Abstract:
A broadband power amplifier is embodied by realizing a substantially two-section output matching circuit or a substantially two-section input matching circuit using a quarter wave transformer itself as the input matching circuit or the output matching circuit. The broadband power amplifier is advantageous in view of integration and miniaturization due to the low characteristic impedance of the quarter wave transformer and enables both reduction of sizes of chips and circuits due to its simple circuit structure and reduction in cost due to the reduced number of passive devices.

Description:
[0001]    This application is a Continuation Application of PCT International Application No. PCT/KR2008/004518 filed on Aug. 4, 2008, which designated the United States. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to a broadband power amplifier and, more particularly, to a broadband power amplifier employing a simple circuit structure while having broadband characteristics and being advantageous in view of integration and miniaturization. 
       BACKGROUND OF THE INVENTION 
       [0003]    In recent years, due to miniaturization and multi-functionality of mobile communication terminals, relays, and base stations, many studies on improvement in the efficiencies of RF power amplifiers that consume almost all consumption power are being carried out in an effort to increase battery time, and, among them, studies on Doherty power amplifier, which is representative of solution for increasing efficiency of a RF power amplifier, are mainly carried out. 
         [0004]    The Doherty amplifier was provided by W. H. Doherty in 1936, in which a carrier amplifier and a peaking amplifier are connected in parallel to each other using a quarter wave transformer (λ/4 line). The Doherty amplifier varies the amount of current supplied to a load by the peaking amplifier according to power level to control the load line impedance of a carrier amplifier, thereby increasing efficiency thereof. 
         [0005]    A Doherty power amplifier for used in a handset, i.e. a Doherty amplifier for a mobile communication terminal according to the prior art replaces a quarter wave transformer by a circuit such as a π-network including an inductor L and a capacitor C. Input matching circuits are connected to input terminals of the carrier amplifier and the peaking amplifier, and output matching circuits are connected to output terminals of the carrier amplifier and the peaking amplifier. 
         [0006]    In realization of Doherty power amplifiers for mobile communication terminals, due to increased importance of their size, they are needed to be miniaturized, in which case the Doherty amplifiers are being realized on PCB packages due to their sizes and inductor losses in spite of their needs for integration on chips. 
         [0007]    In order to widen the bandwidth of a frequency and obtain broadband characteristics, reference impedance is generally matched to desired impedance through a matching circuit conventionally including two or three inductors, a capacitor, and/or a micro-strip line. In this case, the circuit structure becomes complex as the number of matching circuits increases, causing the sizes of chips and circuits to become larger and causing increase in cost owing to additional use of passive devices. Then, as the number of passive devices increases, the matching circuit loss in the entire circuit increases due to loss in the passive devices, thereby causing lowering of the efficiency of the circuit. 
         [0008]      FIG. 1  is a circuit diagram illustrating the first example of a Doherty power amplifier according to the prior art, and  FIG. 2  is a circuit diagram illustrating an example of widening the bandwidth of a frequency of the Doherty power amplifier in  FIG. 1 . The reference numeral  11  in  FIGS. 1 and 2  denotes a carrier amplifier, the reference numerals  13 ,  14 ,  23 , and  24 , output matching circuits, the reference numeral  15 , a micro-strip line, and the reference numeral  21 , a peaking amplifier. The micro-strip line  15  functions as a quarter wave transformer. A load impedance R L  and a characteristic impedance R 0  do not need to be 50 Ohm and 100 Ohm, respectively, but need to satisfy the condition of R 0 =R L ×2.  FIG. 2  realizes a process of matching the output impedances of the carrier amplifier  11  and the peaking amplifier  21  from the load impedance R L  to an optimum power source impedance R opt  of a power device with two-section output matching circuits  13 ,  14 ,  23 , and  24  to thereby widen the bandwidth of a frequency. 
         [0009]      FIG. 3  is a circuit diagram illustrating the second example of a Doherty power amplifier according to the prior art, and  FIG. 4  is a circuit diagram illustrating an example of widening the bandwidth of a frequency of the Doherty power amplifier of  FIG. 3 . In  FIGS. 3 and 4 , the reference numeral  31  denotes a carrier amplifier, the reference numeral  33 , a micro-strip line, the reference numeral  41 , a peaking amplifier, the reference numeral  43 , an offset line, and the reference numerals  51  and  53 , output matching circuits. The Doherty power amplifier of  FIG. 3  has a configuration that is advantageous in view of miniaturization and integration by lowering the characteristic impedance of the micro-strip line  33  functioning as a quarter wave transformer, and  FIG. 4  represents a structure for widening the bandwidth of a frequency by realizing a process of matching output impedances of the carrier amplifier  31  and the peaking amplifier  41  from a load impedance R L  to an optimum power source impedance R opt  of a power device with two-section output matching circuits  51  and  53 . 
