Abstract:
An amplification device includes: a first circuit configured to: split an input signal into a first signal and a second signal, and adjust the first signal so that an amplitude of the first signal is less than an amplitude of the second signal by a reduced value, a first amplifier configured to amplify the adjusted first signal, a second amplifier configured to amplify the second signal, and a second circuit configured to: determine a reflection coefficient in case where the amplified second signal is a travelling wave and the amplified first signal is a reflected wave, and determine the reduced value based on the reflection coefficient.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-018788, filed on Feb. 3, 2014, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an amplification device and an amplification method. 
     BACKGROUND 
     Amplification devices configured to amplify transmission power have heretofore been used in various electronic apparatuses including base stations in a mobile communication system. In particular, with an increase in the speed of telecommunications in recent years, there is a demand for amplifying transmission power with higher efficiency from the viewpoint of saving power consumption and the like. It is known that an amplification device achieves the highest efficiency in a power saturation state (a non-linear state). An amplification device which employs outphasing (hereinafter referred to as an “outphasing amplification device”) is proposed as an amplification device adapted to achieve such high efficiency. Examples of a combiner used in the outphasing amplification device include a Chireix combiner. The Chireix combiner has an asymmetrical configuration (transmission line (TL)=90±0 deg, for example) to improve power efficiency characteristics at an output back-off power point. 
     Such a technique is described, for example, in Japanese Laid-open Patent Publication No. 2007-174148. 
     SUMMARY 
     According to an aspect of the invention, an amplification device includes: a first circuit configured to: split an input signal into a first signal and a second signal, and adjust the first signal so that an amplitude of the first signal is less than an amplitude of the second signal by a reduced value, a first amplifier configured to amplify the adjusted first signal, a second amplifier configured to amplify the second signal, and a second circuit configured to: determine a reflection coefficient in case where the amplified second signal is a travelling wave and the amplified first signal is a reflected wave, and determine the reduced value based on the reflection coefficient. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a configuration of an amplification device of an embodiment; 
         FIG. 2  is a view illustrating loci of reflection coefficients when input amplitude values of amplifiers are equal to each other; 
         FIG. 3  is a view illustrating loci of reflection coefficients when the input amplitude values of the amplifiers are different from each other; 
         FIG. 4  is a block diagram illustrating a configuration of an amplification device of a modified example; 
         FIG. 5  is a view illustrating aspects of variations in load on the amplifiers caused by the amplification device; 
         FIG. 6  is a view illustrating an aspect of improvement in dynamic range of output power caused by the amplification device; and 
         FIG. 7  is a view illustrating an application example of the amplification devices of the embodiment and the modified example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Conventional outphasing amplification devices have a problem of a narrow dynamic range of output power because input impedance of the Chireix combiner viewed from the two amplifiers is asymmetrical due to high output amplitude from the Chireix combiner. Specifically, while such outphasing amplification device amplifies and transmits a radio frequency (RF) signal, reproducibility of a transmission signal depends on the size of the dynamic range and thus a reduction in the dynamic range (by about 20 dB, for example) becomes a factor of reduction in reproducibility of the transmission signal. Such reduction in reproducibility affects improvement in modulation accuracy. Further, since the outphasing amplification device includes two amplifiers located upstream of the Chireix combiner, the reduction in dynamic range and the consequent deterioration in modulation accuracy become particularly noticeable when an input phase difference between the amplifiers is large (that is, when the amplitude after a synthesis is small). 
     The technique disclosed herein has been made in view of the above-described circumstances and aims to provide an amplification device and an amplification method which are capable of improving modulation accuracy of a transmission signal. 
     Hereinafter, an embodiment of an amplification device and an amplification method disclosed in this application are described in detail with reference to the drawings. It is to be noted, however, that the amplification device and the amplification method disclosed in this application are not limited to the following embodiment. 
