Therefore, the drain-to-source on resistance of the output transistors used to implement the amplifier may be reduced and amplifier efficiency increased by increasing the channel width of the transistors. Increasing the width of the channel for the output transistors may require additional IC real estate. Therefore, the desired efficiency of a CMOS implementation of the disclosed amplifier may be traded off with the amount of IC area required. Typical values of a fraction of an OHM are readily achievable for Rdson.  
         [0033]     When the output of the amplifier reaches the level of supply voltage  160  minus approximately 0.15V, the amplifier may again be supplying the load at a high efficiency. For example, if the supply voltage  160  were 7 volts, the amplifier might supply the load current with around 98% efficiency. Although three output stages are depicted in  FIG. 1 , the advantages of this design are readily extensible to amplifiers with any number of supply voltages and corresponding output stages greater than or equal to two.  
         [0034]      FIG. 1A  presents one embodiment of the amplifier with a more detailed depiction of the output stage control functionality. Input voltage  180  may be applied to the positive input of differential amplifier  181 . The output voltage  175  may be divided by resistors  182  and  183 , and a portion applied to the negative input of differential amplifier  181 . In this configuration, the output of the differential amplifier may drive NMOS transistor  184  so as to draw enough current from current mirrors  191  included in output stages  190  to satisfy the relationship Vout=Vin(1+R2/R1), under normal operating conditions.  
         [0035]     If Vin starts at zero and increases monotonically, current mirror  191 A may supply the total amplifier output current  175  until the amplifier output voltage reaches the level of the lowest supply voltage  120  minus delta V. Differential amplifier  181  may be driving transistor  184  to attempt to draw current from transistors  108  and  106 . The gate of transistor  108  may be biased from supply voltage  120  such that it is always on and attempting to draw current from the input side of current mirror  191 A. Current mirrors  191  may be configured to contribute current to Vout  175  until the point that Vout reaches the level of their input supply, as will be explained in more detail with regard to  FIG. 3 . Therefore, in the stated range of Vout, current mirror  191 A of output stage  190 A may be capable of contributing current to the amplifier output  175 . On the other hand, the gate of transistor  106  is biased from Vout and therefore, no significant current may flow through transistor  106  until Vout reaches a level of voltage at the drain of transistor  184 , plus the threshold voltage for transistor  106 . In some embodiments, this voltage may be designed to be equal to the lowest supply voltage  120  minus delta V.  
         [0036]     At this point transistor  111 , biased by supply voltage  140  is able to draw current from current monitor  191 B, but Vout is not great enough to turn on transistor  112 , therefore, output stages  190 A and  190 B may contribute current to amplifier output  175 . As Vout rises toward supply voltage  120 , the proportion of output current contributed by current mirror  191 B as compared with that contributed by current mirror  191 A may increase rapidly due to differences in the channel geometries of transistors  108  and  106 , such that when Vout is equal to supply voltage  120  minus delta V, current mirror  191 A may be contributing almost the entire output current of the amplifier, but by the time Vout reaches supply voltage  120 , current mirror  191 B may be supplying nearly all of the output current. At this point, protective circuitry may isolate current mirror  191 A from the output node of the amplifier as described below, causing current mirror  191 B to provide all of the output current for the amplifier.  
         [0037]     Current mirror  191 B may supply the entire output current of the amplifier as Vout rises toward supply voltage  140 . When Vout reaches supply voltage  140  minus delta V, a hand off may occur between current mirrors  191 B and  191 C similarly to the one previously described between current mirrors  191 A and  191 B. This hand off of the amplifier output current supply may be initiated when the gate bias voltage of transistor  112  reaches a level that is the NMOS threshold voltage above the voltage at the drain of transistor  106 . In some embodiments, this point may be designed to be when Vout reaches a level that is delta V below supply voltage  140 . At Vout equal to supply voltage  140 , current mirror  191 B may be isolated from the amplifier output and current mirror  191 C may assume the role of sole provider of amplifier output current as Vout rises to its maximum value.  
         [0038]     As stated previously, the methods described above for smoothly transferring the load from one output stage to another dependent on the value of the amplifier output voltage are readily extensible to embodiments of the disclosed amplifier including any number of output stages, even though  FIG. 1A  depicts a particular embodiment with three output stages.  
