Abstract:
An amplification device includes a first amplifier configured to amplify an input signal in accordance with a first gate voltage, and a second amplifier configured to amplify the input signal in accordance with a second gate voltage, wherein at least one of the first gate voltage and the second gate voltage are controlled on the basis of a current ratio of a first drain current of the first amplifier to a second drain current of the second amplifier.

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. 2016-081298, filed on Apr. 14, 2016, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to an amplification device. 
       BACKGROUND 
       [0003]    Amplification circuits, which amplify transmission power, have been used in various radio apparatuses including a base station of a mobile communication system. Particularly, in recent years, along with high-speed communication, it is desired to amplify the transmission power with higher efficiency from a viewpoint of reducing the power consumption. It is known that the efficiency of an amplification circuit is the highest in an output saturation state (nonlinear state), and as an amplification circuit having this characteristic, a Doherty amplification circuit (hereinafter referred to as a “Doherty circuit”) may be used. A Doherty circuit has a carrier amplifier (CA) and a peak amplifier (PA) which are coupled in parallel, and a gate voltage applied to the CA and PA is normally fixed to an optimal operation point at which the efficiency has a maximum. 
         [0004]    However, it is known that the optimal operation point varies with change in temperature. When the gate voltage deviates from the optimal operation point due to a change in temperature, the input/output characteristics of the Doherty circuit changes. Consequently, a signal outputted from the Doherty circuit is distorted. 
         [0005]    In order to reduce such a deviation of the gate voltage, a technology in related art controls the gate voltage applied to the CA and PA using, for instance, a temperature variable resistor. Also, a technology in related art detects a drain current of the CA and PA which varies according to a change in temperature, and controls the gate voltage so that the detected drain current falls within a predetermined range. 
         [0006]    Related techniques are disclosed in, for example, Japanese Laid-open Patent Publication Nos. 2006-279707 and 2007-129492. 
       SUMMARY 
       [0007]    According to an aspect of the invention, an amplification device includes a first amplifier configured to amplify an input signal in accordance with a first gate voltage, and a second amplifier configured to amplify the input signal in accordance with a second gate voltage, wherein at least one of the first gate voltage and the second gate voltage are controlled on the basis of a current ratio of a first drain current of the first amplifier to a second drain current of the second amplifier. 
         [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  is a block diagram illustrating an example base station including a radio apparatus in a first embodiment; 
           [0011]      FIG. 2  is a block diagram illustrating an example amplification device in the first embodiment; 
           [0012]      FIG. 3  is a graph illustrating an example relationship between current ratio and distortion of output signal due to change in temperature; 
           [0013]      FIG. 4  is a flowchart illustrating example gate voltage control processing in the first embodiment; 
           [0014]      FIG. 5  is a flowchart illustrating another example gate voltage control processing in the first embodiment; 
           [0015]      FIG. 6  is a block diagram illustrating an example amplification device in a second embodiment; 
           [0016]      FIG. 7  is a table illustrating an example conversion table in which a range of temperature difference and a current difference are associated with each other; and 
           [0017]      FIG. 8  is a flowchart illustrating example gate voltage control processing in the second embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    A deviation of the gate voltage is caused not only by a change in temperature, but also by a mechanical error (hereinafter referred to as an “individual difference of parts”) in the CA and PA. Specifically, the optimal operation point of the CA and PA, and the amount of change in the drain current vary with the CA and PA, and the amount of deviation of the gate voltage also varies according to the variation. 
         [0019]    However, in the above-mentioned technology in related art, reduction of deviation of the gate voltage caused by the individual difference of parts is not taken into consideration. 
         [0020]    Specifically, in a technology in related art which controls the gate voltage using a temperature variable resistor, an uniform gate voltage is applied at a certain temperature, and thus when the optimal operation point varies due to the individual difference of parts, deviation of the gate voltage still occurs. Also, in a technology in related art which controls the gate voltage so that the drain current falls within a predetermined range, when the amount of change in the drain current varies due to the individual difference of parts, the accuracy of control of the gate voltage is reduced, and thus deviation of the gate voltage occurs. 
