Abstract:
There is disclosed a power supply stage, and a corresponding method, comprising: a plurality of amplifiers for amplifying an input signal, each amplifier receiving a power supply voltage; a common selection means for selecting one of a plurality of power supply voltages in dependence on a reference signal representing a desired power supply voltage; and a plurality of adjusting means, corresponding to the plurality of amplifiers, adapted to generate an adjusted selected power supply voltage for a respective amplifier tracking the reference signal in dependence on the one selected power supply voltage and the reference signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    U.S. patent application Ser. No. 12/988,400, with a 371(c) date of Dec. 17, 2010, is incorporated herein by reference. 
       BACKGROUND TO THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to amplification stages for providing a modulated supply voltage, and particularly to such amplification stages organised to provide higher power than can be achieved in a single stage by using an arrangement known as a ‘corporate structure’ amplifier. 
         [0004]    2. Description of the Related Art 
         [0005]    It is known to provide a modulated power supply for providing a supply voltage to an amplification stage, such as a radio frequency (RF) amplification stage. An example of a particularly advantageous modulated power supply stage can be found in United Kingdom Patent No. 2398648. 
         [0006]    In this advantageous modulated power supply stage there is provided an efficient technique for tracking the supply voltage to an RF amplifier in dependence on the RF input signal to be amplified by the amplifier. A first control loop tracks the envelope of the input signal, representing a desired supply voltage for the amplification stage, and selects one of a plurality of available supply voltages in dependence thereon. A second control loop tracks the envelope of the input signal and the actual output signal, and generates an error signal representing the difference there between. This error signal is combined with the selected supply voltage to provide an adjusted selected supply voltage for the amplification stage. The first control loop is a low frequency loop, and the second control loop is a high frequency loop. 
         [0007]    Such an improved modulated supply stage offers significant advantages and improvements in delivering a highly accurate tracking (or tracked) power supply to an amplifier, with associated efficiency improvements. 
         [0008]    It is also known, in amplifying signals such as RF signals, to split the input signal into portions, and then amplify each portion in a separate power amplifier, before recombining the amplified portions. The use of multiple power amplifiers to amplify a signal in this way enables a more powerful signal to be delivered than could otherwise be delivered. Typically such an amplified signal is delivered to an antenna of an RF transmitter. 
         [0009]    Arrangements in which a signal is split in this way for amplification by multiple parallel amplifiers, before recombining, are known as ‘corporate structures’. 
         [0010]    In the prior art highly accurate tracking power supply arrangement found in United Kingdom Patent No. 2398648, if the power supply stage is adapted for a corporate structure to deliver a power supply to n power amplifiers, then the power of each of the low frequency loop and the high frequency loop must be increased by a factor n. This has a negative effect on performance. The loops must be made physically larger. Layout problems may arise on the circuit board, with longer tracks needed to some power amplifiers. The maximum number of amplifiers which may be combined may also be restricted due to physical constraints of the power amplifiers on the feed network through which the modulated supply is delivered. 
         [0011]    Alternatively a single modulated power supply may be provided for each single amplifier stage. This requires a large amount of replication of circuits. This adds to the cost and size of the implementations, and thus introduces of its own inefficiencies. In this arrangement, to achieve the benefits of an efficient, highly accurate tracking power supply for each single amplifier requires a high component count and associated cost. 
         [0012]    As noted above, in a corporate structure with n amplifiers and n associated modulated supplies, regardless of the specific modulated supply implementations, disadvantages of performance, cost and size may be encountered. 
         [0013]    It is an aim of the invention to provide a technique to enable some or all of the benefits of high accuracy tracking power supplies to be obtained in arrangements in which multiple power amplifiers are implemented in parallel. 
       SUMMARY OF THE INVENTION 
       [0014]    In accordance with the invention there is provided a power supply stage, comprising: a plurality of amplifiers for amplifying an input signal, each amplifier receiving a power supply voltage; a common selection means for selecting one of a plurality of power supply voltages in dependence on a reference signal representing a desired power supply voltage; and a plurality of adjusting means, corresponding to the plurality of amplifiers, adapted to generate an adjusted selected power supply voltage for a respective amplifier tracking the reference signal in dependence on the one selected power supply voltage and the reference signal. 
         [0015]    The common selection means may be a switched voltage supply. The plurality of adjusting means may each comprise an error correction means. Each adjusting means may be adapted to receive the adjusted selected power supply and compare it with the reference signal, to thereby generate an error signal to be added to the selected power supply voltage. 
