Abstract:
The present invention has for its object to provide, in an environment where plural radio systems coexist, a feed forward amplifier for multiple frequency bands, capable of adaptively selecting the frequency band which is used. 
     The feed forward amplifier of the present invention comprises a distortion detection circuit and a distortion elimination circuit and has first and second variable frequency band extractors  25   a  and  25   b  provided in series with respective vector adjustment paths  21   a  and  21   b.  Also, the feed forward amplifier comprises a frequency band controller which varies the frequency band of variable frequency band extractors  25   a  and  25   b  and has been designed, by changing the frequency band of first and second variable frequency band extractors  25   a  and  25   b  in response to a frequency switching request from the outside, to be able to adaptively control the frequency band in which distortion is compensated.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention pertains to a power amplifier for mobile communications which adaptively changes the frequency band among a plurality of frequency bands. In particular, it pertains to a feed forward amplifier for multiple frequency bands which collectively amplifies a plurality of frequency bands. 
   2. Description of Related Art 
   The base configuration of a conventionally used feed forward amplifier is shown in  FIG. 1 . The feed forward amplifier includes two signal processing circuits. One is a distortion detection circuit  150  and the other is a distortion elimination circuit  151 . Distortion detection circuit  150  is composed of a main amplifier signal path  153  and a linear signal path  154 . Distortion elimination circuit  151  is composed of a main signal path  158  and a distortion injection path  159 . Main amplifier signal path  153  (also called a vector adjustment path) is composed of a vector adjuster  155  and a main amplifier  156 . Vector adjuster  155  is composed of a variable phase shifter  155   a  and a variable attenuator  155   b.  Linear signal path  154  is composed of delay lines. Also, main signal path  158  is composed of delay lines. Distortion injection path  159  (also called a vector adjustment path) is composed of a vector adjuster  200  and an auxiliary amplifier  201 . Vector adjuster  200  is composed of a variable phase shifter  200   a  and a variable attenuator  200   b.  Here, a divider  152 , a power combiner/divider  157 , and a combiner  202  are simple lossless power dividers and power combiners composed of transformer circuits and hybrid circuits. 
   First, an explanation of the basic operation of the feed forward amplifier will be given. The signal input into the feed forward amplifier is divided into main amplifier signal path  153  and linear signal path  154  by means of divider  152 . At this point, variable phase shifter  155   a  and variable attenuator  155   b  of main amplifier signal path  153  are adjusted so that the signals of main amplifier signal path  153  and linear signal path  154  have equal amplitude and opposite phase. As methods for bringing the paths to opposite phases, there is the method wherein divider  152  or power combiner/divider  157  sets a phase shift appropriately between the input and output terminals or the method wherein main amplifier  156  inverts the phase. 
   Since distortion detection circuit  150  is configured in this way, power combiner/divider  157  can output the differential component of the signal passing through main amplifier signal path  153  and the signal passing through linear signal path  154 . This differential component is precisely the distortion component generated in main amplifier  156 . Due to this fact, the block from divider  152  to power combiner/divider  157  shown in  FIG. 1  is called a distortion detection circuit. 
   Next, an explanation regarding distortion elimination circuit  151  will be given. The output of distortion elimination circuit  150  is divided, via power combiner/divider  157 , into main signal path  158  and distortion injection path  159 . The output of main amplifier  156  from main amplifier signal path  153  (the signal passing through main amplifier signal path  153 ) is input into main signal path  158 . Also, the differential component of main amplifier  156  detected in distortion detection circuit  150  (the differential component of the signal passing through main amplifier signal path  153  and the signal passing through linear signal path  154 ) is input into distortion injection path  159 . As for variable phase shifter  200   a  and variable attenuator  200   b  of distortion injection path  159 , the distortion components of the signal passing through main signal path  158  and the signal passing through distortion injection path  159  are adjusted so as have equal amplitude and opposite phase. By making an adjustment in this way, combiner  202  can combine the signal passing through main amplifier signal path  153  with the distortion component of main amplifier  156  having equal amplitude and opposite phase. And then, combiner  202  outputs a signal in which the distortion components of the whole amplifier are cancelled. Further, even if it is a matter of common knowledge, a linear amplifier is used as an auxiliary amplifier in order to eliminate the distortion component generated in the main amplifier used in a feed forward amplifier. The aforementioned operation is an ideal operation of a feed forward amplifier. In practice, it is not simple to completely maintain a balance of the distortion detection circuit and the distortion elimination circuit. Also, even if tentatively the initial settings are perfect, since the properties of the amplifier change due to fluctuations in ambient temperature, power supply, and the like, it is extremely difficult to preserve an excellent balance which is stable over time. 
   As methods for maintaining a highly accurate balance of the distortion detection circuit and the distortion elimination circuit of this feed forward amplifier, there is known an self-adjusting method using a pilot signal. E.g., there exist the Japanese Patent Application Laid-Open Publication No. 1 (1989)-198809 (Patent Reference 1) and the like. As devices putting these methods into practical use, there is known the article “Extremely Low-Distortion Multi-Carrier Amplifier For Mobile Communication Systems—Self-adjusting Feed-Forward Amplifier (SAFF-A)” by Toshio Nojima and Shoichi Narahashi, Institute of Electronics, Information, and Communication Engineers, Wireless Communication Systems Society, RCS90-4, 1990 (Non-patent Reference 1). These feed forward amplifiers were put into practice in the 800 MHz band and the 1.5 GHz band of the PDC (Personal Digital Cellular) mobile communications standard in Japan. This kind of feed forward amplifier is generally designed and adjusted to amplify separately for each frequency band. 
