Abstract:
There is disclosed an arrangement comprising: a driver stage connected to receive an input signal and generate a drive signal; a transformer comprising: a first winding of a first side of the transformer, across which winding a voltage signal is developed in dependence on the drive signal; and a second winding of the first side of the transformer, coupled to the first winding, which exhibits across it a voltage signal related to the voltage across the first winding, by swingback; and a first controller for comparing the voltage exhibited in the second winding to a first threshold voltage, and for selecting a first or a second supply voltage for the arrangement in dependence on the comparison.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the provision of a supply voltage to a drive amplifier, the drive amplifier providing a drive signal to a winding of a transformer. The invention is concerned particularly, but not exclusively, to an arrangement in which the drive amplifier is a correction amplifier forming a correction path of a control loop, a transformer being used to combine the output of the correction path with an output of a main path. The invention is further particularly, but not exclusively, concerned with such an arrangement providing a modulated supply voltage to an RF (radio frequency) amplifier, the modulated supply voltage being provided by the combined output of the transformer. 
     2. Background to the Invention 
     Key amplifier characteristics such as load impedance, supply voltage and peak efficiency are determined by an amplifier&#39;s maximum output power requirements. If a power amplifier, such as an RF amplifier, is operated at less than maximum output power its efficiency is reduced. When amplifying a high dynamic range signal, the power amplifier typically achieves maximum output power only rarely and frequently operates at significantly lower power. Hence, the power amplifier may exhibit low overall efficiency. 
     Various techniques are known in the art for enhancing power amplifier efficiency based on the supply voltage. One broad classification of solution is envelope tracking. 
     In a known envelope tracking technique an efficient switched mode supply stage, comprising a variable pulse width modulator, may be combined with a linear correction amplifier. The efficient switched mode supply provides a coarse approximation of the output signal, containing a majority of the required power, and the linear correction amplifier provides a high bandwidth correction signal which is combined with the coarse approximation signal. A modulated power supply with high bandwidth and generally good efficiency is thereby provided. 
     An example of an RF amplification stage incorporating a particularly advantageous technique in accordance with these principles is disclosed in British Patent No. 2398648. 
     In general, in such a control system, a coarse or low frequency path is fed by a control signal which is low-pass filtered and then used to control a switched mode power supply that provides the coarse output. In a correction path, a feedback signal is received from the output. The feedback signal is compared with an ideal reference to produce an error signal. A linear correction amplifier in the correction path provides a high frequency correction output from the error signal. The correction output is combined with the coarse output to provide an error-corrected output. The error-corrected output provides the modulated supply voltage. 
     Overall such an arrangement, particularly when used for providing a modulated power supply, provides high efficiency and high bandwidth amplification simultaneously. 
     However inefficiencies arise in respect of certain components within the linear correction amplifier. The linear correction amplifier must operate over a wide range of potential voltages, up to a possible peak voltage. The linear correction amplifier has to provide a large negative correction voltage during periods of high voltage from the coarse output. Conversely a large positive correction voltage is required during periods of low voltage from the coarse output. If the supply voltage in the linear correction amplifier is fixed, it has to be set to a sufficient value for the amplifier to provide any peak output levels without clipping. This requires the amplifier to have a higher supply voltage than necessary most of the time, and consequently its efficiency is degraded. 
     It is an aim of the invention to address the above-stated problem. 
     In particular it is an aim of the invention to provide a technique in which the voltage drop across the output device of the correction amplifier is reduced. It is an aim of the invention particularly to provide such an improvement in an arrangement in which a transformer is used to combine the outputs of a coarse path and a correction path. 
     SUMMARY OF THE INVENTION 
     In one aspect the invention provides an arrangement comprising: a driver stage connected to receive an input signal and generate a drive signal; a transformer comprising: a first winding of a first side of the transformer, across which winding a voltage signal is developed in dependence on the drive signal; and a second winding of the first side of the transformer, coupled to the first winding, which exhibits across it a voltage signal related to the voltage across the first winding; and a first controller for comparing the voltage exhibited in the second winding to a first threshold voltage, and for selecting a first or a second supply voltage for the arrangement in dependence on the comparison. 