         [0016]    The power supply stage may further comprise means for providing the reference signal to each adjusting means, which means may provide a plurality of copies of the reference signal for the respective adjusting means. 
         [0017]    The power supply stage may further comprise a plurality of said common selection means each associated with a plurality of amplifiers and a plurality of adjusting means. 
         [0018]    The power supply stage may further comprise: means for distributing the reference signal to the plurality of common selection means; means for distributing the reference signal to the plurality of adjusting means associated with each common selection means; and means for distributing the reference signal distributed to each plurality of adjusting means to the individual adjusting means. 
         [0019]    The power supply stage may further comprise means for controlling the timing of the reference signal at each of the adjusting means such that the adjusted supply voltage for each amplifier is time-aligned to the signal being carried by each amplifier. The signal being carried by each amplifier refers to the signal which each amplifier is amplifying, i.e. it&#39;s input signal. The signal being carried by each amplifier is, in a preferred embodiment, a radio frequency (RF) signal, each amplifier being an RF amplifier. 
         [0020]    The means may be adapted to further control the timing of the reference signal at the common selection means. 
         [0021]    In accordance with the invention there is further provided a method for providing a power supply voltage to each of a plurality of amplifiers arranged to amplify an input signal, comprising: selecting one of a plurality of power supply voltages in dependence on a reference signal representing a desired power supply voltage; and generating a plurality of adjusted selected power supply voltages for the respective plurality of amplifiers, each tracking the reference signal in dependence on the one selected power supply voltage and the reference signal. 
         [0022]    The step of generating a plurality of adjusted selected power supply voltages may comprise receiving the adjusted selected power supply and comparing it with the reference signal, and in dependence thereon generating an error signal to be added to the selected power supply voltage. 
         [0023]    The method may further comprise providing a plurality of copies of the reference signal for generating a plurality of adjusted selected power supply voltages. 
         [0024]    The method may further comprise a plurality of said selecting one of a plurality of power supply voltages steps, and an associated plurality of generating a plurality of adjusted selected power supply voltages steps. 
         [0025]    The method may further comprise: distributing the reference signal for the plurality of selecting step; distributing the reference signal for the plurality of generating steps associated with each selecting step; and distributing the reference signal distributed to each plurality of generating steps to the individual generating steps. 
         [0026]    The method may further comprise controlling the timing of the reference signal for each of the adjusting steps such that the adjusted supply voltage for each amplifier is time-aligned to the input signal for each amplifier. 
         [0027]    An advantage of the invention is that cost and space savings are achieved by separating out the two control loops. The low frequency control loop, which is relatively slow, is separated out from the high frequency control loop. The low frequency control loop is provided as a common control loop for a set of amplifiers. The common control loop can be adapted to be high-powered, to deliver an output to multiple amplifiers, without suffering a loss of performance. An individual high frequency loop is provided for each amplifier of the set. The provision of an individual high frequency loop for each amplifier allows the high accuracy benefits provided by advantageous arrangements to be substantially maintained, without incurring significant overheads in consuming space and an increased component count by replicating circuitry. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0028]    The invention will now be described by way of example with reference to embodiments and the following Figures in which: 
           [0029]      FIG. 1  illustrates a preferred exemplary dual-loop control system; 
           [0030]      FIG. 2  illustrates schematically a dual-loop control system for a multi-stage amplifier in accordance with the principles of the invention; 
           [0031]      FIG. 3  illustrates an implementation of a further part of the dual-loop control system of  FIG. 2  for a multi-stage amplifier in accordance with the principles of the invention; 
           [0032]      FIG. 4  illustrates an implementation of part of the dual-loop control system of  FIG. 5  for a multi-stage amplifier in accordance with the principles of the invention; 
           [0033]      FIG. 5  illustrates schematically a dual-loop control system for a multi-stage amplifier in accordance with the principles of the invention; and 
           [0034]      FIG. 6  illustrates aspects of the timing control of a dual-loop control system for a multi-stage amplifier in accordance with the principles of the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0035]    The invention is now described below by way of example with reference to non-limiting embodiments. 
         [0036]      FIG. 1  illustrates an exemplary dual-loop control system in accordance with the principles set out in United Kingdom Patent No. 2398648. A difference block  102  and a low frequency amplifier  104  define a first path  130 . The first path may also be referred to as a first control path, or a main path. A difference block  106  and a high frequency amplifier  108  define a second path  132 . The second path may also be referred to as a second control path or an error correction path. In general, the second path removes an error from the first path, as will be understood from the following description. 