   The feed forward amplifiers of Japanese Patent Application Laid-Open Publication No. 2000-223961 (Patent Reference 2) and Japanese Patent Application Laid-Open Publication No. 2001-284975 (Patent Reference 3) fragment a single transmission band, e.g. 20 MHz inside the 2 GHz band, by means of a plurality of band pass filters, and amplify the fragmented and extracted signals. And then, the same amplifier compensates, separately for each fragmented frequency, the amplitude divergence and the phase divergence generated in the amplifier to raise the distortion compensation accuracy. 
   In the radio systems developed this far, a single system in accordance with any one of PDC, GSM (Global System for Mobile communications), IMT-2000 (International Mobile Telecommunications 2000), and the like, was used. As against this, there is the technology of carrying out a transfer to software of some functionality of radio devices so that it becomes possible for a single hardware to handle a plurality of radio systems. If it is possible for a single hardware to handle a plurality of radio systems, the user can use the mobile communication environment without any awareness of the radio system or the core network in the background thereof. However, a single hardware actually handling a plurality of radio systems is something that has not reached implementation. 
   Also, it can be considered that, for each region or operator, the services offered with the radio system will be different and that the radio systems will also gradually become diversified. For this reason, it can be considered that, in the future, there will arise a need to make radio systems coexist which are optimal for each purpose, at the same time and in the same place. 
   As methods of using these plural radio systems, there is the multiband radio system. This radio system adaptively changes the frequency band used or the number of frequency bands used in response to the propagation environment and the traffic conditions. Also, in order to ensure a prescribed transmission quality or transmission volume, multiband transmission using frequency bands not in use is effective. Consequently, in a multiband radio system, in order to ensure the transmission quality or transmission volume to be guaranteed by the same radio system, the number of frequency bands is changed. Moreover, changes are also carried out in the same way within the same frequency band. Further, a multiband radio system, in case there coexist frequency bands used by several operators, can raise the frequency utilization efficiency by carrying out adaptive control using available frequency bands by means of interference recognition technology, frequency sharing technology, interference cancellation technology, produced interference reduction and avoidance technology, multiband control technology, and the like. 
   The feed forward amplifier is used as a linear amplifier for base stations handling multiband radio systems like this. However, in case the plural frequency bands to be amplified are widely separated, compared to the bandwidth of each frequency band, the adjustment levels of the variable phase shifter and the variable attenuator for keeping the balance of the distortion detection circuit and the distortion elimination circuit within a designated range vary with the frequency band to be amplified, because the electrical length of the delay line for each frequency band differs. 
   To put it in concrete terms, in case a delay line is used in common for all frequency bands, there is, due to the frequency differences of the input signals, ordinarily a need for the setting value of the vector adjuster to track a signal rotating with the angular velocity of the frequency difference. However, in the vector adjusters developed this far, it has not been possible to track a signal rotating at a velocity like that. Also, as for the vector adjusters discussed this far, it has not been possible to simultaneously set an optimal amplitude and phase, with respect to plural input signals, for structural reasons. 
   E.g., in case 800 MHz band and 1.5 GHz band signals are input into the same vector adjuster, it is possible to carry out optimal vector adjustment with respect to any one of the frequency bands. However, it is not possible to carry out optimal vector adjustment which tracks a frequency difference of 700 MHz. Consequently, the conventional feed forward amplifier has not been able to simultaneously amplify the 800 MHz band signal and the 1.5 GHz band signal at or below a prescribed distortion compensation level. 
   As a method of resolving this, a dual-band feed forward amplifier is proposed in the article “A Dual-Band Feed-Forward Amplifier” by Yasunori Suzuki and Shoichi Narahashi, the 2005 General Meeting of the Institute of Electronics Information and Communication Engineers, C-2-2, March 2005 (Non-patent Reference 2). With this configuration, there is proposed, for each frequency band, a vector adjuster having a band extraction means. In other words, this dual-band feed forward amplifier extracts the signal of the vector adjusted frequency band from the input signals of two frequency bands by means of a filter provided in a pre-stage of the vector adjuster. And then, vector adjustment is carried out for each frequency band. This dual-band feed forward amplifier configuration is capable of distortion compensation in a plurality of frequency bands. Further, the compensated band is fixed by the filter. 
   In multiband radio systems having a plurality of transmission bands, it can be considered to change the frequency band due to the service situation of the radio system, interference of other radio systems, and the like. However, as mentioned above, the bandwidth of the distortion compensation of the feed forward amplifier is determined by the adjustment accuracy of each loop of the distortion detection circuit and the distortion elimination circuit. Consequently, in the conventional feed forward amplifier, the adjustment of distortion compensation could not be made to correspond with the frequency band changes. Also, it was not possible for the conventional dual-band feed forward amplifier in which the distortion compensated frequency band was fixed to adaptively change the operating frequency. For a feed forward amplifier used over a long time, the change in frequency band accompanies repairs or a change of the feed forward amplifier in the base station. Consequently, an enormous amount of labor and time is required to readjust a large number of feed forward amplifiers. A feed forward amplifier configuration making this kind of labor and time expense unnecessary was required. 
