PATENT ABSTRACT
An amplifier includes a first amplification element configured to amplify a first signal in one of first and second operation classes, a second amplification element configured to amplify a second signal in one of first and second operation classes, a first transmission line through which the amplified first signal is transferred, and a coupler configured to couple the transferred first signal and the amplified second signal, wherein the first amplification element amplifies the first signal in the first operation class and the second amplification element amplifies the second signal in the second operation class, when the first signal and the second signal have a first frequency band, and wherein the first amplification element amplifies the first signal in the second operation class and the second amplification element amplifies the second signal in the first operation class, when the first signal and the second signal have a second frequency band.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2011-267539, filed on Dec. 7, 2011, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to an amplifier. 
       BACKGROUND 
       [0003]    The number of frequency bands utilized in a mobile communication system has been increased. Recently, in the mobile communication system, a service is provided in multiple bands (multiband). More specifically, the service is provided in a 700 MHz band, an 800 MHz band, a 1.5 GHz band, a 1.7 GHz band, a 2.1 GHz band, and a 2.5 GHz band in the mobile communication system. 
         [0004]    An amplifier used for a base station in the mobile communication system is requested to have a high efficiency performance. To satisfy the request of the high efficiency performance, the Doherty amplifier is adopted in many cases. The Doherty amplifier includes a carrier-amplifier and a peak-amplifier arranged in parallel. The carrier-amplifier regularly operates, and the peak-amplifier operates only at the time of a high output. 
         [0005]    In the base station, an amplifier is prepared for each frequency band. However, the preparation of the amplifier for each frequency band is not preferable from viewpoints of a design, a cost, and an amount of resources. Therefore, a Doherty amplifier that can cope with the multiband by a single amplifier is desired. 
         [0006]    A technique for achieving the high efficiency performance with respect to the multiband by switching, using a switch, an electrical length of an output power combining circuit of the Doherty amplifier in accordance with the frequency band is proposed (for example, see Japanese Laid-open Patent Publication No. 2006-345341). 
       SUMMARY 
       [0007]    According to an aspect of the invention, an amplifier includes a first amplification element configured to amplify a first signal in one of a first operation class and a second operation class, a second amplification element configured to amplify a second signal in one of a first operation class and a second operation class, a first transmission line through which the amplified first signal is transferred, and a coupler configured to couple the first signal transferred through the first transmission line and the amplified second signal so as to transfer the coupled signal to a second transmission line, wherein the first amplification element amplifies the first signal in the first operation class and the second amplification element amplifies the second signal in the second operation class, when the first signal and the second signal have a first frequency band, and wherein the first amplification element amplifies the first signal in the second operation class and the second amplification element amplifies the second signal in the first operation class, when the first signal and the second signal have a second frequency band. 
         [0008]    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. 
         [0009]    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 
         [0010]      FIG. 1  illustrates a base station according to a first embodiment; 
           [0011]      FIG. 2  illustrates another base station according to the first embodiment; 
           [0012]      FIG. 3  illustrates a transmission unit according to the first embodiment; 
           [0013]      FIG. 4  illustrates an amplifier according to the first embodiment; 
           [0014]      FIG. 5  is a Smith chart illustrating an example of an impedance matching by input matching circuits; 
           [0015]      FIG. 6  is a Smith chart illustrating an example of an impedance matching by output matching circuits; 
           [0016]      FIG. 7  is a flowchart of an operation by the amplifier according to the first embodiment; 
           [0017]      FIG. 8  illustrates a transmission unit according to a second embodiment; 
           [0018]      FIG. 9  illustrates an amplifier according to the second embodiment; and 
           [0019]      FIG. 10  is a flowchart of an operation by the amplifier according to the second embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0020]    Hereinafter, embodiments will be described based on the drawings. In all the drawings for describing the embodiments, the same reference sign is used for elements having the same function, and a repeated description thereof will be omitted. 
       First Embodiment 
     &lt;Base Station  100 &gt; 
       [0021]      FIG. 1  illustrates a base station  100  according to an embodiment. The base station  100  includes amplification units  102  ( 102   1  to  102   6 ), modulation units  104  ( 104   1  to  104   6 ), control units  106  ( 106   1  to  106   6 ), and power units  108  ( 108   1  to  108   6 ). A base station may be configured by including units represented by the same suffix. 
         [0022]      FIG. 1  illustrates a case in which the base station  100  includes six amplification units  102   1  to  102   6 . However, the number of the amplification units  102  is not limited to six, and one or two to five amplification units, or seven or more amplification units may be included.  FIG. 1  illustrates a case in which the base station  100  includes six modulation units  104   1  to  104   6 . However, the number of the modulation units  104  is not limited to six, and one or two to five modulation units, or seven or more modulation units may be included.  FIG. 1  illustrates a case in which the base station  100  includes six control units  106   1  to  106   6 . However, the number of the control units  106  is not limited to six, and one or two to five control units, or seven or more control units may be included.  FIG. 1  illustrates a case in which the base station  100  includes six power units  108   1  to  108   6 . However, the number of the power units  108  is not limited to six, and one or two to five power units, or seven or more power units may be included. 
         [0023]    For example, the amplification units  102   1  to  102   6 , the modulation units  104   1  to  104   6 , the control units  106   1  to  106   6 , and the power units  108   1  to  108   6  may have a card-like shape. The base station is configured by storing the respective units in a casing of the base station. 
