TECHNICAL FIELD 
     This disclosure relates generally to power amplifiers. More specifically, this disclosure relates to an efficient envelope tracking power supply for radio frequency or other power amplifiers. 
     BACKGROUND 
     Wireless base stations are routinely used to support wireless communications with various wireless devices. As base station technology has evolved, base stations have often required the use of much more complex transmission schemes. These transmission schemes usually employ complex modulation techniques that often require linear power amplification. Linear power amplification is often performed by one or more power amplifiers (PAs). 
     Power amplifiers typically consume a significant amount of power in base stations. For example, in the majority of cases, this may account for more than half of the total power consumed by a base station. This typically increases the cost of operating the base station. Moreover, linear power amplification may require one or more radio frequency (RF) power amplifiers to operate in a “backed off” state. This state decreases a base station&#39;s overall efficiency because RF power amplifiers are much less efficient when in the backed off state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure and its features, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates an example linear-assisted architecture for power amplifier (PA) envelope tracking according to this disclosure; 
         FIGS. 2 through 5  illustrate example embodiments of a linear amplifier used in the architecture of  FIG. 1  according to this disclosure; 
         FIG. 6  illustrates an example method for envelope tracking according to this disclosure; and 
         FIG. 7  illustrates a number of waveforms according to this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 through 7 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any type of suitably arranged device or system. 
       FIG. 1  illustrates an example linear-assisted architecture  100  for power amplifier (PA) envelope tracking according to this disclosure. In the context of a PA, envelope tracking (ET) refers to a process in which a dynamically changing voltage is applied to a PA to ensure that the PA remains in a linear operating region. 
     In the example embodiment shown in  FIG. 1 , the efficiency of a PA  102  is improved using an envelope tracking power supply  120 . The PA  102  can operate in a linear mode, and its supply voltage V dd  is provided by the envelope tracking power supply  120 . The power supply  120  dynamically adjusts the supply voltage V dd  to increase or maximize the efficiency of the PA  102  while maintaining a sufficient linearity of the PA  102  in the linear mode. The supply voltage V dd  can be proportional to the envelope of an input signal coming into the PA  102  with a minimum supply voltage being clamped so that the gain variation of the PA  102  is tolerable. By continuously operating the PA  102  close to its saturation (but not too close to lose linearity), high efficiency can be maintained across all power levels. In the linear-assisted architecture  100  of  FIG. 1 , the power supply  120  is current controlled, DC power is provided to the PA  102  by a switching converter (“switcher”) in the power supply  120 , and AC power of the envelope is provided to the PA  102  by a linear amplifier in the power supply  120 . 
     The PA  102  represents any suitable structure for amplifying an input signal. In this example, the PA  102  represents a radio frequency (RF) power amplifier capable of receiving an input signal RF in  and generating an output signal RF out . The power supply  120  represents any suitable structure for supplying power to a PA and implementing envelope tracking. Note that the embodiment of the envelope tracking power supply  120  shown in  FIG. 1  is for illustration only. Other embodiments of the envelope tracking power supply  120  could be used without departing from the scope of this disclosure. 
     In the example shown in  FIG. 1 , the power supply  120  receives a power supply voltage V dc  and an input voltage V in . The power supply voltage V dc  may represent a voltage received from a power supply. The input voltage V in  may represent a voltage used to control the supply voltage V dd  generated by the power supply  120 . A linear amplifier  104  receives the input voltage V in  as an input and is powered by the power supply voltage V dc . The amplifier  104  amplifies the input voltage V in  and operates to regulate the supply voltage V dd  sent to the PA  102 . The amplifier  104  includes any suitable structure for amplifying an input voltage in a substantially linear manner. 
     An output of the amplifier  104  is coupled to a resistor  106 , which is coupled to a comparator  108 . The resistor  106  represents any suitable resistive structure having any suitable resistance. A voltage drop across the resistor  106  is compared by the comparator  108 , which could determine whether the voltage drop is greater than some threshold value. The comparator  108  represents any suitable structure for comparing inputs. 