         [0021]    Like this, in the technologies in related art, the optimal operation point and the amount of change in the drain current vary due to the individual difference of parts, and thus deviation of the gate voltage occurs. As a result, the signal outputted from the Doherty circuit is distorted. 
         [0022]    The technology of the present disclosure has been made in view of the above-mentioned problems, and provides an amplification device and a radio apparatus that are capable of reducing an occurrence of distortion in the Doherty circuit. 
         [0023]    Hereinafter, embodiments of an amplification device and a radio apparatus disclosed in the present application will be described in detail based on the drawings. It is to be noted that the disclosed technology is not limited by the embodiments. In the embodiments, components having the same function are labeled with the same symbol, and redundant description is omitted. 
       First Embodiment 
       [0024]    [Configuration Example of Base Station] 
         [0025]      FIG. 1  is a block diagram illustrating an example base station including a radio apparatus in a first embodiment. As illustrated in  FIG. 1 , a base station  10  has a control device  11  and a radio apparatus  12 . The control device  11  and the radio apparatus  12  are coupled by an optical fiber, for instance. Specifically, for instance, in an long term evolution (LTE) system which is standardized by a 3rd generation partnership project (3GPP), a base band unit (BBU) corresponds to the control device  11 , and a remote radio head (RRH) corresponds to the radio apparatus  12 . 
         [0026]    The control device  11  performs predetermined baseband transmission processing such as encoding of transmission data to generate a transmission signal in the baseband, and outputs the generated transmission signal to the radio apparatus  12 . 
         [0027]    The radio apparatus  12  performs processing such as modulation, up-convert, amplification on a transmission signal inputted from the control device  11 , and transmits the signal via an antenna A. The radio apparatus  12  has an amplification device  50 , which amplifies the above-mentioned transmission signal. 
         [0028]    [Configuration Example of Amplification device] 
         [0029]      FIG. 2  is a block diagram illustrating an example amplification device in the first embodiment. As illustrated in  FIG. 2 , the amplification device  50  has an amplification unit  51 , a power supply  52 , current detection units  53 ,  54 , a current ratio calculation unit  55 , and a gate voltage control unit  56 . 
         [0030]    The amplification unit  51  has a distributor  61 , CA  62 , PA  63 , output matching units  64 ,  65 , and a compositor  66 . Specifically, the amplification unit  51  is a Doherty amplification unit (amplification circuit). 
         [0031]    When the power value of a transmission signal inputted from an input terminal is less than a predetermined threshold value, the distributor  61  outputs the transmission signal only to the CA  62 . On the other hand, when the power value of a transmission signal is greater than or equal to a predetermined threshold value, the distributor  61  outputs the transmission signal to both the CA  62  and the PA  63 . 
         [0032]    The CA  62  is an amplifier that has linearity when an input power is low, operates with a power supply voltage supplied from the power supply  52 , amplifies the power of a transmission signal inputted from the distributor  61 , and outputs the amplified signal to the compositor  66  via the output matching unit  64 . On the other hand, the PA  63  is an amplifier that is used only when the input power is high, operates with a power supply voltage supplied from the power supply  52 , amplifies the power of a transmission signal inputted from the distributor  61 , and outputs the amplified signal to the compositor  66  via the output matching unit  65 . 
         [0033]    The output matching unit  64  adjusts the output-side impedance of the CA  62 . The output matching unit  65  adjusts the output-side impedance of the PA  63 . 
         [0034]    The compositor  66  combines a signal inputted from the CA  62  via the output matching unit  64  with a signal inputted from the PA  63  via the output matching unit  65 , and outputs the obtained composite signal as an output signal from an output terminal. 
