Abstract:
The present invention relates generally to the field of RF transmitters, and more particularly to a method and apparatus for increasing the efficiency of transmitters which are capable of transmitting at different power levels in each of at least two frequency bands. An inventive method is presented as well as an inventive transmitter comprising at least one power amplifier in which the load in the transmission line is varied as the output power is varied, in order to keep the efficiency of the power amplifier at a high level.

Description:
This patent application claims priority from and incorporates by reference the entire disclosure of U.S. Provisional Patent Application No. 60/284,507, filed on Apr. 19, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of RF technology, and more particularly to the field of RF transmitters. 
     BACKGROUND 
     In many applications of RF transmitters, it is desirable to introduce the possibility of the RF transmitter transmitting at different power levels. This is e.g. the case in mobile radio communications, where the output radio power of the mobile stations as well as of the base stations should advantageously be variable depending on signal quality and distance between transmitter and receiver. 
     If the change in output power level is effectuated simply by changing the input power level to the power amplifier of the RF transmitter, the power amplifier will, at some power levels, have to work at combinations of power supply voltage and input power level which yields poor efficiency of the power amplifier. The power supply voltage can be adjusted and the efficiency of the power amplifier optimised for the highest of the power levels, while the efficiency will be impaired as the input power level to the power amplifier is lowered. Since in many applications of RF transmitters, such as in mobile stations, one has to rely on a battery as the power supply, the battery having limited energy, poor efficiency of the RF transmitter is a severe problem. 
     This problem has previously been solved by introducing a settable switched power supply to the power amplifier, so that the power supply voltage can be lowered as the input power level to the power amplifier is lowered, the efficiency of the power amplifier thus staying at a high level. Another solution that has been found is to introduce a transformer with switched steps at the output of the power amplifier, so that, without changing the input power level tot he power amplifier, the output signal power can be varied as different parts of the transformer is connected to the transmitter output at different times. However, there are certain drawbacks with each of these solutions. A settable switched power supply generates disturbances which interferes with the RF signal to be transmitted. Further, it is difficult to design transformers for ultra high frequencies covering large frequency ranges. As many RF transmitters today are designed for large frequency ranges, as in e.g. mobile radio telephony, where transmitters often are designed to operate in more than one frequency band, the frequency bands being far apart, the transformer solution is not optimal. Thus, it is desirable to find a transmitter which can transmit RF signals at different power levels in a wide frequency range without impairing the efficiency as the output power is changed, that does not introduce disturbances to the RF signal as the output power is changed. 
     SUMMARY 
     An object of the present invention is to provide a method and apparatus by which the transmission power level of an RF transmitter, capable of transmitting in more than one frequency band, can be varied without significantly impairing the efficiency of the transmitter and without introducing disturbances to the transmitted RF signal. 
     This object is met by transmitter for transmitting RF signals at at least two power levels in each of at least two frequency bands. The transmitter comprises at least one power amplifier connected to a working load and to a first electronic circuit. The first electronic circuit is connected, in parallel to the working load, to the at least one power amplifier at a line length from the at least one power amplifier. The first electronic circuit comprises a reactive impedance and a switching device connected in series, the reactive impedance and the line length having been chosen as to provide, when the switching device is closed, a load selected with regard to the efficiency of the at least one power amplifier when the transmitter transmits at one of the at least two power levels. If the transmitter comprises more than one power amplifier, the line lengths at which the first electronic circuit is connected to the power amplifiers is different for each power amplifier. 
     The object of the invention is further met by an inventive method for improving the efficiency of a transmitter upon changing the output power level at which the transmitter transmits. The transmitter is capable of transmitting at at least two power levels in each of at least two frequency bands, and the transmitter comprises a power amplifier connected to a working load. In the inventive method, at least one electronic circuit is connected, in parallel with the working load, to the power amplifier at a line length from the power amplifier, the electronic circuit comprising a switching device and a reactive impedance, the line length and the reactive impedance being chosen to form an optimal load with regard to the efficiency of the power amplifier when the transmitter transmits in one of the at least two frequency bands at one of the at least two power levels. If the transmitter transmits at the power level and in the frequency band for which the line length and the reactive impedance has are chosen to form an optimal load, then said switching device is closed. If the RF transmitter transmits at a power level and/or in a frequency band for which the line length and the reactive impedance are not chosen to form an optimal load, then said switching device is opened. 