     First, an amplification device according to an embodiment disclosed in this application is described.  FIG. 1  is a block diagram illustrating a configuration of an amplification device  10  of the embodiment. As illustrated in  FIG. 1 , the amplification device  10  includes a signal processor  11 , an amplitude-phase converter  12 , an amplitude variable unit  13 , amplifiers  141  and  142 , directional couplers  151  and  152 , reflection coefficient calculators  161  and  162 , a comparator  17 , and a combiner  18 . These constituents are connected such that signals and data can be inputted and outputted unidirectionally or bidirectionally. 
     The signal processor  11  splits an input signal into two signals each having a certain amplitude value. In addition, the signal processor  11  controls the amplitude variable unit  13  by instructing the amplitude variable unit  13  as to how much amplitude of the input signal (input amplitude) to the amplifier  141  is to be reduced by the amplitude variable unit  13 . The signal processor  11  performs waveform shaping, peak processing, and the like of a transmission signal. For example, the signal processor  11  instructs the amplitude variable unit  13  to reduce the input amplitude such that a value c to be described later becomes equal to 1. 
     The amplitude-phase converter  12  separates the transmission signal into two signals having the same amplitude but different phases, and outputs the signals obtained by the separation to upper and lower systems (a system of the amplifier  141  and a system of the amplifier  142 ), respectively. 
     The amplitude variable unit  13  adjusts (reduces, for example) the amplitude of the input signal after the amplitude-phase conversion in accordance with the instruction from the signal processor  11 . 
     The amplifier  141  includes an input matching unit  141   a , an amplifier element  141   b , and an output matching unit  141   c . The input matching unit  141   a  achieves matching of impedance on an input side of the amplifier element  141   b . The amplifier element  141   b  amplifies the input signal. The output matching unit  141   c  achieves matching of impedance on an output side of the amplifier element  141   b . The amplifier  142  has the same configuration as the amplifier  141 . Accordingly, constituents of the amplifier  142  common to those of the amplifier  141  are denoted by reference numerals having the same suffixes and descriptions thereof are omitted. Specifically, an input matching unit  142   a , an amplifier element  142   b , and an output matching unit  142   c  of the amplifier  142  correspond to the input matching unit  141   a , the amplifier element  141   b , and the output matching unit  141   c  of the amplifier  141 , respectively. 
     The directional coupler  151  outputs an output from the upper system (the amplifier  141 ) at a predetermined coupling amount (such as 30 dB) from a signal line  151   a  as a traveling wave, and outputs an output from the lower system (the amplifier  142 ) from a signal line  151   b  as a reflected wave. In contrast, the directional coupler  152  outputs the output from the lower system (the amplifier  142 ) at a predetermined coupling amount (such as 30 dB) from a signal line  152   a  as a traveling wave, and outputs the output from the upper system (the amplifier  141 ) from a signal line  152   b  as a reflected wave. 
     The reflection coefficient calculator  161  calculates a value by dividing the reflected wave inputted from the directional coupler  151  by the traveling wave inputted from the directional coupler  151 , and outputs the calculated value to the comparator  17  as a reflection coefficient b/a. Likewise, the reflection coefficient calculator  162  calculates a value by dividing the reflected wave inputted from the directional coupler  152  by the traveling wave inputted from the directional coupler  152 , and outputs the calculated value to the comparator  17  as a reflection coefficient b′/a′. 
     The comparator  17  outputs a value c (=ab′/a′b), which is obtained by dividing the reflection coefficient b/a inputted from the reflection coefficient calculator  161  by the reflection coefficient b′/a′ inputted from the reflection coefficient calculator  162 , to the signal processor  11 . Here, the value c may be a value calculated by dividing the reflection coefficient b′/a′ by the reflection coefficient b/a depending on the configuration of the combiner  18 . The signal processor  11  instructs the amplitude variable unit  13  to reduce the input amplitude such that the value c becomes equal to 1. 
     The combiner  18  performs a vector synthesis of a signal outputted from a TL  181  of the upper system and a signal outputted from a TL  182  of the lower system, and reproduces an amplified transmission signal. 