         [0037]    A summer or combiner  110  is provided to combine the two control paths. The objective of the control system is to provide on an output line  120  a signal which is an accurate replica of an input signal provided on line  112 . The control system preferably provides an output signal on line  120  having a much larger current available than is associated with the input signal on line  112 . Such a system may be advantageously used as a high efficiency modulated or tracking power supply, with a load connected to the output signal line  120 . 
         [0038]    The input signal on line  112  provides a first input to the difference block  102 . The difference block  102  forms an output on line  114  to the low frequency amplifier  104 . The output of the low frequency amplifier  104  on line  116  forms a first input to the combiner  110 , and is also fed back to form a second input to the difference block  102  on line  118  via a scaling block  150 . 
         [0039]    The input signal on line  112  provides a first input to the difference block  106  on line  129  via a delay block  131 . The difference block  106  forms an output on line  124  to provide an input to the high frequency amplifier  108 . The high frequency amplifier  108  provides an output on line  126  which forms a second input to the combiner  110 . The combiner  110  combines the signals on lines  116  and  126  to form the output signal on line  120 . The output signal on line  120  is fed back to form the second input to the difference block  106  on line  122  via a scaling block  152 . 
         [0040]    In an example application where the input signal on line  112  is an envelope derived from a RF signal to be amplified, the signal has a wide frequency spectrum compared to the operating frequency bandwidth of the low frequency amplifier  104 . In this system the low frequency amplifier  104  provides a large portion of the output power delivered on the output signal line  120 , but is incapable of operating at the higher frequency range of the input signal. The high frequency amplifier  108  effectively operates as an error correcting or clean-up loop to provide the missing part of the output signal on line  120 . The error correction or clean-up is provided by summing the signal on line  126  with the signal on line  116  to deliver a desired output signal on line  120 . 
         [0041]    In the arrangement of  FIG. 1 , the high frequency amplifier  108  must be able to operate over almost the full frequency range of the input signal. This creates demands on the dynamic range and fractional bandwidth of the high frequency amplifier  108 , and particularly creates demands on the design of the combiner  110  which must be capable of operating at a very high fractional bandwidth. 
         [0042]    To mitigate these problems, the delay block  131  is preferably provided in accordance with the principles discussed in United Kingdom patent application number 0803711.1. 
         [0043]    The provision of the delay block  131  reduces the low frequency content of the signal provided to the high frequency amplifier  108  as described further below. 
         [0044]    The delay block  131  introduces a delay into the second control path equivalent to the delay through the first control path. In the arrangement of  FIG. 1  a finite delay is introduced by the control loop  130 . The delay block  131  of the arrangement of  FIG. 2  operates as a balancing delay, delaying the signal applied to the first input of the difference block  106  by an amount corresponding to the delay of the first control loop  130 , which delay is present in the signal delivered to the second input of the difference block  106  on line  122 . The balancing delay afforded by the delay block  131  is preferably substantially constant with frequency. 
         [0045]    Thus the provision of the delay block  131  ensures that the difference block  106  provides an output on line  124  which has reduced low frequency signals. 
         [0046]    The cancellation of the low frequency signals in this way means that the high frequency amplifier  108  is not required to amplify those signals, and the combiner  110  is not required to handle those signals on the input line  126 . Thus the removal of the low frequency content in this way allows for signal coupling in the combiner  110  using, for example, a transformer or a capacitor. The use of a transformer for the combiner  110  is a particularly advantageous, and preferable, arrangement. 
         [0047]    Preferably the delay provided by the delay block  131  is a digital delay. A digital delay is preferable as this provides a constant delay at all frequencies. A digital delay is particularly appropriate where the input signal is in digital form. The invention, and embodiments thereof, are not limited to digital delays. The delay can be implemented as an analogue network. 
         [0048]    With reference to  FIG. 2 , there is now illustrated schematically an exemplary dual-loop control system for a multi-stage amplifier arrangement in accordance with the principles of the invention, which utilises the principles of the preferred dual-loop control system of  FIG. 1 . 
         [0049]    It should be noted that for ease of illustration, in the following Figures the scaling blocks  150  and  152  of  FIG. 1  are not illustrated. One skilled in the art will appreciate, however, that such scaling blocks may preferably be provided. 
         [0050]    Referring to  FIG. 2 , the multi-stage amplifier arrangement is generally denoted by reference numeral  210 , and includes a plurality n of RF amplifiers. In  FIG. 2  there is illustrated a first RF amplifier  202   1 , a second RF amplifier  202   2 , and an n th  RF amplifier  202   n . The respective RF amplifiers  202   2  to  202   n  receive an input signal on a respective input  204   1  to  204   n  and generate an output signal on a respective output  206   1  to  206   n . Each of the amplifiers  202   1  to  202   n  of the multi-stage amplifier arrangement  210  receives equivalent input signals on their respective inputs  204   1  to  204   n . The input signals may be provided by a splitter stage. The output signals on outputs  206   1  to  206   n  may be combined in a subsequent combiner stage. 
