[0001]    Great Britain Patent Application No. 0920869.5, filed on Nov. 27, 2009, is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention is directed to a modulated supply stage comprising a switched supply and a correction stage for correcting an error in the switched supply to generate a modulated supply voltage. The invention is particularly, but not exclusively, concerned with the provision of a modulated supply voltage to an RF amplifier. 
       BACKGROUND OF THE INVENTION 
       [0003]    United Kingdom Patent No. 2398648 describes a particularly advantageous modulated supply stage comprising a switched supply stage and an error correction stage. The switched supply stage selects one of a plurality of supply voltages in dependence on a reference signal representing, in the preferred implementation, a signal to be amplified by an RF (radio frequency) amplifier. The error correction stage comprises an error correction amplifier and provides an error correction voltage for a correction of an error in a switched supply voltage, to deliver a more accurate supply voltage to the RF amplifier. 
         [0004]    The error correction stage is provided to enable fast correction of the switched supply stage. There is a trade-off between output power and bandwidth. An increase in output power tends to result in a reduction in bandwidth due to increased parasitic elements of the higher power correction amplifier and combining components. 
         [0005]    Further, when a transformer is advantageously used to combine the switched supply voltage and the error correction voltage, the increase in transformer size necessary for increased power handling results in increased leakage inductance and inter-winding capacitance, and a consequent loss of bandwidth. 
         [0006]    It is an aim of the invention to provide an improvement to the advantageous modulated power supply described in United Kingdom Patent No. 2398648, and particularly to provide improvements in the error correction stage. 
       SUMMARY OF THE INVENTION 
       [0007]    In accordance with the invention there is provided a power supply stage comprising: a means for generating an intermediate supply signal in dependence on a reference signal representing a desired power supply; and a plurality of adjusting means, each adapted to generate an adjusted supply signal tracking the reference signal, in dependence on the generated intermediate supply signal and the reference signal. 
         [0008]    There may be further provided a current summing means for summing the plurality of generated adjusted supply signals to provide an output power supply voltage with higher output current capability. The higher output current capability is preferably a current capability which is higher than the current capability of any individual adjusted supply signal 
         [0009]    The output power supply stage may be provided as a power supply signal to an RF amplifier. 
         [0010]    Each adjusted supply signal may be provided as a power supply signal to an RF amplifier stage comprising a corresponding plurality of RF amplifiers, each adjusted supply signal providing a power supply for one RF amplifier. 
         [0011]    The means for generating a supply signal may be adapted to select one of a plurality of power supply voltages in dependence on the reference signal. 
         [0012]    There may be provided a plurality of combining means for combining the supply signal with each of a plurality of correction signals in order to generate the plurality of adjusted supply signals. Each adjusting means may include a correction amplifier for generating a correction signal, wherein each correction amplifier receives as an input a signal representing a difference between the reference signal and the sum of the adjusted supply signals. Each combining means may comprise a transformer. The supply signal may be connected to a tap of a primary winding of each transformer, and a respective correction signal is connected to a tap of a secondary winding of each transformer, an adjusted supply signal being formed on another tap of the secondary winding of each transformer. 
         [0013]    The summing means may comprise a connection between the taps of the transformers on which the adjusted supply signals are formed. 
         [0014]    The intermediate supply signal may be a voltage supply signal and the adjusted supply signal is a voltage supply signal. 
         [0015]    In accordance with the invention there is provided a method of generating an output supply signal comprising: generating an intermediate supply signal in dependence on a reference signal representing a desired power supply; and generating a plurality of adjusted supply signals tracking the reference signal in dependence on the intermediate supply signal and the reference signal. 
         [0016]    The method may further comprising the step of providing the plurality of adjusted supply voltages as power supply signals to a corresponding plurality of parallel connected amplifiers. 
         [0017]    The method may further comprise the step of summing the plurality of adjusted supply signals to provide an output power supply voltage. 
         [0018]    The method may further comprise the step of providing the summed adjusted supply voltages as a power supply signal to an amplifier. 
         [0019]    The step of generating an intermediate supply signal may comprise selecting one of a plurality of power supply voltages in dependence on the reference signal. 