   E.g., in case, for a dual-band feed forward amplifier which simultaneously compensates the distortion of a signal in a frequency band f 1  and a signal in a frequency band f 2 , the frequency band was changed from f 2  to f 3 , it has not been possible to simultaneously compensate the distortion of the signal in frequency band f 1  and the signal in frequency band f 3 . This was so because loop adjustment by the frequency difference of f 1  and f 3  was not possible, as mentioned above, due to the fact that the operating frequencies of a conventional dual-band feed forward amplifier are fixed. 
   Also, there can be considered the method of providing, in the dual-band feed forward amplifier, fixed filter and vector adjusters handling all the frequency bands that may be thought to be available for future service. However, having fixed filters and vector adjusters able to handle all the frequency bands amounts to having fixed filters and vector adjusters which are not used something which runs counter to configuring a cost-effective feed forward amplifier. There was demanded a feed forward amplifier with no need for the exchange of constituent parts and having no redundancy to accompany in this way the changes in frequency band or the increase and/or decrease in the number of carrier waves. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention pertains to a feed forward amplifying device comprising a distortion detection circuit and a distortion elimination circuit. First variable frequency band extractors, extracting specific frequency bands, and first vector adjusters are respectively provided in N first vector adjustment paths of the distortion detection circuit. Also, N second variable is frequency band extractors, extracting specific frequency bands, and second vector adjusters are respectively provided in N second vector adjustment paths of the distortion elimination circuit. And then, the frequency bands extracted with the N first variable frequency band extractors and the N second variable frequency band extractors are adaptively controlled by a frequency control part. 
   According to the present invention, it is possible to implement a feed forward amplifier making possible adaptive distortion compensation, even with respect to changes among a plurality of frequency bands. The configuration of a feed forward amplifier collectively amplifying a plurality of frequency bands is simplified, so a reduction in power consumption can be implemented. According to the configuration of the present invention, it is possible to make an adjustment to a specific distortion compensation level for each frequency band, independently of the electric length difference of the delay lines constituting a linear signal path. By the fact that it is possible to change the center frequency or the bandwidth of the frequency band extractor, an adjustment can be made to a specific compensation level for each frequency band. 
   In this way, the feed forward amplifier of the present invention is capable of linearly amplifying a frequency band corresponding to the service situation of the radio system. Consequently, the present invention can make unnecessary additional equipment that would accompany a change in the frequency band and an increase in carrier waves. 
   Also, the present invention uses variable filters in the feed forward amplifier in order to make changes in the frequency band simple and inexpensive. Also, the pass band of the variable filter is controlled so as to match the used band. Consequently, even with one feed forward amplifier, it is possible to handle the frequency changes of the radio system. Also, since the feed forward amplifier of the present invention can also switch the operating band due to an instruction from an operations center to switch the frequency bands, the need to spend an enormous amount of labor on radio system adjustments disappears. In addition, since the feed forward amplifier of the present invention can detect the frequency band of the received signal and automatically switch the frequency band, it is also possible to change the frequency band dynamically, even if the transmitter side changes the frequency band. Also, rather than configuring a feed forward amplifier for each frequency band, the case of making an implementation with a single feed forward amplifier is more advantageous from the viewpoints of device scale and power consumption. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing the basic configuration of a conventional feed forward amplifier. 
       FIG. 2  is a diagram showing a configuration example of a multiple frequency band signal processing circuit used in the feed forward amplifier of the present invention. 
       FIG. 3  is a diagram showing a conceptual diagram of distortion compensation in the case of configuring a variable frequency band extractor with a variable band pass filter. 
       FIG. 4  is a diagram showing an example of the attenuation level in the case where the variable frequency band extractor is configured with a variable band elimination filter. 
       FIG. 5  is a diagram showing the cascade connection of a band elimination filter. 
       FIG. 6  is a diagram showing a configuration example of filtering based on four filters. 
       FIG. 7  is a diagram showing the frequency characteristics of the filtering. 
       FIG. 8  is a diagram showing the frequency characteristics of the filtering. 
       FIG. 9  is a diagram showing a concrete configuration example of a multiple frequency band processing circuit. 
       FIG. 10  is a diagram showing a second configuration example of a multiple frequency band processing circuit. 
       FIG. 11  is a diagram showing the first embodiment of a feed forward amplifier in accordance with the present invention. 
       FIG. 12  is a diagram showing the second embodiment of a feed forward amplifier in accordance with the present invention. 
       FIG. 13  is a diagram showing the third embodiment of a feed forward amplifier in accordance with the present invention. 
       FIG. 14  is a diagram showing the fourth embodiment of a feed forward amplifier in accordance with the present invention. 
       FIG. 15  is a diagram showing the fifth embodiment of a feed forward amplifier in accordance with the present invention. 