       &lt;Base Station  200 &gt; 
       [0024]      FIG. 2  illustrates a base station  200  according to the embodiment. The base station  200  includes Remote Radio Heads (RRHs)  202  ( 202   1  to  202   3 ) and a Base Band Unit (BBU)  208 .  FIG. 2  illustrates a case in which the base station  200  includes three RRHs  202   1  to  202   3 . The number of the RRH  202  is not limited to three, and one or two, or four or more remote radio heads may be included. 
         [0025]    The RRH  202  is a wireless unit of the base station. More specifically, the RRH  202  includes modulation units  206  ( 206   1  to  206   3 ) configured to perform modulation processing on transmission data. The RRH  202  also includes amplification units  204  ( 204   1  to  204   3 ) configured to amplify a signal on which the modulation processing is performed by the modulation unit  206 . The BBU  208  is configured to perform base band signal processing. 
       &lt;Transmission Unit  300 &gt; 
       [0026]      FIG. 3  illustrates a transmission unit  300  according to the embodiment. The transmission unit  300  may be mainly included in the amplification unit  102 , the modulation unit  104  in the base station  100  illustrated in  FIG. 1  and the RRH  202  in the base station  200  illustrated in  FIG. 2 . 
         [0027]    The transmission unit  300  includes modulation circuits  302   1  and  302   2 , digital-to-analog (D/A) converters  304   1  and  304   2 , preamplifiers  306   1  and  306   2 , an amplifier  308 , a filter  310 , an antenna  312 , and a phase conversion circuit  314 . The amplifier  308  is achieved by the Doherty amplifier. 
         [0028]    The modulation circuits  302   1  and  302   2  are configured to modulate the transmission signal. The modulation circuit  302   1  sends out the modulated transmission signal to the D/A converter  304   1 . The modulation circuit  302   2  sends out the modulated transmission signal to the D/A converter  304   2 . 
         [0029]    The D/A converters  304   1  and  304   2  are respectively connected to the modulation circuits  302   1  and  302   2 . The D/A converters  304   1  and  304   2  convert the modulated transmission signal from a digital signal to an analog signal. The D/A converter  304   1  sends out the signal converted into the analog signal to the preamplifier  306   1 . The D/A converter  304   2  sends out the signal converted into the analog signal to the preamplifier  306   2 . 
         [0030]    The preamplifiers  306   1  and  306   2  are respectively connected to the D/A converters  304   1  and  304   2 . The preamplifiers  306   1  and  306   2  amplify the analog signals from the D/A converters  304   1  and  304   2 . The preamplifier  306   1  sends out the amplified analog signal to the amplifier  308 . The preamplifier  306   2  sends out the amplified analog signal to the phase conversion circuit  314 . 
         [0031]    The phase conversion circuit  314  is connected to the preamplifier  306   2 . The phase conversion circuit  314  shifts a phase of the signal from the preamplifier  306   2  by 90 degrees. The phase conversion circuit  314  sends out the signal from the preamplifier  306   2  the phase of which is shifted by 90 degrees to the amplifier  308 . Specifically, the phase conversion circuit  314  delays the phase of the signal from the preamplifier  306   2  by 90 degrees. The phase of the signal from the preamplifier  306   2  is delayed by 90 degrees because the signal from a carrier-amplifier and the signal from a peak-amplifier are coupled to each other at a phase difference by 90 degrees in the Doherty amplifier. 
         [0032]    The amplifier  308  is connected to the preamplifier  306   1  and the phase conversion circuit  314 . The amplifier  308  utilizes the signal from the preamplifier  306   1  and the signal from the phase conversion circuit  314  to amplify the power up to an average power level by the carrier-amplifier and operate the peak-amplifier from a point in the middle of the power elevation. The amplifier  308  synthesizes the signal amplified by the carrier-amplifier with the signal amplified by the peak-amplifier. With the amplification by the carrier-amplifier, it is possible to improve the amplification efficiency. With the operation by the peak-amplifier, it is also possible to obtain the maximum power. The amplifier  308  sends out the amplified signal obtained by utilizing the signal from the preamplifier  306   1  and the signal from the phase conversion circuit  314  to the filter  310 . 
         [0033]    The filter  310  is connected to the amplifier  308 . The filter  310  performs a band limitation on the signal from the amplifier  308  and sends out the signal to the antenna  312 . 
         [0034]    The antenna  312  is connected to the filter  310 . The antenna  312  wirelessly transmits the signal on which the filter  310  performs the band limitation. 
       &lt;Amplifier  308 &gt; 
       [0035]      FIG. 4  illustrates the amplifier  308  according to the embodiment. The amplifier  308  operates corresponding to plural bands, that is, multiband. Specifically, the amplifier  308  operates corresponding to the 700 MHz band, the 800 MHz band, the 1.5 GHz band, the 1.7 GHz band, the 2.1 GHz band, and the 2.5 GHz band. 
         [0036]    A case will be described in which the amplifier  308  according to the embodiment operates corresponding to the 700 MHz band and the 2.1 GHz band among the plural bands. The present embodiment can also similarly be applied to a case in which the amplifier  308  operates corresponding to other frequencies without a limitation on the case in which the amplifier  308  operates corresponding to 700 MHz band and the 2.1 GHz band. 
