in the following , a number of telecommunication standards are mentioned and referenced by acronyms and / or abbreviations , such as 3g ( third generation ), 4g ( fourth generation ), hsdpa ( high speed downlink packet access ), gsm ( global system for mobile communications or telecommunications ), edge ( enhanced data rates for global evolution ), gprs ( general packet radio service ), umts ( universal mobile telecommunications system ( service )), wcdma ( wideband code division multiple access ), and cdma ( code division multiple access ). it should be understood that the present invention is not limited to the above stated standards only , but the invention is also applicable in analogue and digital information transmission over a medium in general and in other non - mentioned close related standards within wireless telecommunication . as mentioned above , the present invention relates generally to power amplifiers and amplifying methods , and more specifically to high efficiency power amplifiers . to theoretically illustrate how drain - efficiency and output power , respectively , depends on variation of load , a drain - efficiency characteristic ( continuous graph ) η and a load power graph ( dotted graph ) p l , respectively , is illustrated in fig3 a . for example , an ab amplifier has a maximum efficiency η of 60 %, where η = p l / p in ( p in is the supplied power ). the power p l out decreases asymptotically with increasing r l . the amplifier &# 39 ; s drain efficiency η increases from zero to a maximum efficiency at r l = r η . when linear power amplifiers like class a , class ab , push pull b and linear class f is operated close to saturation , the efficiency is at its maximum level . however , the efficiency will slowly drop for increasing load r l . the inventive idea according to the present invention is to adjust the load r l to the input power level , which is depending on the amplitude of the input drive signal , and thereby keeping linearity and high efficiency . an adjustment of the load r l will also cause a change of the slope of the load line , which will be discussed in the following with reference to fig3 b - 3 c . in fig3 b , the vi - characteristics of the amplifier circuit is illustrated including two theoretical load lines having different slopes . if the load is changed from r 2 to r 1 , where r 1 & lt ; r 2 , the slope of the load line will accordingly change from − 1 / r 2 to − 1 / r 1 . further , a theoretical boarder line between the saturation region and the active region is drawn . said boarder line is defined as the saturation stop for each level of power input , i . e . amplitude of the drive signal . for any swing , at the saturation stop , the output power is at maximum . in the active region , the linearity of the amplification is secured . along the load line , the input amplitude s 1 swings between points a and b . as an example , for a given load r 2 , a part of the swing , close to the peak point a , will enter into the non - linear saturation region . by decreasing the load enough , to r 1 , the load line will tilt and the whole swing s 1 will be situated in the active region between peak points a ′ and b ′. fig3 c is a diagram showing a graph illustrating the load output power p l as a function of input power . the abscissa is input power and the ordinate is the resulting power level . the input power value corresponds to the amplitude level a in . the graph is almost linear up to saturation stop , where the maximum , p lmax . beyond said level , p l will decrease slowly for increasing p in . as shown , the function is not increasing linearly in an interval before and close to p insat . saturation stop is depending on the load and the peak - value of the swing . maximum efficiency is desirable , but linearity is a necessity . the inventive idea is to give up some efficiency to secure linearity . the inventive idea may be described as the load to the amplifier is tuned and tracked by using an offset adjusted input power value resulting in a linear dependence between input power p in . as illustrated in fig3 c , the output power p l is limited from the maximum p lmax at saturation stop p insat with a value δp l corresponding to a decrease in input power δp in . the loss of efficiency η is than η loss = δp l / δp in . according to the present invention , see fig4 a - 4 e said idea is achieved by an amplifier system and a method , wherein variable impedance of a load is tracked to the amplitude of a drive signal that is fed to the amplifier system comprising at least one amplifier . said system is generating and feeding a power amplified output signal to said load , comprising the following steps of : determining an instant value related to the envelope / amplitude of the drive signal ( step 402 ); determining an adjusted value using an offset value s off and the instant value ( step 404 ); generating an adjusted value depending control signal ( v control ) for controlling and adjusting the variable load impedance ( step 406 ); adjusting the variable load impedance by means of the control signal ( step 408 ). the steps of the invented method are illustrated in fig4 a , and will be further described below . fig4 b presents an amplifier system 400 comprising a load tracking arrangement 410 according to the present invention . the system may be used for power amplification of signals , especially modulated radio frequency ( rf ) signals in the ghz frequency band , to be transmitted over the air interface in a cellular mobile radio telecommunication network , which will be further described in reference to fig6 and 7 . such telecommunication networks are designed to support at least one of a number of to different telecommunication standards such as edge , 3g , 4g , hsdpa , etc . the present invention may therefore be suitable for power amplification in transmitters in base stations of telecommunication networks . the amplifier system 400 comprises at least one linear power amplifier 420 . it is sometimes necessary to back off the input signal , or drive signal , when a part of the input signal &# 39 ; s swing will appear in the non - linear saturated region of the amplifier . however , the system will lose