         [0035]    The power supply  52  is a power supply that supplies a power supply voltage to the amplification unit  51 . The power supply  52  is coupled to the gate terminal of the CA  62  via a gate bias line  52   a,  and applies a predetermined gate voltage to the CA  62  using the gate bias line  52   a.  In addition, the power supply  52  is coupled to the gate terminal of the PA  63  via a gate bias line  52   b,  and applies a predetermined gate voltage to the PA  63  using the gate bias line  52   b.  Also, the power supply  52  is coupled to the drain terminal of the CA  62  via a drain bias line  52   c,  and applies a predetermined drain voltage to the CA  62  using the drain bias line  52   c.  Also, the power supply  52  is coupled to the drain terminal of the PA  63  via a drain bias line  52   d,  and applies a predetermined drain voltage to the PA  63  using the drain bias line  52   d.    
         [0036]    The current detection unit  53  is disposed in the drain bias line  52   c  to detect a drain current of the CA  62  in the drain bias line  52   c,  and outputs the detected drain current of the CA  62  to the current ratio calculation unit  55 . The drain current of the CA  62  varies due to a temperature change of the CA  62  or a temperature change around the CA  62 . Also, the amount of change in the drain current of the CA  62  due to such a temperature change varies with the CA  62 . The drain current of the CA  62  increases as the gate voltage applied to the CA  62  is increased, and the drain current of the CA  62  decreases as the gate voltage applied to the CA  62  is decreased. The current detection unit  53  is an example of the first detection unit. 
         [0037]    The current detection unit  54  is disposed in the drain bias line  52   d  to detect a drain current of the PA  63  in the drain bias line  52   d,  and outputs the detected drain current of the PA  63  to the current ratio calculation unit  55 . The drain current of the PA  63  varies due to a temperature change of the PA  63  or a temperature change around the PA  63 . Also, the amount of change in the drain current of the PA  63  due to such a temperature change varies with the PA  63 . The drain current of the PA  63  increases as the gate voltage applied to the PA  63  is increased, and the drain current of the PA  63  decreases as the gate voltage applied to the PA  63  is decreased. The current detection unit  54  is an example of the second detection unit. 
         [0038]    The current ratio calculation unit  55  calculates a current ratio which is a ratio of the drain current of the PA  63  to the drain current of the CA  62  by dividing the drain current of the PA  63  inputted from the current detection unit  54  by the drain current of the CA  62  inputted from the current detection unit  53 . Since the amount of change in the drain current of the CA  62  and the PA  63  due to a temperature change varies with the CA  62  and the PA  63 , the current ratio calculated by the current ratio calculation unit  55  varies with the CA  62  and the PA  63 . 
         [0039]    The gate voltage control unit  56  controls a gate voltage applied to the PA  63  using a reference value and a current ratio calculated by the current ratio calculation unit  55 . The aforementioned reference value is a predetermined value obtained by pre-measuring a current ratio when distortion of an output signal outputted from the amplification unit  51  is less than or equal to in a predetermined standard value. Specifically, when the current ratio is greater than or equal to a predetermined value, the gate voltage control unit  56  controls the gate voltage applied to the PA  63  until the current ratio falls below the predetermined value. For instance, when the gate voltage applied to the PA  63  is decreased, the drain current of the PA  63 , which is the numerator of the current ratio, decreases. Thus, the gate voltage control unit  56  gradually decreases the gate voltage applied to the PA  63  until the current ratio falls below the predetermined value. 
         [0040]    Here, an example of control of the gate voltage by the gate voltage control unit  56  will be described using  FIG. 3 .  FIG. 3  is a graph illustrating an example relationship between current ratio and distortion of an output signal due to change in temperature. In  FIG. 3 , the horizontal axis indicates the temperature (° C.) in the amplification unit  51 , and the vertical axis indicates the distortion (dBm) which occurs in the output signal of the amplification unit  51 . In  FIG. 3 , a graph  101  illustrates a change in distortion of the output signal due to change in temperature when the current ratio is 0.4. In  FIG. 3 , a graph  102  illustrates a change in distortion of the output signal due to change in temperature when the current ratio is 0.33. In  FIG. 3 , a graph  103  illustrates a change in distortion of the output signal due to change in temperature when the current ratio is 0.25. 
