Abstract:
A circuit is disclosed for operating Doherty amplifiers in parallel in a small size circuit and at a low cost while reducing transmission loss and preventing a narrowed band. The circuit has a plurality of Doherty amplifiers and a signal combiner. Each of the plurality of Doherty amplifiers is applied with a distributed input signal which is amplified and delivered by the Doherty amplifier. The signal combiner is made up of a transmission line transformer, is connected to the outputs of the Doherty amplifiers and to its output terminal. The signal combiner has an impedance as viewed from the Doherty amplifiers, which represents an optimal load for the Doherty amplifiers, and an impedance as viewed from the output terminal, which is equal to the characteristic impedance of a transmission line connected to the output terminal. The signal combiner combines the outputs of the Doherty amplifiers and delivers the resulting output from the output terminal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a Doherty amplifier which combines outputs of a carrier amplifier and a peak amplifier that are different in operation class from each other, to deliver the combined output.  
         [0003]     2. Description of the Related Art  
         [0004]     Power amplifiers utilized in radio communications systems are required to provide linearity and high efficiency.  
         [0005]     Also, in recent communications systems which employ multi-value digital modulation, often handle signals having a signal amplitude whose average value is different from a maximum value of the same. For amplifying such a signal, in a conventional amplifier, an operating point of the amplifier must be set such that the signal can be amplified without distortions up to a maximum amplitude. For this reason, the amplifier fails to operate near a saturated output, which would allow the amplifier to maintain a relatively high efficiency. This situation (i.e. amplifier barely operates near a saturated output) forces the amplifier to be used at a low efficiency.  
         [0006]     To solve this problem, a variety of techniques have been proposed for improving the efficiency of an amplifier while maintaining the linearity. One of such techniques is a Doherty amplifier. The Doherty amplifier, which is aimed at improving the efficiency of a high output power amplifier, delivers a combination of outputs of a carrier amplifier and a peak amplifier which are different in operation class from each other. The basic operation and configuration of the Doherty amplifier was published in an article entitled “A New High Efficiency Power Amplifier for Modulated Waves” by W. H. Doherty, 1936 Proc. of IRE, Vol. 24, No. 9, pp 1163-1182, and thus is well known in the art.  
         [0007]     The Doherty amplifier operates with the carrier amplifier maintaining saturation near saturated output power, a higher efficiency can be provided than in normal class-A and class-AB amplifiers as a whole even if the Doherty amplifier delivers an output with a backoff taken from the saturated power. The backoff refers to the difference between average output power and the saturated power. A large backoff state thus refers to a state in which the average output power is smaller than the saturated power.  
         [0008]     The general operating principles of the Doherty amplifier are well known, for example, from an article entitled “Advanced Technique in RF Power Amplifiers” by Steve C. Cripps, Artech House 2002, pp 33-56, relevant to the present invention will be described below.  
         [0009]     A combiner circuit for combining the output of the carrier amplifier with the output of the peak amplifier is made up a transformer or an impedance converter, and often includes a transmission line of one-quarter wavelength when it handles a signal in a micro-wave band. For an ideal operation of the Doherty amplifier, the impedance of a load, as viewed from an output combination point, is typically set to Z 0 /2, where Z 0  represents the characteristic impedance of a transmission line. Typically, Z 0  is set to 50 Ω in high frequency circuits.  
         [0010]     As a circuit for covering the load impedance Z 0 /2 to the characteristic impedance Z 0  in order to accomplish impedance matching, in a high frequency band, a transmission line of one-quarter wavelength is used between the combination point and output. As an example of such a circuit, a combiner circuit implemented in the micro-wave band was published in an article THE MICROWAVE DOHERTY AMPLIFIER” by R. J. McMorrow et al., 1994 IEEE MTT-S Digest (TH3E-7), pp 1653-1656).  
         [0011]     Amplifiers have also been increasingly required to produce higher power, and this requirement has conventionally been met by operating a plurality of amplifiers in parallel to produce higher power. Well-known methods for operating a plurality of amplifiers in parallel include an in-phase distribution combination type using a Wilkinson distribution combiner, a push-pull type, a balance type using a hybrid circuit, and the like. For example, an article “MICROWAVE TRANSISTOR AMPLIFIERS Analysis and Design second edition” by Guillermo Gonzalez, pp 327-333, Prentice-Hall, 1997, a general method of operating a plurality of amplifiers in parallel employs the balance type using a hybrid circuit, a Wilkinson distribution combiner, and the like.  
