Abstract:
The invention relates to improving the performance of load modulation power amplifiers through the use of coupled transmission line-based power combiners. Exemplary embodiments disclosed include a power amplifier comprising an input connected to first and second amplifier stages and an output stage configured to combine phase shifted amplified outputs from the first and second amplifier stages and to provide an amplified signal at an output of the power amplifier, wherein the output stage comprises coupled first and second transmission lines connected between the output of the first amplifier stage and an output load connection.

Description:
This application claims the priority under 35 U.S.C. §119 of European patent application no. 10250822.3, filed on Apr. 23, 2010, the contents of which are incorporated by reference herein. 
     FIELD OF INVENTION 
     The invention relates to improving the performance of load modulation power amplifiers through the use of coupled transmission line-based power combiners. 
     BACKGROUND OF THE INVENTION 
     Power amplifiers (PAs) that use load line modulation techniques to improve overall amplifier efficiency have been known for some time. Two well-known examples are in the form of a two-way Doherty amplifier, illustrated in  FIG. 1 , and a Chireix outphasing amplifier, illustrated in  FIG. 2 . A two-way Doherty amplifier comprises two amplifier stages  101 ,  102 , a first of these being a peak amplifier  101  and a second being a main amplifier  102 . The peak amplifier  101  amplifies a phase-shifted version of the input signal, while the main amplifier  102  amplifies an unshifted version. A combiner stage  103  combines the output signals from the amplifier stages  101 ,  102  and provides an output amplified signal to a load  104 . A Chireix outphasing amplifier operates according to a similar principle, with two amplifier stages  201 ,  202  providing amplified versions of the input signal to a combiner stage  203 , which combines the outputs to provide an amplified output signal to a load  204 . 
     N-way Doherty power amplifiers, where N&gt;2, are also known, one example being a three-way Doherty power amplifier, an example of which is illustrated in  FIG. 3 . In this type of Doherty amplifier, outputs from a main amplifier  301  and two peak amplifiers  302   a ,  302   b  are combined in a combiner stage  303  to provide an amplified output signal to a load  304 . The arrangement of phase shifts on the input and output stages of the amplifier can be varied. WO 2009/081341 discloses further alternative examples of such amplifiers, one of which is illustrated in  FIG. 4 , with a similar arrangement of amplifier stages  401 ,  402   a ,  402   b  but with a different arrangement of phase shifts on the inputs to the amplifier stages and in the combiner stage  403 . 
     All of the above described power amplifier concepts share one similarity, which is the use of an output stage power combiner  103 ,  203 ,  303 ,  403  in various different arrangements and with single or multiple λ/4 lines. 
     A λ/4 (i.e. quarter wavelength) line, when used as combiner, has some limitations related to its frequency properties that can adversely impact the power amplifier, in particular by reducing to a certain extent the operational frequency bandwidth. With increasing complexity of the power combiner this effect becomes more pronounced. 
     Contemporary power amplifiers of the above type are typically used to amplify digitally modulated signals with a high peak to average ratio (PAR). The efficiency of the power amplifier at moderate power back off levels determines the overall amplifier performance. Based on typical signal statistics, most of the time such power amplifiers will operate with a value of output load significantly deviating from a nominal load of 50 Ω. For instance the main amplifier stage of a 2-way symmetric Doherty power amplifier, when amplifying digitally modulated signals, will tend to experience dynamic excursions of the load that may change from the nominal value to two times the nominal value. For a Chireix outphasing amplifier the load variations tend to be even larger. It is of significant importance that the load conditions do not change with frequency in order to preserve the optimum power amplifier performance over the entire frequency band of operation. 
       FIG. 5  shows the impedance transformation properties of a λ/4 line, as used as 2-way Doherty combiner centred at 2 GHz. When the λ/4 line transforms the load from the nominal value R 1  to the double load 2×R 1  the transformation is exact only at the centre frequency. The transformation bandwidth at a voltage standing wave ratio of 1.1:1 is from 1.85 GHz to 2.25 GHz, or a relative bandwidth of 20%. The deviation from the desired load impedance with the change of the operation frequency effectively compromises all important power amplifier parameters such as gain, output power and efficiency. 
     It is an object of the invention to address one or more of the above mentioned problems. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention there is provided a power amplifier comprising an input connected to first and second amplifier stages and an output stage configured to combine phase shifted amplified outputs from the first and second amplifier stages and to provide an amplified signal at an output of the power amplifier, wherein the output stage comprises coupled first and second transmission lines connected between the output of the first amplifier stages and an output load connection. 
     The use of coupled first and second transmission lines in this way significantly improves the above mentioned deficiency of the λ/4 line power combiner, allowing the power amplifier to achieve improved operation over a comparable frequency band, or alternatively a similar or better operation over a significantly wider frequency bandwidth. 