         [0041]    As illustrated in the graphs  101  to  103 , distortion which occurs in the output signal of the amplification unit  51  increases as the temperature increases. In addition, an amount of increase in distortion relative to change in temperature increases as the current ratio increases. In the example of  FIG. 3 , when the current ratio is 0.25, although distortion which occurs in the output signal of the amplification unit  51  increases along with increase of the temperature, the distortion falls within a range of a predetermined standard value (−20.5 dBm) or less. On the other hand, when the current ratio increases to 0.33 or 0.4, distortion which occurs in the output signal of the amplification unit  51  increases along with increase of the temperature, and exceeds the predetermined standard value. In other words, by maintaining the current ratio less than a predetermined value (for instance, 0.3), distortion which occurs in the output signal of the amplification unit  51  falls within a range of a predetermined standard value (for instance, −20.5 dBm) or less. 
         [0042]    Thus, when the current ratio is greater than or equal to a predetermined value, the gate voltage control unit  56  controls the gate voltage applied to the PA  63  until the current ratio falls below the predetermined value. In the example of  FIG. 3 , a case is assumed where the current ratio is 0.4. In this case, since the current ratio is greater than or equal to a predetermined value (0.3), the gate voltage control unit  56  gradually decreases the gate voltage applied to the PA  63  until the current ratio reaches 0.25 which falls below the predetermined value. Thus, as illustrated in the graph  103  of  FIG. 3 , distortion which occurs in the output signal of the amplification unit  51  falls within a range of a predetermined standard value (−20.5 dBm) or less regardless of the increase in temperature. 
         [0043]    [Operation Example of Amplification Device] 
         [0044]    An example of gate voltage control processing in the amplification device  50  having the aforementioned configuration will be described.  FIG. 4  is a flowchart illustrating example gate voltage control processing in the first embodiment. The gate voltage control processing illustrated in  FIG. 4  is repeatedly performed with a predetermined period (for instance, 30 seconds). 
         [0045]    As illustrated in  FIG. 4 , the current detection unit  53  detects a drain current Ica of the CA  62 , and the current detection unit  54  detects a drain current Ipa of the PA  63  (step S 101 ). When the drain current Ica of the CA  62  is the same as the initial value Ica° and the drain current Ipa of the PA  63  is the same as the initial value Ipa 0  (No in step S 102 ), the gate voltage control processing is completed. The initial values Ica 0 , Ipa 0  are the values of drain current pre-measured at a reference temperature at the time of factory shipment of the amplification device  50 , for instance. 
         [0046]    On the other hand, when the drain current Ica of the CA  62  is not the same as the initial value Ica 0  or the drain current Ipa of the PA  63  is not the same as the initial value Ipa 0  (Yes in step S 102 ), the current ratio calculation unit  55  calculates current ratio α (step S 103 ). Specifically, the current ratio calculation unit  55  calculates the current ratio α by dividing the drain current Ipa of the PA  63  by the drain current Ica of the CA  62 . 
         [0047]    The gate voltage control unit  56  determines whether or not the current ratio α is less than a predetermined value α 0  (step S 104 ). When it is determined by the gate voltage control unit  56  that the current ratio α is less than the predetermined value α 0  (Yes in step S 104 ), the gate voltage is not controlled by the gate voltage control unit  56  because the distortion which occurs in the output signal of the amplification unit  51  falls within a range of a predetermined standard value or less. 
         [0048]    On the other hand, when the current ratio α is greater than or equal to the predetermined value α 0  (No in step S 104 ), the gate voltage Vgp applied to the PA  63  is decreased by a predetermined step voltage ΔVgp (step S 105 ). Thus, the drain current Ipa of the PA  63 , which is the numerator of the current ratio α, is reduced by a step current according to the step voltage ΔVgp, and the decreased drain current Ipa of the PA  63  is detected by the current detection unit  54  (step S 101 ). Consequently, the current ratio α calculated by the current ratio calculation unit  55  is decreased (step S 103 ). Subsequently, the gate voltage control unit  56  gradually decreases the gate voltage Vgp applied to the PA  63  by a predetermined step voltage ΔVgp at a time until the current ratio a falls below the predetermined value α 0 . The step voltage ΔVgp is, for instance, 0.1 V. 