         [0012]     From the foregoing, it is thought that Doherty amplifiers, having a high efficiency, are effectively operated in parallel for producing a higher output in order to satisfy the requirements for linearity and high efficiency and to accomplish a higher output.  
         [0013]     As described above, in general the ideal impedance of a load, viewed from a Doherty amplifier, is different from characteristic impedance Z 0  of a system, and is one half the characteristic impedance, i.e., Z 0 /2. Therefore, for operating a plurality of Doherty amplifiers in parallel in a push-pull configuration or in a balanced configuration, a circuit is required at the output to convert impedance Z 0  of a combiner for combining outputs of the plurality of Doherty amplifiers, to load impedance Z 0 /2 of the Doherty amplifiers.  
         [0014]     As an example of the parallel operation of Doherty amplifiers, “RF HIGH POWER DOHERTY AMPLIFIER FOR IMPROVING THE EFFICIENCY OF A FEEDFORWARD LINEAR AMPLIFIER” by Kyoung-Joon Cho et al, 2004 IEEE MTT-S digest (WE6C-3), pp 847-850 has proposed a balanced-type amplifier which couples two Doherty amplifiers using a hybrid circuit for parallel operation.  FIG. 1  is a circuit diagram illustrating a conventional circuit for operating Doherty amplifiers in parallel. Referring to  FIG. 1 , at the output of each of Doherty amplifiers  51 ,  52 , there is connected quarter-wavelength impedance conversion line  53 ,  54  for converting the impedance from Z 0 /2 to Z 0 . Hybrid  55  is connected at the outputs of quarter-wavelength impedance conversion lines  53 ,  54  that for combines the outputs of two Doherty amplifier  51 ,  52 . These components make up a balanced amplifier based on the Doherty amplifiers that operate in parallel.  
         [0015]     However, the foregoing prior art example suffers from the following problems.  
         [0016]     The conventional combiner circuit illustrated in  FIG. 1 , presents a problem of larger signal transmission loss and lower amplifier efficiency due to the extended transmission line from each Doherty amplifier to the output.  
         [0017]     Further, since the impedance conversion lines and power combiner such as the hybrid circuit are both constructed by one-quarter wavelength transmission lines, the band is limited so that a desired operation can only be performed in a limited narrow band.  
         [0018]     Furthermore, a terminator is required in the combiner both in a configuration using a hybrid combiner and in a configuration using a Wilkinson combiner. Thus, such a configuration is disadvantageous in point of a reduction in size and cost because the line portion including the impedance conversion line is large.  
       SUMMARY OF THE INVENTION  
       [0019]     It is an object of the present invention to provide a compact and low-cost circuit for operating Doherty amplifiers in parallel, which reduces transmission loss, prevent a band from being narrowed.  
         [0020]     To achieve the above object, a Doherty amplifier parallel operation circuit according to the present invention that amplifies an input signal to deliver the amplified signal at an output terminal, includes a plurality of Doherty amplifiers and a signal combiner.  
         [0021]     Doherty amplifiers each receives and amplifies a distributed input signal for output. The signal combiner is made up of a transmission line transformer, is connected to the outputs of the Doherty amplifiers and to the output terminal. The signal combiner has an impedance as viewed from the Doherty amplifiers, which represents an optimal load for the Doherty amplifiers, and an impedance as viewed from the output terminal, which matches the characteristic impedance of a transmission line connected to the output terminal. The signal combiner combines the outputs of the Doherty amplifiers and delivers the resulting output from the output terminal.  
         [0022]     The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a diagram illustrating a conventional circuit for operating Doherty amplifiers in parallel;  
         [0024]      FIG. 2  is a diagram illustrating the configuration of a Doherty amplifier parallel operation circuit according to one embodiment of the present invention;  
         [0025]      FIGS. 3A  to  3 C are diagrams for describing a transmission line transformer;  
         [0026]      FIG. 4  is a diagram illustrating the configuration of a Doherty amplifier parallel operation circuit according to another embodiment of the present invention; and  
         [0027]      FIG. 5  is a diagram illustrating the configuration of a Doherty amplifier parallel operation circuit according to yet another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0028]      FIG. 2  is a diagram illustrating the configuration of a Doherty amplifier parallel operation circuit according to one embodiment of the present invention.  