         [0037]    The amplifier  308  includes input matching circuits  402   1  and  402   2 , amplification elements  404   1  and  404   2 , the output matching circuits  406   1  and  406   2 , and transmission lines  408  and  410 . For example, the amplification elements  404   1  and  404   2  may be a semiconductor device such as an LD-MOS (Lateral Double-Diffused MOS), a GaAs-FET, an HEMT, or an HBT. 
         [0038]    In the amplifier  308 , between a bias voltage operating in Class AB and a bias voltage operating in Class C, voltages applied to the amplification elements  404   1  and  404   2  are switched. More specifically, in a case where the input signal is a signal in the 700 MHz band, the bias voltage to operate in Class AB is applied to the amplification element  404   1 , and the bias voltage to operate in Class C is applied to the amplification element  404   2 . Since the amplification element  404   1  receives the bias voltage to operate in Class AB, the amplification element  404   1  functions as the carrier-amplifier of the Doherty amplifier. Since the amplification element  404   2  receives the bias voltage to operate in Class C, the amplification element  404   2  functions as the peak-amplifier of the Doherty amplifier. The input signals are the signals from the preamplifier  306   1  and the phase conversion circuit  314 . 
         [0039]    Further, in a case where the input signal is a signal in the 2.1 GHz band, the bias voltage to operate in Class C is applied to the amplification element  404   1 , and the bias voltage to operate in Class AB is applied to the amplification element  404   2 . Since the amplification element  404   1  receives the bias voltage to operate in Class C, the amplification element  404   1  functions as the peak-amplifier of the Doherty amplifier. Since the amplification element  404   2  receives the bias voltage to operate in Class AB, the amplification element  404   2  functions as the carrier-amplifier. 
         [0040]    By switching the bias voltages to be applied to the amplification elements  404   1  and  404   2 , the amplification elements  404   1  and  404   2  can switch the functions between the function as the carrier-amplifier and the function as the peak-amplifier of the Doherty amplifier. 
         [0041]    The input matching circuit  402   1  is connected to the preamplifier  306   1 . The input matching circuit  402   1  converts an impedance of the signal from the preamplifier  306   1  to be matched with an input impedance of the amplification element  404   1 . The input matching circuit  402   1  sends out the impedance-converted signal to the amplification element  404   1 . 
         [0042]    The amplification element  404   1  is connected to the input matching circuit  402   1 . The amplification element  404   1  is an amplification element configured to amplify the signal. The amplification element  404   1  is biased to Class AB or Class C. That is, the amplification element  404   1  receives, as the bias voltage, the voltage that is applied for operating the amplification element  404   1  in Class AB or the voltage that is applied for operating the amplification element  404   1  in Class C. Since the amplification element  404   1  receives the voltage to an extent where the operation class is changed, the amplification element  404   1  can be operated as the Class AB amplifier or the Class C amplifier. The amplification element  404   1  sends out the amplified signal to the output matching circuit  406   1 . 
         [0043]    The output matching circuit  406   1  is connected to the amplification element  404   1 . The output matching circuit  406   1  including the transmission line  408  converts a load impedance of the signal from the amplification element  404   1 . 
         [0044]    The input matching circuit  402   2  is connected to the phase conversion circuit  314 . The input matching circuit  402   2  converts the impedance of the signal from the phase conversion circuit  314  to be matched with the input impedance of the amplification element  404   2 . The input matching circuit  402   2  sends out the impedance-converted signal to the amplification element  404   2 . 
         [0045]    The amplification element  404   2  is connected to the input matching circuit  402   2 . The amplification element  404   2  is an amplification element configured to amplify the signal. The amplification element  404   2  is biased to Class AB or Class C. That is, the amplification element  404   2  receives, as the bias voltage, the voltage that is applied for operating the amplification element  404   2  in Class AB or the voltage that is applied for operating the amplification element  404   2  in Class C. Since the amplification element  404   2  receives the voltage to an extent where the operation class is changed, the amplification element  404   2  can be operated as the Class AB amplifier or the Class C amplifier. The amplification element  404   2  sends out the amplified signal to the output matching circuit  406   2 . 
         [0046]    The output matching circuit  406   2  is connected to the amplification element  404   2 . The output matching circuit  406   2  converts a load impedance of the signal from the amplification element  404   2 . 
         [0047]    The transmission line  408  is connected to the output matching circuit  406   1 . The transmission line  408  is a transmission line configured to carry out the impedance conversion of the signal from the output matching circuit  406   1 . More specifically, in the transmission line  408 , an impedance conversion is carried out based on an electrical length of λ/4 with respect to a frequency of the signal that is input to the amplifier  308 . The signal input to the amplifier  308  is the signal from the preamplifier  306   1 . Herein, the electrical length regulates the length of the transmission line while a wavelength (λ) in the transmission line is set as a reference. With the regulation while the wavelength in the transmission line is set as a reference, it is possible to take a line constant into account. The line constant includes a specific inductive capacity of a dielectric or the like. By carrying out the impedance conversion of the signal from the output matching circuit  406   1  based on the electrical length of λ/4 with respect to the frequency of the signal that is input to the amplifier  308 , it is possible to ensure the matching with the signal from the output matching circuit  406   2 . 