     An output of the comparator  108  is coupled to a gate driver  110 , which drives two transistors  112 - 114 . The transistor  112  is coupled to the power supply voltage V dc , and the transistor  114  is coupled to ground. By turning the transistors  112 - 114  on and off, the gate driver  110  can control the voltage provided to an inductor  118 , which generates the supply voltage V dd  for the PA  102 . The inductor  118  is also coupled to the resistor  106 . The gate driver  110  represents any suitable structure for driving one or more transistors. The transistors  112 - 114  represent any suitable switching devices, such as NMOS transistors. The inductor  118  represents any suitable inductive device having any suitable inductance. 
     Although  FIG. 1  illustrates an example linear-assisted architecture  100  for PA envelope tracking, various changes may be made to  FIG. 1 . For example, while described as being used to amplify RF signals, the architecture  100  could be used to amplify any other suitable signals. Also, other embodiments of each circuit within  FIG. 1  could be used without departing from the scope of this disclosure. 
       FIGS. 2 through 5  illustrate example embodiments of a linear amplifier used in the architecture of  FIG. 1  according to this disclosure. In particular,  FIGS. 2 through 4  illustrate example embodiments of the linear amplifier  104  in  FIG. 1 . 
     As shown in  FIG. 2 , an input voltage V in  is supplied to an amplifier  202  and to level shifters  208 - 210 . An amplifier  206  receives a shifted input voltage from the level shifter  210  and outputs a signal to an amplifier  204 . The amplifier  204  receives a shifted input voltage from the level shifter  208  and outputs a signal to the amplifier  202 . The amplifier  202  receives the input voltage V in  and generates a supply voltage V dd . It is understood that the output from amplifier  206  is used as the lower supply rail for the amplifier  204 . The output from amplifier  204  is used as the lower supply rail for the amplifier  202 . 
     The input voltage for each amplifier  204 - 206  is DC shifted by the level shifters  208 - 210 , and the supply voltage V dd  is generated by the amplifier  202 . The amplifier  206  has input (from the level shifter  210  denoted V ls1 ), with supply rails V sup  and V min  that may be governed by the following equations when the supply headroom is ignored:
 
 V   sup =( V   max   −V   min )/ N+V   min   (1)
 
 V   ls,i =( V   max   −V   min )/ N *( N−i )  (2)
 
     Amplifier  204  works similarly to Amplifier  206 . Here, V sup  denotes the supply voltage for the amplifier  206 , i denotes the level, Vmin and Vmax are the min and max levels of V dd , and N refers to the stacked levels of the amplifiers. It is understood that the offsets from level shifters  208 ,  210  are preferably different. If an amplifier output is used as the supply rail, it is considered stacked. For example, V 1  is the output of amplifier  206 , and is being used as the lower supply rail for amplifier  204 . V 2  is the output of amplifier  204 , and is being used as the lower supply rail for amplifier  202 . Note that any number of additional amplifiers may be added to the linear amplifier  200  consistent with  FIG. 2 . 
     In  FIG. 3 , a linear amplifier  300  is similar to the linear amplifier  200  in  FIG. 2 . Here, the linear amplifier  300  includes the three amplifiers  202 - 206  and the two level shifters  208 - 210 . In addition, the linear amplifier  300  includes diodes  302 - 304  and capacitors  306 - 308 . The supply voltage V sup  is coupled to the diodes  302 - 304 . The diode  302  is coupled to the amplifier  202  and the capacitor  306 , and the capacitor  306  is connected to the output of the amplifier  204 . The diode  304  is coupled to the amplifier  204  and the capacitor  308 , and the capacitor  308  is connected to the output of the amplifier  206 . 
     This architecture may provide various benefits depending on the implementation. For example, the architecture in  FIG. 3  may be more efficient as the loss would be effectively 1/N of the single amplifier case if the dominant energy is provided by the amplifier  202 . This is because the supply voltage difference of the amplifier  202  can be 1/N of the full rail V max −V min . Also, the architecture in  FIG. 3  may allow the use of lower breakdown voltage processes, which could have higher bandwidths (due to less capacitance) and smaller areas for the same function. Further, the architecture in  FIG. 3  could deliver more output power with higher output voltages. In addition, the architecture in  FIG. 3  may increase the output power capability of the amplifiers  202 - 206 . 
     In some embodiments, the voltage difference between the two rails of each level may be constant ((V max −V min )/N). It is understood that the voltage difference of the two rails in each level could be set differently with a different clamp voltage in each level during the capacitor charging phase. 