         [0049]    When the current ratio α is greater than or equal to a predetermined value α 0  like this, the gate voltage Vgp applied to the PA  63  is gradually decreased by a predetermined step voltage ΔVgp at a time until the current ratio α falls below the predetermined value α 0 . Here, the predetermined value α 0  is a measured value of the current ratio α when distortion of the output signal outputted from the amplification unit  51  is less than or equal to in a predetermined standard value. Therefore, when the current ratio α falls below the predetermined value α 0 , the distortion of the output signal outputted from the amplification unit  51  is less than or equal to in a predetermined standard value, and thus deviation of the gate voltage caused by the individual difference in the CA  62  and PA  63  is reduced. As a consequence, even when the individual difference in the CA  62  and PA  63  is present, the distortion which occurs in the output signal outputted from the amplification unit  51  is reduced. 
         [0050]    As described above, in this embodiment, the amplification device  50  has the amplification unit  51 , the current detection unit  53 , the current detection unit  54 , the current ratio calculation unit  55 , and the gate voltage control unit  56 . The amplification unit  51  is a Doherty amplification unit (amplification circuit) which has the CA  62  and the PA  63 . The current detection unit  53  detects a drain current of the CA  62 . The current detection unit  54  detects a drain current of the PA  63 . The current ratio calculation unit  55  calculates a current ratio which is a ratio of the drain current of the PA  63  to the drain current of the CA  62 . The gate voltage control unit  56  controls the gate voltage applied to the PA  63  using the reference value and the current ratio calculated by the current ratio calculation unit  55 . The aforementioned reference value is a predetermined value obtained by pre-measuring a current ratio when distortion of an output signal outputted from the amplification unit  51  is less than or equal to in a predetermined standard value. For instance, when the current ratio is greater than or equal to a predetermined value, the gate voltage control unit  56  decreases the gate voltage applied to the PA  63  until the current ratio falls below the predetermined value. 
         [0051]    With this configuration of the amplification device  50 , when the current ratio falls below a predetermined value, the distortion of the output signal outputted from the amplification unit  51  is less than or equal to in a predetermined standard value, and thus deviation of the gate voltage caused by the individual difference in the CA  62  and PA  63  is reduced. As a consequence, even when the individual difference in the CA  62  and PA  63  is present, the distortion which occurs in the output signal outputted from the Doherty circuit is reduced. 
         [0052]    In the amplification device  50 , the current detection unit  53  detects a drain current of the CA  62  in the drain bias line  52   c  coupled to the drain terminal of the CA  62 . The current detection unit  54  then detects a drain current of the PA  63  in the drain bias line  52   d  coupled to the drain terminal of the PA  63 . 
         [0053]    With this configuration of the amplification device  50 , the drain current of the CA  62  and the drain current of the PA  63  are detected with high accuracy, and thus the gate voltage is controlled with high accuracy using the current ratio, and as a consequence, the occurrence of distortion in the Doherty circuit can be further reduced. 
         [0054]    It is to be noted that in the above description, when the current ratio is greater than or equal to a predetermined value, the gate voltage applied to the PA  63  is decreased until the current ratio falls below the predetermined value. However, the gate voltage applied to the CA  62  may be increased until the current ratio falls below the predetermined value.  FIG. 5  is a flowchart illustrating another example gate voltage control processing in the first embodiment. It is to be noted that in  FIG. 5 , the same portion as in  FIG. 4  is labeled with the same symbol, and detailed description thereof is omitted. The gate voltage control processing illustrated in  FIG. 5  is repeatedly performed with a predetermined period (for instance, 30 seconds). 