     The principle of the invention may be applied to a power amplifier configured as a Doherty amplifier or as a Chireix outphasing amplifier. In the case of a Doherty amplifier, the first transmission line may be connected between the output of the first amplifier stage and the output of the second amplifier stage and the output load connection of the power amplifier, and the second transmission line connected to the output of the second amplifier stage and coupled to the first transmission line. The first and second transmission lines preferably have a length of a quarter of a wavelength of an operating frequency of the power amplifier. The second transmission line may be connected to the output load connection via a third transmission line having a length of at least a quarter wavelength of the operating frequency of the power amplifier, or alternatively directly connected to the output load connection and the ground. The power amplifier may comprise a third amplifier stage having an output connected to the output load connection and coupled third and fourth transmission lines connected between the output of the second amplifier stage and the output load connection. The power amplifier may further comprise coupled fifth and sixth transmission lines connected between the output of the third amplifier stage and the output load connection. 
     Where the power amplifier is configured as a Chireix outphasing amplifier, the power amplifier may comprise coupled third and fourth transmission lines connected between the output of the second amplifier stage and the output load connection. The first to fourth transmission lines each preferably have a length of a quarter of a wavelength of an operating frequency of the power amplifier. The input of the second transmission line may be connected to the output load connection via a fifth transmission line and the input of the fourth transmission line connected to the output load connection via a sixth transmission line, the fifth and sixth transmission lines each having a length of at least a quarter wavelength of the operating frequency of the power amplifier. 
     The operating frequency of the power amplifier, for example as defined by the centre operating frequency, may be within the range 1 to 3 GHz. 
     In accordance with a second aspect of the invention there is provided a method of operating a power amplifier according to the first aspect, the method comprising: 
     receiving a signal at an input of the power amplifier; 
     dividing the signal between the first and second amplifier stages; 
     amplifying the divided signal by the first and second amplifier stages; 
     combining the resulting amplified signals from outputs of the first and second amplifiers in the output stage; and 
     providing an amplified output signal at an output of the power amplifier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments according to the invention are described in further detail below with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic circuit diagram of a conventional two-way Doherty amplifier; 
         FIG. 2  is a schematic circuit diagram of a conventional two-way Chireix amplifier; 
         FIG. 3  is a schematic circuit diagram of a known three-way Doherty amplifier; 
         FIG. 4  is a schematic circuit diagram of an alternative known three-way Doherty amplifier; 
         FIG. 5  is a plot of impedance as a function of frequency for the combiner stage of a conventional two-way Doherty amplifier; 
         FIGS. 6A and 6B  are schematic circuit diagrams of a two-way Doherty amplifier according to a first exemplary embodiment; 
         FIG. 7  is a plot of impedance as a function of frequency for a Doherty amplifier according to the first exemplary embodiment compared with a conventional two-way Doherty amplifier; 
         FIG. 8  is a further plot of impedance as a function of frequency for a Doherty amplifier according to the first exemplary embodiment compared with a conventional two-way Doherty amplifier; 
         FIG. 9  is a schematic circuit diagram of a two-way Chireix amplifier according to a second exemplary embodiment; 
         FIG. 10  is a plot of efficiency as a function of phase for a Chireix amplifier according to the second exemplary embodiment compared with a conventional Chireix amplifier; 
         FIG. 11  is a further plot of efficiency as a function of phase for a Chireix amplifier according to the second exemplary embodiment compared with a conventional Chireix amplifier; 
         FIG. 12  is a schematic circuit diagram of three-way Doherty amplifier according to a first further alternative exemplary embodiment; and 
         FIG. 13  is a schematic circuit diagram of a three-way Doherty amplifier according to a second further alternative exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A main principle according to embodiments of the invention disclosed herein is based on a combination of electrically connected coupled and single transmission lines in an output stage of a power amplifier comprising two or more amplifier stages providing phase shifted amplified outputs of an input signal. Coupled transmission lines are known for use as impedance transforming elements in impedance matching structures, but not as combining structures for load modulation power amplifiers such as Doherty or Chireix power amplifiers, where amplifier operation has to be guaranteed under dynamically varying load conditions. 
       FIG. 6A  illustrates a two-way symmetric Doherty power amplifier according to a first exemplary embodiment of the invention. As with the Doherty amplifier shown in  FIG. 1 , the amplifier comprises first and second amplifier stages  601 ,  602  having inputs connected to a common input signal, the second amplifier stage  602  being provided with a phase-shifted version of the input signal. The outputs of the amplifier stages  601 ,  602  are combined in a power combiner stage  603 . The power combiner  603  comprises a pair of coupled transmission lines  605 , each transmission line  605   a ,  605   b  having a length λ/4, i.e. a quarter wavelength based on the centre frequency of the amplifier. 
     As used herein, the term coupled lines, or coupled transmission lines, refers to two transmission lines that share common electrical and magnetic field. 