     A high output voltage V dd  can be provided with the use of all three stacked amplifiers  202 - 206 . When a high output voltage V dd  is not present, the output of the amplifier  206  could be near its lower rail (near V min ) because its input is near V min  due to the DC shift performed by the level shifter  210 . 
     It is understood that all amplifiers may be in use during the operation of the circuit. The lower amplifier ( 204  and/or  206 ) may be in a clamp mode (e.g., a capacitor is being charged). The amplifier can be in the amplification mode, i.e., it is the supply for the higher level through the capacitor (instead of the higher level own supply through a diode). 
     In particular embodiments, the sizes of the amplifiers  202 - 206  can be similar. This may be done so that the amplifiers  202 - 206  provide the same peak current in a peak power condition during stacking. Also, in particular embodiments, the diodes  302 - 304  are of a high-voltage type. 
     In the example shown in  FIG. 3 , the floating supplies in the high levels are being generated by the charge pumps. In particular embodiments, the floating supply is generated similarly to a class-H amplifier. The voltage difference between the two supply rails may be constant, which is different from class-H supplies for digital subscriber line (DSL) applications where a lower rail is moved lower when an upper rail is moved higher. The capacitors  306 - 308  are charged most of the time through the diodes  302 - 304 , respectively, using the supply voltage V sup  when the lower rail of each level is low. When the input voltage V in  is higher, the lower rail can be moved upwards to follow the input. During the delivery of low power levels, both capacitors  306 - 308  may be charged. During the delivery of medium power levels, the capacitor  308  may be charged, while the capacitor  306  may be discharged to supply the output voltage. During the delivery of high power levels, the capacitors  306 - 308  may be discharged to supply the output voltage. 
     In  FIG. 4 , a linear amplifier  400  includes a diode  402 , a capacitor  404 , a level shifter  406 , and amplifiers  408 - 410 . This arrangement is similar to that shown in  FIG. 3 , but the arrangement in  FIG. 4  has two levels instead of three. Here, the level shifter  406  generates a voltage V il  provided to the amplifier  410 , which generates a voltage V l  (such as 10-25V). Supply voltage V sup2  is from V sup  through the diode  402  when the capacitor is in the charging phase, and V sup2  is generated by the lower amplifier  410  through the capacitor  404 . The amplifier  408  generates the output voltage V dd  (such as 10-40V). The lower rail for the amplifier  410  could be, for example, 10V. The gain from V in  to V dd  is 1 shown in the waveforms of  FIG. 4 . However, it is understood that the gain could be much higher for a low V in . 
     Instead of using a single power buffer, two levels are stacked up in  FIG. 3 . The power loss from the linear amplifier  400  is effectively cut in half in the first order. This is because the supply voltage of the main amplifier  408  can be reduced to half, while the current consumption can remain the same. Most of the time, the amplifier  408  is the one that delivers the output power. During peak power levels, the capacitor  404  could supply approximately half of the peak power, and the amplifier  410  could supply approximately the other half of the peak power. In particular embodiments, the amplifier  410  delivers the same current as the amplifier  408  in the peak power case, so its chip area can be doubled, but the linear amplifier&#39;s power loss is cut by half. In further embodiments, when the input power is higher, the lower rail is moving upward following the input, the output power is being delivered partly by the lower level amplifier through the capacitor, and partly by the stored energy in the capacitor. 
     When a charge pump is used, it may require peak power occurring infrequently so that capacitors can be replenished before being depleted too much for intolerable voltage drops. This is normally the case for envelope signals with large peak-to-average ratios. 
     In the above description, it has been assumed that all amplifiers have rail-to-rail outputs. This, however, is not necessary. In many situations, a Darlington source-follower can be used for high output currents, so the output voltage swing may be within ±3V of its supply rails. In order to have an output with the required swing, the supply headroom can be increased to accommodate the output, and the DC level shifting offsets can be optimized, as well. 
     The efficiency of a linear amplifier can be further improved with the consideration that its main amplifier output source-follower supplies do not have to be the same as the amplifier&#39;s drivers. This is because the drivers may need a higher voltage for their upper rails and a lower voltage for their lower rail than the output devices. By optimizing the rails of the driver and output devices, the efficiency is further improved. This may be considered a special case of a class-G amplifier. When the output swing is small, the efficiency can be significant. This is particularly true for the multi-level architecture in  FIG. 2  since the full swing is divided over multiple levels. The multi-level architecture of amplifiers permits the operation of the power supply without a full swing. 