         [0048]    The transmission line  410  is connected to the transmission line  408  and the output matching circuit  406   2 . In the transmission line  410 , the impedance conversion is carried out on the signal obtained by synthesizing the signal from the transmission line  408  with the signal from the output matching circuit  406   2 . A point where the signal from the transmission line  408  is coupled to the signal from the output matching circuit  406   2  is set as a coupling part A. The signal from the transmission line  408  is synthesized with the signal from the output matching circuit  406   2  at the coupling part A. More specifically, in the transmission line  410 , the impedance conversion is carried out based on the electrical length of λ/4 with respect to the frequency of the signal that is input to the amplifier  308 . By carrying out the impedance conversion based on the electrical length of λ/4 with respect to the frequency of the signal that is input to the amplifier  308 , it is possible to ensure the matching with the filter  310 . 
       &lt;Input Matching Circuits  402   1  and  402   2 &gt; 
       [0049]    Under the condition for matching the impedance between the devices, an impedance point at which the efficiency is optimized may be different from an impedance point at which the electric power is maximized in some cases. Depending on the frequency of the input signal, also, an impedance point at which the efficiency is optimized may be different from an impedance point at which the electric power is maximized in some cases. 
         [0050]    An amplifier that operates corresponding to plural frequency bands will be described. For example, the amplifier can be used for input signals in two frequency bands. In the above-mentioned amplifier, it may be difficult, in some cases, to obtain a matching circuit that can ensure the impedance matching so that the efficiency is optimized in both frequency bands of a frequency band f 1  and a frequency band f 2 . Herein, the frequency band f 1  may be the 700 MHz band, and the frequency band f 2  may be the 2.1 GHz band. According to the present embodiment, the amplifier is designed so that the matching can be ensured to optimize the efficiency in the frequency band f 1  and the matching can be ensured to maximize the electric power in the frequency band f 2 . The impedance matching point at which the efficiency is optimized may be different from the impedance matching point at which the electric power is maximized. In the amplifier  308 , since the two types of impedance points can be selected, it is possible to increase the impedance points that can be selected at the time of designing. 
         [0051]      FIG. 5  is a Smith chart illustrating an example of an impedance matching by the input matching circuits  402   1  and  402   2 . 
         [0052]    In  FIG. 5 , “Pf 1 AB” is a point representing the impedance where the electric power is maximized when the input signal in the frequency band f 1  is amplified by the amplification element that receives the bias voltage to operate in Class AB. “Pf 1 C” is a point representing the impedance where the electric power is maximized when the input signal in the frequency band f 1  is amplified by the amplification element that receives the bias voltage to operate in Class C. “γf 1 AB” is a point representing the impedance where the efficiency is optimized when the input signal in the frequency band f 1  is amplified by the amplification element that receives the bias voltage to operate in Class AB. “γf 1 C” is a point representing the impedance where the efficiency is optimized when the input signal in the frequency band f 1  is amplified by the amplification element that receives the bias voltage to operate in Class C. 
         [0053]    Further, “Pf 2 AB” is a point representing the impedance where the electric power is maximized when the input signal in the frequency band f 2  is amplified by the amplification element that receives the bias voltage to operate in Class AB. “Pf 2 C” is a point representing the impedance where the electric power is maximized when the input signal in the frequency band f 2  is amplified by the amplification element that receives the bias voltage to operate in Class C. “γf 2 AB” is a point representing the impedance where the efficiency is optimized when the input signal in the frequency band f 2  is amplified by the amplification element that receives the bias voltage to operate in Class AB. “γf 2 C” is a point representing the impedance where the efficiency is optimized when the input signal in the frequency band f 2  is amplified by the amplification element that receives the bias voltage to operate in Class C. 
         [0054]    In the example illustrated in  FIG. 5 , “Pf 1 AB” as the impedance matching point at which the electric power is maximized in the frequency band f 1  or “γf 1 AB” as the impedance matching point at which the efficiency is optimized in the frequency band f 1  can be set in the input matching circuit  402   1 . Furthermore, in accordance with the impedance matching point set in the input matching circuit  402   1 , it is possible to set the impedance matching point in the input matching circuit  402   2 . More specifically, “γf 1 C” as the impedance matching point at which the efficiency is optimized in the frequency band f 1  or “Pf 1 C” as the impedance matching point at which the electric power is maximized in the frequency band f 1  can be set in the input matching circuit  402   2 . 
         [0055]    Further, “Pf 2 AB” as the impedance matching point at which the electric power is maximized in the frequency band f 2  or “γf 2 AB” as the impedance matching point at which the efficiency is optimized in the frequency band f 2  can be set in the input matching circuit  402   1 . Furthermore, in accordance with the impedance matching point set in the input matching circuit  402   1 , it is possible to set the impedance matching point in the input matching circuit  402   2 . More specifically, “γf 2 C” as the impedance matching point at which the efficiency is optimized in the frequency band f 2  or “Pf 2 C” as the impedance matching point at which the electric power is maximized in the frequency band f 2  can be set in the input matching circuit  402   2 . Accordingly, when the multiband of the amplifier is to be achieved, it is possible to improve the degree of freedom in the design of the matching circuit. 
