Patent Abstract:
There is disclosed a method of generating a supply voltage ( 230 ) for a power amplifier ( 114 ) arranged to amplify an input signal ( 228 ), comprising: generating ( 104 ) a target supply voltage tracking the input signal ( 228 ); predistorting ( 400 ) the target supply voltage to compensate for effects on a supply voltage in the amplifier ( 114 ); and generating ( 102 ) the supply voltage for the amplifier ( 114 ) in dependence on the predistorted target supply voltage ( 414 ).

Full Description:
BACKGROUND TO THE INVENTION 
     1. Field of the Invention 
     The present invention relates to envelope tracking power supplies, and particularly but not exclusively to envelope tracking power supplies for radio frequency (RF) amplifiers. 
     2. Description of the Related Art 
     Envelope tracking power supplies are well-known in the art. The principle behind such power supplies is that the supply voltage delivered to an amplifier tracks the input signal to be amplified by the amplifier, such that the supply voltage is at a level sufficient to amplify the instantaneous input signal. In this way, the power supply to the power amplifier does not need to be maintained at a level which corresponds to the peak input signal, but can track the input signal to provide an efficient supply voltage which improves the overall efficiency of the amplifier. A particularly advantageous technique for providing an envelope tracking voltage supply is disclosed by Nujira Limited in UK Patent No. 2398648. 
     Power amplifiers, such as RF power amplifiers, require a network through which the DC supply voltage is fed to the active element of the amplifier. In a simple arrangement this may comprise an inductor, choke, or other passive network, which is designed to provide a low impedance at DC but which represents a high impedance at the RF operating frequency. Alternatively, the supply feed network may also form part of the RF matching network at the amplifier output. 
     Prior art techniques implemented in efficient envelope tracking power supplies are adapted to minimise the misalignment of the power supply delivered to the amplifier and the input signal to be amplified, with the intention that the instantaneous voltage supply is based on the instantaneous input signal to be amplified. 
     However, the presence of the supply feed network causes errors in the supply voltage presented to the active device of the amplifier itself. Thus whilst the voltage supply delivered to a terminal or node of the amplifier may be correctly aligned with the input signal to be amplified, the supply feed network results in this signal being misaligned at the point it is delivered to the active device of the amplifier. These errors become more pronounced as the supply feed inductance increases. At each instant in time, the active device is thus not working at its intended operating point and this degrades the RF output spectrum and other metrics of transmission quality. 
     It is an aim of the invention to provide an improvement in an envelope tracking power supply, which addresses the above-stated problem. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a technique for pre-distorting the supply voltage fed to the RF amplifier, so that the active device itself of the RF amplifier receives the correctly aligned supply voltage at each time instant. This ensures the RF amplifier always operates with the correct degree of compression which, for mobile handset implementations in particular, gives constant RF gain irrespective of RF output power. 
     Simulations and measurements have shown the invention gives a beneficial improvement in RF output spectrum. 
     In one aspect the invention provides a method of generating a supply voltage for a power amplifier arranged to amplify an input signal, comprising: generating a target supply voltage tracking the input signal; pre-distorting the target supply voltage to compensate for effects on a supply voltage in the amplifier; and generating the supply voltage for the amplifier in dependence on the pre-distorted target supply voltage. 
     The step of pre-distorting may comprise applying the inverse effect of the effect on the supply voltage in the amplifier. 
     The step of pre-distorting may compensate for the effects applied to the supply voltage between an input terminal of the amplifier and a node of an active element of the amplifier to which the modulated supply voltage is applied. 
     The step of pre-distorting may compensate for the effects of a supply feed inductance. 
     The step of pre-distorting may compensate for the effects of at least one capacitor in the supply feed network of the amplifier. 
     The step of pre-distorting may compensate for shunt capacitance presented by the active element of the amplifier. 
     The step of pre-distorting may include estimating the current flowing in the active element of the amplifier. 
     The step of estimating the current flowing in the active element of the amplifier may comprise modelling the active element as a variable resistor, and mapping a non-linear relationship between resistor values for the variable resistor and the supply voltage at the active element, and estimating the current by dividing the target supply voltage by the resistor value associated with a supply voltage at the active element corresponding to that target supply voltage. 
     The step of generating the target supply voltage may comprise generating an envelope signal representing the envelope of the signal to be amplified. 
     The step of generating the target supply voltage may comprise shaping the generated envelope signal. Shaping may be a non-linear mapping between the RF signal envelope or power, and the power amplifier voltage. 
     The step of generating the target supply voltage may comprise modulating the generated envelope signal. 