         [0010]      FIG. 5  is a circuit diagram illustrating the third example of a Doherty power amplifier according to the prior art. The reference numeral  61  of  FIG. 5  is a carrier amplifier; the reference numerals  63 ,  73 , and  83  are output matching circuits; the reference numeral  65  is a micro-strip line functioning as a quarter wave transformer; the reference numeral  71  is a peaking amplifier; and the reference numeral  75  is an offset line. In  FIG. 5 , the one-staged output matching circuits  63  and  73  among the two-section output matching circuits  63 ,  73 , and  81  are used for widening the bandwidth of a frequency and are located on the output lines of the carrier amplifier  61  and the peaking amplifier  71 . The Doherty power amplifier of  FIG. 5  has almost the same characteristics as that of  FIG. 4 . 
         [0011]    The first to third examples of  FIG. 2 ,  FIG. 4 , and  FIG. 5  use two-section output matching circuits to widen the bandwidths of frequencies, thereby obtaining the broadband characteristics, in which case the structures thereof are complex, causing large sizes of chips and circuits, and the matching circuit loss of all the circuits increases due to the losses in passive devices, causing lowering of the efficiencies of the circuits. In particular, the first example of the prior art is vulnerable in integration and miniaturization due to a high characteristic impedance of a micro-strip line functioning as a quarter wave transformer when it is used as a power amplifier for a mobile communication terminal. 
         [0012]    Further, even when a Doherty power amplifier is employed on a package of a printed circuit board in order to realize a power amplifier for a mobile communication terminal, the size of the package and the manufacturing cost increase due to the use of several passive devices. 
         [0013]    In addition to disadvantages in view of miniaturization of such a power amplifier, the broadband characteristics of a power amplifier may limit the adoption of power amplifiers that are to be used in the next generation mobile communication system. 
       SUMMARY OF THE INVENTION 
       [0014]    It is, therefore, an object of the present invention is to provide a power amplifier that is advantageous in view of integration and miniaturization due to the low characteristic impedance of a quarter wave transformer and has a simple circuit structure while showing the broadband characteristics. 
         [0015]    Another object of the present invention is to provide a power amplifier that has a simple circuit structure while showing the broadband characteristics by realizing a substantially two-section input matching circuit using a quarter wave transformer itself as the input matching circuit. 
         [0016]    In accordance with an aspect of the present invention, there is provided a broadband power amplifier including: a first amplifier and a second amplifier that are connected in parallel; a first quarter wave transformer connected to an output terminal of the first amplifier to perform an output matching function by controlling a load line impedance of the first amplifier; a second quarter wave transformer connected to an output terminal of the second amplifier to perform an output matching function by controlling a load line impedance of the second amplifier; and an output matching circuit connected to rear ends of the first quarter wave transformer and the second quarter wave transformer. 
         [0017]    In accordance with another aspect of the present invention, there is provided a broadband power amplifier including: a first amplifier and a second amplifier that are connected in parallel; a first impedance control line connected in series to an output terminal of the first amplifier; a second impedance control line connected in series to an output terminal of the second amplifier; an output line connecting in parallel output sides of the first impedance control line and the second impedance control line; an output matching circuit located on the output line to match a load impedance to R out //R out ′ 
         [0000]    
       
         
           
             
               ( 
               
                 
                   
                     where 
                      
                     
                         
                     
                      
                     
                       R 
                       out 
                     
                   
                   // 
                   
                     R 
                     out 
                     ′ 
                   
                 
                 = 
                 
                   
                     
                       R 
                       out 
                     
                     × 
                     
                       R 
                       out 
                       ′ 
                     
                   
                   