         [0051]    Each RF amplifier receives, in accordance with the principles of the present invention, a modulated power supply voltage on a respective power supply line  120   1  to  120   n . In the example illustrated, the RF amplifiers  202   1  to  202   n  are further connected to ground. 
         [0052]    In accordance with the principles of the present invention, the multi-stage amplifier arrangement  210  is provided with a single low frequency amplification stage, and n high frequency amplification stages. The single low frequency amplification stage is denoted by reference numeral  234 , and comprises the low frequency amplification stage  130  of  FIG. 1  together with the delay stage  131  of  FIG. 1 . Thus the delay stage  131  of  FIG. 1  is additionally provided as a single, common stage for the multi-stage amplifier arrangement  210 . 
         [0053]    Although referred to as the low frequency amplification stage, it will be appreciated that the stage  234  includes the delay  131  which is not associated with the low frequency amplification. The stage  234  is in general a common stage for the multi-stage amplifier arrangement, but is referred to herein as the low frequency amplification stage in view of its main functional purpose. 
         [0054]    As illustrated in  FIG. 2 , for the multi-stage amplifier arrangement  210  a plurality n of high frequency amplification stages denoted by reference numerals  232   1  to  232   n  are provided. Each of the high frequency amplification stage  232   1  to  232   n  correspond to the high frequency amplification stage  132  of  FIG. 1 . In addition each of the high frequency amplification stages  232   1  to  232   n  includes a combiner equivalent to the combiner  110  of  FIG. 1 . 
         [0055]    Although referred to as high frequency amplification stages, it will be appreciated that the stages  232   1  to  232   n  include the combiners  110   1  to  110   n  which are associated also with the low frequency amplifications outputs. The stages  232   1  to  232   n  are in general dedicated stages for the multi-stage amplifier arrangement, but are referred to herein as high frequency amplification stages in view of their main functional purpose. 
         [0056]    As illustrated in  FIG. 2 , the common low frequency amplification stage  234  receives the input reference signal on line  112 . The common low frequency amplification stage  234  provides the output signal on line  116  in accordance with the arrangement of  FIG. 1 . 
         [0057]    Each of the high frequency amplification stages  232   1  to  232   n  receive two inputs, being the output on line  116  of the low frequency amplification stage  234 , and the output on line  129  which is the delayed version of the input reference signal. As noted above, each of the high frequency amplification stages  232   1  to  232   n  generates a respective modulated supply voltage on lines  120   1  to  120   n  in dependence on the signals on lines  116  and  129 . 
         [0058]    Each of the high frequency amplification stages  232   1  to  232   n  includes a difference block  106   1  to  106   n  respectively corresponding to the difference block  106  of  FIG. 1 , and a high frequency amplifier denoted  108   1  to  108   n  respectively corresponding to the high frequency amplifier  108  of  FIG. 1 . In addition each of the high frequency amplification stages includes a respective combiner denoted  110   1  to  110   n , corresponding to the combiner  110  of  FIG. 1 . The interconnection of the difference blocks, high frequency amplifiers, and combiners of the high frequency amplification stages  232   1  to  232   n  is the same as that as illustrated in  FIG. 1 . 
         [0059]    The low frequency amplification stage  234  operates in combination with each of the high frequency amplification stages  232   1  to  232   n  to provide the same functional effect as  FIG. 1 , to provide a modulated power supply voltage on the respective lines  120   1  to  120   n  for the respective individual RF amplifiers. 
         [0060]    Thus in accordance with the principles of the invention, as set out with reference to  FIG. 2 , a single low frequency amplification stage in combination with multiple high frequency amplification stages allows the advantages of a highly accurate tracking system such as illustrated in  FIG. 1  to be maintained in a corporate structure. 
         [0061]    The single low frequency amplification stage  234  can be made appropriately high powered for delivering signals to multiple low frequency amplification stages. No disadvantage is involved in this, since the low frequency amplification stage is in any event a low-speed stage, due to its characteristics as a switched voltage stage. 
         [0062]    However, by separating out the high frequency amplification stages, they may be maintained as small-sized and fast stages, which allows the advantages of high efficiency, highly accurate tracking to be maintained. 