     The step of generating the supply voltage may comprise modulating the pre-distorted target supply voltage. 
     In another aspect the supply voltage stage for a power amplifier arranged to amplify an input signal, comprising: a tracking voltage generator for generating a target supply voltage tracking the input signal; a pre-distortion stage for pre-distorting the target supply voltage to compensate for effects on a supply voltage in the amplifier; and a supply voltage generator for generating the supply voltage for the amplifier in dependence on the pre-distorted target supply voltage. 
     The supply voltage for the amplifier may be connected at a supply voltage terminal of the amplifier, the pre-distortion stage being configured to compensate for any effect on the supply voltage in the amplifier path between the supply voltage terminal and an active element of the amplifier which provides the amplification of the input signal. 
     The active element may be a transistor, and the amplifier path is between the supply voltage terminal of the amplifier and a supply mode of the transistor. 
     The amplifier path may attenuate or delay the supply voltage delivered to the active element. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention is now described by way of example with reference to the accompanying figures, in which: 
         FIG. 1  illustrates an exemplary implementation of an envelope tracking power supply for a power amplifier in accordance with the prior art; 
         FIGS. 2   a  and  2   b  illustrate exemplary simplified models of an RF amplifier output; 
         FIGS. 3   a  and  3   b  illustrate exemplary output models of a transistor of a power amplifier; 
         FIG. 4  illustrates a modification to an envelope tracked power supply in accordance with a first embodiment of the invention; 
         FIG. 5  illustrates a modification to an envelope tracked power supply in accordance with a second embodiment of the invention; and 
         FIG. 6  illustrates a mapping function in accordance with an implementation of a preferred embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The 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 an RF amplification stage. However more generally the invention may apply to any arrangement where it is necessary to provide a tracked power supply for an amplification stage. The invention is described herein as particularly advantageous when applied to an RF amplification stage of a portable device, such as a portable device including telephone functionality, such as a mobile telephone handset. 
     With reference to  FIG. 1 , there is illustrated an exemplary arrangement of a power amplifier provided with an envelope tracking power supply. The arrangement  100  includes a baseband system  108 , an envelope detector  106 , a shaping table or function  104 , an envelope tracking modulator  102 , a baseband converter  110 , one or more pre-amplifier stages  112 , and an output envelope tracking power amplifier  114 . 
     The baseband system  108  generates a complex I-Q source waveform on a line  118 . This waveform is provided as an input to the baseband converter  110 . The baseband converter  110  converts the signal provided by the baseband system  108  to a signal for amplification by the RF amplifier. The baseband converter may comprise an RF modulator, an RF up-converter, or a vector modulator for example. The baseband converter modulates the baseband I-Q waveforms onto an RF carrier as known in the art. The output of the baseband converter  110  is provided as an input to the one or more pre-amplifier stages  112 , which boost the RF signal to the required transmit output power. The output of the amplification stages  112  on line  228  forms an input to the power amplifier  114 , which itself comprises one or more power amplification stages. 
     The power supply to the power amplifier  114  is delivered on a line  230  from the envelope tracking modulator  102 , and is a power supply voltage which tracks the envelope of the input signal to be amplified on line  228 . The invention is not limited to any particular implementation of an envelope tracking modulator. Envelope tracking modulators for providing a modulated power supply are well-known in the art. 
     The envelope tracking is provided by providing the baseband I-Q data on line  118  to the envelope detector  106 . The envelope detector includes a magnitude calculation function which derives the baseband envelope signal. Various techniques for implementing an envelope detector are known in the art. 
     The thus derived envelope signal is fed into the shaping function  104 , which preferably comprises a shaping table, for mapping the baseband signal to an amplifier supply voltage. The shaping table of the shaping function  104  is normally a mathematical equation or a look-up table, possibly including an interpolation. The shaping function provides shaping which is a mapping between the RF signal envelope or power, and the power amplifier voltage, which mapping may typically be a non-linear mapping. 
     The shaped envelope at the output of the shaping function  104  is provided as an input to the envelope tracking modulator  102 , which provides the modulated tracking supply voltage on line  230  to the power amplifier  114 . 
     In the examples described herein, it is assumed that the power amplifier  114  is the only amplification stage which is subject to envelope modulation. However one skilled in the art will appreciate how the principles of the invention as described herein may be extended to other arrangements in which a modulated power supply is delivered to more than one output stage amplifier. 
     With reference to  FIGS. 2   a  and  2   b , there are illustrated two alternative models of an exemplary amplifier, such as the RF power amplifier  114 . 