                     
                       R 
                       out 
                     
                     + 
                     
                       R 
                       out 
                       ′ 
                     
                   
                 
               
               ) 
             
             , 
           
         
       
     
         [0000]    wherein R out  denotes an output impedance of the first impedance control line, and R out ′ denotes an output impedance of the second impedance control line; a first quarter wave transformer whose characteristic impedance R o  satisfies the condition of R opt &lt;R o &lt;R out  or R opt &gt;R o &gt;R out  is located on the first impedance control line to match the output impedance R out  of the first impedance control line to R opt , wherein R opt  denotes an optimum power source impedance of the carrier amplifier; and a second quarter wave transformer whose characteristic impedance R o ′ satisfies the condition of R opt ′&lt;R o ′&lt;R out ′ or R opt ′&gt;R o ′&gt;R out ′ is located on the second impedance control line to match the output impedance R out ′ of the second impedance line to R opt ′, wherein R opt ′ denotes an optimum power source impedance of the peaking amplifier. 
         [0018]    In accordance with still another aspect of the present invention, there is provided a broadband power amplifier including: a first amplifier and a second amplifier that are connected in parallel; a first quarter wave transformer connected to an input terminal of the first amplifier to perform an input matching function by controlling an input impedance of the carrier amplifier; a second quarter wave transformer connected to an input terminal of the second amplifier to perform an input matching function by controlling a load line impedance of the carrier amplifier; and an input matching circuit connected to front ends of the first quarter wave transformer and the second quarter wave transformer. 
         [0019]    In accordance with yet another aspect of the present invention, there is provided a broadband power amplifier including: a first amplifier and a second amplifier connected in parallel; a first impedance control line connected in series to an input terminal of the first amplifier; a second impedance control line connected in series to an input terminal of the second amplifier; an input line connecting in parallel input sides of the first impedance control line and the second impedance control line; an input matching circuit located on the input line to match an input impedance to R in //R in ′ 
         [0000]    
       
         
           
             
               ( 
               
                 
                   
                     where 
                      
                     
                         
                     
                      
                     
                       R 
                       in 
                     
                   
                   // 
                   
                     R 
                     in 
                     ′ 
                   
                 
                 = 
                 
                   
                     
                       R 
                       in 
                     
                     × 
                     
                       R 
                       in 
                       ′ 
                     
                   
                   
                     
                       R 
                       in 
                     
                     + 
                     
                       R 
                       in 
                       ′ 
                     
                   
                 
               
               ) 
             
             , 
           
         
       