     By the method and the transmitter of the invention is achieved that the load of a power amplifier of a transmitter can, in a simple manner, without introducing disturbances to the transmitted RF signal, be adjusted to the power level at which the transmitter transmits. Thus, the efficiency of the power amplifier and hence of the transmitter will not have to be impaired by a change in the output power of the transmitter. 
     In one embodiment of the inventive transmitter, the at least one power amplifier is two separate power amplifiers, each operable on RF signals in one of the at least two frequency bands. The two power amplifiers are in this embodiment connected to the first electronic circuit at different line lengths. The reactive impedance of the first electronic circuit and the two line lengths are selected in a manner as to provide said load, when the switching device is closed, when the transmitter transmits in one of the at least two frequency bands. In one aspect of this embodiment, the reactive impedance is selected in a manner as to provide said load, when the switching device is closed, no matter in which of the at least two frequency bands transmitter transmits. The at least two power levels at which the transmitter is designed for transmitting could then be the same for the at least two frequency bands, or different. The reactive impedance of the first electronic circuit could in this embodiment e.g. be formed by a capacitor and an inductor connected in parallel or in series. Furthermore, the two power amplifiers could be capable of operating on RF signals of different modulation modes. 
     In a second embodiment of the inventive transmitter, the at least one power amplifier is one single power amplifier operable on RF signals of the at least two frequency bands, to which a second electronic circuit is connected. The second electronic circuit is connected in parallel to the working load and the first electronic circuit, at a second line length from the power amplifier. The second electronic circuit comprises a second reactive impedance and a second switching device. The reactive impedance and the line length of the first electronic circuit are adjusted to provide said load selected with regard to the efficiency of the power amplifier when the dual mode RF transmitter transmits at one of the at least two power levels in the first of the at least two frequency bands. The second reactive impedance and the second line length have been chosen to provide a load selected with regard to the efficiency of the power amplifier when the dual mode RF transmitter transmits at one of the at least two power levels in a second of the at least two frequency bands. The power level with regard to which the reactive impedance and the line length of the first electronic circuit has been chosen and the power level with regard to which the reactive impedance and the line length of the second electronic circuit has been chosen could be the same, or different. 
     In a third embodiment of the inventive transmitter, wherein the at least one power amplifier is one single power amplifier operable on RF signals of the at least two frequency bands, the transmitter comprises a filter arrangement connected at the output of the power amplifier. The filter arrangement provides different electrical lengths for the at least two frequency bands. These electrical lengths, the line length and the reactive impedance of the first electronic circuit has been chosen in a manner as to provide said load, when the switching device is closed, when the transmitter transmits in at least one of the at least two frequency bands. The electrical lengths, the line length and the reactive impedance of the first electronic circuit could have been chosen in a manner as to provide said load, when the switching device is closed, no matter in which of the at least two frequency bands the transmitter transmits. 
     In one aspect of the invention, the line length(s) and the reactive impedance of the first electronic circuit have been chosen so that ratio of the load when the switching device is open and the load when the switching device is closed equals the ratio of the power level for which the transmitter is designed to transmit when the switching device is closed and the power level for which the transmitter is designed to transmit when the switching device is open. 
     In one aspect of the inventive method, the at least one electronic circuit is one single electronic circuit and the dual mode RF transmitter comprises two separate power amplifiers connected at different line lengths to the electronic circuit. Each power amplifier is operable on RF signals in one of the frequency bands. The reactive impedance of the electronic circuit and the two line lengths are selected in a manner as to provide said load, when the switching device is closed, no matter in which of the at least two frequency bands the transmitter transmits. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be discussed in more detail with reference to preferred embodiments of the present invention, given only by way of example, and illustrated in the accompanying drawings, in which: 
         FIG. 1  is a dual band RF transmitter for transmitting at two different power levels, the dual band RF transmitter comprising two power amplifiers. 
         FIG. 2  is a dual band RF transmitter for transmitting at two different power levels, the dual band transmitter comprising one single power amplifier. 
         FIG. 3  is another dual band RF transmitter for transmitting at two different power levels comprising one single power amplifier. 
     