     With reference to  FIG. 2   a , there is illustrated a first model  200 . The model  200  includes a transistor  204  which is the active element of the power amplifier. The transistor  204  receives as an input, at an input terminal or node  235  of the amplifier, the input signal to be amplified on line  228 . The tracked supply voltage is provided on line  230  to a power supply terminal or node  231  of the amplifier. The output of the amplifier is provided on line  116  at an output terminal or node  233  of the amplifier. 
     A node  226  represents the terminal of the transistor  204  at which the supply voltage, V drain , is applied to the active element of the amplifier. 
     Where elements shown in different Figures correspond to elements shown in other Figures, like reference numerals are used. 
     The model  200  of  FIG. 2   a  additionally includes a capacitor  218  connected between the node  230  and electrical ground, an inductor  216  connected between nodes  231  and  233 , a capacitor  212  having a connection at node  233 , a matching network  206  connected between the nodes  226  and  233  including a transmission line  208  and a capacitor  210 , and a load  214  connected between the capacitor  212  and electrical ground. 
     A typical RF amplifier output stage, such as that modelled in  FIG. 2   a , may be utilised in a mobile communications application, the frequencies of which are in the band 700 MHz to 2.6 GHz. The impedance of the load  214 , which may be of the order of 50 Ohms, is transformed by a number of passive components to an impedance value required by the active element  204 . In the example illustrated the active element  204  is a GaAsFET transistor. The matching network  206  in the example of  FIG. 2   a  includes the shunt capacitor  210  and the short series transmission line  208 . The supply voltage at node  231  on line  230  is delivered to the amplifier via the series inductor  216 . 
     The inductor  216  is a supply feed inductor and, in general, forms part of the amplifier. One approach involves feeding in the supply voltage through a supply feed inductor which is chosen to have a value to present a high impedance at the RF operating frequency, but which is low impedance at the baseband envelope frequencies. An alternative approach involves making the supply feed inductor part of the RF matching network, which transforms the RF load (e.g. 50 Ohms) to the correct impedance for the active element. 
       FIG. 2   b  shows an alternative model  202 . In the alternative of  FIG. 2   b , the short transmission line  208  of the matching network  206  of  FIG. 2   a  is implemented by its lumped element equivalent. Thus in  FIG. 2   b  a matching network  208  includes an inductor  220  connected between nodes  221  and  223  which connect to nodes  236  and  233  respectively. The matching network  208  further includes capacitors  222  and  224  respectively connected between nodes  221  and  223  and electrical ground. 
     At baseband frequencies (approximately 0-30 MHz), rather than the RF operating frequencies, the impedance ‘looking into’ the active device  204  at its output terminal  226  can be modelled as a shunt resistance and a capacitance as illustrated in  FIG. 3   a .  FIG. 3   a  represents a simple two-element model of the baseband impedance looking into the active devices. Such a simple two-element model is shown and described herein for the purpose of providing a simple example to help understand the principles of the invention. One skilled in the art will appreciate that different models may be utilised, and in particular more complex models may be utilised. The resistor/capacitor model of  FIG. 3   a  is a simple, illustrative example. 
     As illustrated in  FIG. 3   a , the transistor  204  is equivalent to a parallel resistor  302  and capacitor  304  arrangement. One terminal of each of the capacitor  304  and resistor  302  is connected to electrical ground, and the other terminals are connected to node  226 . 
       FIG. 3   b  shows an alternative topology where the shunt resistance  302  of  FIG. 3   a  is replaced with a voltage dependent current source  306 . 
       FIGS. 3   a  and  3   b  each illustrate a simplified output equivalent circuit for the RF power amplifier active element. That is, they represent the load the envelope tracking power amplifier presents, when running, at baseband frequencies to the matching and drain feed network. This is not necessarily the same as the impedance which would be seen (or exhibited) looking into the output node (or pin)  226  with an RF network analyser. The capacitor  304  is the transistor output capacitance, and may include the capacitance due to the active device itself, bond pad capacitance, and package capacitance. The model, in  FIG. 3   a , of a resistor in parallel with a capacitor is simple, but provides a good approximation of the baseband load which the transistor presents to the matching and drain feed network, and thence to the power supply. 
     It can be noted that in an envelope tracking system, an RF amplifier is fed with a modulated RF drive signal on line  228 , and is powered from a time-varying power supply on line  230 . The power supply is arranged to follow the RF envelope. The shaping function  104  controls the relationship between supply voltage and RF power. When the RF amplifier is running, it draws a supply current from the power supply. Since the RF drive and the supply voltage are varying with time, the supply current will also be time varying. Assuming that the RF amplifier has RF decoupling inside, the current flowing from the modulator  102  into the power amplifier via line  230  will be varying at the modulation rate, e.g. 0-20 MHz, rather than the RF frequency. 