     
         [0000]    wherein R in ′ denotes an input impedance of the second impedance control line and R in  denotes an input impedance of the first impedance control line; a first quarter wave transformer whose characteristic impedance R i  satisfies the condition of “R in &lt;R i &lt;R in     —     out ” or “R in &lt;R i &lt;R in     —     out ” is located on the first impedance control line to match the input impedance R in  of the first impedance control line to R in     —     opt , wherein R in     —     opt  denotes an optimum input impedance of the first amplifier; and a second quarter wave transformer whose characteristic impedance R i ′ satisfies the condition of “R in ′&lt;R i ′&lt;R in     —     opt ′” or “R in ′&gt;R i ′&gt;R in     —     opt ′” is located on the second impedance control line to match the input impedance R in ′ of the second impedance line to R in     —     opt ′, wherein R in     —     opt ′ denotes an optimum input impedance of the second amplifier. 
         [0020]    In accordance with the present invention, the broadband power amplifier is advantageous in view of integration and miniaturization due to the low characteristic impedance of a quarter wave transformer and enables both reduction of sizes of chips and circuits due to its simple circuit structure and reduction in cost due to the reduced number of passive devices, by realizing a substantially two-section output matching circuit or a substantially two-section input matching circuit using the quarter wave transformer itself as the input matching circuit or the input matching circuit. Further, the broadband power amplifier enables improvement in the efficiency of a circuit by reducing the matching circuit loss of the entire circuit due to losses in passive devices. 
         [0021]    In addition, the broadband power amplifier that is advantageous in view of efficiency and linearity can be operated over a wide range of frequencies and can be miniaturized and integrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The above and other objects and features of the present invention will become apparent from the following description of embodiments given in conjunction with the accompanying drawings, in which: 
           [0023]      FIGS. 1 and 2  are circuit diagrams illustrating a first example of a Doherty power amplifier according to the prior art; 
           [0024]      FIGS. 3 and 4  are circuit diagrams illustrating a second example of a Doherty power amplifier according to the prior art; 
           [0025]      FIG. 5  is a circuit diagram illustrating a third example of a Doherty power amplifier according to the prior art; 
           [0026]      FIGS. 6 to 8  are circuit diagrams illustrating broadband power amplifiers in accordance with a first to third embodiments of the present invention; 
           [0027]      FIGS. 9 to 11  are circuit diagrams illustrating broadband power amplifiers in accordance with a fourth to sixth embodiments of the present invention; 
           [0028]      FIGS. 12 and 13  illustrate graphs representing frequency band characteristics of a Doherty power amplifier according to the prior art and a broadband power amplifier in accordance with the present invention, respectively, when load impedance is 50 ohm; 
           [0029]      FIG. 14  is a circuit diagram illustrating a broadband power amplifier in accordance with a seventh embodiment of the present invention; and 
           [0030]      FIG. 15  is a circuit diagram illustrating a broadband power amplifier in accordance with an eighth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0031]    Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
         [0032]      FIGS. 6 to 8  are circuit diagrams of broadband power amplifiers for output matching in accordance with a first to third embodiments of the present invention. 
         [0033]    Referring first to  FIG. 6 , a broadband power amplifier according to the first embodiment of the present invention includes a first amplifier  111  and a second amplifier  121  that are connected in parallel, a first quarter wave transformer  113  connected to an output terminal of the first amplifier  111  to perform an output matching function through control of the load line impedance of the first amplifier  111 , a second quarter wave transformer  123  connected to an output terminal of the second amplifier  121  to perform an output matching function through control of the load line impedance of the second amplifier  121 , and an output matching circuit  131  connected to rear ends of the first quarter wave transformer  113  and the second quarter wave transformer  123 . 
         [0034]    According to the first embodiment of the present invention, the first and the second quarter wave transformers  113  and  123  includes first and second micro-strip lines, respectively. In addition, the first amplifier  111  and the second amplifier  121  may include a carrier amplifier and a peaking amplifier, respectively, or vice versa. 
         [0035]    Referring to  FIG. 7 , a broadband power amplifier according to the second embodiment of the present invention is identical to that of the first embodiment, except that the quarter wave transformers are substituted with π-networks  115  and  125 , respectively. Each of the π-networks  115  and  125  has an inductor-capacitor circuit having an inductor L and capacitors C. Although one inductor L and two capacitors C are employed to realize each π-network  115  and  125  in  FIG. 7 , two inductors L and one capacitor C may be employed to realize a π-network. For example, a capacitor C may be disposed at the locations of the inductors L of the π-network  115  and inductors L may be disposed at the locations of the capacitors C of the π-network  115 . 
         [0036]    Referring to  FIG. 8 , a broadband power amplifier according to the third embodiment of the present invention is identical to that of the first embodiment, except that the quarter wave transformers are substituted with T-networks  117  and  127 , respectively. Each of the T-networks  117  and  127  has an inductor-capacitor circuit having an inductors L and a capacitor C. Although one inductor L and two capacitors C are employed to realize each T-network  115  and  125  in  FIG. 8 , two inductors L and one capacitor C may be employed to realize a T-network. For example, a capacitor C may be disposed at the locations of the inductors L of the T-network  117  and inductors L may be disposed at the locations of the capacitors C of the π-network  115 . 
         [0037]    Meanwhile, although the quarter wave transformers are realized by π-networks or T-networks in the embodiments of  FIGS. 7 and 8 , any circuits that ensure the characteristics of the quarter wave transformers using inductors and capacitors may be applied to a broadband power amplifier according to the present invention, instead of the π-networks and T-networks. 
         [0038]    In order to help understand the operational characteristics of the respective broadband power amplifiers according to the first to third embodiments of the present invention, a broadband power amplifier will be described in detail with reference to  FIG. 6  only. 
         [0039]    A line connected in series to an output terminal of the first amplifier  111  functions as a first impedance control line, a line connected in series to an output terminal of the second amplifier  121  functions as a second impedance control line, and a line connecting in parallel the first impedance control line and the second impedance control line functions as an output line. 
         [0040]    Assuming that R opt  denotes an optimum power source impedance of the first amplifier  111 , R opt ′, an optimum power source impedance of the second amplifier  121 , R out , an output impedance of the first impedance control line, and R out ′, an output impedance of the second impedance control line, an output matching circuit matching a load impedance to R out //R out ′ 
         [0000]    
       