     The voltage exhibited across the second winding related to the voltage across the first winding can be referred to as a swingback voltage. 
     The second supply voltage may be greater than the first supply voltage, and the controller may be adapted to select the first supply voltage if the drive signal voltage is less than the maximum voltage which can be linearly amplified using the first supply voltage, and select the second supply voltage if the drive signal voltage is greater than said maximum voltage. 
     The first winding of the first side of the transformer may be arranged to be connected to the amplified signal at a first end thereof, and to be connected to the selected one of the first or second supply voltage at a second end thereof. 
     The first controller may include a voltage translator for translating the voltage exhibited across the second winding into a modified voltage. 
     The voltage translator may apply a predetermined offset to the voltage exhibited across the second winding to generate the modified voltage. The voltage translator may be a Zener diode. 
     The controller may include a switch for selecting the second supply voltage. The switch may selectively connect the second supply voltage to the second end of the first winding of the first side of the transformer. The switch may be a transistor. 
     The controller may include an isolation circuit for selectively isolating the first winding of the transformer from the first supply voltage. The isolation circuit may isolate the first supply voltage from the other end of the first winding of the transformer when the second supply voltage is selected. The isolation circuit may be a Schottky diode. 
     There may further be provided a winding of a second side of the transformer, wherein one end of the winding of the second side of the transformer is connected to a voltage signal, and the other end provides a sum of said voltage signal and a voltage induced across the second side of the transformer as a result of voltages across the windings of the first side of the transformer. 
     The first controller may comprise: a first Zener diode; and a first transistor, wherein: the cathode of the first Zener diode is connected to receive the voltage signal across the second winding, the anode of the first Zener diode is connected to the gate of the first transistor, and the channel of the first transistor is connected between the second supply voltage and the other end of the first winding. 
     The arrangement may further comprise a first Schottky diode having an anode connected to the first voltage supply and a cathode connected to the first winding. 
     The first threshold voltage may be determined by the breakdown voltage of the Zener diode and the threshold voltage of the transistor. 
     If the swingback voltage is less than the threshold voltage the transistor may be off, and power may be drawn from the first supply voltage. 
     If the voltage exhibited in the second winding is greater than the threshold voltage the transistor may be on, and power may be drawn from the second supply voltage. 
     The second supply voltage may be greater than the first supply voltage. 
     The amplifier may include a transistor arranged to receive the signal to be amplified at a gate thereof, and having its channel connected to one side of the first winding to deliver the amplified signal thereto. 
     The stage may be double-sided, the first winding of the side of the transformer being arranged to receive one side of the amplified signal, and the second winding of the second side of the transformer being arranged to receive the other side of the amplified signal. 
     A third and a fourth supply voltage may be provided for the other side of the amplifier. The third and the fourth supply voltage may be provided by the first and second supply voltages respectively. 
     The first winding of the first side of the transformer may exhibit the voltage signal in the second winding. This is achieved by swingback. 
     The arrangement may further comprise a second controller for comparing the voltage exhibited in the first winding to a second threshold voltage, and for selecting the third or fourth supply voltage in dependence on the comparison. 
     The second threshold voltage may be derived from the first threshold voltage. 
     The second controller may comprise a voltage translator for translating the voltage exhibited across the first winding into a modified voltage. 
     The second controller may include a switch for selecting the fourth supply voltage. 
     The second controller may include an isolation circuit for selectively isolating the second winding of the transformer from the third supply voltage when the fourth supply voltage is connected. 
     The magnitude of the fourth supply voltage may be greater than the magnitude of the third supply voltage. 
     The arrangement may further comprise: a third supply voltage for the amplification stage; and a second controller for comparing the voltage exhibited in the second winding to a second threshold voltage, and for selecting the third supply voltage in dependence on the comparison. 
     The second controller may include a voltage translator for translating the voltage exhibited across the second winding into a second modified voltage. 
     The second controller may include a switch for selecting the third supply voltage. 
     The second, controller may further include an isolation circuit for selectively isolating the first winding of the transformer from the second supply voltage. 
     The second controller may isolate the first winding from the second supply voltage on selection of the third supply voltage. 