         [0063]    With reference to  FIG. 3 , there is now illustrated a preferred implementation of the high frequency amplification stage of  FIG. 2 , in accordance with the principles of UK patent application no. 0803821.8. The described preferred implementation is a transformer based power supply which is used for modulating the power supplied to a plurality of power amplifiers in an envelope tracking system. 
         [0064]    Each of the high frequency stages  232   1  to  232   n  is implemented in the same way. 
         [0065]    With reference to  FIG. 3 , a filtered output from the low frequency stage  234  on line  116  is coupled to a first tap  403   n  of a secondary winding  410   n  of a transformer  404   n  via a resistor  418   n  and capacitor  416   n  connected in parallel. A second tap  405   n  of the secondary winding  410   n  is coupled to the amplifier  202   n  on output line  120   n . 
         [0066]    A bypass inductor  420   n has first and second terminals, the first terminal being coupled to the first tap  403   n  of the secondary winding  410   n , and the second terminal being coupled to the second tap  405   n  of the secondary winding  410   n  via a resistor  440   n  representing the resistance of the inductor  420   n . The illustration of the resistor  440   n  is for the purposes of later discussion. 
         [0067]    The delayed version of the input reference signal on line  129  is coupled to a first input of a subtractor  412   n , the subtractor  412   n  having a second input coupled to the second tap  405   n  of the secondary winding  410   n . The output of the subtractor  412   n  is coupled to the input of a correction amplifier  406   n . The output of correction amplifier  406   n  is coupled to the first tap  407   n  of the primary winding  408   n  of the transformer  404   n . The second tap  409   n  of the primary winding  408   n  is coupled to ground. 
         [0068]    In overview, the voltage on line  116  is applied to the first tap  403   n  of the secondary winding and the first terminal of the bypass inductor  420   n . The subtractor  412   n  receives the delayed version of the input reference signal and subtracts the value of the voltage present at the second tap  405   n  of the secondary winding  110 , i.e. the output voltage, to produce a voltage error signal. This voltage error signal is then amplified in the correction amplifier  406   n  and applied to the first tap  407   n  of the primary winding  408   n . The voltage on line  116  and the voltage provided by the correction amplifier  408   n  are then combined by the transformer  404   n  to provide a corrected voltage output at the second tap  405   n  of the secondary winding  410  being the output voltage. 
         [0069]    In the arrangement of  FIG. 3 , the low frequency (switched), or coarse, voltage signal on line  116  is applied to the secondary winding  410   n  of the transformer, and may cause a significant DC current to flow in the secondary winding. This DC current may generate a significant magnetic flux in the transformer core, and may lead to magnetic saturation of the core. 
         [0070]    In order to address this problem, the amplification stage  232   n  is preferably provided with the bypass inductor  420   n . The bypass inductor  420   n  preferably comprises a high power inductor and therefore presents a high impedance to high frequency signals, but a very low impedance to DC current and low frequency signals. Therefore, the bypass inductor presents a low impedance DC current bypass path around the transformer, and a large proportion of the DC current on line  116  will flow through the bypass inductor and not through the transformer. The flux in the transformer core due to the DC current flowing through the transformer windings will be reduced as less DC current flows through the secondary winding. Thus, the susceptibility of the core to magnetic saturation is reduced. This allows a physically smaller transformer, compared with the transformer that would otherwise be required. This is advantageous as a smaller transformer may have an improved high frequency response. 
         [0071]    Each amplification stage  232   1  to  232   n  may thus be implemented using a transformer as a combiner, but with a reduced size. 
         [0072]    The purpose of the capacitor  416   n  and the resistor  418   n , is to further provide for the desirable flow of DC current in the bypass inductor. 
         [0073]    Bypass inductor  420   n  has an associated resistance value, R L , due to the length of wire in the inductor coil, represented by resistor  440   n . This resistance value R L  is normally small. The resistor  418   n  is preferably chosen to have a resistance value R trans  that is greater than R L , preferably much greater. Therefore, DC current output on line  116  will flow preferentially through the low resistance path provided by bypass inductor  420   n , with its inherent low resistance, R L . The ratio of R trans  to R L  then determines the reduction in DC current flowing in the transformer secondary  410   n . 
         [0074]    The resistor  418   n  thus advantageously provides a means for directing additional current through the bypass path rather than through the transformer. 
         [0075]    The capacitor  416   n  provides a low impedance path for AC current flow through the secondary winding  410   n  of the transformer  404   n . By providing a low impedance path for AC currents, excessive dissipation of high frequency signals in resistor  418   n  may be avoided. 