         
           
             
               ( 
               
                 
                   
                     where 
                      
                     
                         
                     
                      
                     
                       R 
                       out 
                     
                   
                   // 
                   
                     R 
                     out 
                     ′ 
                   
                 
                 = 
                 
                   
                     
                       R 
                       out 
                     
                     × 
                     
                       R 
                       out 
                       ′ 
                     
                   
                   
                     
                       R 
                       out 
                     
                     + 
                     
                       R 
                       out 
                       ′ 
                     
                   
                 
               
               ) 
             
             , 
           
         
       
     
         [0000]    is located on the output line, and a first quarter wave transformer  113  whose characteristic impedance R o  satisfies the condition of R opt &lt;R o &lt;R out  or R opt &gt;R o &gt;R out  is located on the first impedance control line to match the output impedance R out  of the first impedance control line to R opt . 
         [0041]    A second quarter wave transformer  123  whose characteristic impedance R o ′ satisfies the condition of R opt ′&lt;R o ′&lt;R out ′ or R opt ′&gt;R o ′&gt;R out ′ is located on the second impedance control line to match the output impedance R out ′ of the second impedance line to R opt ′. 
         [0042]    The broadband power amplifiers according to the first to third embodiments of the present invention are advantageous in integration and miniaturization due to their low characteristic impedances of the quarter wave transformers. 
         [0043]    The quarter wave transformers, which include the micro-strip lines  113  and  123 ; the π-networks  115  and  125 ; or the T-networks  117  and  127 , are employed in output matching circuits to realize substantially two-section output matching circuits, thereby widening the bandwidths of frequencies so as to acquire wide bandwidth characteristics. Further, the two-section output matching circuits are substantially the same as a single section output matching circuit in view of complexity and costs of the circuits and losses of the matching circuits. 
         [0044]    Alternatively, one or more inductors L in the π-networks  115  and  125  and the T-networks  117  and  127  of the second embodiment may be realized by small inductors such as micro-strip lines or by bondwire inductors (for example, bonding wire inductors) essential for connection of MMIC chips to modules, in order to improve the complexity of the circuits, costs, and losses of the matching circuits. 
         [0045]      FIGS. 9 to 11  are circuit diagrams of broadband power amplifiers for output matching in accordance with fourth to sixth embodiments of the present invention. 
         [0046]    Referring first to  FIG. 9 , a broadband power amplifier of the fourth embodiment includes a first amplifier  211  and a second amplifier  221  that are connected in parallel to induce a Doherty operation, a first quarter wave transformer  213  connected to an output terminal of the first amplifier  211  to perform an output matching function through control of the load line impedance of the first amplifier  211 , and a second quarter wave transformer  225  connected to an output terminal of the second amplifier  221  to perform an output matching function through control of the load line impedance of the second amplifier  121 . The broadband Doherty power amplifier of the fourth embodiment further includes an offset line  223  connected between the output terminal of the second amplifier  221  and the second quarter wave transformer  225  to make the output impedance to the second amplifier  221  sufficiently high as compared with an impedance of an output terminal and an impedance of the first amplifier  211 , and an output matching circuit  231  connected to rear ends of the first quarter wave transformer  213  and the second quarter wave transformer  223 . 
         [0047]    According to the fourth embodiment of the present invention, the first and the second quarter wave transformers  213  and  225  include first and second micro-strip lines, respectively. In addition, the first amplifier  211  and the second amplifier  221  may include a carrier amplifier and a peaking amplifier, respectively, or vice versa. 