       FIG. 16  is a diagram showing a functional configuration example of a band detector. 
       FIG. 17  is a diagram showing an example of the spectrum of the input signal of a feed forward amplifier. 
       FIG. 18  is a diagram showing the relationship between the sweep frequency and the frequency of an input signal. 
       FIG. 19  is a diagram showing the time variation of a signal output from a local oscillator. 
       FIG. 20  is a diagram showing the time variation of a signal output from a low-pass filter. 
       FIG. 21  is a diagram showing that the bandwidth of a detected frequency band becomes narrower in the case where a threshold value is set in the output of the low-pass filter. 
       FIG. 22  is a diagram showing the sixth embodiment of a feed forward amplifier in accordance with the present invention. 
       FIG. 23  is a diagram showing the seventh embodiment of a feed forward amplifier in accordance with the present invention. 
       FIG. 24  is a diagram showing the eighth embodiment of a feed forward amplifier in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 2 , there is shown the principle of the multiple frequency band signal processing circuit of a feed forward amplifier of the present invention. This multiple frequency band signal processing circuit includes a linear signal path  20  constituted by a delay line, respective variable frequency band vector adjustment paths  21   a  and  21   b,  multiple frequency band amplifying parts  22   a  and  22   b  amplifying the signals of the respective variable frequency band vector adjustment paths  21   a  and  21   b,  a dividing part  23  dividing input signals into the linear signal path and the respective variable frequency band vector adjustment paths, and a combining part  24  combining the output of multiple frequency band amplifying parts  22   a  and  22   b  and the output of linear signal path  20 . 
   First variable frequency band vector adjustment path  21   a  includes a first variable frequency band extractor  25   a  extracting a first frequency band signal at a center frequency f 1  and a vector adjuster  26   a  adjusting the amplitude and phase of the first frequency band signal. Second variable frequency band vector adjustment path  21   b  includes a second variable frequency band extractor  25   b  extracting a second frequency band signal at a center frequency f 2  and a vector adjuster  26   b  adjusting the amplitude and phase of the second frequency band signal. The outputs of these vector adjusters  26   a  and  26   b  are amplified by multiple frequency band amplifying parts  22   a  and  22   b.    
   In  FIG. 2 , it is indicated with dotted lines that yet other variable frequency band vector adjustment paths may be provided. Each of these vector adjusters is left not illustrated but, similarly to variable frequency band vector adjustment paths  21   a  and  21   b,  they are constituted by a series connection of a variable frequency band extractor, a vector adjuster, and a multiple frequency band amplifying part. Dividing part  23  divides the input signal into linear signal path  20  and first and second variable frequency band vector adjustment paths  21   a,    21   b,  etc. Combining part  24  combines the outputs of the same paths. It is possible to configure the feed forward amplifier of this invention by applying the multiple frequency band signal processing circuit shown in  FIG. 2  to distortion detection circuit  150  and distortion elimination circuit  151  of the feed forward amplifier explained in  FIG. 1 . 
   An explanation will be given for the case where e.g. frequency band f 1  corresponds to the 800 MHz band, frequency band f 2  corresponds to the 1.5 GHz and where further the 2 GHz band is used as frequency band f 3  and the 5 GHz band is used as frequency band f 4 . These frequency bands are sufficiently separated from one another, compared to the bandwidth of each frequency band, so a variable frequency band extractor is provided for each respective frequency band. Variable frequency band extractors  25   a,    25   b,    25   c,  and  25   d  respectively extract the signals of each frequency band. Vector adjusters  26   a,    26   b,    26   c,  and  26   d  respectively adjust the vectors of the signals of each frequency band. Multiple frequency band amplifying parts  22   a,    22   b,    22   c,  and  22   d  respectively amplify the signals of each frequency band. Combining part  24  combines the outputs from multiple frequency band amplifying parts  22   a,    22   b,    22   c,  and  22   d  and the output of linear signal path  20 . 
   In  FIG. 3 , there is shown conceptually the distortion compensation in the case where the first and second variable frequency band extractors are constituted by variable band pass filters. The frequency bands respectively having center frequencies of f 1  and f 2  are respectively sufficiently separated, making distortion compensation possible in the respective frequency bands. 
   First and second variable frequency band extractors  25   a  and  25   b  extract signals in the respective first frequency band and second frequency band so that the bands have desired bandwidths with respective center frequencies f 1  and f 2 . Each variable frequency band extractor of this kind may e.g. be constituted by a variable band pass filter (BPF) or may be constituted by a variable band elimination filter (BEF). 
     FIG. 4  shows an example of the frequency characteristics of the attenuation level in the case where the first variable frequency band extractor is constituted by a variable band elimination filter. This example conceptually shows the characteristics required of first variable frequency band extractor  25   a  in the case where frequency bands f 3  and f 4  are also added to the multiple frequency band signal processing circuit of  FIG. 2 . These characteristics can, as shown in  FIG. 5 , be formed with three band elimination filters BEF 2 , BEF 3 , and BEF 4  eliminating the second, third, and fourth frequency bands, all the frequency bands except for the first frequency band. It is desirable for each band elimination filter to have sufficient elimination characteristics in the band thereof and to have sufficiently low-loss pass characteristics in the other bands. A band elimination filter of that kind can e.g. be constituted by a notch filter. When it comes to notch filters, there are band elimination filters using dielectric resonators and filters using stubs based on micro strip lines. In the same way, second variable frequency band extractor  25   b  can be formed with three band elimination filters eliminating the first, third, and fourth frequency bands. It is similar for the third and fourth frequency band extractors as well. In this way, the multiple frequency band signal processing circuit is not one in which the number of applicable frequency bands is limited to two, but in order to simplify the explanation below, the case where the number of frequency bands is 2 will be explained. 