         [0055]    As illustrated in  FIG. 5 , the current detection unit  53  detects a drain current Ica of the CA  62 , and the current detection unit  54  detects a drain current Ipa of the PA  63  (step S 101 ). When the drain current Ica of the CA  62  is the same as the initial value Ica 0  and the drain current Ipa of the PA  63  is the same as the initial value Ipa 0  (No in step S 102 ), the gate voltage control processing is completed. The initial values Ica 0 , Ipa 0  are the values of drain current pre-measured at a reference temperature at the time of factory shipment of the amplification device  50 , for instance. 
         [0056]    On the other hand, when the drain current Ica of the CA  62  is not the same as the initial value Ica 0  or the drain current Ipa of the PA  63  is not the same as the initial value Ipa 0  (Yes in step S 102 ), the current ratio calculation unit  55  calculates current ratio α (step S 103 ). Specifically, the current ratio calculation unit  55  calculates the current ratio α by dividing the drain current Ipa of the PA  63  by the drain current Ica of the CA  62 . 
         [0057]    The gate voltage control unit  56  determines whether or not the current ratio α is less than a predetermined value α 0  (step S 104 ). When it is determined by the gate voltage control unit  56  that the current ratio α is less than the predetermined value α 0  (Yes in step S 104 ), the gate voltage is not controlled by the gate voltage control unit  56  because the distortion which occurs in the output signal of the amplification unit  51  falls within a range of a predetermined standard value or less. 
         [0058]    On the other hand, when the current ratio α is greater than or equal to the predetermined value α 0  (No in step S 104 ), the gate voltage Vgc applied to the CA  62  is increased by a predetermined step voltage ΔVgc (step S 105   a ). Thus, the drain current Ica of the CA  62 , which is the denominator of the current ratio α , is increased by a step current according to the step voltage ΔVgc, and the increased drain current Ica of the CA  62  is detected by the current detection unit  53  (step S 101 ). Consequently, the current ratio α calculated by the current ratio calculation unit  55  is decreased (step S 103 ). Subsequently, the gate voltage control unit  56  gradually increases the gate voltage Vgc applied to the CA  62  by a predetermined step voltage ΔVgc at a time until the current ratio a falls below the predetermined value α 0 . The step voltage ΔVgc is, for instance, 0.1 V. 
         [0059]    When the current ratio α is greater than or equal to a predetermined value α 0  like this, the gate voltage Vgc applied to the CA  62  is gradually increased by a predetermined step voltage ΔVgc at a time until the current ratio α falls below the predetermined value α 0 . Here, the predetermined value α 0  is a measured value of the current ratio α when distortion of the output signal outputted from the amplification unit  51  is less than or equal to in a predetermined standard value. Therefore, when the current ratio α falls below the predetermined value α 0 , the distortion of the output signal outputted from the amplification unit  51  is less than or equal to in a predetermined standard value, and thus deviation of the gate voltage caused by the individual difference in the CA  62  and PA  63  is reduced. As a consequence, even when the individual difference in the CA  62  and PA  63  is present, the distortion which occurs in the output signal outputted from the amplification unit  51  is reduced. 
         [0060]    It is to be noted that although the gate voltage applied to one of the CA  62  and the PA  63  is controlled by the above description, the gate voltage applied to both of the CA  62  and the PA  63  may be controlled. 
       Second Embodiment 
       [0061]    When a current detection unit is directly disposed in a line coupled to the drain terminal of each of the CA  62  and the PA  63 , the power may be lost in the line. Thus, in a second embodiment, the temperature of the line coupled to the drain terminal of each of the CA  62  and the PA  63  is detected in a non-contact manner, and the temperature is converted into a drain current of each of the CA  62  and the PA  63 . It is to be noted that the basic configuration of a base station in the second embodiment is the same as the configuration of the base station  10  in the first embodiment. 
         [0062]    [Configuration Example of Amplification Device] 
         [0063]      FIG. 6  is a block diagram illustrating an example amplification device in the second embodiment. In  FIG. 6 , the same portion as in  FIG. 2  is labeled with the same symbol, and a description thereof is omitted. As illustrated in  FIG. 6 , an amplification device  50 A has temperature sensors  74 ,  75  and current detection units  76 ,  77 . 