         [0048]    Referring now to  FIG. 10 , a broadband power amplifier according to the fifth embodiment of the present invention is identical to that of the fourth embodiment, except that the quarter wave transformer are realized by π-networks  215  and  227 , respectively. Each of the π-networks  215  and  227  includes an inductor-capacitor circuit having an inductor L and capacitors C. Although one inductor L and two capacitors C are employed to realize each π-network  215  and  227  in  FIG. 10 , two inductors L and one capacitor C may be employed to realize a π-network. For example, a capacitor C may be disposed at the locations of the inductors L of the π-network  215  and inductors L may be disposed at the locations of the capacitors C of the π-network  215 . 
         [0049]    Referring to  FIG. 11 , a broadband power amplifier according to the sixth embodiment of the present invention is identical to that of the fourth embodiment, except that the quarter wave transformer is realized with T-networks  217  and  229 , respectively. Each of the T-networks  217  and  229  has an inductor-capacitor circuit having an inductor L and a capacitor C. Although one inductor L and two capacitors C are employed to realize each T-network  217  and  229  in  FIG. 11 , two inductors L and one capacitor C may be employed to realize a T-network. For example, a capacitor C may be disposed at the locations of the inductors L of the T-network  117  and inductors L may be disposed at the locations of the capacitors C of the T-network  217 . 
         [0050]    Meanwhile, although the quarter wave transformers are realized by π-networks or T-networks in the embodiments of  FIGS. 10 and 11 , any circuits that ensure the characteristics of quarter wave transformers using inductors and capacitors may be applied to a broadband power amplifier according to the present invention, instead of π-networks and T-networks. 
         [0051]    In order to help understand the operational characteristics of the respective broadband power amplifiers according to the fourth to sixth embodiments of the present invention, the descriptions of the broadband power amplifiers will be made with reference to  FIG. 9  only. 
         [0052]    A line connected in series to an output terminal of the first amplifier  211  functions as a first impedance control line, a line connected in series to an output terminal of the second amplifier  221  functions as a second impedance control line, and a line connecting in parallel the first impedance control line and the second impedance control line functions as an output line. 
         [0053]    Assuming that R opt  denotes an optimum power source impedance of the first amplifier  211 ; R apt ′, an optimum power source impedance of the second amplifier  221 ; R out , an output impedance of the first impedance control line; and R out ′; and an output impedance of the second impedance control line, an output matching circuit matching a load impedance to R out //R out ′ is located on the output line, and a first quarter wave transformer  213  whose characteristic impedance R o  satisfies the condition of R opt &lt;R o &lt;R out  or R opt &gt;R o &gt;R out  is located on the first impedance control line to match the output impedance R out  of the first impedance control line to R opt . 
         [0054]    A second quarter wave transformer  225  whose characteristic impedance R o ′ satisfies the condition of R opt ′&lt;R o ′&lt;R out ′ or R opt ′&gt;R o ′&gt;R out ′ is located on the second impedance control line to match the output impedance R out ′ of the second impedance line to R opt ′. 
         [0055]    The broadband power amplifiers according to the fourth to sixth embodiments of the present invention are advantageous in integration and miniaturization due to the low characteristic impedances of the quarter wave transformers, which may include the micro-strip lines  213  and  225 ; the π-networks  215  and  217 ; or T-networks  217  and  229 . 
         [0056]    The quarter wave transformers are employed in output matching circuits to realize substantially two-section output matching circuits, thereby widening the bandwidths of frequencies so as to acquire wide bandwidth characteristics. Further, the two-section output matching circuits are substantially the same as a single section output matching circuit in view of complexity and manufacturing costs of the circuits and losses of the matching circuits. 
         [0057]    Alternatively, one or more inductors L in the π-networks  215  and  227  and the T-networks  217  and  229  of the fifth and sixth embodiments may be realized by small inductors such as micro-strip lines or by bondwire inductors (for example, bonding wire inductors) essential for connection of MMIC chips to modules, in order to improve the complexity of the circuits, costs, and losses of the matching circuits. 
         [0058]    In the broadband Doherty power amplifier of the prior art that is illustrated in  FIG. 3 , when a load impedance R L  is determined, impedance conversion (or impedance matching) for converting the impedance of R L  to R opt /2 needs to be carried out to the output matching circuit  51  in order to make the impedance viewed from the output terminals of the first amplifier  31  and the second amplifier  41  be R opt . 