       FIG. 2  is a view illustrating loci of reflection coefficients R 1  when input amplitude values A 1  of the amplifiers  141  and  142  are equal to each other. In  FIG. 2 , an upper view is a graph in which the amplitude (voltage) A 1  of the signal after the amplitude-phase conversion by the amplitude-phase converter  12  and the phase are represented in polar coordinates. A lower view in  FIG. 2  is a Smith chart representing the loci of the reflection coefficients R 1  (loads) when an input side of the combiner  18  is viewed from output sides of the amplifiers  141  and  142 . As illustrated in  FIG. 2 , the reflection coefficients R 1  become unbalanced between the two systems when the inputs of the amplifiers  141  and  142  are of equal amplitude at an arbitrary phase difference (0 to ±90 deg). Specifically, the reflection coefficients R 1  become asymmetrical in regions where the phase difference exceeds ±60 deg. 
       FIG. 3  is a view illustrating loci of reflection coefficients R 2  when input amplitude values A 2  of the amplifiers  141  and  142  are different from each other. The amplitude variable unit  13  reduces the amplitude of one of the systems (the system on the amplifier  141  side in  FIG. 1 ) until the reflection coefficients R 2  of both of the systems become equal. The amplitude variable unit  13  repeatedly executes this processing for each phase. At this time, the amplitude variable unit  13  controls the input amplitude of the amplifier  141  in accordance with the instruction from the signal processor  11  in a phase difference region S 1  or S 2  in which the difference is greater than a phase difference P 1  or P 2  (60 deg in  FIG. 3 ) where the loci of the reflection coefficients cross each other. As a result, amplitude control information indicated with the input amplitude A 2  in the phase difference region S 1  is obtained, and load impedance of the amplifiers  141  and  142  at that time marks the values indicated as the reflection coefficient R 2  therein. 
     As described above, the amplification device  10  includes the signal processor  11 , the amplifier  142 , the amplifier  141 , the reflection coefficient calculator  162 , and the amplitude variable unit  13 . The signal processor  11  splits the input signal into a first signal and a second signal. The amplifier  142  amplifies and outputs the first signal. The amplifier  141  amplifies and outputs the second signal. The reflection coefficient calculator  162  calculates the reflection coefficient determined by the output (the traveling wave) from the amplifier  142  and the output (the reflected wave) from the amplifier  141 . For example, the reflection coefficient calculator  162  calculates the reflection coefficient when the combiner  18  is viewed from the output from the amplifier  142 . The reflection coefficient is determined by the output (the traveling wave) from the amplifier  142  and the output (the reflected wave) from the amplifier  141 . This reflection coefficient is, for instance, the reflection coefficient of the output (the reflected wave) from the amplifier  141  relative to the output (the traveling wave) from the amplifier  142 . The amplitude variable unit  13  reduces the amplitude of the input signal (the second signal) of the amplifier  141  by using the reflection coefficient calculated by the reflection coefficient calculator  162  and a reference value. Here, in the amplification device  10 , the reference value may be the reflection coefficient on the amplifier  141  side. In this case, the amplitude variable unit  13  reduces the amplitude of the input signal of the amplifier  141  such that the reflection coefficient on the amplifier  142  side and the reflection coefficient on the amplifier  141  side become equal to each other. 
     As described above, when the input phase difference between the amplifiers  141  and  142  is large, the amplification device  10  reduces the input amplitude of only one of the systems such that load impedance values (the reflection coefficients) become balanced between the amplifiers  141  and  142 . In this way, output amplitude of the amplification device  10  becomes sufficiently small even when the input phase difference is large, and a dynamic range of the amplification device  10  is expanded accordingly. Thus, reproducibility of the transmission signal is enhanced. As a result, modulation accuracy is improved. 