       &lt;Output Matching Circuits  406   1  and  406   2 &gt; 
       [0056]    With regard to the output matching circuits  406   1  and  406   2 , similarly as in the input matching circuits  402   1  and  402   2 , under the condition for matching the impedance between the devices, an impedance point at which the efficiency is optimized may be different from an impedance point at which the electric power is maximized in some cases. Depending on the frequency of the input signal, also, an impedance point at which the efficiency is optimized is different from an impedance point at which the electric power is maximized. 
         [0057]    For example, in the amplifier that operates corresponding to the plural frequency bands, in both the frequency of the frequency band f 1  and the frequency band f 2 , it may be difficult to achieve a matching circuit that can ensure the impedance matching so that the efficiency is optimized in some cases. In this case, the design is made such that the matching can be ensured to optimize the efficiency in the frequency band f 1  and the matching can be ensured to maximize the electric power in the frequency band f 2 . The impedance matching point at which the efficiency is optimized may be different from the impedance matching point at which the electric power is maximized. In the amplifier  308 , since the two types of impedance points can be selected, it is possible to increase the impedance points that can be selected at the time of designing. 
         [0058]      FIG. 6  is a Smith chart illustrating an example of an impedance matching by the output matching circuits  406   1  and  406   2 . 
         [0059]    In  FIG. 6 , “Pf 1 AB” is a point representing the impedance where the electric power is maximized when the input signal in the frequency band f 1  is amplified by the amplification element that receives the bias voltage to operate in Class AB. “Pf 1 C” is a point representing the impedance where the electric power is maximized when the input signal in the frequency band f 1  is amplified by the amplification element that receives the bias voltage to operate in Class C. “γf 1 AB” is a point representing the impedance where the efficiency is optimized when the input signal in the frequency band f 1  is amplified by the amplification element that receives the bias voltage to operate in Class AB. “γf 1 C” is a point representing the impedance where the efficiency is optimized when the input signal in the frequency band f 1  is amplified by the amplification element that receives the bias voltage to operate in Class C. 
         [0060]    Further, “Pf 2 AB” is a point representing the impedance where the electric power is maximized when the input signal in the frequency band f 2  is amplified by the amplification element that receives the bias voltage to operate in Class AB. “Pf 2 C” is a point representing the impedance where the electric power is maximized when the input signal in the frequency band f 2  is amplified by the amplification element that receives the bias voltage to operate in Class C. “γf 2 AB” is a point representing the impedance where the efficiency is optimized when the input signal in the frequency band f 2  is amplified by the amplification element that receives the bias voltage to operate in Class AB. “γf 2 C” is a point representing the impedance where the efficiency is optimized when the input signal in the frequency band f 2  is amplified by the amplification element that receives the bias voltage to operate in Class C. 
         [0061]    In the example illustrated in  FIG. 6 , “Pf 1 AB” as the impedance matching point at which the electric power is maximized in the frequency band f 1  or “γf 1 AB” as the impedance matching point at which the efficiency is optimized in the frequency band f 1  can be set in the output matching circuit  406   1 . Furthermore, in accordance with the impedance matching point set in the output matching circuit  406   1 , it is possible to set the impedance matching point in the output matching circuit  406   2 . More specifically, “γf 1 C” as the impedance matching point at which the efficiency is optimized in the frequency band f 1  or “Pf 1 C” as the impedance matching point at which the electric power is maximized in the frequency band f 1  can be set in the output matching circuit  406   2 . 
         [0062]    Further, “Pf 2 AB” as the impedance matching point at which the electric power is maximized in the frequency band f 2  or “γf 2 AB” as the impedance matching point at which the efficiency is optimized in the frequency band f 2  can be set in the output matching circuit  406   1 . Furthermore, in accordance with the impedance matching point set in the output matching circuit  406   1 , it is possible to set the impedance matching point in the output matching circuit  406   2 . More specifically, “γf 2 C” as the impedance matching point at which the efficiency is optimized in the frequency band f 2  or “Pf 2 C” as the impedance matching point at which the electric power is maximized in the frequency band f 2  can be set in the output matching circuit  406   2 . Accordingly, when the multiband of the amplifier is to be achieved, it is possible to improve the degree of freedom in the design of the matching circuit. 
       &lt;Operation by the Amplifier  308 &gt; 
       [0063]      FIG. 7  is a flowchart of an operation by the amplifier  308  according to the embodiment. Herein, a case where the switching is conducted so that the signal in the 700 MHz band is amplified and a case where the switching is conducted so that the signal in the 2.1 GHz band is amplified will be described. 
         [0000]    &lt;Case where Switching is Conducted so that Signal in 700 MHz band is Amplified&gt; 
         [0064]    The switching is conducted to cause the amplifier  308  that has been set to amplify the signal in the 2.1 GHz band to amplify the signal in the 700 MHz band. 
         [0065]    The matching condition is set in the amplifier  308  (step S 702 ). More specifically, in the input matching circuit  402   1 , the impedance matching point at which the efficiency is optimized at 700 MHz is set. Also, in the output matching circuit  406   1 , the impedance matching point at which the efficiency is optimized at 700 MHz is set. 
         [0066]    On the other hand, in the input matching circuit  402   2 , the impedance matching point at which the electric power is maximized at 700 MHz is set. Also, in the output matching circuit  406   2 , the impedance matching point at which the electric power is maximized at 700 MHz is set. 