       FIG. 5  may be an implementation of the efficient envelope tracking power supply using a class-G driver supply (i.e., the output rails having less headroom than the drivers). In  FIG. 5 , a linear amplifier  500 , a level shifter  506 , and amplifiers  508 - 510  are in the same arrangement as shown in  FIG. 4 . In this example, however, the amplifier  510  includes four power supply inputs, including V sup , V sup +, V min − and V min . The V sup + could represent the V sup  voltage plus an additional voltage (such as 3V). The V min − could represent the V min  voltage minus the additional voltage. 
     The linear amplifier  500  also includes a diode  512  coupled to the supply voltage V sup  and a capacitor  514  coupled to the diode  512 . The linear amplifier  500  can further include a diode  518  coupled to the amplifier  508  and a capacitor  520  coupled across the supply voltages V sup + and V 1 − of the amplifier  508 . The output from the amplifier  510  is fed into a node connected to the amplifier  508  and to the capacitor  514 . In this embodiment, dropout losses can be reduced to a secondary level by reducing headroom. Also shown in  FIG. 5  are inputs for amplifier  508  output device and the amplifier  508  driver referred to as V 1  and V 1 −. V 1 − and V 1  have a relationship such that V 1 −=V 1 −V headroom , where V headroom  could be a voltage similar to 3V. Once the headroom is reduced, the quiescent current loss is dominant over other losses, except for switcher loss. A more complicated implementation with additional levels may be used to remove substantially all of the headroom. 
     In the embodiment shown in  FIG. 5 , the voltages on the driver (upper) rails of the amplifiers  508 - 510  can be generated using one or more charge pump DC/DCs or one or more inductive floating supplies. When the top amplifier driver supply is implemented by a charge pump similar to the main supply, it needs a separate smaller amplifier with a bigger offset. This separate smaller amplifier should be very small and require low power as no significant power is required for the driver rail. 
     Although  FIGS. 2 through 5  illustrate example embodiments of a linear amplifier used in the architecture of  FIG. 1 , various changes may be made to  FIGS. 2 through 5 . For example, other linear amplifiers having any number of levels could be used. 
       FIG. 6  illustrates an example method  600  for envelope tracking according to this disclosure. In step  602 , an input voltage is received. In step  604 , the input voltage is split into two signals. One signal is transmitted to a voltage shift device, and another signal is transmitted to a first amplifier. In step  606 , an output from the voltage shift device is transmitted to a second amplifier. In step  608 , an output from the second amplifier is transmitted to an input of the first amplifier. In step  610 , an output signal is generated using the first amplifier that maintains a PA in a linear mode of operation. 
     Although  FIG. 6  illustrates an example method  600  for envelope tracking, various changes may be made to  FIG. 6 . For example, while shown as a series of steps, various steps in  FIG. 6  could overlap, occur in parallel, or occur multiple times. 
       FIG. 7  is an example of a plurality of waveforms  700  that may be present within the linear-assisted architecture  100  for power amplifier (PA) envelope tracking according to this disclosure. The signal names in the waveforms may be referring to the signals illustrated in  FIG. 4 . As shown in  FIG. 7 , when V in  increases from 25V to 40V, V sup2  increases from 25V to 40V at approximately the same time. In addition, V il /V l  and V dd  also increase at approximately the same time as the increase in Vin and reflect the change in V in . The level shifter  406  in  FIG. 4  has a built-in clamp, as demonstrated from the V il  waveform. With the clamp, the capacitor voltage will be maintained relatively constant independent of the input signal V in . 
     It may be advantageous to set forth definitions of certain words and phrases that have been used within this patent document. The term “couple” and its derivatives refer to any direct or indirect communication between two or more components, whether or not those components are in physical contact with one another. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like. 
     While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.
An apparatus includes an envelope tracking power supply configured to control a power amplifier. The power supply includes a first amplifier configured to receive an input voltage and generate a supply voltage for the power amplifier. The power supply also includes a second amplifier configured to receive a shifted input voltage. An output of the second amplifier is coupled to the first amplifier. The first amplifier is configured to maintain an operational mode of the power amplifier. The power supply could further include a third amplifier. An output of the third amplifier is coupled to an input of the second amplifier, and the third amplifier is configured to receive a second shifted input.