         [0029]     As shown in  FIG. 2 , the Doherty amplifier parallel operation circuit of this embodiment comprises two Doherty amplifiers  11 ,  12 , signal distributor  13 , and transmission line transformer  14 . Signal distributor  13  distributes an input signal from input terminal  15  to two Doherty amplifiers  11 ,  12 . Doherty amplifier  11  includes carrier amplifier  111 , peak amplifier  112 , branch circuit  113 , and combiner circuit  114 . Similarly, Doherty amplifier  12  comprises carrier amplifier  121 , peak amplifier  122 , branch circuit  123 , and combiner circuit  124 . By way of example, branch circuits  113 ,  123  and combiner circuits  114 ,  124  are each constituted by a one-quarter wavelength transmission line.  
         [0030]     Outputs of two Doherty amplifiers  11 ,  12  are combined by transmission line transformer  14 , and delivered from output terminal  16  to a transmission line (not shown) connected thereto. Signal distributor  13  on the input side is of a conventional general type.  
         [0031]     The transmission line transformer may be constituted by a one-quarter wavelength coaxial line, by way of example. As another example, the transmission line transformer may be constituted by an equivalent lumped-constant circuit or a parallel two-wire line. Transmission line transformer  14  in the present embodiment is a coaxial cable having a characteristic impedance Z 0  and a length of one-quarter wavelength, where Z 0  is the characteristic impedance of the transmission line.  
         [0032]      FIGS. 3A  to  3 C are diagrams for describing the transmission line transformer. As illustrated in  FIG. 3A , the transmission line transformer has impedance Z 0 , as viewed from terminal T 1 , and impedance Z 0 /2, as viewed from terminals T 2 , T 3 . Specifically, as illustrated in  FIG. 3B , assuming that the impedances of terminals T 2 , T 3  are each Z 0 /2, the impedance of terminal T 1  is Z 0 . As illustrated in  FIG. 3C , assuming that the impedance of terminal T 1  is Z 0 , the impedance of terminals T 2 , T 3  are each Z 0 /2. An opposite-phase condition of a signal for Z 0 /2 of impedance can be readily established by distributing signals in opposite phase at signal distributor  13  on the input side of Doherty amplifiers  11 ,  12 .  
         [0033]     Therefore, when transmission line transformer  14  is used for a signal combiner to combine the outputs of Doherty amplifiers  11 ,  12  that operate in parallel, as illustrated in  FIG. 2 , the impedance is Z 0 /2 when the load is viewed from each Doherty amplifier  11 ,  12 . This is an ideal or optimal load condition for the Doherty amplifier. On the other hand, the impedance is Z 0  when the circuit of  FIG. 2  is viewed from the output side of transmission line transformer  14 , thus establishing the matching to the impedance of the system.  
         [0034]     As described above, in the present embodiment, a short transmission line length from each Doherty amplifier  11 ,  12  to the output reduces transmission loss and accordingly improves amplification efficiency. For example, the construction of this embodiment can reduce transmission loss to approximately one half as much as that which has been produced by a conventional circuit illustrated in  FIG. 1 , which has an impedance converter that is separate from a hybrid combiner.  
         [0035]     Also, since the circuit of this embodiment eliminates a circuit portion which would cause a limited band on the output side, the band can be extended as compared with before. For example, a portion composed of two circuits, i.e., the impedance converter and hybrid combiner in the conventional circuit illustrated in  FIG. 1  is replaced with single transmission line transformer  14  in this embodiment. This result in reduction to one-half of the circuit which may causes a limited band.  
         [0036]     Also, since the circuit of this embodiment is simple in configuration with only transmission line transformer  14  arranged on the output side of Doherty amplifiers  11 ,  12 , the circuit may be simplified and reduced in size.  
         [0037]     Also, because the circuit of this embodiment eliminates a terminator which has been required by a conventional circuit, the cost and size of the circuit is reduced.  
         [0038]     Moreover, in the foregoing embodiment, transmission line transformer  14  has a characteristic impedance that is identical to the characteristic impedance of the system, so that transmission line transformer  14  can be constituted by an inexpensive coaxial cable, thus reducing the cost of the circuit.  