     If the voltage exhibited is greater than the second threshold voltage, the third supply voltage is delivered to the amplifier. 
     The third supply voltage may be greater than the second supply voltage. 
     The arrangement may be arranged to combine two inputs signals using the transformer, wherein the drive signal comprises the first input signal, and a second input signal is connected to a side of a winding of a second side of the transformer, the other side of the winding of the second side of the transformer providing a combined output signal. 
     The driver stage may be an amplifier, the driver signal being an amplified version of the input signal. 
     In another aspect the invention provides a method comprising: generating a drive signal; developing a voltage signal across a first winding of a first side of a transformer in dependence on the drive signal; and exhibiting a voltage signal, corresponding to the voltage signal developed across the first winding, across a second winding of the first side of the transformer; comparing the swingback voltage developed in the second winding to a first threshold voltage; and selecting a first or a second supply voltage in dependence on the comparison. 
     The exhibited voltage may be developed by swingback. 
     The second supply voltage may be greater than the first supply voltage, and the selecting step comprises selecting the first supply voltage if the drive signal voltage is less than the maximum voltage which can be linearly amplified using the first supply voltage, and selecting the second supply voltage if the drive signal voltage is greater than said maximum voltage. 
     The method may further comprise connecting one end of the first winding to the amplified voltage signal, and connecting the selected one of the first or second supply voltage at the other end thereof. 
     The method may further comprise translating the swingback voltage generated across the second winding into a modified voltage. 
     The method may further comprise applying a predetermined offset to the exhibited voltage to generate the modified voltage. 
     The method may further comprise selecting the second supply voltage by controlling a switch. The switch may selectively connects the second supply voltage to the other end of the first winding of the first side of the transformer. 
     The method may further comprise selectively isolating the first winding of the transformer from the first supply voltage. 
     The step of isolating may comprise isolating the first supply voltage from the other end of the first winding of the transformer when the second supply voltage is selected. 
     The method may further comprise connecting one end of a second winding of the transformer to a voltage signal, the other end providing a sum of the voltage signal with the drive signal voltage. 
     The method may further comprise providing one side of a double-sided drive signal at one side of the first winding of the first side, and providing the other side of the double-sided drive signal at the second winding of the first side of the transformer. 
     The method may further comprise providing a third and a fourth supply voltage for the other side of the transformer. 
     The method may, further comprise providing a third supply voltage and comparing the exhibited voltage in the second winding to a second threshold voltage, and selecting the third supply voltage in dependence on the comparison. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present invention is now described by way of example with reference to the accompanying Figures, in which: 
         FIG. 1  illustrates a control loop in which embodiments of the invention may be implemented; 
         FIG. 2  illustrates an exemplary implementation of an AC amplifier and transformer-combiner with which embodiments of the invention may be implemented; 
         FIG. 3  illustrates a modification to the AC amplifier and transformer in accordance with a first embodiment of the invention; 
         FIG. 4  illustrates the voltages formed in the arrangement of  FIG. 3  in an exemplary operation; 
         FIG. 5  illustrates a modification to the AC amplifier and transformer in accordance with a second embodiment of the invention; and 
         FIG. 6  illustrates a modification to the AC amplifier and transformer in accordance with a third embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described herein by way of particular examples and specifically with reference to preferred embodiments. It will be understood by one skilled in the art that the invention is not limited to the details of the specific embodiments given herein. In particular the invention is described herein by way of reference to the provision of a power supply for an RF amplification stage. Whilst this represents a particularly advantageous implementation of the principles of the invention, the invention may more generally apply to any arrangement where it is necessary to improve the efficiency of a driver stage providing a drive signal to the winding of a transformer. 
     In the following description with reference to the Figures, where common reference numerals appear between different Figures they refer to the same elements. 
     With reference to  FIG. 1  there is illustrated in schematic form the main elements of a control loop in which preferred embodiments of the invention may be implemented. Such a control loop is suitable for generating a modulated supply voltage, which modulated supply voltage may provide the supply voltage for an RF amplifier. The control system of  FIG. 1  is suitable for use as an envelope tracking power supply for an RF power amplifier. 