    
    
     DETAILED DESCRIPTION 
     In order to increase the efficiency of a power amplifier as the output power of the RF signal transmitted by an RF transmitter is varied, one could use a variable load in the transmission path, additional to the working load. When the power level of the RF signal which is fed into the power amplifier is changed in order to change the power level of the transmitted RF signal, the load of the transmission path could be changed into a load for which the efficiency of the power amplifier is kept at a high value. The output voltage of the power amplifier is advantageously kept constant as the transmitter output power level is varied, while the load experienced by the power amplifier is varied. The energy consumed by the power amplifier would then be less than if the RF signal which is fed into the power amplifier would be changed without adjusting the load. 
     An example of a dual band RF transmitter  100 , or simply transmitter  100 , operating according to the principle of varying the load in accordance with at which power level the transmitter  100  is transmitting is schematically shown in  FIG. 1 , the transmitter  100  for transmitting RF signals in two different frequency bands f 1  and f 2 . These frequency bands could e.g. be 890 Mz to 915 MHz and 1710 MHz and 1785 MHz, corresponding to the uplink frequency bands of GSM (Global System for Mobile Communication) and DCS (Digital Communication System), or any other frequency bands. The dual band RF transmitter  100  of  FIG. 1  comprises two power amplifiers  105  and  110 . Power amplifier  105  operates on RF signals in one of the two frequency bands, f 1 , while power amplifier  110  operates on RF signals in the other frequency band, f 2 . Two transmission paths  115  and  120  connect power amplifier  105  and  110 , respectively, to a combiner  125  which connects the two power amplifiers  105  and  110  to the same output port. The line lengths l 1  and l 2  of the transmission paths  115  and  120  could be adjusted separately. The combiner  125  could e.g. be a filter diplexer or a switch. The output of combiner  125  is then connected to an electronic circuit  130  and a working load  135 , the electronic circuit  130  and the working load  135  being connected in parallel. The working load  135  could e.g. be another amplifier or an antenna, where the antenna could e.g. comprise an antenna array. 
     The electronic circuit  130  comprises a switch  140  connected in series to a reactive impedance, the reactance of the reactive impedance being frequency dependent and the reactive impedance thus providing different reactances in the two frequency bands. In  FIG. 1 , a capacitor  145  of capacitance C 145  and an inductor  150  of inductance L 150  connected in parallel is used as an exemplary reactive impedance. The reactive impedance of electronic circuit  130  could alternatively comprise a capacitance and an inductance connected in series, or a more complex network. When the switch  140  is open, the impedance that an RF signal experiences at the output of power amplifier  105  or  110  is set by the impedance of the working load  135 , while when the switch  140  is closed, the impedance experienced by an RF signal will be determined by the impedance of the working load  135  and the impedance of electronic circuit  130 . 
     The reflection coefficient ρ of a transmission path with characteristic admittance Y 0  and a connected load of admittance Y L  is defined as: 
               ρ   =           Y   0     -     Y   L           Y   0     +     Y   L         =         1   -       Y   L       Y   0           1   +       Y   L       Y   0           =       1   -     y   L         1   +     y   L               ,           (   1   )             
 
where y L  is the normalised load admittance. The impedance Z experienced at an electrical length φ from the point where the load is connected is then 
             Z   =       Z   0     ⁢       1   +     ρ   ⁢           ⁢     ⅇ       -   2     ⁢           ⁢   j   ⁢           ⁢   φ             1   -     ρ   ⁢           ⁢     ⅇ       -   2     ⁢           ⁢   j   ⁢           ⁢   φ                       (   2   )             
 
where Z 0 =1/Y 0  and the electrical length φ is measured in wavelengths of the RF signal on the transmission path expressed in angular terms.
 
     In  FIG. 1 , the electronic circuit  130  and the working load  135  are both connected at the connection point  155 . If the working load  135  is assumed to be only resistive, the admittance of the total load when the switch  140  is closed, Y L   closed , can be written as 
               Y   L   closed     =         G   WL     +     j   ⁢           ⁢     B   130         =       G   WL     +     j   ⁡     (       ω   ⁢           ⁢     C   145       -     1     ω   ⁢           ⁢     L   150           )                   (   3   )             
 
where G WL  is the conductance of the working load, B 130  is the susceptance of electronic circuit  130  and ω is the angular frequency of the RF signal. When the switch  140  is open, the admittance of the total load Y L   open  is simply
 
{Y L   open   =G   WL   (4)
 