         [0020]    The method may further comprise the step of correction signals representing an error between the reference signal and an output signal, generating a plurality of amplified versions of the reference signal, and combining each amplified version with the intermediate supply signal to generate the plurality of adjusted supply signals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention will now be described with reference to the accompanying drawings in which: 
           [0022]      FIG. 1  illustrates an exemplary modulated power supply stage incorporating a correction stage including parallel correction amplifiers in accordance with the invention; 
           [0023]      FIG. 2  illustrates an example implementation of the correction stage of  FIG. 1  for delivering a power supply voltage using transformer combiners; and 
           [0024]      FIG. 3  illustrates an example implementation of the correction stage of  FIG. 1  for delivering power supply voltages to parallel RF amplification stages 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The present invention is now described by way of example with reference to exemplary embodiments. One skilled in the art will appreciate that embodiments are described for ease of understanding the invention, and the invention is not limited to details of any embodiment described. The scope of the invention is defined by the appended claims. 
         [0026]    In the following description where the same reference numerals are used in different Figures, they denote an element in one Figure which corresponds to an element in another Figure. 
         [0027]      FIG. 1  illustrates an exemplary embodiment of a modulated power supply in accordance with the principles of the invention. The exemplary embodiment is based on the modulated power supply architecture disclosed in United Kingdom Patent No. 2398648. In general the exemplary modulated power supply, generally denoted by reference numeral  100 , generates a coarse supply voltage from a switched supply stage. A correction stage, including a correction amplifier, provides high bandwidth correction for the coarse supply voltage, such that the output voltage of the modulated power supply is a corrected voltage. 
         [0028]    In accordance with the principles of the invention, as will be discussed further hereinbelow, the correction amplifier of the correction stage is implemented as two or more parallel amplifiers. This allows for improvements in the operation and implementation of the modulated power supply stage, and also improvements in the operation and implementation of the overall architecture in which the modulated power supply stage is implemented. 
         [0029]    With further reference to  FIG. 1 , the modulated power supply stage  100  includes a difference block  102 , a switched supply stage  104  and a scale block  150 , which in general can be considered a coarse supply stage. 
         [0030]    The difference block  102  receives as a first input a reference signal REF on an input line  112 . The reference signal REF is representative of a signal to be amplified by an RF amplifier for which the modulated power supply stage is to generate a supply voltage. The reference signal REF may, for example, be a signal representing the envelope of a signal to be amplified by the RF amplifier. 
         [0031]    An output of the difference block  102  forms an input to the switched supply stage  104 . As known in the art, the switched supply stage is adapted to generate at its output a voltage generated from one of a plurality of fixed voltage levels, in dependence on the signal at its input. The invention is not, however, limited to the use of a switched supply. 
         [0032]    The output of the switched supply stage  104  on line  116  is a switched voltage supply V SW . The switched voltage supply V SW  forms an input to a scale block  150  which scales the signal before providing it as a second input to the difference block  102 . Thus the difference block  102  generates an output to the switched supply which represents an error between the (ideal) reference signal and the actual signal at the output of the switched supply. In dependence on this error signal, the switched supply  104  switches the supply signal at its output. 
         [0033]      FIG. 1  does not represent a complete implementation of a coarse supply stage, not does it necessarily represent essential elements of a coarse supply stage. The difference block  102 , switched supply stage  104  and scale block  150  are illustrative of an exemplary implementation, as known in the art, and which may also require the inclusion of additional functional elements. For example, an output filter may be required at the output of the switched supply stage  104 . The invention is not concerned with the implementation of the coarse supply stage, and one skilled in the art will understand that alternatives, modifications or enhancements to the illustrated coarse supply stage are possible. 
         [0034]    With further reference to  FIG. 1 , the modulated power supply stage  100  further includes a difference block  106 , a correction stage  160  (described in further detail below), a current combiner stage  178 , and a scale block  152 , which in general can be considered an error correction path. 
         [0035]    The difference block  106  receives as a first input a delayed version of the reference signal REF, denoted REFDELAY, on line  129 . The delayed reference signal REFDELAY on line  129  is generated by a delay block  131  which receives as its input the reference signal on line  112 . 
         [0036]    The output of the difference block  106  forms an input to the correction stage  160 , which in accordance with the principles of the invention comprises two or more parallel connected correction amplifiers. In the example illustrated, for clarity and simplicity, two parallel correction amplifiers are shown. However in general it will be understood by one skilled in the art that the principles of the invention extend to any number of parallel correction amplifiers n. 
         [0037]    The high frequency bandwidth of each of the n amplifiers is greater than the bandwidth of a single amplifier having n times the power handling capabilities. 
         [0038]    The exemplary correction stage  160  illustratively comprises a first correction amplifier  162 , a second correction amplifier  164 , a first combiner  170  and a second combiner  172 . 