         [0064]    The temperature sensor  74  measures the temperature of the drain bias line  52   c  in a non-contact manner, and outputs the measured temperature of the drain bias line  52   c  to the current detection unit  76 . The temperature sensor  74  is an example of the first measurement unit. 
         [0065]    The temperature sensor  75  measures the temperature of the drain bias line  52   d  in a non-contact manner, and outputs the measured temperature of the drain bias line  52   d  to the current detection unit  77 . The temperature sensor  75  is an example of the second measurement unit. 
         [0066]    The current detection unit  76  converts the temperature of the drain bias line  52   c  inputted from the temperature sensor  74  into a drain current of the CA  62 , and outputs the converted drain current of the CA  62  to the current ratio calculation unit  55 . 
         [0067]      FIG. 7  is a table illustrating an example conversion table in which a range of temperature difference and a current difference are associated with each other. For instance, the current detection unit  76  converts the temperature of the drain bias line  52   c  inputted from the temperature sensor  74  into a drain current of the CA  62 , using the conversion table illustrated in  FIG. 7 . Specifically, the current detection unit  76  calculates a difference in temperature by subtracting the initial value of the temperature of the drain bias line  52   c  from the temperature of the drain bias line  52   c  inputted from the temperature sensor  74 . Here, it is assumed the difference in temperature calculated by the current detection unit  76  is “6.0° C. In this case, the current detection unit  76  uses the conversion table to obtain a current difference “40 mA” corresponding to a range of “to 7.5° C. ” to which difference in temperature “6.0° C. ” belongs. The current detection unit  76  then calculates a drain current of the CA  62  by adding the obtained current difference “40 mA” to the initial value of the drain current of the CA  62 . 
         [0068]    The current detection unit  77  converts the temperature of the drain bias line  52   d  inputted from the temperature sensor  75  into a drain current of the PA  63 , and outputs the converted drain current of the PA  63  to the current ratio calculation unit  55 . For instance, the current detection unit  77  uses the aforementioned conversion table illustrated in  FIG. 7  to convert the temperature of the drain bias line  52   d  into a drain current of the PA  63  by the same technique as used by the current detection unit  76 . 
         [0069]    [Operation Example of Amplification Device] 
         [0070]    An example of gate voltage control processing in the amplification device  50 A having the aforementioned configuration will be described.  FIG. 8  is a flowchart illustrating example gate voltage control processing in the second embodiment. It is to be noted that in  FIG. 8 , the same portion as in  FIG. 4  is labeled with the same symbol, and detailed description thereof is omitted. The gate voltage control processing illustrated in  FIG. 8  is repeatedly performed with a predetermined period (for instance, 30 seconds). 
         [0071]    As illustrated in  FIG. 8 , the temperature sensor  74  measures the temperature Tca of the drain bias line  52   c  in a non-contact manner, and the temperature sensor  75  measures the temperature Tpa of the drain bias line  52   d  in a non-contact manner (step S 111 ). When the temperature Tca of the drain bias line  52   c  is the same as the initial value Tca 0  and the temperature Tpa of the drain bias line  52   d  is the same as the initial value Tpa 0  (No in step S 112 ), the gate voltage control processing is completed. The initial values Tca 0 , Tpa 0  are the values of temperature pre-measured at a reference temperature at the time of factory shipment of the amplification device  50 A, for instance. 
         [0072]    On the other hand, when the temperature Tca of the drain bias line  52   c  is not the same as the initial value Tca 0  or the temperature Tpa of the drain bias line  52   d  is not the same as the initial value Tpa 0  (Yes in step S 112 ), the current detection unit  76  and the current detection unit  77  perform the following processing. That is, the current detection unit  76  converts the temperature Tca of the drain bias line  52   c  into the drain current Ica of the CA  62 , and the current detection unit  77  converts the temperature Tpa of the drain bias line  52   d  into the drain current Ipa of the PA  63  (step S 113 ). For instance, the current detection unit  76  and the current detection unit  77  perform conversion from the temperature to a drain current using the conversion table illustrated in  FIG. 7 . 