     If the impedance of the working load  135  has a reactive component, expressions (3) and (4) will have to be adjusted accordingly. 
     When designing the transmitter  100  for different load impedances at different output power levels, the different load impedances are advantageously chosen so that the output voltage of power amplifiers  105  and  110  stays the same as the transmitter output power level is varied. The ratio of Z closed  and Z open  could hence advantageously equal the ratio of the power levels P open  and P closed  for which the switching device  140  is designed to be open and closed, respectively. By inserting equation (3) and (4) into equation (1), equations for the reflection coefficients ρ closed  and ρ open  can be found, respectively. By inserting ρ closed  into equation (2), a value for Z closed  at an electrical length φ from the connection point  155  can be obtained, while inserting ρ open  into the same equation similarly yields an expression for Z open . If the admittance of the working load, G WL , equals the characteristic admittance, Y 0 , then ρ open =0 and the load experienced by the power amplifiers  105  and  110  when switch  140  is open is Z 0 , independent of the line lengths l 1  and l 2 . The expression for the ratio Z closed /Z open , set equal to the desired value of P open /P closed , is then: 
                 Z   closed       Z   0       =         1   -         j   ⁢           ⁢     B   130           2   ⁢     Y   0       +     j   ⁢           ⁢     B   130           ⁢     ⅇ       -   2     ⁢           ⁢   j   ⁢           ⁢   φ             1   +         j   ⁢           ⁢     B   130           2   ⁢     Y   0       +     j   ⁢           ⁢     B   130           ⁢     ⅇ       -   2     ⁢           ⁢   j   ⁢           ⁢   φ             =       P   open       P   closed                 (   5   )             
 
     Expression (5) is solved by two different sets of values of the susceptance B 130  and electrical length φ. The power amplifiers  105  and  110  each has an independent electrical length φ 105  and φ 110  to the connection point  155 , the electrical lengths being independently adjustable as the length of the transmission paths  115  and  120  are varied. Furthermore, the susceptance B 130 (ω) is a function of frequency. Since the power amplifiers operate on RF signals in different frequency bands, each power amplifier operating on RF power signals in only one of the frequency bands, the values of φ 105  and B 130 (ω 1 ) as well as φ 110  and B 130 (ω 2 ) can be chosen so that the value of Z closed /Z open  stays the same, no matter in which of the two frequency bands the transmitter  100  is transmitting. By choosing C 145  and L 150  so that the resonance frequency of electronic circuit  130  lies between the two frequency bands f 1  and f 2 , the susceptance B 130  could e.g. take one value at the frequency band f 1 , and another value, the modulus being the same but the sign being the opposite, at frequency band f 2 . 
     For illustrative purposes, a numerical example will be given below, based on an exemplary dual band RF transmitter  100  operating in two exemplary frequency bands centred around the frequencies 900 MHz and 1800 MHz, respectively. The desired power level of the example, when the switch  140  is closed, is half that of when the switch is open, i.e. the desired value of ratio P open /P closed  is 2. The characteristic admittance Y 0  in the example is 0.02 Ω −1 . The RF signal travels on the exemplary transmission line with a speed of 0.6 c, where c is the speed of light. By inserting the values applicable to the example in expression (5), one gets the following expression, where b=B 130 /Y 0 : 
             2   =       1   -       j   ⁢           ⁢   b   ⁢           ⁢     ⅇ       -   2     ⁢           ⁢   j   ⁢           ⁢   φ           2   +     j   ⁢           ⁢   b             1   +       j   ⁢           ⁢   b   ⁢           ⁢     ⅇ       -   2     ⁢           ⁢   j   ⁢           ⁢   φ           2   +     j   ⁢           ⁢   b                     (   6   )             
 
     Expression (6) is satisfied by the following sets of b and φ: 
                     b   1     =     -       1   2                                                       b   2     =     +       1   2                       sin   ⁢           ⁢   2   ⁢           ⁢     φ   1       =     +       2   ⁢     2       3                                                     sin   ⁢           ⁢   2   ⁢           ⁢     φ   2       =     -       2   ⁢     2       3                     cos   ⁢           ⁢   2   ⁢           ⁢     φ   1       =     -     1   3                                                     cos   ⁢           ⁢   2   ⁢           ⁢     φ   2       =     -     1   3                     φ   1     =       54.5   ⁢   °     +     n   ⁢           ⁢   180   ⁢   °                                                     φ   2     =       125.5   ⁢   °     +     n   ⁢           ⁢   180   ⁢   °                     (   7   )             
 
     In the calculations of the numerical example, one frequency in each of the two frequency bands is selected and inserted in the above expressions: the frequency 900 MHz is used to represent the first frequency band, and 1800 MHz is used to represent the second frequency band. By inserting the two values of the angular frequencies ω 1  and ω 2 , corresponding to the two frequencies 900 MHz and 1800 MHz, into the expression for b, 
         b   =         B   130     /     Y   0       =       1     Y   0       ⁢     (       ω   ⁢           ⁢     C   145       -       1   /   ω     ⁢           ⁢     L   150         )           ,       
 
and setting the expression equal to the roots b 1  and b 2 , one can obtain suitable values for C 145  and L 150 . In the example presently discussed, it is found that C 145  should be chosen to 2.5 pF and L 150  should be chosen to 6.3 nH.
 