         [0039]    Each of the first and second correction amplifiers  162  and  164  receive as an input the output of the difference block  106 . Each of the first and second correction amplifiers generates an output on lines  166  and  168  respectively, which form first inputs to combiners  170  and  172  respectively. In general, for n parallel correction amplifiers, there is provided n combiners. A second input for each of the combiners  170  and  172  is provided by the switched voltage supply V SW  on line  116 . Each combiner  170  and  172  thus combines the output of a respective correction amplifier  170  and  172  with the switched supply voltage V SW  to provide identical corrected supply voltages on output lines  174  and  176  at the outputs of the combiners  170  and  172 . The correction stage  160  thus provides a plurality of identical corrected switched supply voltages at its outputs. In general, the correction stage  160  provides n corrected switched supply voltages. 
         [0040]    It should be understood that whilst the correction stage generates a plurality of identical corrected supply voltages, this represents an ideal scenario. In practice the plurality of corrected supply voltage may not be identical due to component tolerances or operating conditions for example. The corrected supply voltages can therefore be understood to be substantially identical. 
         [0041]    In general, it can be considered that the correction stage provides a plurality of adjusting means each adapted to generate an adjusted selected power supply voltage tracking the reference signal in dependence on the power supply signal from the switched supply and the reference signal. 
         [0042]    In a first embodiment, a combiner stage  178  is provided as shown in  FIG. 1 , which receives as inputs the corrected switched supply voltages on lines  174  and  176 . The identical corrected switched supply voltages form respective inputs to a current combiner  180  of the combiner stage  178 . The current combiner  180  combines the corrected switched supply voltages to provide as an output a high power corrected switched supply voltage V OUT  on line  120 . In general the combiner stage  178  combines the current from n corrected switched supply voltages to provide a single output voltage with high current capability. 
         [0043]    The output voltage V OUT  on line  120  is fed back through the scale block  152  to provide a second input to the difference block  106 . Thus the correction amplifiers of the correction stage operate to amplify the error in the output voltage compared with the delayed (ideal) reference signal. 
         [0044]    The provision of two or more parallel amplifiers in the correction stage allows for a higher output power without a high frequency bandwidth penalty. To produce the same output power with a single correction amplifier, there would be a high frequency bandwidth penalty. 
         [0045]    By the same principle, the provision of two or more parallel amplifiers in the correction stage allows for an extended high frequency bandwidth for the same power. In general, the benefit may be a mix of increased bandwidth and power. 
         [0046]    Thus, it is possible (i) to increase power without reducing the high frequency bandwidth, or (ii) increase the high frequency bandwidth without reducing power, or (iii) a combination of both. 
         [0047]    In a preferred arrangement, the combiners  170  and  172  are implemented as transformers. The invention has particular advantages when the combiners are implemented as transformers. Each transformer may be made smaller and have increased high frequency bandwidth than if a single high power correction amplifier were used with a single higher power transformer-combiner. 
         [0048]    An exemplary implementation using transformers for the combiners  170  and  172 , and showing how in such an implementation the function of the combiner  180  may be implemented, is described with reference to  FIG. 2 . 
         [0049]    The output of the correction amplifier  162  on line  166  is provided as an input to a first tap of a first winding of a transformer  202 . The second tap of the first winding of the transformer  202  is connected to electrical ground. A first tap of a second winding of the transformer  202  is connected to the switched supply voltage V SW  on line  116 . The second tap of the second winding of the transformer  202  is connected to the output signal line  174 . The provision and connection of the transformer in this way results in the corrected switched supply voltage being generated at the second tap of the second winding, and thus on line  174 . 
         [0050]    The output of the correction amplifier  164  on line  168  is provided as an input to a first tap of a first winding of a transformer  204 . The second tap of the first winding of the transformer  204  is connected to electrical ground. A first tap of a second winding of the transformer  204  is connected to the switched supply voltage V SW  on line  116 . The second tap of the second winding of the transformer  204  is connected to the output signal line  176 . The provision and connection of the transformer in this way results in the corrected switched supply voltage being generated at the second tap of the second winding, and thus on line  176 . 
         [0051]    In this way the correction stage  160  generates the two identical corrected switched supply voltages on lines  174  and  176 . 
         [0052]    The exemplary combining stage  178  of  FIG. 2 , which receives the corrected switched supply voltages, combines the current from the corrected switched supply voltages by electrically connecting the outputs of the transformers. Thus it can be seen that the two output lines are connected together, and the corrected switched supply voltage output V OUT  is provided on line  120 . This principle as illustrated in  FIG. 2  extends to n transformers, and the electrical connection of n transformer outputs to provide an overall output. 