     With reference to  FIG. 1  block  100  represents a digital control block for providing control signals. A first control signal is provided on output line  129  to a first signal path  130 , and a second control signal is provided on output line  131  to a second signal path  132 . These control signals on lines  129  and  131  are the appropriate reference signals required by each path. The generation of such reference signals is not described in detail herein. The generation of such signals, by a variety of known techniques, will be within the scope of one skilled in the art. In general, the generated reference signals are representative of a signal to be amplified by the exemplary RF amplifier. The digital control block  100  provides a first control signal on line  129  as an input to a digital-to-analogue converter  102  of the first signal path  130 . The digital control block  100  provides a further control signal on line  131  as an input to a digital-to-analogue converter  104  of the second signal path  132 . 
     In general terms, the first signal path  130  can be termed a coarse signal path, as the path generates a signal (as will be described further hereinbelow) which is a coarse representation of the reference signal on line  129 . The coarse signal path may also be referred to as a low frequency path. Also in general terms, the second signal path  131  can be termed a correction path, as the path generates a signal (as will be described further hereinbelow) which represents an error in the coarse representation generated by the first path, in dependence on the reference signal on line  131  and a signal fed back from the output. The correction signal path may also be referred to as a high frequency path. In the further description below, the first and second signal paths are therefore generally referred to as a coarse signal path and a correction signal path. 
     The control loop of  FIG. 1  further includes a combiner  120 . The combiner receives at a first input the coarse output signal on a line  124  from the coarse signal path. The combiner also receives at a second input the correction output signal on a line  122  from the correction signal path. The combiner  120  combines these two signals to provide an output of the control loop on line  126 , which corresponds to the coarse output signal with an error removed therefrom. 
     The coarse signal path  130  includes the digital-to-analogue converter  102 , a low-pass filter  106 , a switched-mode power supply  110 , and a low-pass filter  116 . The low-pass filter  106  filters the control signal from the digital-to-analogue converter  102  to provide a control signal to the switched-mode power supply  110 . The appropriate switched voltage is generated at the output of the switched-mode power supply  110 , and filtered by the low-pass filter  116 . The output of the low-pass filter  116  comprises the output voltage of the coarse signal path  130  on line  124 , which output voltage is denoted V sw . 
     The output voltage of the coarse signal path, V SW , on line  124  forms a first input to a combiner  120 . 
     The correction signal path  132  includes the digital-to-analogue converter  104 , a low-pass filter  108 , a reference amplifier  112 , a subtractor  114 , and a linear correction amplifier  118 . 
     The signal generated by the digital-to-analogue converter  104  is provided as an input to the low-pass filter  108 . The low-pass filter  108  has a much higher cut-off frequency than the low-pass filter  106 , and operates as a reconstruction filter. The output of the low-pass filter  108  is provided to the reference amplifier  112 . 
     The output of the reference amplifier  112  provides a first input to the subtractor  114 . The second input to the subtractor  114  is provided on a line  117 . The signal on line  117  represents the output signal generated by the combiner  120  on line  126 . The output signal on line  126  is fed back to a scaler  115 , which scales the output signal and provides it on line  117  at its output. 
     The subtractor  114  subtracts the signal representing the output on line  117  from the signal provided by the reference amplifier  112 . The subtractor  114  thus provides at its output an error signal representing the error in the output signal compared to an ideal reference signal. 
     The error signal at the output of the subtractor  114  is amplified by the correction amplifier  118  and delivered to the second input of the combiner  120  on line  122 . The signal on line  122  represents the error voltage V ERROR  (or correction voltage), between the output signal V OUT  and the coarse signal path output voltage V. As mentioned above, the combiner  120  combines the voltage signal V SW  at the output of the coarse signal path with the error voltage, V ERROR  at the output of the correction signal path to generate the output voltage V OUT  on line  126 . 
     As discussed in the background section hereinabove, inefficiencies arise in respect of the correction amplifier  118 . In prior art arrangements the correction amplifier  118 , which is an AC amplifier, is provided with a fixed voltage supply, which fixed voltage supply must be set at a level sufficient to handle all possible peaks. This results in inefficiencies when the voltage being amplified is below such peak. 