     By, in a similar manner, using the roots φ 1  and φ 2  of expression (7) and the values of the frequencies f 1  and f 2 , one can obtain values of the line lengths l 1  and l 2  that should be parting the power amplifiers  105  and  100 , respectively, and the connection point  155 . Since the RF signal of the example is transmitted by 0.6 c, where c is the speed of light, the expression for l can be written as 
       l   =         0.6   ⁢   c     f     ⁢       φ     360   ⁢   °       .           
 
It is then found that l 1 , corresponding to the lower frequency band, should be 3.0 cm, while l 2 , corresponding to the frequency band, should be 3.5 cm. Any number of half wavelengths of the respective RF signal could be added to these lengths.
 
     The numerical example given above is merely an illustrative example used for illustration purposes, and the inventive transmitter of  FIG. 1  could be used in any two frequency bands, with any ratio between the two output power levels and with any impedance on the working load. The characteristic admittance Y 0  of the transmission lines could take any value. Furthermore, the inventive transmitter shown in  FIG. 1  could be a dual mode transmitter, i.e. the power amplifiers  105  and  110  could not only be operable on RF signals of different frequencies, but also of different modulation modes. Power amplifier  105  could e.g. be operable on an AM modulated signal of frequency f 1 , while power amplifier  110  was operable on an FM modulated signal of frequency f 2 . The capacitor  145  and inductor  150  of electronic circuit  130  could be connected in series instead of in parallel as shown in  FIG. 1 , or a more complex network could be used as the reactive impedance of electronic circuit  130 . Expression (3) would then have to be adjusted accordingly. 
     The RF signals transmitted by transmitter  100  in frequency band f 1  could either be transmitted at the same power as the RF signals in frequency band f 2 , or at a different power. If the reactive impedance of electronic circuit  130  and the line lengths l 1  and l 2  are chosen so that the admittance Y L   closed  in frequency band f 1  is the same as the admittance Y L   closed  in frequency band f 2 , then the ratio between the two power levels for which the loads are adjusted would be the same. One could also choose to select the reactive impedance of electronic circuit  130  and the line lengths l 1  and l 2  in a manner so that the admittance Y L   closed  is different for the two frequency bands. 
     In  FIG. 2 , a dual band RF transmitter  200 , or simply transmitter  200 , is schematically shown, the transmitter  200  capable of transmitting RF signals in two different frequency bands f 1  and f 2 . Transmitter  200  comprises a power amplifier  205 , operable on RF signals in both frequency bands f 1  and f 2 , connected to a working load  135 . Connected in parallel to the working load  135  are two electronic circuits  215  and  220 , connected at the connection points  225  and  230  at a line length l 1  and l 2  from the power amplifier, respectively. Electronic circuit  215  comprises reactive impedance  235  of impedance X 235 (ω) and a switch  240 , while electronic circuit  220  comprises reactive impedance  245  of impedance X 245 (ω)) and switch  250 . The electrical lengths between the output of the power amplifier and the connection points  225  and  230 , φ 225  and φ 230 , could be adjusted independently since the line lengths l 1  and l 2  could be chosen independently. The values of X 235 (ω), X 245 (ω), φ 225  and φ 230  could be chosen according to the principles of expressions (1) and (2) so that having switch  240  closed when transmitting in frequency band f 1  would give the same impedance experienced by the power amplifier  205  as having switch  250  closed when transmitting in frequency band f 2 . Each electronic circuit  215  and  220  can hence be adjusted for operating in one of the frequency bands, and the electronic circuits  215  and  220  can be adjusted to provide the same, or similar, load when the switch  240  (in case of f 1 ) or switch  250  (in case of f 2 ) is closed. Thus, by having both switches open, one load adjusted for transmission at a first power level is obtained, and by having that switch closed which has been adjusted for the frequency band at which the transmitter presently is transmitting, another load, adjusted for transmission at a second power level, is obtained. Alternatively, one could design the transmitter  200  for transmission at different power levels in the two frequency bands f 1  and f 2 , so that the reactive impedance  235  of electronic circuit  215  provides an impedance in the frequency band f 1  which is different to the impedance provided by reactive impedance  245  in the frequency band f 2 . 