         [0053]    For completeness, in  FIG. 2 , there is illustrated an RF amplifier  210  to which the modulated supply voltage stage may be arranged to provide a supply voltage. The supply voltage is provided by the corrected switched supply voltage on line  120 . The RF amplifier, illustratively, amplifies an input signal on line  212  and provides an amplified output on line  214 . 
         [0054]    Each correction amplifier  162  and  164  (and associated transformer windings) may be a push-pull arrangement fed from two halves of a supply rail. However for the purposes of illustration and simplicity, the invention is described in  FIG. 2  on the basis of a single supply rail. 
         [0055]    In accordance with the general benefits of the invention as mentioned above, the arrangement of  FIG. 2 , where respective transformer output signals on lines  174  and  176  are combined to feed a single RF amplifier, allows increased bandwidth for a given power level to be achieved due to reduced parasitic elements (the leakage inductance of the transformer and inter-winding capacitance), since the transformer can be made physically smaller. Alternatively, it is possible to provide more power without degrading bandwidth. To achieve this, n transformers (driven by n correction amplifiers) of the same size and bandwidth as a single stage design can be used in parallel to feed a single high-powered RF amplifier. 
         [0056]    However, as known in the art, there is a limit as to how much power a single RF amplifier can handle, because of limitations associated with a transistor on which the RF amplifier is based. For this reason, it is known in the art of high power amplification to split the RF amplifier into multiple stages, and provide two or more RF amplifiers in parallel, connected to amplify the same input signal and have their outputs combined. Such a parallel amplifier arrangement can be advantageously combined with the present invention. 
         [0057]    In a second embodiment, the arrangement of  FIG. 1  and described above is modified. This is illustrated with reference to  FIG. 3 . Only the portions of  FIG. 1  necessary to understand the modification are illustrated in  FIG. 3 . 
         [0058]    With reference to  FIG. 3 , the combining stage  178  of  FIG. 1  is dispensed with. The RF amplifier to which the modulated power supply stage provides a power supply voltage is modified, in accordance with known principles, as two-stage parallel RF amplifiers  302  and  304 . The two RF amplifiers  302  and  304  are connected to amplify the same input signal on line  306 , and combine (by means not shown but within the scope of a skilled person) their outputs onto an output line  308 . 
         [0059]    The two identical error corrected switched supply voltages on lines  174  and  176  are respectively connected to the power supply terminals of the RF amplifiers  302  and  304 . Thus in this second embodiment the corrected supply voltages from the two correction amplifiers are not combined, but delivered directly to respective RF amplifiers. In general, n corrected supply voltages may deliver supply voltages to n parallel RF amplifiers. Further modifications may be possible, e.g. with one or more sub-sets of the n corrected supply voltages being combined for delivery to one of the plurality of RF amplifiers. 
         [0060]    In an arrangement in accordance with the second embodiment as shown in  FIG. 3 , it is necessary for the individual corrected voltage signals from the n correction amplifiers to be combined in order to provide the signal for the feedback path to provide the second input to the difference block  106 . Thus there is illustrated in  FIG. 3  a voltage combiner  310  which receives as inputs the corrected voltage signals on lines  174  and  176 , and provides a combined signal to the scale block  152  (referring to the example of  FIG. 1 ). 
         [0061]    In the exemplary arrangement of  FIG. 3  the RF amplifiers  302  and  304  are preferably identical. 
         [0062]    A further advantage can be achieved by the arrangement of the second embodiment as illustrated in  FIG. 3 . It is desirable for the path length between the output of the voltage supply modulator and the supply input to the RF transistors to be as short as possible. By using a distributed correction amplifier, as described herein, the path length to the RF amplifier can be kept shorter than if a single high power correction amplifier were used. This leads to improved high frequency performance. 
         [0063]    The reduction in path length can be achieved by controlling where the physical outputs and physical inputs of the devices are located, for example, in an arrangement where the modulated power supply and the amplification stage are provided on separate ICs. 
         [0064]    The invention has been described herein by way of reference to particular examples and embodiments, for the purposes of illustrating the invention and its embodiments. The invention is not limited to the specifics of any embodiment descried herein. Any feature of any embodiment may be implemented in combination with features of other embodiments, no embodiment being exclusive. The scope of the invention is defined by the appended claims.
A power supply stage and a method of controlling the same comprising: a means for generating an intermediate supply signal in dependence on a reference signal representing a desired power supply; and a plurality of adjusting means, each adapted to generate an adjusted supply signal tracking the reference signal, in dependence on the generated intermediate supply signal and the reference signal.