     To further understand the invention and how it addresses the above-stated problem, reference is made to  FIG. 2  where there is shown an example implementation of the correction amplifier  118  and combiner  120  of  FIG. 1  in simplified schematic form, where the implementation of the combiner  120  is by way of a transformer. A transformer is an advantageous way to implement the combiner  120 . 
     The example implementation of the correction amplifier and combiner in  FIG. 2  is generally denoted by reference numeral  240 . The correction amplifier  118  is formed of a differential amplifier  230  and an output or driver stage  221 . The combiner  120  is formed of a transformer  256 . 
     The error signal at the output of the subtractor  114  is provided as an input to the differential amplifier  230 . 
     The exemplary differential amplifier stage  230  includes a pair of bipolar transistors  206  and  208  (which could also be implemented as field effect transistors), and resistors  212 ,  214  and  210 . The emitters of the transistors  206  and  208  are connected together and connected to one terminal of a resistor  210 , the other terminal of the resistor  210  being connected to a negative supply voltage. The base of the transistor  206  is connected to the output of the subtractor  114 , and the base of the transistor  208  is connected to a voltage reference selected to suit the bias requirements of transistors  216  and  218 . The collector of the transistor  206  is connected to a supply voltage V S  via the resistor  212 . The collector of the transistor  208  is connected to the supply voltage V S  via the resistor  214 . 
     The differential amplifier  230  provides complementary output signals on lines  217  and  219  to drive the output stage  221  of the correction amplifier  118 . The complementary output signals on lines  217  and  219  are taken from the respective collectors of the transistors  206  and  208  (possibly via buffer amplifiers). 
     The output stage  221  comprises transistors  216  and  218 . Referring further to  FIG. 2 , the output at the collector of the transistor  206  on line  217  is connected to the gate of field effect transistor  216 . The output at the collector of the transistor  208  on line  219  is connected to the gate of field effect transistor  218 . The transistors  216  and  218  could alternatively be implemented as bipolar transistors. As will be described further hereinbelow, the transistor  216  is used to generate positive correction voltages at the output of the correction amplifier, and the transistor  218  is used to generate negative correction voltages at the output of the correction amplifier. 
     The source of the transistor  216  is connected to ground and the drain of the transistor  216  is connected to a tap of a first primary winding  200  of the transformer  256  on line  204 . The other tap of the first primary winding  200  is connected to a high voltage supply V H  denoted by reference numeral  210 . A positive correction to the coarse output voltage signal V SW  on line  124  is generated by transistor  216  when V SW &lt;V OUT . The voltage on line  204  reduces when a positive correction is made. 
     The source of the transistor  218  is connected to ground. The line  202  is connected to a tap of a second winding  203  of the primary side of the transformer  256 . The other tap of the second winding  203  is connected to a voltage supply V L  denoted by reference numeral  208 . A negative correction to the coarse signal V SW  is generated by transistor  218  when V OUT &lt;V SW . The voltage on line  202  reduces when a negative correction is made. 
     The transformer  256  includes the two primary windings, denoted by reference numerals  200  and  203  as referenced hereinabove. As set out above, positive corrections for the coarse signal V SW  are delivered by the first winding  200 , and negative corrections are delivered by the second winding  203 . 
     The secondary side of the transformer  256  includes a single winding  404 . A first tap of the secondary winding  404  is connected to receive the coarse voltage V SW  on line  124  from the coarse signal path. The output voltage, VOUT, including error correction, is then generated at the second tap of the winding  404  on line  126 . 
     In operation, as known in the art, the voltage V SW  is increased or decreased by the voltages generated across the transformer secondary winding  404  from the primary windings  200  and  203  to generate the output voltage V OUT  on line  126 . 
     Each of the supply voltages to the transformer combiner  120 , V H  and V L , must be sized to accommodate any peak (positive or negative respectively) which the output stage  221  of the correction amplifier  118  must handle. In the prior art these supply voltages are therefore set and fixed at peak levels. As noted above, this results in inefficiencies in the operation of the output stage  221  of the correction amplifier  118 , which adversely affects the overall efficiency of the control stage. 