     The invention and its embodiments is directed at compensating for a baseband effect which occurs in the RF tuning and matching components in the RF amplifier. As the supply voltage and supply current can be varying at up to 20 MHz or so, voltage drops occur between the supply feed line  230  and the active device prior art node  226 . At a particular time instant, the goal is to apply a particular supply voltage to the active device. The supply voltage to the amplifier is therefore—in accordance with the invention—pre-distorted so that, after this time-varying voltage has passed through the drain feed and power amplifier network, the correct voltage is delivered at node  226 . 
     The embodiments of the present invention provide for the supply voltage delivered to the output power amplifier at node  231  to be pre-distorted to take into account the distortion effects caused by the components within the amplifier  114  itself before the supply voltage is applied at node  226 . In this way, the invention operates toward an objective of delivering a tracked supply voltage at the drain of the active element  204 , at node  226 , which is aligned to the input signal to be amplified on line  228 . Thus the embodiments of the invention seek to address the distortion caused by the elements within the amplifier  114  which modify the voltage delivered at node  230  such that the supply voltage delivered to the active device at node  226  is not a faithful copy of the voltage delivered at node  230 . 
     The transistor supply voltage at node  226 , denoted as V drain  in  FIGS. 2 and 3 , and the RF drive signal into the transistor at node  235  are related via the shaping table within the shaping function  104 . 
     In the following analysis, it is assumed that the capacitance of the capacitor  304  of  FIGS. 3   a  and  3   b  is independent of the transistor supply voltage V drain  on line  226 , and remains a constant value. This assumption is made for simplicity, and ease of explanation. The mathematics can easily be extended to make the capacitance value vary with voltage. This is more realistic for active devices fabricated in certain semiconductor technologies. 
     With reference to  FIG. 4 , there is shown the implementation of a pre-distortion technique in accordance with a first embodiment of the invention. 
     A pre-distortion block  400  in accordance with the first embodiment is located between the shaping function  104  and the modulator  102 . The pre-distortion block  400  includes a mapping function  404 , a capacitor  402 , an inductor  410 , a divider  406 , a combiner  408 , and a combiner  412 . 
     The pre-distortion block  400  receives the output of the shaping function  104  as its input, and generates the input to the modulator  102  on a line  414 . The modulator  102  then generates the supply voltage line  230  to the amplifier  114 . 
     The output of the shaping function  104  provides a target drain voltage, i.e. the desired drain voltage to be applied at the node  226 . 
     In the pre-distortion block  400  of  FIG. 4 , pre-distortion is applied to take into account the effect on the supply voltage between nodes  231  and  226  of the resistance of the resistor  302  of  FIG. 3   a , the capacitance of the capacitor  210  of  FIG. 2   a , and the inductance of the inductor  216  of  FIG. 2   a . This is now further explained with reference to  FIG. 4 . 
     The mapping function  404  includes a mapping table which is formed based on pre-characterisation of the active device  204  as illustrated in  FIG. 6 . The pre-characterisation of the active device  204 , with reference to the model of  FIG. 3   a , establishes the relationship between the value of the resistor  302  for a given drain voltage V drain  at node  226 . As can be seen, the resistance of the resistor  302  (denoted R fet  in  FIG. 6 ) varies in a predetermined way in accordance with variation of the drain voltage (denoted by V fet  in  FIG. 6 ) at node  226 . Thus, with reference to  FIG. 4 , the mapping function  404  provides a resistor value at its output in accordance with the target drain voltage value at its input provided by the shaping function  104 . 
     The target drain voltage is also provided as one input to the divider  406 , and the resistor value from the mapping table  404  is provided as the other input to the divider  406 . The divider  406  thus divides the target drain voltage by the associated resistor value to generate a current value, which is representative of the baseband current in the active device of the amplifier, and hence also the baseband current in the transistor  204  for the given target drain voltage value. This current is provided as a first input to the combiner  408 . 
     The target drain voltage is also provided as an input to the capacitor block  402 . The capacitor function  402  takes the capacitance values of the device output capacitance  304  of the model of  FIG. 3   a , and the capacitance of the matching capacitor  210  of the model of  FIG. 2   a , and sums them, to provide a capacitance value C. The small current in the capacitors depends on the rate of change of V drain . The capacitor function determines a current value I C  associated with these capacitors, in dependence upon the rate of change of the drain voltage. This current value I C  then provides a second input to the combiner  408 . 