         [0039]     In summary, according to the foregoing embodiment, input is distributed into two opposite-phase components by signal distributor  13 , and the two components are applied to Doherty amplifiers  11 ,  12 , respectively. The outputs of Doherty amplifiers  11 ,  12  are combined by transmission line transformer  14  which has an impedance of Z 0 /2 as viewed from Doherty amplifiers  11 ,  12  and an impedance Z 0  as viewed from the output side. Consequently, the resulting Doherty amplifier parallel operation circuit can reduce transmission loss and band limitation, and combine the outputs of the Doherty amplifiers, while satisfying an ideal operation condition of the Doherty amplifiers, in a small, inexpensive, and simple configuration.  
         [0040]     Next, a description will be given of another embodiment of the present invention which shows a circuit for operating four Doherty amplifiers in parallel.  
         [0041]      FIG. 4  is a block circuit diagram illustrating the configuration of the Doherty amplifier parallel operation circuit according to another embodiment of the present invention. Referring to  FIG. 4 , the Doherty amplifier parallel operation circuit of this embodiment comprises four Doherty amplifiers  31 - 34 , and transmission line transformers  35 ,  36 . Four Doherty amplifiers  31 - 34  are operated in parallel, while transmission line transformers  35 ,  36  function as a signal combiner. Assume that the characteristic impedance of transmission line transformers  35 ,  36  is chosen to be Z 0 /√{square root over (2)}. In  FIG. 4 a  signal distributor is omitted and is therefore not shown.  
         [0042]     The output of Doherty amplifier  31  is connected to a central conductor of transmission line transformer  35 , while the output of Doherty amplifier  33  is connected to an external conductor of transmission line transformer  35 . Similarly, the output of Doherty amplifier  32  is connected to a central conductor of transmission line transformer  36 , while the output of Doherty amplifier  34  is connected to an external conductor of transmission line transformer  36 .  
         [0043]     Doherty amplifiers  31 ,  32  are driven in phase, while Doherty amplifiers  33 ,  34  are also driven in phase. In addition, Doherty amplifiers  31   32  are driven in opposite phase to Doherty amplifiers  33 ,  34 .  
         [0044]     On the output side, the central conductor of transmission line transformer  36  is connected to the external conductor of transmission line transformer  35 . The central conductor of transmission line transformer  35  and the external conductor of transmission line transformer  36  constitute the output of the Doherty amplifier parallel operation circuit.  
         [0045]     Since the Doherty amplifier parallel operation circuit is configured such that Doherty amplifier  33  is driven at opposite phase to Doherty amplifier  31 , the impedance is one-half of the impedance Z 0  of the transmission line when the load is viewed from Doherty amplifier  31 . Similarly, the impedance is also one-half of the impedance Z 0  of the transmission line when the load is viewed from each of the remaining Doherty amplifiers  32 - 34 .  
         [0046]     Also, when transmission line transformers  35 ,  36  are connected in a manner shown in this embodiment, the impedance is Z 0  when transmission line transformer  35  is viewed from the input side of transmission line transformer  35 . Similarly, the impedance is Z 0  when transmission line transformer  36  is viewed from the input side of transmission line transformer  36 .  
         [0047]     As described above, the impedance present when the load is viewed from Doherty amplifiers  31 - 34 , is Z 0 /2, which represents an ideal load condition of the Doherty amplifiers. On the other hand, since the impedances are each Z 0 /2, when transmission line transformers  35 ,  36  are viewed from the output, the impedance is Z 0  when the Doherty amplifier parallel operation circuit is viewed from the output side, thus accomplishing the matching to the impedance of the system.  
         [0048]     Thus, according to the foregoing embodiment, the outputs of Doherty amplifiers  31 - 34  are combined by the signal combiner composed of transmission lines  35 ,  36 , the impedance of which is Z 0 /2 when viewed from the Doherty amplifiers  31 - 34 , and is Z 0  when viewed from the output side, the resulting Doherty amplifier parallel operation circuit can reduce transmission loss and band limitation, and combine the outputs of the Doherty amplifiers, while satisfying an ideal operation condition of the Doherty amplifiers in a small, inexpensive, and simple configuration.  
         [0049]     While the foregoing embodiment has shown an example in which the transmission line transformers are each composed of a coaxial wire, the transmission line transformers may each be composed of an equivalent lumped-constant circuit or a parallel two-wire line.  
         [0050]     Next, a description will be given of a further embodiment of the present invention which shows a circuit for operating k 2  (k is a positive even number) Doherty amplifiers in parallel.  