     In accordance with the invention, the combiner stage  120  is modified such that efficiency improvements are obtained by controlling either one or both of the supply voltages to the transformer combiner such that inefficiencies due to the need to accommodate the peak voltage supply is reduced. The control of one or both of the supply voltages to the transformer combiner reduces the power dissipated in the output device of the output stage  221  of the correction amplifier. The voltage across the transistors  216  and  218  is thus reduced. 
     With reference to  FIGS. 3 ,  5  and  6 , first, second and third embodiments of the invention are respectively described. It should be noted that in each of  FIGS. 3 ,  5  and  6  only those elements of previous Figures are shown which are necessary for understanding the principles of the invention. As such the transistors  216  and  218  of the correction amplifier, providing the drive signals to the transformer  256 , are illustrated together with the transformer  256 . Further modifications associated with embodiments of the invention affect only these portions of the control loop. 
     In accordance with the first embodiment of the invention, and with reference to  FIG. 3 , the arrangement of  FIG. 2  is modified to provide a voltage translator (in the exemplary arrangement a Zener diode  304 ), a switch (in the exemplary arrangement a transistor  406 ), and an isolation circuit (in the exemplary arrangement a Schottky diode  302 ). In addition a second high voltage supply, V H2 , is added as denoted by reference numeral  308 . The high voltage supply V H  of  FIG. 2  is denoted in  FIG. 3  as V H1 , a first high voltage supply. 
     Although a Zener diode is illustrated as providing the voltage translator in the example, in general any means of providing a voltage offset may be utilised, which may in general be represented by a voltage source. 
     The cathode of Zener diode  304  is connected to the drain of the transistor  218  at node  202 , and the anode is connected to the gate of transistor  406 , via connection  312 . 
     Although a transistor is illustrated in the example, in general any means for selectively connecting the supply voltage V H2  to the transformer winding  200  may be provided. 
     The drain of the transistor  406  is connected to the second high voltage supply, V H2 . The source of the transistor  406  is connected to a node  306 , being one side, the second tap, of the primary transformer winding  200 . This node  306  is the opposite side of the transformer winding  200  to which the drain of the transistor  216  on line  204  is connected. 
     Although a Schottky diode is illustrated as providing the isolation circuit in the example, in general any means or circuit for isolating the supply voltage V H1  from the transformer winding  200  may be provided. 
     The Schottky diode  302  is connected between the node  306  and the first high voltage supply V H1 . The cathode of the Schottky diode  302  is connected to the node  306 , and the anode of the Schottky diode  302  is connected to the high voltage supply V H1 . 
     In general the voltage translator translates a swingback voltage (explained further hereinbelow) to provide a modified voltage for controlling the switch. In the example, the voltage translator applies a predetermined offset to a swingback voltage to generate the modified voltage. In the specific example, the offset is determined by the characteristics of the Zener diode. 
     The switch provides for the higher voltage to be connected in to provide the supply voltage when the modified voltage reaches a threshold level. In the specific example, the threshold level is defined by the threshold voltage of the transistor  406  implementing the switch. The voltage translator and the switch in combination operate to ensure that the higher voltage is connected in when the swingback voltage reaches a predetermined level. 
     The isolation circuit ensures that the lower supply voltage is isolated when the higher supply voltage is connected in. As these two supply voltages are connected to a common node, the lower supply voltage must be isolated when the higher supply voltage is connected. When the lower supply voltage is connected, the higher supply voltage is effectively isolated by the opening of the switch. 
     For the purposes of describing an example, it is assumed that the values of the first high voltage supply V H1  and the low voltage supply VL are 10 volts, and the value of the second high voltage supply V H2  is 30 volts. In general the second high voltage supply is greater than the first high voltage supply. It is assumed that the transistor  406  has a threshold voltage of 4 volts. It is further assumed that the Zener diode  304  has a breakdown voltage of 4 volts. In general the breakdown voltage of the Zener diode, and the threshold voltage of the transistor  406 , are chosen to suit any given implementation. 
     In general, the transistor  406  may be an enhancement mode FET, a depletion mode FET, or a bipolar transistor. 