         [0059]    In order to compare the broadband Doherty amplifier according to the fifth embodiment of the present invention to the aforementioned prior art, it is assumed that the impedance characteristics of the first amplifier and the second amplifier are the same in the broadband Doherty amplifier according to the fifth embodiment of the present invention that is illustrated in  FIG. 10 . 
         [0060]    In the broadband power amplifier according to the fifth embodiment of the present invention, when a load impedance R L  is determined, impedance conversion (or impedance matching) for converting the impedance of R L  to R out /2 needs to be carried out to the output matching circuit  231  in order to make the impedance viewed from the output terminals of the first amplifier  211  and the second amplifier  221  be R opt . The reason why is that the π-networks  215  and  227  functioning as quarter wave transformers carry out conversion from the impedance of R out  to R opt . 
         [0061]    In order to realize the impedance matching of the above-mentioned ratio, in the embodiments of the present invention, two-section matching is carried out. The matching ratios in the respective sections are R opt /2R L  in  FIG. 3  and are R out /2R L  and R opt /R out  if R opt =R opt ′ and R out =R out ′ in  FIG. 10 . In general, the frequency characteristics in  FIG. 10  are more excellent than in  FIG. 3  due to the relationship of R opt &lt;R out &lt;R L . The reason why is that, in designing a micro-circuit, the frequency characteristics of an impedance matching circuit, i.e., the desired frequency range within which an impedance converter is operated becomes narrower as the matching ratio becomes larger. 
         [0062]      FIGS. 12 and 13  illustrate graphs representing frequency band characteristics of power amplifiers shown in  FIG. 4  and  FIG. 10 , respectively. It can be seen, from the graphs illustrating simulation results of frequency characteristics of both power amplifiers, that the broadband power amplifier of  FIG. 10  is more excellent than that of  FIG. 4 . 
         [0063]    Meanwhile, in an RF circuit as in a power amplifier, the bandwidth of a frequency, in which the circuit is operated, is determined by a portion of the circuit where the ratio for impedance matching is largest. Therefore, it is necessary to pay attention to the impedance matching ratios of an input matching circuit and an internal matching circuit as well as an output matching circuit. 
         [0064]      FIG. 14  is a circuit diagram illustrating a broadband power amplifier for input matching in accordance with a seventh embodiment of the present invention. 
         [0065]    Referring first to  FIG. 14 , a broadband power amplifier according to the seventh embodiment of the present invention includes a first amplifier  323  and a second amplifier  333  that are connected in parallel, a first quarter wave transformer  321  connected to an output terminal of the first amplifier  323  to perform an output matching function through control of the load line impedance of the first amplifier  323 , a second quarter wave transformer  331  connected to an input terminal of the second amplifier  333  to perform an input matching function through control of the load line impedance of the second amplifier  333 , and an input matching circuit  311  connected to front ends of the first quarter wave transformer  321  and the second quarter wave transformer  331 . 
         [0066]    In the seventh embodiment, the first and the second quarter wave transformers  321  and  331  include first and second micro-strip lines, respectively. In addition, the first amplifier  323  and the second amplifier  333  may include a carrier amplifier and a peaking amplifier, respectively, or vice versa. 
         [0067]    The impedance matching characteristic of the broadband power amplifier of the seventh embodiment will be described in detail. 
         [0068]    A line connected in series to an input terminal of the first amplifier  323  functions as a first impedance control line, a line connected in series to an input terminal of the second amplifier  333  functions as a second impedance control line, and a line connecting in parallel the first impedance control line and the second impedance control line functions as an input line. 
         [0069]    Assuming that R in     —     opt  denotes an optimum input impedance of the first amplifier  323 ; R in     —     opt ′, an optimum input impedance of the second amplifier  333 ; R in , an input impedance of the first impedance control line; and R in ′, an input impedance of the second impedance control line, an input matching circuit matching an input impedance to R in //R in ′ is located on the input line, and the first quarter wave transformer  321  whose characteristic impedance R o  satisfies the condition of R in     —     opt &lt;R i &lt;R in  or R in     —     opt &gt;R i &gt;R in  is located on the first impedance control line to match the input impedance R in  of the first impedance control line to R in     —     opt . 