         [0076]    In the advantageous arrangement of  FIG. 3 , as the flux in the transformer is further reduced, the transformer itself may be further reduced in size whilst avoiding magnetic saturation of the core. 
         [0077]    The presence of bypass inductor  420   n  in the amplification stages  232   1  to  232   n  may provide still further benefits. 
         [0078]    Assuming a lossless transformer, the average power delivered on line  120   n  to the amplifier  202   n  is the combination of the average power delivered by (i) a switchable main voltage source of the low frequency amplifier  104  and (ii) the correction amplifier  406   n . However, the instantaneous power delivered to the amplifier  202   n  may not be equal to the sum of the instantaneous power delivered by (i) and (ii). This is because energy is stored in the bypass inductor  420   n  and in the transformer magnetising inductance (not shown) and this energy may be released during periods of high instantaneous output power. 
         [0079]    The majority of the power being delivered to the amplifier  202   n  at any one point in time is provided by the low frequency amplifier  104 . However, during peaks in output power, a significant amount of power is provided by the bypass inductor  420   n . 
         [0080]    The net average power output of the (ideal) bypass inductor must be zero, in order for energy to be conserved. The inductor  420   n  is ‘charged’ during periods of low instantaneous output power, and releases the stored-energy during periods of high instantaneous output power. Advantageously, this significantly reduces the peak power requirement of the correction amplifier  406   n . 
         [0081]    Energy which would otherwise be stored in the magnetising inductance of the transformer  404   n  is instead stored in a physically separate inductor  420   n , which unlike the transformer  404   n , does not need to be optimised for high frequency operation. The stored energy may then be delivered to the amplifier  202   n  during periods of high instantaneous output power, thereby reducing the peak power requirement of the correction amplifier  406   n . 
         [0082]    With reference to  FIG. 4 , there is illustrated the implementation detail of the amplification stage  210  of  Figure 2  in a corporate structure. The modified amplification stage  210  of  FIG. 2  is denoted by reference numeral  460  in  FIG. 4 . Reference numerals used earlier in the application are reused in  FIG. 4  where like elements are illustrated. 
         [0083]    As illustrated in  FIG. 4 , the delayed version of the reference signal is delivered on a line  532 , common to the amplification arrangement  460 . The delayed version of the reference signal on line  532  is provided as an input to a splitter stage  512 , comprising a plurality of n buffers  514   1  to  514   n . Each of the n buffers  514   1  to  514   n  receive the delayed version of the reference signal on a line  532 , and generate a copy of that signal on their respective output lines  515   1  to  515   n . The copies of the delayed version of the reference signal on lines  515   1  to  515   n  provide the first inputs of the difference blocks  106   1  to  106   n  of  FIG. 2  of the high frequency amplification stages  232   1  to  232   n . Thus the signal on line  129  shown in  FIG. 2  is replaced by n versions of that signal on lines  515   1  to  515   n . 
         [0084]    The arrangement of  FIG. 4  is preferred in a corporate structure, where it is necessary to distribute the reference signal not only to one or more low frequency amplification stages, but to one or more sets of high frequency amplification stages. Due to the distribution of the reference signal in a hierarchical manner in a corporate structure it is necessary to split and then buffer the signal in order to ensure it is delivered to each amplification stage  104  or  232  at an appropriate level. 
         [0085]    With reference to  FIG. 5 , there is illustrated an overall schematic of the distribution of the reference signal in a corporate structure containing a plurality m of amplification stages corresponding to the amplification stage  460  of  FIG. 4 . 
         [0086]    Turning now to  FIG. 5 , there is illustrated schematically an implementation of a large corporate structure architecture, comprising multiple amplifiers. In the example of  FIG. 5 , there is disclosed a plurality m of groups of n amplifiers. n may vary for each group. One of the m group of n amplifiers is preferably arranged as illustrated in  FIG. 4 . 
         [0087]    Each group of n corresponds, in structure, to the amplification stage of  FIG. 4 . Thus there is shown m amplification arrangements  460   1  to  460   m , each corresponding to the arrangement  460  of  FIG. 4 . 
         [0088]    The number of amplifiers n, and consequently the number of high frequency amplification stages n, in each of the stages  460   1  to  460   n  may vary. 
         [0089]    In general, with reference to  FIG. 5 , the reference signal on line  112  is provided as an input, and may be denoted the master reference signal, REF MAS . The reference signal on line  112  forms an input to a digital-to-analogue converter  524  to provide a digitised version of the analogue reference signal. The digitised version of the reference signal of the output of the digital-to-analogue converter may be considered to be the digitised master reference signal for the low frequency amplification stages, denoted REF LF     —     MAS , and provides an input to a low frequency splitter/buffer stage  516 . 