   The advantage of configuring a frequency band extractor with a band pass filter is that it is easy to extract the band periphery of the center frequency and also that it is comparatively easy to obtain isolation from the center frequency. However, since the center frequency becomes the oscillation frequency of the band pass filter, the delay of the signal increases. Consequently, there is a need to extend the delay line constituting linear signal path  20  of  FIG. 2  to match the delay quantity thereof, so there is the disadvantage that the attenuation level also increases. In the case that the frequency band extractor is configured with a band elimination filter, the extracted frequency band is not the center frequency of the band elimination filter. Consequently, the delay in the extracted frequency band is small. Accordingly, there are the advantages that the line length of linear signal path  20  becomes shorter and has lower loss. Further, the design of band elimination filters is also simple. 
   As for variable band pass filters and variable band elimination filters, it is possible to change the center frequency or the bandwidth thereof. In the case of filters made with micro strip lines, there is the method of changing the resonator length by means of a diode, a MEMS (Microelectromechanical System) switch, or the like, to change the center frequency. As a method of varying the bandwidth of a band pass filter, there is the method of switching to a filter bank with a different number of center frequencies. In  FIG. 6 , an example of a filter bank having four filters is shown. A frequency band controller  32  controls the number of operated filters by turning on and off switches  30  and  31  located before and after the filters. In  FIG. 7 , the frequency characteristics of the filter bank in the cases that only filter BPF 1  is operated are shown. In  FIG. 8 , the frequency characteristics of the filter bank in the case that filter BPF 1  and filter BPF 2  are operated are shown. Since BPF 1  and BPF 2  have adjacent frequency characteristics, the result is that the frequency characteristics of the filter bank are the combined frequency characteristics of BPF 1  and BPF 2 . In this way, by using a filter bank, the pass bandwidth can be modified. As a method of varying the bandwidth of a band elimination filter, there is the method of switching, with a diode, a MEMS switch, or the like, a resonator based on micro strip lines. 
   The line length of linear signal path  20  is designed so that, on the input side of combining part  24 , the signal delay quantity due to linear signal path  20  and the delay quantity due to variable frequency band vector adjustments paths  21   a  and  21   b  become equal. First vector adjuster  26   a  controls the phase and amplitude of the signal of first variable frequency band vector adjustment path  21   a  so that the first frequency band f 1  component of the output signal of linear signal path  20  and the output of multiple frequency band amplifying part  22   a  have equal amplitude and opposite phase. In the same way, second vector adjuster  26   b  controls the phase and amplitude of the signal of second variable frequency band vector adjustment path  21   b  so that the second frequency band f 2  component of the output signal of linear signal path  20  and the output of multiple frequency band amplifying part  22   b  have equal amplitude and opposite phase. By means of this adjustment, combining part  24  can output the differential component and the additive component of the output of linear signal path  20  and the outputs of variable frequency band vector adjustment paths  21   a  and  21   b.    
   Vector adjusters  26   a  and  26   b  of first and second variable frequency band vector adjustment paths  21   a  and  21   b  of the multiple frequency band signal processing circuit of  FIG. 2  are respectively adjusted taking linear signal path  20  as a reference. As a result of this, it is possible to perform vector adjustment independently with respect to frequency band f 1  and frequency band f 2 . 
   Below, an explanation is given of a more specific example of a multiple frequency signal processing circuit. Further, in the explanation below, the parts given names ending by -er/-or can of course be constituted by physical circuits, but it is also possible to implement the same by means of arithmetic processors and software. 
     FIG. 9  is a specific first configuration example of the multiple frequency band signal processing circuit shown in  FIG. 2 . This first configuration example is composed of multiple frequency band amplifying part  22  of  FIG. 2 , individual amplifiers  80   a  and  80   b  for the respective frequency bands, and a combiner  81  combining the outputs of the same amplifiers and taking this to be the output of the multiple frequency band amplifying part. Also, a divider  82  is composed of a divider  82   a  and a divider  82   b.  Divider  82   a  divides the input signal into two, distributing one to linear signal path  20  and the other to divider  82   b.  Divider  82   b  further divides the signal distributed from divider  82   a  into signals for each variable frequency band vector adjustment circuit. The adjustment of the signal vector due to the vector adjustment path of each variable frequency band and the differential component and the additive component obtained thereby at the output terminal of combiner  24  are the same as in the case of  FIG. 2 , so an explanation thereof is omitted. As for combiner  24 , a directional coupler, a Wilkinson power combiner, or the like, can be used. Frequency band controller  32  controls variable band pass filters  25   a  and  25   b  by a control signal from an operations center or a band detector. 