     In  FIG. 3 , a dual band RF amplifier  300  comprising only one power amplifier  205  operable on RF signals in both frequency bands f 1  and f 2  is shown. The RF dual band transmitter  300  comprises a filter arrangement  305 , providing different line lengths for signals of the two frequency bands f 1  and f 2 . The filter arrangement  305  is connected in series between the output of the power amplifier  205  and the working load  135 . At the output of the filter arrangement  305 , an electronic circuit  130 , similar to the electronic circuit  130  in  FIG. 1 , is connected in parallel to the working load  135 . The exemplary filter arrangement  305  shown in  FIG. 3  comprises a diplexer  310 , which divides the incoming RF signal so that the part of the RF signal which lies in the frequency band f 1  is transmitted on the transmission path  315 , and the part of the RF signal which lies in the frequency band f 2  is transmitted on the transmission path  320 . The line lengths of the transmission paths  315  and  320  could be adjusted independently to satisfy the desired electrical lengths for the relevant frequency band, f 1  or f 2 . The transmission paths  315  and  320  are then connected to a combiner  325 , which connects the two transmission paths  315  and  320  to the same output port. The filter arrangement  305  could e.g. be replaced by a single filter capable of providing different line lengths for the signals of the two different frequency bands. By introducing filter arrangement  305  which provides different electrical lengths for RF signals in the two frequency bands, it is achieved that despite having one single power amplifier  205  operable on both frequency bands, one single electronic circuit  130 , comprising one switching device connected in series to a reactive impedance, can be used. In the exemplary electronic circuit  130  shown in  FIG. 3 , the reactive impedance is shown as a capacitor  330  connected in series to an inductor  335 , but the reactive impedance of electronic circuit  130  could consist of an inductance and a capacitance being connected in parallel, or of a more complex network. 
     The capacitor  145  and inductor  150  of  FIG. 1  could be replaced by suitable lengths of line, or by any other arrangement which has a suitable impedance with the desired frequency dependence. The same would be valid for capacitor  330  and inductor  335  of FIG.  3 . Reactive impedances X 235 (ω) and X 245 (ω) of  FIG. 2  could consist of capacitors and inductors connected in a suitable manner, or suitable lengths of line, or of any other arrangement which gives a reactive impedance of the desired value. The switches  140 ,  240  and  250  could be diode switches, relays, or any other type of switching arrangement. The impedance of electronic circuit  130 , together with value of the electrical lengths φ 105  and φ 110 , can be chosen so that a closed switch  140  gives either a higher or a lower output power level than an open switch  140 . Analogously, the values of X 235 (ω), X 245 (ω) and φ 225 , φ 230  can be chosen so that a closed switch gives either a higher or a lower output power level than an open switch. In  FIGS. 1 and 2 , the switches  140 ,  240  and  250  have been shown to be connected between the power amplifier and the reactive impedances of the electronic circuits. The switches  140 ,  240  and  250  could very well be positioned on the other side of the reactive impedances, between the reactive impedance of the relevant electronic circuit and the ground. 
     The above discussed invention could be useful in many different applications of RF transmitters, among which dual band RF transmitters in dual band mobile stations is an important application. The exemplary transmitters  100 ,  200  and  300  shown in  FIG. 3  above are all dual band RF transmitters capable of transmitting RF signals in two separate frequency bands. It should however be understood that the invention is not limited to dual band RF transmitters, but could be applied to transmitters capable of transmitting RF signals in more than two frequency bands. 
     The invention could be used for transmission at other power levels than the power levels for which the load has been adjusted. If e.g. the transmitter  100  of  FIG. 1  would be used for transmission at a power level that is not one of the power levels for which the load has been adjusted, the switch  140  could either be open or closed. Whether the switch should be open or closed could be made dependant on which of the two loads that would yield the best efficiency of the two power amplifiers  105  and  110  at a particular power level. Similarly, the switches  240  and  250  of  FIG. 2  as well as switch  140  of  FIG. 3  could be either open or closed when transmitting at a power level which is not one of the power levels for which the load has been adjusted. 
     One skilled in the art will appreciate that the present invention is not limited to the embodiments disclosed in the accompanying drawings and the foregoing detailed description, which are presented for purposes of illustration only, but it can be implemented in a number of different ways, and it is defined by the following claims.