         [0067]    The bias voltage is applied to the amplifier  308  (step S 704 ). More specifically, the amplification element  404   1  receives the bias voltage to operate in Class AB. On the other hand, the amplification element  404   2  receives the bias voltage to operate in Class C. 
         [0000]    &lt;Case where Switching is Conducted so that Signal in 2.1 GHz Band is Amplified&gt; 
         [0068]    The switching is conducted to cause the amplifier  308  that has been set to amplify the signal in the 700 MHz band to amplify the signal in the 2.1 GHz band. 
         [0069]    The matching condition is set in the amplifier  308  (step S 702 ). More specifically, in the input matching circuit  402   1 , the impedance matching point at which the electric power is maximized at 2.1 GHz is set. Also, in the output matching circuit  406   1 , the impedance matching point at which the electric power is maximized at 2.1 GHz is set. 
         [0070]    On the other hand, in the input matching circuit  402   2 , the impedance matching point at which the efficiency is optimized at 2.1 GHz is set. Also, in the output matching circuit  406   2 , the impedance matching point at which the efficiency is optimized at 2.1 GHz is set. 
         [0071]    The bias voltage is applied to the amplifier  308  (step S 704 ). More specifically, the amplification element  404   1  receives the bias voltage to operate in Class C. On the other hand, the amplification element  404   2  receives the bias voltage to operate in Class AB. 
         [0072]    According to the present embodiment, when the multiband of the Doherty amplifier is to be achieved, it is possible to improve the degree of freedom in the design of the matching circuit. That is, it is possible to select the impedance matching point to be set from the plural impedance matching points. Since the impedance matching point can be selected from the plural impedance matching points, it is possible to easily achieve the multiband of the Doherty amplifier. Also, without switching the transmission lines or the like, it is possible to achieve the multiband of the Doherty amplifier. 
         [0073]    Further, in the amplifier that operates corresponding to the 700 MHz band and the 2.1 GHz band, as described above, the transmission lines  408  and  410  can be achieved by the lines of λ/4. 
         [0074]    In the Doherty amplifier, the signal from the carrier-amplifier and the signal from the peak-amplifier are to be coupled to each other at a phase difference of 90 degrees. In the amplifier  308  illustrated in  FIG. 4 , the signal from the carrier-amplifier and the signal from the peak-amplifier can be coupled to each other at the phase difference of 90 degrees by the transmission line  408 . 
         [0075]    Further, at the time of the operation by the peak-amplifier, since the carrier-amplifier and the peak-amplifier are operated in parallel, the impedance conversion is to be conducted on the signal obtained by synthesizing the signal from the peak-amplifier with the signal from the carrier-amplifier. In the amplifier  308  illustrated in  FIG. 4 , the impedance conversion is conducted by the transmission line  410  on the signal obtained by synthesizing the signal from the peak-amplifier with the signal from the carrier-amplifier. 
         [0076]    For example, the transmission lines  408  and  410  can be replaced by lines of 90 degrees (λ/4). However, since the line has a frequency characteristic, a case of a certain frequency corresponds to the phase of 90 degrees. 
         [0077]    According to the present embodiment, the amplifier  308  is set to correspond to the 700 MHz band and the 2.1 GHz band that is three times as high as 700 MHz. With the settings for corresponding to the 700 MHz band and the 2.1 GHz band, the transmission lines  408  and  410  can be used in common with the λ/4 line at the low frequency, that is, 700 MHz. Since the transmission lines can be used in common with the λ/4 line at 700 MHz, it is possible to avoid the switching of the line on the output side. 
       Second Embodiment 
     &lt;Base Station&gt; 
       [0078]    The base station  100  and the base station  200  according to a second embodiment are similar to those in  FIG. 1  and  FIG. 2 . 
       &lt;Transmission Unit  300 &gt; 
       [0079]      FIG. 8  illustrates the transmission unit  300  according to the second embodiment. The transmission unit  300  according to the second embodiment is different from the transmission unit described with reference to  FIG. 3  in that the transmission unit  300  according to the second embodiment includes a phase conversion circuit  316 . 
         [0080]    The phase conversion circuit  316  is connected to the preamplifier  306   1 . The phase conversion circuit  316  shifts the phase of the signal from the preamplifier  306   1 . The phase conversion circuit  316  sends out the signal from the preamplifier  306   1  the phase of which is shifted to the amplifier  308 . More specifically, the phase conversion circuit  316  delays the phase of the signal from the preamplifier  306   1 . 
         [0081]    Further, the phase conversion circuit  314  adds 90 degrees and more to shift the phase of the signal from the preamplifier  306   2 . The phase conversion circuit  314  sends out the signal from the preamplifier  306   2  the phase of which is shifted by being added with 90 degrees and more to the amplifier  308 . More specifically, the phase conversion circuit  316  shifts the phase of the signal from the preamplifier  306   1  by 90 degrees and further delays the phase. &lt;Amplifier  308 &gt; 
         [0082]      FIG. 9  illustrates the amplifier  308  according to the embodiment. The amplifier  308  according to the second embodiment is different from the amplifier  308  described with reference to  FIG. 4  in that the amplifier  308  according to the second embodiment includes phase compensation lines  412  and  414 . 
         [0083]    A signal from the phase conversion circuit  316  is input to the input matching circuit  402   1 . 