   In  FIG. 10 , a second configuration example of a multiple band signal processing circuit is shown. The difference between  FIG. 10  and  FIG. 9  is that after the outputs of vector adjusters  26   a  and  26   b  have been combined with a combiner  90 , the output is amplified with a common amplifier  91 . The other parts are the same as the corresponding parts in  FIG. 9 , so an explanation thereof will be omitted. 
   In case the frequency is changed e.g. from frequency band f 1  to frequency band f 2 , frequency band controller  32  changes the pass band of variable band pass filter  25   a  from f 1  to f 2  by a control signal from an operations center or a band detector. At this point, variable band pass filter  25   a  changes the center frequency by means of a change in the resonator structure. By proceeding in this way, it is possible to adaptively change a once set operating band of the power amplifier. In other words, the configuration of the power amplifier of the present invention makes unnecessary new equipment investment accompanying a change in the frequency band. 
   A base station handling a plurality of radio systems is provided with transmitters and receivers handling a plurality of radio systems. A plurality of transmitter output signals are power amplified by means of the multiband feed forward amplifier of the present invention. In the case that, within the area in which the base station is offering mobile communication services, cell interference increases because the number of subscribers increases or for some other reason, the operations center monitoring the concerned base station carries out an instruction to the concerned base station to change some of the radio systems. 
   Also, accompanying an increase or a reduction in the number of carrier waves used in frequency band f 1 , frequency band controller  32  increases or reduces the pass bandwidth of variable band pass filters  25   a,    25   b  based on a control signal from the operations center or the band detector. An increase/reduction of the pass bandwidth like this can be implemented, as shown in  FIG. 6 , by changing the number of filters in the filter banks of variable band pass filters  25   a  and  25   b.  In this way, it becomes possible, even in the case of carrying out an increase or reduction in the number of carrier waves in response to fluctuations in the communication traffic, to suppress to a minimum cell interference occurring due to the increase in the number of carrier waves. 
   1. FIRST EMBODIMENT 
   In  FIG. 11 , Embodiment 1 of a feed forward amplifier according to the present invention is shown. In order to simplify the drawings and explanations for all the embodiments below, the explanation will be given taking the number of used frequency bands to be 2, but in general, two or more frequency bands may be used. 
   In the explanation below, “1-” is attached in front of reference numerals of the multiple frequency band signal processing circuit forming the distortion detection circuit and “2-” is attached in front of reference numerals of the multiple frequency band signal processing circuit forming the distortion elimination circuit. However, in the case of intrinsic numerals, the notation is not used. 
   A combiner  1 - 24  of the multiple frequency band signal processing circuit constituting distortion detection circuit  150  functions as a combiner/divider  100 , together with a divider  2 - 23  of the multiple frequency band signal processing circuit constituting distortion elimination circuit  151 . Also, a multiple frequency band amplifying part composed of individual amplifiers  1 - 80   a  and  1 - 80   b  of distortion detection circuit  150  constitutes a main amplifier  1 - 156  in the feed forward amplifier. Each individual amplifier  1 - 80   a  and  1 - 80   b  is a power amplifier. The multiple frequency band amplifying part of distortion elimination circuit  151  constitutes an auxiliary amplifier  101  of the feed forward amplifier. Individual amplifiers  2 - 80   a  and  2 - 80   b  are linear amplifiers. 
   Combiner/divider  100  obtains, at an output terminal thereof, the differential component of the output of linear signal path  1 - 20  and the combined output of vector adjustment paths  1 - 21  and outputs it to a divider  102  of distortion elimination circuit  151 . Also, combiner/divider  100  obtains the additive component of the output of linear signal path  1 - 20  and the output of combiner  1 - 24  and outputs the same to linear signal path  2 - 20  of distortion elimination circuit  151 . Since main amplifier  1 - 156 , which is composed of individual amplifiers  1 - 80   a  and  1 - 80   b,  generates intermodulation distortion when amplifying the signal, the differential component output by combiner/divider  100  to the divider  102  side works out to the distortion component occurring due to individual amplifiers  1 - 80   a  and  1 - 80   b.  Moreover, as for the additive component output by combiner/divider  100  to linear signal path  2 - 20  (main signal path) side, the multiple frequency band input signal and the combined signal of the output signals of the individual amplifiers are output. 
   A combiner  104  of distortion elimination circuit  151  outputs the output of linear signal path  2 - 20  and the differential component of the combined output of the respective frequency band vector adjustment paths. Consequently, the distortion component generated by the main amplifier and included in the output of the linear signal path is cancelled by the combined output of vector adjustment paths  2 - 26   a  and  2 - 26   b,  so the signal component of the multiple frequency bands is output to the terminal. 
   In order to implement a distortion elimination quantity with a distortion elimination circuit  151  like this, distortion detection circuit  150  and distortion elimination circuit  151  may perform vector adjustment based on the multiple frequency band signal processing circuit explained in  FIG. 2 . 
   The feed forward amplifier of Embodiment 1 uses a vector adjuster for each frequency band. Consequently, it is possible to carry out distortion compensation independently for each frequency band. The vector adjusters adjust the amplitude and phase of the signals passing through each vector adjuster so that the signals have the same amplitude, opposite phase, and same delay with respect to the delay lines of distortion detection circuit  150  and distortion elimination circuit  151 . 