         [0039]      FIG. 2  is a flowchart of a method for operating a class G amplifier with three output stages, according to one embodiment. At  200 , if the amplifier output voltage, Vout, is less than the power supply voltage, V 1 , associated with the lowest voltage output stage, then output current may be sourced from the lowest voltage power supply. For example, if the lowest voltage power supply is 2.5 volts, then the output stage associated with this power supply may contribute output current until the output voltage reaches 2.5 volts. As shown at decision block  220 , if the amplifier output voltage rises to within a small voltage difference, d, of the first output stage supply voltage, V 1 , then the second output stage associated with power supply V 2  may start to contribute current to the amplifier output, as indicated by block  230 .  
         [0040]     During the time that the amplifier output is between V 1 - d  and V 1 , both the first and second output stages may contribute current to the amplifier output. By the time the amplifier output reaches V 1 , the second output stage associated with power supply V 2  may be supplying the total output current for the amplifier. Further increases in Vout may be brought about by corresponding increases in current output to the amplifier load from power supply V 2 .  
         [0041]     When the amplifier output reaches a level of V 2 - d , a similar transitioning of the load current may take place between the second and third output stages associated with power supplies V 2  and V 3  respectively. As shown at blocks  240  and  250 , when Vout rises to V 2 - d , power supply V 3  may begin to contribute current to the output of the amplifier and this contribution may increase relative to the contribution of power supply V 2  until V 3  is supplying all of the amplifier output current when the amplifier output voltage reaches V 2 . When Vout exceeds V 2 , the third output stage may supply all of the amplifier output current. Note that this method may be extended to operate an amplifier with more than three output stages.  
         [0042]      FIG. 3  illustrates a mechanism for protecting the power supply of an output stage of a class G amplifier from reverse current flow, according to one embodiment. Voltage-controlled current sink  305  may draw current proportional to the amplifier input voltage signal. The current through sink  305 , or a portion thereof, may be drawn from power supply  325  with voltage Vn through transistor  315 . Transistor  320  may source current to the output of the amplifier  330  in proportion to the current through transistor  315 . This current may be combined with the output currents from the other output stages of the amplifier  335  to form the complete output current for the amplifier.  
         [0043]     As the current sourced by this stage and the other output stages  335  of the amplifier increases, the amplifier output voltage  330  may increase. The level of the amplifier output voltage  330  is monitored with respect to the power supply voltage, Vn, for output stage, n, by comparator  300 . When Vout  330  is less than Vn  325 , comparator  300  may output a high voltage level to the gate of transistor  310 . This high voltage level may keep transistor  310  turned off isolating Vout  330  from the gates of transistors  315  and  320 . When Vout  330  is equal to or exceeds Vn  325 , comparator  300  may output a low voltage level to the gate of transistor  310 . This low voltage level may turn on transistor  310  and apply Vout to the gates of transistors  315  and  320 . When transistor  310  is on, the gate of transistor  320  will be at a voltage level greater than or equal to the voltage level at its source. Since transistor  320  is an enhancement mode PMOSFET in  FIG. 3 , this bias condition may prevent current from flowing into power supply  325  when Vout  330  exceeds Vn  325 .  
         [0044]     This mechanism is significant in that it allows the power supply associated with an output stage to contribute current to the amplifier output up until the point that the amplifier output reaches the level of the power supply for that stage. In an output stage of a typical class G amplifier, a diode is used to prevent current from flowing from the amplifier output into the power supply associated with the output stage when the amplifier output exceeds the level of the supply. This limits the efficiency of the typical class G output stage because all current output from the stage must pass through the diode in addition to a gating device such as a power transistor. The combined voltage drop from the supply voltage to the amplifier output level is a minimum of about one volt in the typical case, and the power wasted in the output stage will be the one volt times the output current. For a typical low-voltage amplifier where the largest supply voltage is, for example, three volts, this wasted power may be nearly as great as the power being supplied to the load. The disclosed mechanism of  FIG. 3  may reduce the power wasted in an output stage by nearly an order of magnitude.  
         [0045]      FIG. 4  is a flowchart of a method for operating an output stage of a class G amplifier, according to one embodiment. At block  400 , the amplifier output voltage may be monitored with respect to the power supply voltage, Vn, associated with a particular output stage. As long as the amplifier output voltage, Vout, remains below Vn, the output device for that output stage may be capable of sourcing current to the amplifier load and may provide the load current for amplifier output voltages in a range below Vn as described previously and indicated by block  410 . When Vout surpasses Vn, the output stage&#39;s power supply may be isolated from the output of the amplifier so that no current may flow from Vout into the power supply, as illustrated at block  420 . This may prevent damage to the power supply for output stage, n, by inhibiting reverse current flow into the supply.  