         [0090]    The reference signal on line  112  additionally forms an input to a delay  528 , which is equivalent to the delay  131  of  FIGS. 1 and 2 . The delayed reference signal at the output of the delay  528  forms an input to a digital-to-analogue converter  526 , which generates at its output an analogue version of the delayed reference signal, which may be considered to be the analogue master reference signal for the high frequency amplification stages, denoted REF HF     —     MAS . The reference signal REF HF     —     MAS  at the output of the digital-to-analogue converter  526  forms an input to a high frequency splitter/buffer stage  518 . 
         [0091]    The low frequency splitter/buffer stage  516  provides a slave reference signal for the low frequency amplification stages of each of the stages  460   1  to  460   m , denoted REF LF     —SL1    to REF LF     —     SLm . 
         [0092]    The high frequency splitter/buffer stage  518  provides a slave reference signal for the high frequency amplification stages of each of the multi-stage amplification stages  510   1  to  510   m , denoted by REF HF     —     SL1  to REF HF     —     SLm . 
         [0093]    The low frequency splitter/buffer stage  516  comprises a plurality m of buffers denoted  520   1  to  520   m . Each buffer respectively provides an output signal on line  530   1  to  530   m  which provides the reference signal REF LF     —     SL1  to REF LF     —     SLm  for the low frequency amplification stages of the respective amplification stages  510   1  to  510   m . 
         [0094]    The high frequency splitter/buffer  518  comprises a plurality m of buffers denoted  522   1  to  522   m . The buffers  522   1  to  522   m  generate output signals on respective output lines  532   1  to  532   m , which respectively provide the high frequency reference signals REF HF     —     SL1  to REF HF     —     SLm  for each of the amplification stages  460   1  to  460   m . 
         [0095]    Each of the high frequency reference signals received on lines  532   1  to  532   m  by the amplification stages  460   1  to  460   m  is received at a respective buffer stage  512   1  to  512   m . In amplification stages  460   1  to  460   3  of  FIG. 5 , as can be seen with further reference to  FIG. 4 , there is generated for each of the n slave reference signals for the high frequency amplification signals on lines  532  sub-slave signals REF HF     —     SUB     —     SL1  to REF HF     —     SUB     —     SLn . 
         [0096]    The number of amplifiers n provided in any amplification stage  460   m  may be dependent upon the number of times the high frequency reference signal can be replicated by any given buffer stage  512 . This limitation may require the hierarchical generation of the high frequency reference voltage, such as illustrated in  FIG. 5  where an initial split of the high frequency amplification signal takes place in block  518 , and a subsequent split takes place in blocks  512 . 
         [0097]    The factors most likely to drive this, however, are space within an amplifier rack, or a conveniently achievable power from the low frequency switched supply, which determines the number of amplifiers an individual low frequency switched supply can support. 
         [0098]    Thus as is illustrated in  FIG. 5 , the advantageous structure for a multi-stage RF amplification stage as illustrated in  FIG. 4  may be replicated a number of times. If the number of high frequency amplification stages  232   n  reaches a maximum, due to performance limitations, then the overall amplification stage  460  of  FIG. 4  can be replicated as denoted in  FIG. 5 . 
         [0099]    In implementing the principles of the present invention, with a distributed arrangement for the implementation of the high frequency amplification stages, it is necessary to ensure that the timing between the various high frequency amplification stages is controlled, such that synchronisation exists between the application of the signals and that in a subsequent combining stage the signals are combined with time synchronicity. 
         [0100]    It is a requirement to (a) accurately align the RF envelope and drain voltage (supply) signals for each amplifier stage used and (b) accurately time-align all power amplifier stages within the corporate structure amplifier. 
         [0101]    This is now further discussed with reference to  FIG. 6 . 
         [0102]      FIG. 6  illustrates in overall schematic the distribution of the high frequency amplification stages of the envelope path of a corporate structure envelope tracking amplifier. In  FIG. 6  the input reference signal is received on a line  718 . This forms an input to a generation and conversion block  702 . This block represents the processing of the input reference signal in order to convert it from digital-to-analogue form, and to generate the “master” high frequency reference voltage. This also includes the application of any appropriate delay in the envelope path (corresponding to the delay of block  131  in  FIG. 1 ). 
         [0103]    It is also necessary to delay the RF path correspondingly to achieve alignment between the RF envelope and drain (supply) voltage on all power amplifier stages. 