efficiency . examples of such amplifiers are class a , class ab , class f or push - pull class b amplifiers . a drive signal s in ( t ) to the system 400 is modulated on a carrier with the frequency w c by a modulator 450 before being amplified by the linear power amplifier 420 and conducted , as an power amplifier signal s out , to a load impedance 440 . e . g . an antenna ( aerial ), here denoted as r l . to overcome the dropping efficiency problem of said power amplifier , an impedance tuner 430 , providing a variable impedance load z var = r var , is provided between the amplifier 420 and the fix load r lfix 440 . the amplifier feeds the tuner 430 and the load r l with an output signal s out . if the tuner is lossless , the load of the tuner is resistive , and the output signal s out will not be distorted . the impedance tuner is preferably lossless , but a low loss impedance tuner giving a neglectable power loss ( due to an existing complex impedance component ) is possible to use . the function of the impedance tuner according to the present invention is to provide dynamically the optimum r ltot = r lfix + r var at each power level to the amplifier . by adjusting the variable load r var , the amplifier 420 to maintain an optimum efficiency over a wider power range . an implementation of said impedance tuner will be presented further down of this specification in reference to fig4 e . the variable load r var of the impedance tuner 430 will be controlled by a load control signal v cntrl that is generated by a load tracking arrangement 410 , which will now be described in more detail with reference to fig4 c . the load tracking arrangement 410 comprises a mathematical operator means 412 , an offset value means 414 , look - up - table lut 416 and a control signal generating means 418 . the drive signal s in ( t ) will be used as input signal to the load tracking arrangement 410 . the instant amplitude , or envelop , of said drive signal will be processed in the following way to generate a load control signal v cntrl for controlling the impedance tuner 430 . the instant amplitude value a in of the drive signal s in is first processed by a mathematical operator means 412 that produces the absolute value abs ( s in ) of the drive signal . said operation may be performed by a transforming means that transform . cartesian coordinates to polar coordinates , i . e . radial and angular coordinate . the radial coordinate corresponds to the amplitude of the drive signal . the angular coordinate will not be used . this absolute value is divided by a pre - determined and pre - stored maximum value of the s in , to determine a nominal value s nom =( abs ( s in )/ max abs ( s in )) 2 in the interval [ 0 , 1 ]. the nominal values s nom is corresponding to the stored values of the lut 416 . examples of such s nom values are indicated in the lut of fig4 c . in the same lut 416 , output voltage steps 1 - 512 are indicated . each s nom value corresponds to an output step and a corresponding wished p l . for each input value received by the lut , a corresponding output voltage step will be fed to the output of the lut and read by a control signal generating means 418 . in accordance with the invention , the nominal value s nom is thereafter adjusted by means of an offset value means 414 that will produce a look - up - table lut input value s lut . depending on the pre - set offset value s off , it will be possible to increase or decrease s lut . said operation will correspond to a change of the output voltage a pre - determined number of steps in the lut 416 . therefore , the offset process is possible to define by the equation by means of the offset adjusting equation , it is possible to determine the correct load tracking control voltage v cntrl in the look - up - table 416 . the load tracking signal v cntrl is controlling the impedance tuner 430 , which impedance is tuned to an impedance value r lfix , for providing an optimum r ltot = r lfix + r var at each power level to the amplifier 420 . said control signal is dependent on the design of the tuner . in the present embodiment , the control signal is a dc signal of which the dc voltage is dependent of the value in the second column of the lut . the lut device 416 and how to program and operate is regarded to be already known to a person skilled in the art . the offset value means 414 may be implemented as an adder / subtracter device , however , other mathematical processes may be used . a person skilled in the art will also be able to design a control signal generating means 418 , for instance by using a voltage source and a transistor circuit controlling the delivered voltage v cntrl in correspondence to the output from the lut 416 . an embodiment of a lossless impedance tuner 430 is presented in fig4 e , which is a circuit diagram . the use and control of impedance tuners are earlier known to a person skilled in the art , for example from the document “ high - efficiency linear amplification by dynamic load modulation ” by frederick h . raab , pp . 1717 - 1720 , 2003 ieee mtt - s digest . the present impedance tuner 430 is a tuneable filter arrangement , which comprises a transmission line 435 with constant impedance z 0 and a diode arrangement 431 . the diode arrangement comprises two diodes 432 and 433 , here varactor diodes , which cathodes are connected , as illustrated in the figure . both diodes are charged in the reverse direction , the first diode by a dc bias source 434 and the second diode by earth potential . their cathodes are connected to an input for a control voltage v cntrl that will be able to control the diode capacitance of the diodes . by controlling the diode capacitances , it is possible to adjust the impedance to compensate for the reactance effect depending on the current power level and achieve a pure optimal resistive load , r var , at each power level to the amplifier output . in this embodiment , the transforming means generates the input control voltage v cntrl . fig5 is a diagram showing the variable load r var of the impedance tuner as a function of the control signal v cntrl . the load - control signal function is linear and the graph has a negative slope . the amendment