         [0046]      FIG. 5  illustrates circuitry for limiting the total output current of an amplifier as a function of the amplifier output voltage, according to one embodiment. Voltage-controlled current source  500  may output a current into resistor  520  that is inversely proportional to the amplifier output voltage, Vout,  550 . Current-controlled current source  510  may also supply current to resistor  520 . Current source  510  may output a current that is a particular fraction of the sum of the currents being output by all output stages of the amplifier. Current sources  500  and  510  may be designed such that the sum of their outputs delivered to resistor  520  is constant or nearly constant over the range of amplifier output voltage.  
         [0047]     The current through resistor  520  may produce a voltage at the gate of transistor  530 . The current through resistor  520  may be set such that the voltage produced at the gate of transistor  530  is insufficient to turn the transistor on under normal operating conditions. If the current produced by the output stages should rise disproportionately with respect to the amplifier output voltage  550 , the sum of the currents from sources  500  and  510  might rise as well. This condition may be caused by a reduction in amplifier load impedance, an extreme example of which might be a short circuit of the amplifier output. Under these circumstances, the current through source  510  may rise more rapidly than the current from source  500  diminishes resulting in a net increase in the current through resistor  520 . This increase in current may produce a corresponding increase in the voltage at the gate of transistor  530 .  
         [0048]     As the current through resistor  520  increases, the gate-to-source voltage of transistor  530  may also increase to the point of turning the transistor on. When transistor  530  is off, amplifier input signal  570  is applied to resistor  540  and a current  580  proportional to the input voltage is used to control the current produced by the output stages of the amplifier as described previously. When transistor  530  turns on, it shunts a portion of the input current through resistor  540  to ground. The reduction in control current  580  may produce a corresponding drop in the current produced by the amplifier output stages. This mechanism may prevent damage to the amplifier resulting from excessive output current.  
         [0049]      FIG. 6  is a flowchart of a method for providing output current limiting in a class G amplifier, according to one embodiment. At  600 , a current corresponding to a fraction of the output current from stage n of the amplifier may be summed with similar fractional currents corresponding to each of the other n−1 output stages of the n-stage class G amplifier. This current is added to a current that is inversely proportional to the amplifier output voltage, Vout, to form the current Isum. As depicted in decision block  610 , Isum is compared to an output current limit, Ilim. If Isum is less than or equal to Ilim, the amplifier output current is within tolerance and no action is necessary. When Isum exceeds Ilim, the current-representation of the input waveform Vin, which is fed to the control section of each output stage may be reduced, as shown at block  620 . Reducing the control current to the output stages may reduce the amplifier output current and eliminate an over-current condition.  
         [0050]     An exemplary application for the disclosed amplifier may be to drive a cooling fan in a personal computer. Efficient utilization of power may be of high importance to designers of personal computers and particularly to those designing portable computers powered from batteries. One well-known method of conserving power in computers is variable speed fan control. This method controls the speed of the cooling fan based on the temperature of the air within the computer, the operational mode of the processor, and/or other operational parameters such that more cooling air is forced through the computer when more heat is being generated by the internal components. If the fan is powered by a fixed-voltage supply capable of operating the fan at its maximum speed, then typically when it is desired to reduce the fan speed, some portion of the supply power is diverted from the fan and wasted as heat. If the fan spends the majority of its time operating at speeds less than maximum, then the efficiency of the fan&#39;s power supply my be less than desired.  
         [0051]     The power supplies of personal computers generally provide several output voltage levels. The processor and associated digital components operate from low voltage levels that decrease as clock speeds increase. Peripheral devices such as disk drives, communications cards, and cooling devices may require power at higher voltages. The availability of a variety of supply voltages may allow for the implementation of a class G amplifier to provide power to a cooling fan. The disclosed class G amplifier may be implemented as an integrated circuit using CMOS technology to power a cooling fan using the several voltages available from the computer power supply. Since each output stage may supply power to the fan until the amplifier output voltage reaches the level of the supply associated with that output stage, as described above, the efficiency of powering the fan may be very high (90% or more) whenever the fan voltage is just below one of the supply voltages.  
         [0052]     Although some examples of the disclosed amplifier have depicted a device with three output stages, it is noted that the same inventive features may be applied to amplifiers with any number of supply voltages and corresponding output stages greater than or equal to two. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.