         [0084]    The phase compensation line  412  is connected to the output matching circuit  406   1 . The phase compensation line  412  is a transmission line configured to compensate for a shift of the phase generated by the amplification element  404   1 . 
         [0085]    Specifically, in a case where the signal in the 700 MHz band is amplified, the phase compensation line  412  compensates the signal from the output matching circuit  406   1  by a phase θ 1 . The phase θ 1  is a shift of the phase supposed to be generated by the amplification element  404   1  when the signal in the 700 MHz band is amplified. The signal the phase of which is compensated by the phase compensation line  412  is input to the transmission line  408 . 
         [0086]    Further, in a case where the signal in the 2.1 GHz band is amplified, the phase compensation line  412  compensates the signal from the output matching circuit  406   1  by a phase θ 3 . The phase θ 3  is a shift of the phase supposed to be generated by the amplification element  404   1  when the signal in the 2.1 GHz band is amplified. The signal the phase of which is compensated by the phase compensation line  412  is input to the transmission line  408 . 
         [0087]    The phase compensation line  414  is connected to the output matching circuit  406   2 . The phase compensation line  414  is a transmission line configured to compensate for a shift of the phase generated by the amplification element  404   2 . 
         [0088]    Specifically, in a case where the signal in the 700 MHz band is amplified, the phase compensation line  414  compensates the signal from the output matching circuit  406   2  by a phase θ 2 . The phase θ 2  is a shift of the phase supposed to be generated by the amplification element  404   2  when the signal in the 700 MHz band is amplified. The signal the phase of which is compensated by the phase compensation line  414  is synthesized with the signal from the transmission line  408  at the coupling part A to be input to the transmission line  410 . 
         [0089]    Further, in a case where the signal in the 2.1 GHz band is amplified, the phase compensation line  414  compensates the signal from the output matching circuit  406   2  by a phase θ 4 . The phase θ 4  is a shift of the phase supposed to be generated by the amplification element  404   2  when the signal in the 2.1 GHz band is amplified. The signal the phase of which is compensated by the phase compensation line  414  is synthesized with the signal from the transmission line  408  at the coupling part A to be input to the transmission line  410 . 
         [0090]    In the Doherty amplifier, the signal from the carrier-amplifier and the signal from the peak-amplifier are to be coupled to each other at a phase difference of 90 degrees. However, since the bias conditions and the matching conditions vary between the carrier-amplifier and the peak-amplifier, the signal that is output from the carrier-amplifier and the signal that is output from the peak-amplifier do not have a same passing phase. Accordingly, on the output side of the output matching circuits  406   1  and  406   2 , the phase compensation lines  412  and  414  are respectively provided, so that the lines configured to compensate the passing phase are inserted. 
         [0091]    However, if the frequency for achieving the multiband is changed, the phase shift amounts are not the same. To compensate for the phase shift amounts supposed to fluctuate in response to the change in the frequency, the phase shift amounts are individually set in the carrier-amplifier and the peak-amplifier. 
         [0092]    More specifically, in a case where the signal in the 700 MHz band is amplified, the phase conversion circuit  316  sets an amount of shifting the phase of the signal from the preamplifier  306   1  as Δθ 1 , and the phase conversion circuit  314  sets an amount of shifting the phase of the signal from the preamplifier  306   2  as Δθ 2 . 
         [0093]    Further, in a case where the signal in the 2.1 GHz band is amplified, the phase conversion circuit  316  sets an amount of shifting the phase of the signal from the preamplifier  306   1  as Δθ 3 , and the phase conversion circuit  314  sets an amount of shifting the phase of the signal from the preamplifier  306   2  as Δθ 4 . 
         [0094]    With this configuration, for achieving the multiband, even in a case where the frequency of the input signal is changed, it is possible to compensate for the phase shift amount that fluctuates when the frequency is changed. 
       &lt;Operation by the Amplifier  308 &gt; 
       [0095]      FIG. 10  is a flowchart of an operation by the amplifier  308  according to the embodiment. Herein, a case will be described where the switching is conducted so that the signal in the 700 MHz band is amplified and a case will be described where the switching is conducted so that the signal in the 2.1 GHz band is amplified. 
         [0000]    &lt;Case where Switching is Conducted so that Signal in 700 MHz Band is Amplified&gt; 
         [0096]    The switching is conducted to cause the amplifier  308  that has been set to amplify the signal in the 2.1 GHz band to amplify the signal in the 700 MHz band. 
         [0097]    The matching condition is set in the amplifier  308  (step S 1002 ). Specifically, in the input matching circuit  402   1 , the impedance matching point at which the efficiency is optimized at 700 MHz is set. Also, in the output matching circuit  406   1 , the impedance matching point at which the efficiency is optimized at 700 MHz is set. 
         [0098]    On the other hand, in the input matching circuit  402   2 , the impedance matching point at which the electric power is maximized at 700 MHz is set. Also, in the output matching circuit  406   2 , the impedance matching point at which the electric power is maximized at 700 MHz is set. 
         [0099]    The phase is set (step S 1004 ). More specifically, Δθ 1  is set as the phase shift amount in the phase conversion circuit  316 , and Δθ 2  is set as the phase shift amount in the phase conversion circuit  314 . Also, the phase compensation line  412  is set to compensate the phase of the signal from the output matching circuit  406   1  by the phase θ 1 . Also, the phase compensation line  414  is set to compensate the phase of the signal from the output matching circuit  406   2  by the phase θ 2 . 