     It is an inherent characteristic of the transformer  256  that any pulse appearing in one or other of the first or second primary windings  200  or  203  is reflected in the other. Thus a pulse appearing in the primary winding  200  is reflected in the primary winding  203 , and conversely a pulse appearing in the primary winding  203  is reflected in the primary winding  200 . Thus with the first and second windings of a first side of a transformer being coupled, a voltage signal is exhibited in one of the windings (e.g. the second) which is related to the voltage across the other winding (e.g. the first). This well-known phenomenon is known as “backswing” and in this description is referred to as backswing. 
     By way of example, and with further reference to  FIG. 3  and also  FIG. 4 , an example implementation is described. 
     It is assumed that at time t 1  a voltage of 7 volts is applied across the first winding  200  of the primary side of the transformer such that the voltage at node  204  is 7V less than the voltage at node  210  as illustrated in  FIG. 4(   a ). As a result of backswing, 7 volts is also developed across the second winding  203  of the primary side of the transformer. This results in the voltage at node  202  being 7V greater than the V L  voltage at node  208  as illustrated in  FIG. 7(   b ). 
     As the voltage across the first winding  200  increases, the voltage across the second winding  203  similarly increases, but with opposite phasing. 
     At time t 1 , the voltage at node  306  is V H1  (the voltage at node  210 ) minus one diode voltage drop, being the drop resulting from the presence of Schottky diode  302 . This voltage is denoted by V H1 −V diode  in  FIGS. 3 and 4 . 
     When the voltage at node  202  reaches approximately 8 volts greater than the voltage at node  306  at time t 2 , the transistor  406  having a threshold voltage of 4V begins to turn-on, assuming a Zener diode voltage of 4V. The voltage at node  306  thus begins to rise as illustrated in  FIG. 4(   c ) and starts to track the voltage at node  202  but with a voltage level approximately 8V lower. As shown in  FIG. 4(   c ), the voltage at node  306  rises toward the voltage V H2 . During this time, the voltage at node  204  does not change, as illustrated in  FIG. 4(   a ). However the voltage across the winding  200  increases by virtue of an increase in the voltage at node  306  whilst the voltage at node  204  is unchanged. As the voltage across the winding  200  increases, the voltage across the winding  203  increases. At the winding  203  this results in a continued increase at node  202  whilst the voltage at node  208  remains unchanged. 
     The voltage at node  202  is equal to V L  plus the voltage at node  204  subtracted from the voltage at node  306 . 
     Once the transistor  406  is fully turned-on, the supply current for transistor  216  is provided from the second supply V H2 . 
     The Schottky diode  302  prevents the current from transistor  406  from flowing into the V H1  supply at node  210 . 
     When the voltage across the first winding  200  of the primary side of the transformer drops again below 8V at time t 3 , the transistor  406  will begin to turn-off, and the supply current for the transistor  216  will be sourced from V H1 . 
     As such, and as can be understood from the foregoing example, the backswing effect of the transformer windings can be used to automatically control which of the two supply voltages is used to provide power to the output stage transistor, such that the correction amplifier does not need to be permanently operated from the maximum supply voltage required for distortion free operation. As such for significant portions of time the correction amplifier can operate at a supply voltage which is significantly less than that required to deliver the demanded peak. 
     The transistor  406  and the Zener diode  304  provide a means for switching the second supply voltage to provide the supply voltage in dependence on a threshold voltage determined by the Zener diode breakdown voltage and the transistor threshold voltage. 
     The technique in accordance with the invention provides an additional level of power supply only when demanded by the signal and without significant additional circuitry. If the drive signal voltage is less than the maximum voltage which can be linearly amplified by the amplifier  216  using the first supply voltage, the first supply voltage is used. If the drive signal voltage is greater than the maximum voltage which can be linearly amplified by the amplifier  216  using the first supply voltage, the second supply voltage is used. 
     In  FIG. 3  the principle of the present invention is shown in the context of only one half of the power supply. Specifically the principle of the present invention is used to provide an efficiency improvement in respect of the high voltage power supply. The techniques of the present invention may be used, in a mirror-fashion, to provide efficiency improvements in the low voltage power supply, associated with the transistor  218  of the correction amplifier. 