of the v cntrl one step ( e . g . from 1 to 2 , as illustrated in the figure ) will cause an amendment of the variable load impedance δr var . if δr var becomes equal to zero , the total load r ltot = r lfix . the steps of v cntrl is stored in the lut and the size of the voltage steps and their corresponding voltage , is defined by the voltage generator 418 . fig4 d is a block diagram showing another embodiment of the amplifier system according to the present invention . this second embodiment differ from the first described . preferred embodiment only regarding a number of features will now be described . the remaining parts id described in said preferred embodiment . in the second embodiment , the modulator means 450 and transforming means that transform cartesian coordinates to polar coordinates ( located in the mathematical operator means 412 in the preferred embodiment ) has been integrated together . this is practical if the modulator 450 serves an out phasing amplifier system . the radial component r ( t ) of the drive signal is generated and fed to the amplifiers 420 for the amplification . said radial component may therefore also be used by the mathematical operator means 412 in the load tracking arrangement 410 , which other means 414 , 416 and 416 operates as described above . further , may a digital pre - distortion ( dpd ) block 460 be connected before the amplifiers 420 . said block is used for improving the linearization in the digital domain of the used amplifiers . in addition , the fixed load r lfix may be integrated with the impedance tuner and implemented as one block r ltot 430 . it must be regarded to a person skilled in the art that the load tracking arrangement 410 and the impedance tuner 430 may be implemented as one single functional block . the described invented amplifier systems are possible to integrate on an asic in different ways . the present invention is preferably adapted for use in radio terminals , e . g . mobile radio terminals or base stations of cellular mobile radio telecommunications systems , or within satellite systems . fig6 is a schematic illustration of a wcdma network architecture 600 comprising base stations 620 , node b . the network comprises a core network 605 to which a number of radio network controllers 610 ( rnc ) are connected and other networks 608 , e . g . public switched telephony network ( pstn ), public data networks , internet , integrated services digital network ( isdsn ), other public land mobile networks ( plmn ). satellite telecommunication systems , etc . the rnc 610 controls at least one dedicated node b 620 ( base station in gsm networks ). the rnc 620 controls and handles the uplink and downlink communications over the air interface between a node b 620 ( connected to the rnc ) and subscriber units , such as radio handset terminals 630 . a node b comprises , among other units 8 not shown ), base band units 622 , radio units 624 , filter units 626 and an antenna system 628 . the base band unit 622 transforms digital messages into i and q vectors , which are transferred to the radio unit 624 . in the radio unit , said vectors are modulated on carriers resulting in signals s in . before transmission , the signals s in are power amplified . therefore , the radio unit is equipped with at least one composite power amplifier system 200 according to the invention . the amplifier system 200 power amplifies s in and provides an output signal s out to the antenna system 628 for transmission over a radio channel to a subscriber unit 630 . e . g . a radio terminal handset . it is also possible to use the composite power amplifier system 200 according to the invention in a satellite 650 for mobile radio telecommunications . the satellite will then operate as a repeater station comprising transmitters and / or transceivers in the radio units 624 and antenna systems 628 for handling the communication with a control station in a satellite telecommunication systems 608 and satellite radio terminals 630 located on the earth . fig7 illustrates schematically a mobile radio terminal , also denoted handset , 700 for mobile radio telecommunications . the terminal comprises a microphone 705 for transforming voice and audio to an electrical signal m ( t ). said signal m ( t ) is processed before being modulated and power amplified by a first signal processing block 710 comprising a / d - converter unit , speech and channel coder units , and digital formatting devices for arranging the signal into a suitable transmitting format for mobile radio telecommunication systems like gsm /( edge ) gprs umts ( wcdma ), and cdma - systems . in the transmitter 715 , or radio unit 720 , the processed signal s in , which is generated by the block 710 , is modulated in a modulator ( not shown ) and thereafter power amplified by the composite power amplifier system 200 according to the present invention , generating an output signal s out , which is transmitted via the aerial / antenna 728 to a base station 620 in a telecommunication network 600 ( see fig6 ). preferably , a filter arrangement ( not shown ) is also inserted between the amplifier system and the antenna . said terminal 700 also comprises at least one controller 730 , such as a microprocessor or central processing unit , for controlling the units of the terminal by using stored , readable and executable software . further , the terminal 700 comprises a receiving unit 735 connected to the antenna 728 for receiving transmitted signals and transform them in a second signal processing block 737 . if the received signal contains voice , the voice signal is decoded by decoder units and converted by a d / a - converter to an audio signal before being transformed to sound by a loudspeaker 707 . the terminal comprises a keyboard 745 , a display 747 and a man - machine - interface block ( mmi ) 740 that allows a user to interactively control the terminal , write and read text messages , initiate telephone calls , etc . the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein : rather , these embodiments are provided so that this disclosure will be thorough and complete , and will fully convey the scope of the invention as defined by the enclosed set of claims .