         [0051]      FIG. 5  is a block circuit diagram illustrating the configuration of the Doherty amplifier parallel operation circuit according to a further embodiment of the present invention. Referring to  FIG. 5 , the Doherty amplifier parallel operation circuit of this embodiment comprises 2n Doherty amplifiers  41   1 - 41   n ,  42   1 - 42   n , and n transmission line transformers  43   1 - 43   n , where n=k 2 /2. Transmission line transformers  43   1 - 43   n  make up a signal combiner. Assume that the characteristic impedance of transmission line transformers  43   1 - 43   n  is chosen to be Z 0 /√{square root over (n)}. In  FIG. 5 , a signal distributor is omitted, and is not therefore shown.  
         [0052]     Each transmission line transformer  43   1 - 43   n  has a central conductor connected to the output of each Doherty amplifiers  41   1 - 41   n , and an external conductor connected to the output of each Doherty amplifiers  42   1 - 42   n .  
         [0053]     Doherty amplifiers  41   1 - 41   n  are all driven in phase, while Doherty amplifiers  42   1 - 42   n  are also operated in phase. In addition, Doherty amplifiers  41   1 - 41   n  are all operated in opposite phase to Doherty amplifiers  42   1 - 42   n .  
         [0054]     On the output side, the central conductor of transmission line transformer  43   i  (i=2−n) is connected to the external conductor of transmission line transformer  43   (i−1) . Also, the central conductor of transmission line transformer  43   1  and the external conductor of transmission line transformer  43   n  constitute the output of the Doherty amplifier parallel operation circuit.  
         [0055]     Since the Doherty amplifier parallel operation circuit is configured such that Doherty amplifiers  41   1 - 41   n ,  42   1 - 42   n  are driven in opposite phase to each other, the impedance is one-half of impedance Z 0  of the transmission line when the load is viewed from Doherty amplifiers  41   1 - 41   n . Similarly, the impedance is also one-half of the impedance Z 0  of the transmission line when the load is viewed from Doherty amplifiers  42   1 - 42   n .  
         [0056]     Also, when transmission line transformers  43   1 - 43   n  are connected in a manner shown in this embodiment, the impedance is Z 0  when transmission line transformers  43   1 - 43   n  are viewed from the input side of transmission line transformers  43   1 - 43   n .  
         [0057]     As described above, the impedance when the load is viewed from Doherty amplifiers  43   1 - 43   n  is Z 0 /2, which represents an ideal load condition of the Doherty amplifiers. On the other hand, since the impedances are each Z 0 /2, when transmission line transformers  43   1 - 43   n  are viewed from the output, the impedance is Z 0  when the Doherty amplifier parallel operation circuit is viewed from the output side, thus accomplishing the matching to the impedance of the system.  
         [0058]     For the transmission line transformers having characteristic impedance Z 0 /√{square root over (n)}, li 6 nes previously so designed and manufactured are preferably used, by way of example. Alternatively, instead of the lines which have been previously designed and manufactured to have characteristic impedance Z 0 /√{square root over (n)}, transmission line transformers, the characteristic impedance of which is Z 0 , may be connected in parallel to make up a transmission line transformer which may be used for the transmission line transformer that has characteristic impedance Z 0 /√{square root over (n)}.  
         [0059]     Now consider, by way of example, that four Doherty amplifiers are operated in parallel with Z 0  being 50 ohms. In this event, since n=2, the characteristic impedance of the transmission lines may be chosen to be 35.35 ohms, as calculated by 50/√{square root over (2)}. Instead of employing so-called dedicated coaxial wires having such a characteristic impedance, two 75-ohm coaxial wires, which are general and inexpensive standard items, may be connected in parallel to make up a transmission line transformer, the characteristic impedance of which is 35 ohms (=75/2).  
         [0060]     While each of the foregoing embodiments has been described for Doherty amplifiers, whose load impedance is Z 0 /2, the present invention is not limited to such Doherty amplifiers. The technical idea underlying the Doherty amplifier parallel operation circuit of the present invention can be applied as well to any Doherty amplifier which is designed for ideal operation under other load conditions.  
         [0061]     Also, in each of the foregoing embodiments so far described, two Doherty amplifiers, which form a pair and are connected to the same transmission line transformer, are driven in opposite phase to each other. The present invention, however, is not limited to this particular configuration. Rather than necessarily driven in opposite phase, two Doherty amplifiers can be operated in parallel by adjusting a phase relationship between them and the design of transmission line transformers connected to the outputs of these Doherty amplifiers (the wire length, impedance and the like when formed of a coaxial wire).  
         [0062]     While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.