   The distortion compensation level in the case of amplifying two frequency band signals by means of the feed forward amplifier of  FIG. 11  has the characteristics shown in  FIG. 3 . In the feed forward amplifier of this invention, the main amplifier distortion components included in the amplified signals of each frequency band with respective center frequencies f 1  and f 2  adjust, for each frequency band, the vector adjusters of distortion detection circuit  150  and distortion elimination circuit  151  so as to respectively be at or below a prescribed value (target value). If the isolation of each vector adjustment circuit is provided sufficiently, no influence is exerted, even if the vector adjuster of one frequency band is adjusted, on the vector adjuster of the other frequency band. In other words, it is possible to independently adjust the vector adjusters of a plurality of frequency bands. Also, by the addition of vector adjustment paths, it is possible to flexibly add frequency bands which are distortion compensated by the feed forward amplifier. 
   Among the first variable frequency band extracting means  1 - 25   a  and  2 - 25   a  and the second variable frequency band extracting means  1 - 25   b  and  2 - 25   b,  of the feed forward amplifier shown in Embodiment 1, it is acceptable to take any one to be a variable frequency extracting means, the others being frequency extracting means not changing the frequency. 
   First and second variable frequency band extracting means  1 - 25   a,    1 - 25   b,    2 - 25   a,  and  2 - 25   b  change the center frequency or the pass bandwidth by means of instructions of frequency band controller  32 . Frequency controller  32 , by a signal from an operations center, changes the center frequency or the bandwidth of the frequency band amplified in the feed forward amplifier. These control periods or control speeds differ depending on the respective radio system. Since the initial retraction operation related to the distortion compensation of the feed forward amplifier has a high speed, it is possible to change the settings of the first and second variable frequency band extracting means if the control period or the control speed is at least equal to or greater than the initial retraction operation time. 
   2. SECOND EMBODIMENT 
   In  FIG. 12 , Embodiment 2 is shown. Embodiment 2 is an embodiment in which the multiple frequency band signal processing circuit shown in  FIG. 10  has been applied as distortion elimination circuit  151 . In the Embodiment 2 feed forward amplifier as well, vector adjustment is performed using vector adjusters  1 - 26   a,    1 - 26   b,    2 - 26   a,  and  2 - 26   b  for each frequency band. If sufficient isolation is provided between the vector adjustment paths, there is no influence exerted on the vector adjusters of the other frequency bands, even if the vector adjuster of one frequency band is adjusted. Consequently, it is possible to carry out distortion compensation independently for each frequency band. Also, if vector adjustment paths are added, it is possible to flexibly add frequency bands which are distortion compensated. 
   An auxiliary amplifier  2 - 156  of the distortion elimination circuit has one common amplifier  2 - 91  which simultaneously amplifies a plurality of frequency bands, as shown in  FIG. 10 . Consequently, there can be expected a simplification and a reduction in power consumption of the device configuration, based on a reduction in the used number of parts in the amplifier. 
   3. THIRD EMBODIMENT 
   In  FIG. 13 , Embodiment 3 is shown. Embodiment 3 is an example using the multiple frequency band signal processing circuit shown in  FIG. 10  as distortion detection circuit  150 . The feed forward amplifier of Embodiment 3 also carries out vector adjustment by using, for each frequency band, vector adjusters  1 - 26   a,    1 - 26   b,    2 - 26   a,  and  2 - 26   b.  If sufficient isolation is provided between the vector adjustment paths, there is no influence exerted on the other vector adjusters, even if the vector adjuster of one frequency band is adjusted. Consequently, it is possible to perform distortion compensation independently for each frequency band. Also, if vector adjustment paths are added, it is possible to flexibly add frequency bands which are distortion compensated. 
   Main amplifier  1 - 156  of distortion detection circuit has one common amplifier  1 - 91  which simultaneously amplifies a plurality of frequency bands, as shown in  FIG. 10 . Consequently, there can be expected a simplification and a reduction in power consumption of the device configuration, based on a reduction in the used number of parts in the amplifier. 
   4. FOURTH EMBODIMENT 
   In  FIG. 14 , Embodiment 4 is shown. Embodiment 4 is an embodiment where the multiple frequency band signal processing shown in  FIG. 10  has been applied both to distortion detection circuit  150  and distortion elimination circuit  151 . Main amplifier  1 - 156  of distortion detection circuit  150  is constituted by one common amplifier  1 - 91  which simultaneously amplifies a plurality of frequency bands, and auxiliary amplifier  2 - 156  of distortion elimination circuit  151  also has one common amplifier  2 - 91  which simultaneously amplifies a plurality of frequency bands. Consequently, there can be expected a simplification and a reduction in power consumption of the device configuration, based on a reduction in the used number of parts in the amplifier. 
   5. FIFTH EMBODIMENT 
   In  FIG. 15 , Embodiment 5 is shown. Embodiment 5 is a configuration with a band detector  33  added to the configuration of  FIG. 11 . In the case of this configuration, a divider  1 - 82   a  distributes a portion of the input signal to band detector  33  as well. Band detector  33  detects the frequency band of the input signal by the method shown below and outputs a control signal to frequency band controller  32 . The operation of the other constituent parts is the same as for Embodiment 1. 