         [0070]    The second quarter wave transformer  331  whose characteristic impedance satisfies the condition of R in     —     opt ′&lt;R i ′&lt;R in ′ or R in     —     opt ′&gt;R i ′&gt;R in ′ is located on the second impedance control line to match the output impedance R out ′ of the second impedance line to R in ′. 
         [0071]    Alternatively, the quarter wave transformer  321  and  331  of the micro-strip lines may be realized by inductor-capacitor circuits such as π-networks and T-networks each having inductors L and capacitors C. 
         [0072]    Moreover, one or more inductors L in the π-networks and the T-networks may be realized by small inductors such as micro-strip lines or by bondwire inductors (for example, bonding wire inductors) essential for connection of MMIC chips to modules, in order to improve the complexity of the circuits, costs, and losses of the matching circuits. 
         [0073]      FIG. 15  is a circuit diagram illustrating a broadband power amplifier for input matching in accordance with an eighth embodiment of the present invention. 
         [0074]    Referring to  FIG. 15 , a broadband power amplifier according to the eighth embodiment of the present invention includes a first amplifier  423  and a second amplifier  433  that are connected in parallel, a first quarter wave transformer  421  connected to an input terminal of the first amplifier  423  to perform an input matching function through control of the load line impedance of the first amplifier  423 , and a second quarter wave transformer  431  connected to an input terminal of the second amplifier  433  to perform an input matching function through control of the load line impedance of the second amplifier  433 , wherein the first and the second quarter wave transformers  421  and  431  include first and second micro-strip lines, respectively. 
         [0075]    The broadband power amplifier according to the eighth embodiment further includes a delay compensation circuit  441  connected to one or all of a position between the first quarter wave transformer  421  and the first amplifier  423  and a position between the second quarter wave transformer  431  and the second amplifier  433  to compensate for a delay between the first amplifier  423  and the second amplifier  433 , and an input matching circuit  411  connected to front ends of the first and the second quarter wave transformers  421  and  431 . 
         [0076]    The impedance matching characteristics of the broadband power amplifier of the eighth embodiment will be described. Here, the structure and wirings of the delay compensation circuit  441  are well known in the art, and thus the detailed description thereof will be omitted. 
         [0077]    A line connected in series to an input terminal of the first amplifier  423  functions as a first impedance control line, a line connected in series to an input terminal of the second amplifier  433  functions as a second impedance control line, and a line connecting in parallel the first impedance control line and the second impedance control line functions as an input line. 
         [0078]    In the eighth embodiment, the first amplifier  323  and the second amplifier  333  may include a carrier amplifier and a peaking amplifier, respectively, or vice versa. 
         [0079]    Assuming that R in     —     opt  denotes an optimum input impedance of the first amplifier  423 ; R in     —     opt ′, an optimum input impedance of the second amplifier  433 ; R in , an input impedance of the first impedance control line; and R in ′, an input impedance of the second impedance control line, an input matching circuit  411  matching an input impedance R s  to R in //R in ′ is located on the input line, and a first quarter wave transformer  421  whose characteristic impedance R i  satisfies the condition of R in     —     opt &lt;R i &lt;R in  or R in     —     opt &gt;R i &gt;R in  is located on the first impedance control line to match the input impedance R in  of the first impedance control line to R in     —     opt . 
         [0080]    A second quarter wave transformer  431  whose characteristic impedance satisfies the condition of R in     —     opt ′&lt;R i ′&lt;R in ′ is located on the second impedance control line to match the input impedance R in ′ of the second impedance line to R in     —     opt ′. 
         [0081]    Alternatively, the quarter wave transformers  421  and  431  may be realized by the inductor-capacitor circuits such as π-networks or T-networks. 
         [0082]    Moreover, one or more inductors in the π-networks and the T-networks may be realized by small inductors such as micro-strip lines or by bondwire inductors (for example, bonding wire inductors) essential for connection of MMIC chips to modules, in order to improve the complexity of the circuits, costs, and losses of the matching circuits. 
         [0083]    While the invention has been shown and described with respect to the exemplary embodiments, it will be understood by those skilled in the art that the system and the method are only examples of the present invention and various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.