     The principles of the present invention may be used to provide an improvement in the efficiency on the high voltage side and the low voltage side either alone or in combination. 
     Where the principle of the invention is used on both the high voltage side and the low voltage side, no extra primary side transformer winding is required. Additional circuitry identical to that illustrated in  FIG. 3  may be connected to the transistor  218  in a mirror arrangement. 
     With reference to  FIG. 5 , a second embodiment of the invention is described. The second embodiment relates to a single-sided correction amplifier, where only one polarity of the error voltage is provided to the transformer. In known arrangements, where only a single error voltage is provided to the transformer, only a single primary winding is required. In order to utilise the principles of the present invention, in accordance with the second embodiment the second primary winding  202  is retained, to provide a swingback voltage for the Zener diode  304  and transistor  406 . As the correction amplifier is single-sided in this embodiment, the transistor  218  of the correction amplifier is not required. The principles of the operation of the embodiment shown in FIG.  5  are identical to those of  FIG. 3 . A backswing voltage is induced from the first winding  200  to the second winding  203 , and when the voltage exceeds a certain level the transistor  406  turns-on to provide an increased supply voltage to the combiner. 
     With respect to  FIG. 6  a third embodiment of the invention is now described. The third embodiment of the invention with respect to  FIG. 6  is based on the first embodiment of  FIG. 3 , and further modified. This embodiment illustrates the cascading of multiple voltage supplies, so that an increased voltage supply may be introduced to the AC amplifier in steps as the voltage required to be amplified increases. 
     In accordance with the third embodiment of the invention there is additionally provided a Zener diode  502 , a transistor  506 , and a Schottky diode  512 . The cathode of the Zener diode  502  is connected to the drain of the transistor  218  on line  202 . The anode of the Zener diode  502  and the gate of the transistor  506  are connected together on a line  516 . The drain of the transistor  506  is connected to a third high level supply voltage V H3 , which has a value greater than the value of V H2 . 
     In this example V H3  is assumed to be 50 volts, the Zener diode  502  is assumed to have a breakdown voltage of 21 volts, and the threshold voltage of the transistor  506  is assumed to be 4 volts. 
     The source of the transistor  506  is connected to a point  514  at the drain of the transistor  406 . A Schottky diode is connected between the point  514  and the supply voltage V H2 . The anode of the Schottky diode  512  is connected to the supply source V H2 , and the cathode of the Schottky diode  512  is connected to the point  514 . 
     The Zener diode  304 , the transistor  406 , and the Schottky diode  302  operate in exactly the way described with reference to  FIG. 3  as the voltage across the transformer winding  200  increases above a level of 8 volts. 
     When the voltage at the drain of the transistor  218  reaches 25 volts greater than the voltage at node  514 , the transistor  506  begins to turn-on and the voltage on line  510  begins to rise and starts to track the voltage at node  202  but with a voltage level approximately 25V less. The Schottky diodes  512  and  302  respectively prevent currents supplied by transistors  406  and  506  from flowing into supplies V H1  and V H2 . The supply current to transistor  216 , is provided from additional supply VH 3  only when the demand is present. 
     Each of the embodiments of the invention described with reference to the  FIGS. 3 ,  5  and  6  may be used in combination or alone. The various modifications may be applied to the high voltage or low voltage side, or to both sides. 
     It is an advantage of the invention in its various embodiments that the voltages applied to the correction amplifier for supply purposes are generally lowered, and current is only drawn from the higher voltage supplies where needed. 
     The invention may particularly advantageously be applied in a control system for an envelope tracking system, such as an envelope tracking power supply. In particular the invention may advantageously be applied for providing a modulated power supply in an RF amplification system. 
     The present invention has been described herein by way of reference to particular preferred embodiments. However the invention is not limited to such embodiments. The present invention has particular application in relation to RF amplifiers, but is not limited to such implementation. The invention can be advantageously utilised in any environment in which a transformer winding is driven by an amplifier. 
     One skilled in the art will appreciate the various modifications and adaptations to the invention and the embodiments described herein are possible within the scope of the invention as defined by the appended claims.