   In  FIG. 16 , there is shown a functional configuration example of band detector  33 . Band detector  33  is composed of a local oscillator frequency controlling part  331 , a local oscillator  332 , a mixer  333 , a low band pass filter  334 , and an analyzing part  335 . Local oscillator frequency controlling part  331  controls local oscillator  332  so as to continuously sweep the frequency from the lower-limit frequency of the input signal to the upper-limit frequency. Following an instruction of local oscillator frequency controlling part  331 , local oscillator  332  oscillates. Mixer  333  multiplies the input signal distributed from divider  1 - 82   a  and the signal from local oscillator  332 . The output signal from mixer  333  includes the frequency component of the difference of the frequency of the input signal and the frequency of the signal from local oscillator  332 . In other words, in the case that the frequency of the input signal and the frequency of the signal from local oscillator  332  are very close, the near-DC component (the low-frequency component) is included in the output from mixer  333 . Low band pass filter  334  only lets through the low-frequency component of the output from mixer  333 . Consequently, only in the case that the frequency of the input signal and the frequency of the signal from local oscillator  332  are very close is the band detector output signal obtained from low band pass filter  334 . Analyzing part  335  compares the frequency sweep signal from local oscillator frequency controlling part  331  with the band detector output signal from low band pass filter  334 , detects the frequency band of the input signal, and outputs the control signal to frequency band controller  32 . 
   In  FIG. 17 , there is shown an example of the input signal spectrum of a feed forward amplifier. The center frequency of the first frequency band is taken to be f 1 , the lower-limit frequency is taken to be f 1 L, and the upper-limit frequency is taken to be f 1 H. The center frequency of the second frequency band is taken to be f 2 , the lower-limit frequency is taken to be f 2 L, and the upper-limit frequency is taken to be f 2 H. In  FIG. 18 , the relationship between the sweep frequency and the input signal frequency is shown. The abscissa axis represents the sweep frequency and the ordinate axis represents the input signal frequency. This diagram shows that, in the case that the sweep frequency lies between frequency f 1 L and f 1 H, or between f 2 L and f 2 H, a near-DC signal from low band pass filter  334  is output. In  FIG. 19 , there is shown the time variation of the signal output from local oscillator  332 . The abscissa axis represents time and the ordinate axis represents the output from local oscillator  332 . In  FIG. 20 , there is shown the time variation of the output from low band pass filter  334 . The abscissa axis represents time and the ordinate axis represents the power from low band pass filter  334 . As shown in  FIG. 20 , in case the frequency of the output from local oscillator  332  corresponds to a range from frequency f 1 L to frequency f 1 H, or a range from frequency f 2 L to frequency f 2 H, the output from low band pass filter  334  is obtained. 
   Further, if a threshold value is set for the output from low band pass filter  334 , the bandwidth of the frequency band becomes narrower, as shown in  FIG. 21 . Consequently, by multiplying, in analyzing part  335 , the obtained lower-limit frequencies f 1 L and f 2 L and the upper-limit frequencies f 1 H and f 2 H by predetermined coefficients, each frequency may be corrected. 
   Also, local oscillator frequency controlling part  331  and analyzing part  335  can be implemented with an analog/digital converter and a microprocessor. As for local oscillator  332 , generally used signal oscillators and the like may be used. Mixer  333  and low band pass filter  334  can be implemented with active filters using LC filters or operational amplifiers. 
   Since band detector  33  operates like this even in the case where the input signal is changed dynamically, the feed forward amplifier can respond adaptively. The time required to change the frequency band processed by the feed forward amplifier depends on the period of the signal swept by local oscillator  332 . In case a high-speed frequency change is required, the period of the signal swept by local oscillator  332  may be shortened. 
   6. SIXTH EMBODIMENT 
   In  FIG. 22 , Embodiment 6 is shown. Embodiment 6 is a configuration in which band detector  33  has been added to the configuration of  FIG. 12 . The configuration and operation of band detector  33  are the same as in Embodiment 5, and otherwise, the embodiment is the same as Embodiment 2. 
   7. SEVENTH EMBODIMENT 
   In  FIG. 23 , Embodiment 7 is shown. Embodiment 7 is a configuration in which band detector  33  has been added to the configuration of  FIG. 13 . The configuration and operation of band detector  33  are the same as in Embodiment 5, and otherwise, the embodiment is the same as Embodiment 3. 
   8. EIGHTH EMBODIMENT 
   In  FIG. 24 , Embodiment 8 is shown. Embodiment 8 is a configuration in which band detector  33  has been added to the configuration of  FIG. 14 . The configuration and operation of band detector  33  are the same as in Embodiment 5, and otherwise, the embodiment is the same as Embodiment 4. 
   INDUSTRIAL APPLICABILITY 
   The feed forward amplifier for multiple frequency bands using a multiple frequency band signal processing circuit of this invention can be utilized in a power amplifier for mobile communications transmitting signals in a plurality of frequency bands.