     Modified Example 
     Next, a modified example is described.  FIG. 4  is a block diagram illustrating a configuration of an amplification device  10  of the modified example. As illustrated in  FIG. 4 , the amplification device  10  of the modified example has the same configuration as the amplification device  10  of the embodiment illustrated in  FIG. 1 , except that an electrical length line  19  is provided in place of the directional coupler  151 . Accordingly, in the modified example, constituents common to those of the embodiment are denoted by the same reference numerals and detailed descriptions thereof are omitted. 
     Difference between the modified example and the embodiment is the value (hereinafter referred to as a “reference value”) to be compared with the reflection coefficient b′/a′. Specifically, in the embodiment, the reference value employs a relative value (the reflection coefficient b/a of the other system), and the amplitude variable unit  13  reduces the input amplitude of the upper system such that the reflection coefficient b′/a′ of the lower system and the reflection coefficient b/a of the upper system become equal to each other (at a ratio of 1:1). On the other hand, in the modified example, the reference value is defined as an absolute value (such as a set value below 1), and the amplitude variable unit  13  adopts a method of reducing the input amplitude of the upper system such that the reflection coefficient b′/a′ of the lower system becomes below 1, for example. In the following, a configuration and an operation of the amplification device  10  of the modified example are described while mainly focusing on the difference from the embodiment. 
     In the amplification device  10 , the directional coupler  152  and the reflection coefficient calculator  162  are provided only in one of the systems. Meanwhile, the electrical length line  19  having the length equivalent to the directional coupler  152  is inserted into the system on the other side. Here, the aspect in which the directional coupler and the reflection coefficient calculator are provided in the lower system is illustrated in  FIG. 4  as an example. However, the directional coupler and the reflection coefficient calculator may be provided in the upper system depending on the design. 
     The absolute value as the reference value is set to the comparator  17 . The signal processor  11  refers to the value set to the comparator  17 , and instructs the amplitude variable unit  13  to reduce the input amplitude of the amplifier  141  such that the reflection coefficient b′/a′ falls below 1, for example. In accordance with the instruction from the signal processor  11 , the amplitude variable unit  13  performs the control to reduce the input amplitude of the one system such that the reflection coefficient b′/a′ falls below 1, for example. 
     Alternatively, the signal processor  11  refers to the value set to the comparator  17 , and instructs the amplitude variable unit  13  to reduce the input amplitude of the amplifier  141  such that the reflection coefficient b′/a′ becomes equal to a predetermined value, for example. In accordance with the instruction from the signal processor  11 , the amplitude variable unit  13  performs the control to reduce the input amplitude of the one system such that the reflection coefficient b′/a′ becomes equal t the predetermined value, for example. 
     Note that the predetermined value is determined in advance at the time of designing or the like. However, the predetermined value might not be a fixed value. The predetermined value may also be a value (a variable value) that varies depending on the phase difference, for example. 
     As described above, in the amplification device  10 , the reference value may be 1, for example. In this case, the amplitude variable unit  13  reduces the amplitude of the input signal of the amplifier  141  such that the reflection coefficient on the amplifier  142  side falls below the predetermined value. The reference value may be a set value at the time of designing, for example. In this case, the amplitude variable unit  13  reduces the amplitude of the input signal of the amplifier  141  such that the reflection coefficient on the amplifier  142  side becomes equal to the predetermined value. 
     Next, effects of the amplification devices  10  of the embodiment and the modified example are described with reference to  FIGS. 5 and 6 .  FIG. 5  is a view illustrating aspects of variations in load on the amplifiers  141  and  142  caused by the amplification devices  10 . As illustrated in  FIG. 5 , the load impedance of the amplifiers  141  and  142  becomes balanced by the amplitude control by the amplification devices  10 . Thus, the dynamic range of the amplification devices  10  subjected to the outphasing control is substantially improved. 
       FIG. 6  is a view illustrating an aspect of improvement in dynamic range of output power P out  by the amplification devices  10 . In  FIG. 6 , the x axis defines a phase P (in the unit of deg) while the y axis defines the output power P out  (in the unit of dBm). As illustrated in  FIG. 6 , while maximum power P max  before and after the amplitude control is about 52 dBm in each case, the dynamic range expands from a previous dynamic range D 1  (about 20 dB) to a dynamic range D 2  (about 50 dB) by the amplitude control by the amplification devices  10 . Thus, the reproducibility and the modulation accuracy of the signal to be transmitted by the amplification devices  10  are improved. 