         [0100]    The bias voltage is applied to the amplifier  308  (step S 1006 ). More specifically, the amplification element  404   1  receives the bias voltage to operate in Class AB. On the other hand, the amplification element  404   2  receives the bias voltage to operate in Class C. 
         [0000]    &lt;Case where Switching is Conducted so that Signal in 2.1 GHz Band is Amplified&gt; 
         [0101]    The switching is conducted to cause the amplifier  308  that has been set to amplify the signal in the 700 MHz band to amplify the signal in the 2.1 GHz band. 
         [0102]    The matching condition is set in the amplifier  308  (step S 1002 ). More specifically, in the input matching circuit  402   1 , the impedance matching point at which the electric power is maximized at 2.1 GHz is set. Also, in the output matching circuit  406   1 , the impedance matching point at which the electric power is maximized at 2.1 GHz is set. 
         [0103]    On the other hand, in the input matching circuit  402   2 , the impedance matching point at which the efficiency is optimized at 2.1 GHz is set. Also, in the output matching circuit  406   2 , the impedance matching point at which the efficiency is optimized at 2.1 GHz is set. 
         [0104]    The phase is set (step S 1004 ). More specifically, Δθ 3  is set as the phase shift amount in the phase conversion circuit  316 , and Δθ 4  is set as the phase shift amount in the phase conversion circuit  314 . Also, the phase compensation line  412  is set to compensate the phase of the signal from the output matching circuit  406   1  by the phase θ 3 . The phase compensation line  414  is set to compensate the phase of the signal from the output matching circuit  406   2  by the phase θ 4 . 
         [0105]    The bias voltage is applied to the amplifier  308  (step S 1006 ). More specifically, the amplification element  404   1  receives the bias voltage to operate in Class C. On the other hand, the amplification element  404   2  receives the bias voltage to operate in Class AB. 
         [0106]    According to the present embodiment, when the multiband of the Doherty amplifier is to be achieved, it is possible to improve the degree of freedom in the design of the matching circuit. That is, it is possible to select the impedance matching point to be set from the plural impedance matching points. Since the impedance matching point can be selected from the plural impedance matching points, it is possible to easily achieve the multiband of the Doherty amplifier. Also, without switching the transmission lines or the like, it is possible to achieve the multiband of the Doherty amplifier. 
         [0107]    Further, in the amplifier that operates corresponding to the 700 MHz band and the 2.1 GHz band, as described above, the transmission lines  408  and  410  can be achieved by the lines of λ/4. 
         [0108]    In the Doherty amplifier, the signal from the carrier-amplifier and the signal from the peak-amplifier are to be coupled to each other at a phase difference of 90 degrees. In the amplifier  308  illustrated in  FIG. 9 , the signal from the carrier-amplifier and the signal from the peak-amplifier can be coupled to each other at the phase difference of 90 degrees by the transmission line  408 . 
         [0109]    Further, at the time of the operation by the peak-amplifier, since the carrier-amplifier and the peak-amplifier are operated in parallel, the impedance conversion is to be conducted on the signal obtained by synthesizing the signal from the peak-amplifier with the signal from the carrier-amplifier. In the amplifier  308  illustrated in  FIG. 9 , the impedance conversion is conducted by the transmission line  410  on the signal obtained by synthesizing the signal from the peak-amplifier with the signal from the carrier-amplifier. 
         [0110]    For example, the transmission lines  408  and  410  can be replaced by lines of 90 degrees (λ/4). However, since the line has a frequency characteristic, a case of a certain frequency corresponds to the phase of 90 degrees. 
         [0111]    According to the present embodiment, the amplifier  308  is set to correspond to the 700 MHz band and the 2.1 GHz band that is three times as high as 700 MHz. With the setting for corresponding to the 700 MHz band and the 2.1 GHz band, the transmission lines  408  and  410  can be used in common with the λ/4 line at the low frequency, that is, 700 MHz. Since the transmission lines can be used in common with the λ/4 line at 700 MHz, it is possible to avoid the switching of the line on the output side. 
         [0112]    Further, according to the present embodiment, since the shift of the passing phases between the signal from the carrier-amplifier and the signal from the peak-amplifier can be compensated for, the shifts of the synthesis points between the signal from the carrier-amplifier and the signal from the peak-amplifier can be reduced. Since the shifts of the synthesis points between the signal from the carrier-amplifier and the signal from the peak-amplifier can be reduced, it is possible to improve the amplification characteristic at a time when the maximum power is obtained, in particular. 
         [0113]    Further, without carrying out the physical switching of the lines or the like when the frequency bands are switched, it is possible to adjust the shift of the phase generated by the frequency characteristic that is different for each amplification element by controlling the phase of the input signal. 
         [0114]    According to the above-mentioned embodiment, the operation classes of the amplification elements  404   1  and  404   2  may be switched between Class AB and Class B and may also be switched between Class A and Class B. Also, the operation classes of the amplification elements  404   1  and  404   2  may be switched between Class AB and Class C and may also be switched between Class A and Class C. 
         [0115]    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 embodiments of the present invention have 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.