         [0104]    Thereafter the “master” version of the high frequency reference signal is delivered to a splitter  704 , which is equivalent to the splitter/combiner  518  of  FIG. 5 . In general, the splitter  704  operates to output m copies of the master high frequency voltage signal, which are “slave” high frequency reference signals, each of which forms an input to a respective further splitter/buffer  706   1  to  706   m , which correspond to the splitter/buffer stages  512   1  to  512   m  of FIG.  5 . Thereafter, each of the splitter stages  706   1  to  706   m  generates an appropriate number of copies n, which are “sub-slave” high frequency reference signals for the high frequency amplification stages in a multi-stage amplifier. Thus, for example, the splitter stage  706   m  generates n copies of the high frequency reference voltage which provides inputs to the high frequency amplification stage  708   m1  to  708   mn . Similarly splitter  706   2  provides copies of the high frequency reference voltage to high frequency amplification stage  708   21  to  708   2n , and splitter  706   1  provides copies of the high frequency voltage reference to high frequency amplification stage  708   11  to  708   1n . 
         [0105]    As shown in  FIG. 6 , each of the high frequency amplification stage  708  provides a modulated supply voltage to an associated RF amplifier, denoted by reference numerals  709   11  to  709   mn . With reference to  FIG. 2 , this corresponds to the provision of the modulated supplies on lines  120 . 
         [0106]    As noted hereinabove, it is important to ensure that the delivery of the reference signal, or copies of the reference signal, to the high frequency RF amplification stages  708  is time-aligned. 
         [0107]    In  FIG. 6 , there is denoted between the various block elements time periods which represent time delays at various points. There is a time delay of t 0  associated with the generation and conversion block  702 . A further time delay of t 1  represents a delay caused by the transmission from the generation and conversion block  702  to the splitter  704 . There is a respective time delay between the splitter  704  and each of the splitters  706   1  to  706   m , each of which time delays is denoted by the times t 21 , t 22 , and t 2m  respectively. Further there is a time delay between each of the splitters  706   1  to  706   m , and the respective amplifiers  708   11  to  708   1n ,  708   21  to  708   2n , and  708   m1  to  708   mn . 
         [0108]    It will be appreciated by one skilled in the art that the number of RF amplifiers  709  and associated high frequency amplification stages  708  is such that additional hierarchical generation of the high frequency voltages may be required, and there may be additional stages incurring time delays.  FIG. 6  is merely representative of the principle of the delivery of the high frequency reference signal from a master version of such through to the individual high frequency amplification stages of the multi-stage environment. There may be more or less splitting stages required, and thus more or less time delay paths. 
         [0109]    In order to ensure time-alignment of the high frequency amplification stages, it is necessary to ensure that the time delay between the receipt of the reference signal on line  718  and the delivery of the copy of the reference signal to an individual high frequency amplification stage  708  are equal. Thus the time delay from the input of the generation and conversion block  702  to the input to any one of the high frequency amplification stages  708  associated with the amplifiers  709  must be equal. For example, this requires t 0 +t 1 +t 21 +t 31n  to be equal to t 0 +t 1 +t 22 +t 222 . 
         [0110]    Further it is important that the envelope of the RF signal of each RF amplifier  709  is time-aligned with the modulated supply voltage provided by each high frequency amplification stage, such that there is time-alignment between the modulated supply provided to an amplifier and the RF signal being carried by the amplifier. It should be noted that the input signal to each amplifier  709  will be derived from a main input signal being split and distributed, in a similar manner to the reference signal for the high frequency amplification stages in  FIG. 6 . 
         [0111]    It is preferable, though not essential, that the reference signal for any low frequency amplification stage be time-aligned with the high-frequency reference signals and the input signals. Such time-alignment is less critical as the high frequency amplification stage is adapted to remove any error in the signal generated by the low frequency amplification stage. 
         [0112]    In order to meet the time-alignment requirements, it may be necessary to take appropriate action to ensure time periods are time-aligned by introducing delays such as by controlling the length of cables for delivering signals. 
         [0113]    The described preferred embodiments utilising an RF amplifier are not limited to any particular load being driven by such RF amplifier. However it is envisaged that such an RF amplifier will typically drive an antenna. As such, the present invention has particular advantageous uses in the field of communications, including the field of mobile communications. 
         [0114]    The present invention is described herein by way of reference to particular preferred embodiments, and particularly by way of reference to an application in a modulated voltage supply. This description is, however, only illustrative of examples. In particular the invention may be implemented more broadly in control systems. Envisaged, but not limiting, applications include dynamic power supplies or any wide frequency range power supply.