     The amplification devices  10  described in the embodiment and the modified example are applicable to a communication apparatus such as a base station.  FIG. 7  is a view illustrating an application example of the amplification devices  10  of the embodiment and the modified example. As illustrated in  FIG. 7 , a base station  100  includes a control apparatus  101 , a transmission apparatus  102 , and a reception apparatus  103 . The control apparatus  101  outputs a transmission signal to the transmission apparatus  102 . The transmission apparatus  102  performs modulation, up-conversion, amplification, and the like on the inputted transmission signal, and transmits the resultant signal via an antenna A. The transmission apparatus  102  includes the above-described amplification device  10 , and the amplification device  10  performs the amplification of the transmission signal. The reception apparatus  103  performs predetermined processing on a signal received via the antenna A, and outputs the reception signal after the processing to the control apparatus  101 . 
     While the amplification devices  10  of the embodiment and the modified example are premised on quadrature amplitude modulation (QAM) as a mode for modulating the transmission signal, the amplification devices  10  may apply other amplitude modulation modes. 
     The reflection coefficient calculators  161  and  162  might not calculate entire parts of the reflection coefficients, but may be configured to calculate only real parts of the reflection coefficients. Likewise, the comparator  17  might not compare the entire parts of the reflection coefficients with the reference value, but may be configured to compare only the real parts thereof with the reference value. 
     Furthermore, the constituents of each amplification device  10  might not be constructed physically as illustrated in the drawings. In other words, specific aspects of integration or disintegration of the devices are not limited only to the illustrated examples. All or part of the devices may be integrated or disintegrated physically or functionally in any arbitrary units depending on various loads, conditions of use, and the like. For example, the signal processor  11  and the comparator  17  may be integrated into one constituent, or the reflection coefficient calculators  161  and  162  and the comparator  17  may be integrated into one constituent. On the other hand, the signal processor  11  may be disintegrated into a part configured to split the input signal into two signals respectively having the predetermined amplitude values, and a part configured to give an instruction of reducing the input amplitude of the amplifier  141 . The latter instruction part may further be disintegrated into a portion configured to perform the instruction to cause the ratio between the reflection coefficients of the two systems to become equal to a predetermined value, and a portion configured to perform the instruction to cause the reflection coefficient of the amplifier  142  to become equal to a predetermined value. 
     The example of hardware of each amplification device  10  is as follows. A signal processor  11  may be achieved by a processor and a memory. The processor may include, for example, at least one of Central Processing Unit (CPU), Digital Signal Processor (DSP), Large Scale Integration (LSI), Field-Programmable Gate Array (FPGA), Application Specific Integrated Circuit (ASIC), and so on, which are digital circuits. The memory may include, for example, at least one of Read Only Memory (ROM), Random Access Memory (RAM), and so on, which are digital circuits. An amplitude-phase converter  12  may be achieved by the processor and the memory. Otherwise, the amplitude-phase converter  12  may be achieved by an analog circuit. An amplitude variable unit  13  may be achieved by the processor and the memory. Otherwise, the amplitude variable unit  13  may be achieved by an analog circuit. Amplifiers  141  and  142  may be achieved by an analog circuit (e.g. a transistor) respectively. Directional couplers  151  and  152  may be achieved by an analog circuit. Reflection coefficient calculators  161  and  162  may be achieved by the processor, the memory, and an analog circuit (e.g. an A/D converter and a D/A converter). A comparator  17  may be achieved by the processor, the memory, and an analog circuit (e.g. A/D converter). A combiner  18  may be achieved by an analog circuit. One or more D/A converter may be inserted between some elements in the  FIG. 4 , although it is not illustrated in the figure. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment of the present invention has been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.