         [0073]    The current ratio calculation unit  55  then calculates the current ratio α (step S 103 ). Specifically, the current ratio calculation unit  55  calculates the current ratio α by dividing the drain current Ipa of the PA  63  by the drain current Ica of the CA  62 . 
         [0074]    The gate voltage control unit  56  determines whether or not current ratio α is less than the predetermined value α 0  (step S 104 ). When it is determined by the gate voltage control unit  56  that the current ratio α is less than the predetermined value α 0  (Yes in step S 104 ), the gate voltage is not controlled by the gate voltage control unit  56  because the distortion which occurs in the output signal of the amplification unit  51  falls within a range of a predetermined standard value or less. 
         [0075]    On the other hand, when the current ratio α is greater than or equal to the predetermined value α 0  (No in step S 104 ), the gate voltage Vgp applied to the PA  63  is decreased by a predetermined step voltage ΔVgp (step S 105 ). Thus, the drain current Ipa of the PA  63 , which is the numerator of the current ratio α , is reduced by a step current according to the step voltage ΔVgp, and the decreased drain current Ipa of the PA  63  is obtained by the conversion of the current detection unit  77  (step S 113 ). Consequently, the current ratio α calculated by the current ratio calculation unit  55  is decreased (step S 103 ). Subsequently, the gate voltage control unit  56  gradually decreases the gate voltage Vgp applied to the PA  63  by a predetermined step voltage ΔVgp at a time until the current ratio α falls below the predetermined value α 0 . The step voltage ΔVgp is, for instance, 0.1 V. 
         [0076]    When the current ratio α is greater than or equal to a predetermined value α 0  like this, the gate voltage Vgp applied to the PA  63  is gradually decreased by a predetermined step voltage ΔVgp at a time until the current ratio α falls below the predetermined value α 0 . Here, the predetermined value α 0  is a measured value of the current ratio α when distortion of the output signal outputted from the amplification unit  51  is less than or equal to in a predetermined standard value. Therefore, when the current ratio α falls below the predetermined value α 0 , the distortion of the output signal outputted from the amplification unit  51  is less than or equal to in a predetermined standard value, and thus deviation of the gate voltage caused by the individual difference in the CA  62  and PA  63  is reduced. As a consequence, even when the individual difference in the CA  62  and PA  63  is present, the distortion which occurs in the output signal outputted from the amplification unit  51  is reduced. 
         [0077]    As described above, in this embodiment, the amplification device  50 A has the temperature sensor  74  and the temperature sensor  75 . The temperature sensor  74  measures the temperature of the drain bias line  52   c  coupled to the drain terminal of the CA  62  in a non-contact manner. The temperature sensor  75  measures the temperature of the drain bias line  52   d  coupled to the drain terminal of the PA  63  in a non-contact manner. The current detection unit  76  then converts the temperature of the drain bias line  52   c  measured by the temperature sensor  74  into the temperature of the CA  62 . Also, the current detection unit  77  converts the temperature of the drain bias line  52   d  measured by the temperature sensor  75  into the temperature of the PA  63 . 
         [0078]    The configuration of the amplification device  50 A allows power loss to be reduced in the line coupled to the drain terminal of each of the CA  62  and the PA  63 . Consequently, the occurrence of distortion in the Doherty circuit can be further reduced and power consumption can be reduced. 
       Other Embodiments 
       [0079]    In the first and second embodiments, the current ratio calculation unit  55  and the gate voltage control unit  56  are implemented, for instance, by a field programmable gate array (FPGA), a large scale integrated circuit (LSI), or a processor as hardware. In addition, the current detection unit  76  and the current detection unit  77  are implemented, for instance, by an FPGA, an LSI, or a processor as hardware. 
         [0080]    Although the radio apparatus  12  is coupled to the control device  11  in the description of the first and second embodiments, the control device  11  and the radio apparatus  12  do not have to be provided separately. For instance, the radio apparatus  12  may perform predetermined baseband transmission processing such as encoding of transmission data to generate a transmission signal in the baseband. 
         [0081]    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.