     The combiner  408  thus combines the current associated with the capacitors  304  and  210  with the current associated with the transistor  204 , and provides the combined current to the inductor function  410 . 
     This combined current represents the current flowing in the inductor  216 . 
     The inductor function  410  then generates a correction voltage, denoted V corr , in dependence upon the known inductor value L of the inductor  216  and the rate of change of current through an inductor having such a value (which current is supplied from the combiner  408 ). 
     This correction voltage V corr  is provided as a first input to the combiner  412 , which receives as its other input the target drain voltage, such that the correction voltage V corr  add to the target drain voltage to provide a voltage on line  414  which is pre-distorted to take into account the effects of the components within the amplifier. 
     In general terms, the pre-distortion block thus applies pre-compensation to the supply voltage delivered to the supply terminal of the amplifier, which compensates for the effects on the supply voltage between the supply terminal of the amplifier and the point (or node) at which it is delivered to the supply node of the active amplifier element of the amplifier. 
     The pre-distortion function may compensate for some or all of the effects of the components in the amplifier. The more effects are compensated for, the greater the improvement achieved. However compensating for one effect still offers some improvement. Compensating for the supply feed inductance offers the single greatest improvement in most implementations. 
     The exemplary pre-distortion technique of  FIG. 4  compensates for circuit elements as illustrated in  FIG. 2   a , relying upon the model of the active element as shown in  FIG. 3   a . Of course many variations will exist depending upon the particular amplifier circuit and the model used. For example the model  208  of  FIG. 2   b , for the transmission line of  FIG. 2   a , may be utilised to further improve the pre-distortion arrangement of  FIG. 4 , to introduce compensation for the transmission line. The pre-distortion technique of  FIG. 4  utilises the model of the active element  204  illustrated in  FIG. 3   a , generating the current flowing in the inductor  216  using the blocks  404  and  406 . Alternatively this current may be estimated in dependence on the model of the active element shown in  FIG. 3   b , replacing the blocks  404  and  406  with a mapping table which generates a current value directly in dependence upon the target voltage. 
     Thus using the model of  FIG. 3   a , the drain current of the transistor can be determined as I drain =V drain /R out =V drain /f(V drain ), where R out =f(V drain ); i.e. R out  is a function of V drain . Using the model of  FIG. 3   b , alternatively, I drain =g(V drain )=V drain /f(V drain ). Thus  FIG. 3   b  provides an alternate way of determining the current in the active element. In practice, the relationship between V drain  and I drain  in envelope tracking operation would be established by measurement and/or simulation. From these results, one would derive the functions f(V drain ) or g(V drain ). Deriving one function readily leads to the other. The pre-distortion equation may then be generated taking the time-varying target V drain  voltage and generating the required power amplifier supply voltage  230 . 
     The arrangement of  FIG. 4  incorporates a feed forward approach. 
     With reference to  FIG. 5 , there is illustrated a second embodiment in accordance with the invention. The arrangement of  FIG. 5  incorporates a feedback approach. 
     In the second embodiment, a pre-distortion block  500  includes a difference amplifier  502 , an inductor function  504 , a capacitor function  506 , and a resistor function  508 . In general, the capacitor function  506  corresponds to the capacitor function  402  of  FIG. 4 , the resistor function  508  corresponds to the resistor function  404  of  FIG. 4 , and the inductor function  504  corresponds to the inductor function  410  of  FIG. 4 . 
     The corrected voltage V corr  is generated on a line  510 , and fed back to a difference input of the difference amplifier  502 . The other input of the difference amplifier is provided by the target drain voltage. The output of the difference amplifier  502  provides the target drain voltage adjusted by the corrected voltage V corr , such that the voltage at the input of the modulator  102  is appropriately pre-distorted. 
     The feedback approach of  FIG. 5  uses a closed loop which forces the voltage across the transistor element  204 , to follow the target supply voltage V drain  provided by the shaping function  104 . The required pre-distorted supply voltage is present at the output of the differential amplifier  502  at node  512 , and this voltage is fed to the modulator  102 . In principle, the pre-distortion function could alternatively be implemented at the output of the modulator. 
     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 applications in relation to RF amplifiers, but is not limited to such implementations. The invention can be advantageously utilised in any environment where a tracked supply signal should be preferably aligned with an input signal. 
     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, and particularly in the field of mobile communication handsets.

Technology Classification (CPC): 7