     An optional third transmission line  606  with a characteristic impedance Z 01  and a length of at least λ/4 is also shown connected between the second one  605   b  of the coupled pair  605  and the output load  604 , which is also connected to the output of the second amplifier stage  602 . The coupled lines  605  may be characterised by their even and odd mode impedances or alternatively by their geometrical dimensions. The transmission lines  605   a ,  605   b  may for example be implemented as symmetric strip lines or asymmetric micro strip lines. The third transmission line  606  may also be implemented in a similar way. In alternative embodiments, the third transmission line may be absent, and replaced by a short circuit and the open end of the second coupled line  605   b  connected to ground. 
     As an illustrative example, the impedances of the transmission lines  605   a ,  605   b ,  606  may be chosen such that the even and odd mode impedances of the coupled lines are 80 Ω(Z oe , or even mode impedance) and 44 Ω(Z oo , or odd mode impedance), and the impedance of the transmission line 75 Ω, and the electrical length of each of the lines selected to be one quarter wavelength. Other impedance values may alternatively be selected. 
       FIG. 6B  depicts an alternate configuration of the embodiment shown in  FIG. 6A . In this configuration, the second transmission line is connected directly to the output load connection and the ground. 
     The performance of the coupled line Doherty power combiner of the embodiment in  FIG. 6A  is shown in  FIG. 7 , as compared to an equivalent conventional Doherty power combiner. The bandwidth of the combiner comprising a coupled pair of transmission lines at double load (indicated by the legend identified as Z 100  in  FIG. 7 , with “Z 100  λ/4” indicating the conventional combiner and “Z 100  cpl” with the coupled transmission lines) shows an improvement in the bandwidth at a VSWR of 1.05 from 0.85 f 0  to around 1.15 f 0 , or a relative BW of 30%, for a centre frequency f 0  of 2 GHz. This is a 50% effective increase of the BW compared with the conventional power combiner. The result of this is that the power amplifier performance at frequencies away from the centre frequency is improved. The centre frequency f 0  of the power combiner may be other than 2 GHz, which is shown in  FIG. 7  by way of example only. 
       FIG. 8  shows the performance of the exemplary combiner  603  further optimized for a maximally flat response at double load by adjusting the parameters of the coupled lines  605   a ,  605   b  and the transmission line  606 . In this case the double load line bandwidth for a VSWR of 1.1 is from 0.6 f 0  to 1.35 f 0  (1.2 GHz to 2.7 GHz with f 0 =2 GHz), equivalent to a relative bandwidth of 75%. For a VSWR of 1.05 the bandwidth is from 0.65 f 0  to 1.3 f 0  (1.3 GHz to 2.6 GHz for f 0 =2 GHz), equivalent to a relative bandwidth of 65%. 
     As shown in the results in  FIGS. 7 and 8 , the performance of the amplifier  600  with a nominal load will deteriorate and become comparable to the performance of the conventional combiner at double load. The overall performance of the power amplifier  600  will however be improved, since most of the time the amplifier operates according to the double load line. 
     The optimal performance of different types of power amplifier may differ from case to case. In the case of a Chireix outphasing amplifier, the output is subject to much bigger load variations than with a two-way Doherty amplifier. The parameters of the coupled line combining structure to optimize the composite power amplifier performance according to predetermined criteria may calculated and adjusted accordingly, for example through the use of known computer simulation techniques. 
     Illustrated in  FIG. 9  is a power amplifier  900  according to a second exemplary embodiment of the invention. The power amplifier  900  is in the form of a Chireix outphasing amplifier, having first and second amplifier stages  901 ,  902  with inputs connected to an SCS (Signal Component Separator) input stage  909 , which performs signal processing of the input signal. Outputs of the amplifier stages  901 ,  902  are connected to a power combiner stage  903  comprising a first pair  905  of coupled first and second transmission lines  905   a ,  905   b  connected between the output of the first amplifier stage  901  and an output load  904 , a second pair  907  of coupled third and fourth transmission lines  907   a ,  907   b  connected between the output of the second amplifier stage  902  and the output load  904 . A second one  905   b  of the first pair  905  of coupled transmission lines is connected to the output load  904  via a fifth transmission line  906  and a second one  907   b  of the second pair  907  of coupled transmission lines is connected to the output load  904  via a sixth transmission line  908 . 
     As with the Doherty amplifier embodiment of  FIG. 6 , the fifth and sixth transmission lines  906 ,  908  have a characteristic impedance Z 01  and a length λ/4 or longer. 
     The efficiency of the combiner  903  of the amplifier  900  of  FIG. 9  is shown as a function of outphasing angle in  FIG. 10 , as compared with a conventional Chireix combiner. The results are shown at several different frequencies, ranging from 2 to 2.3 GHz. For outphasing angles exceeding 40 degrees, the combiner  903  performance (illustrated by lines marked according to the legends CPL 2 GHz, CPL 2.1 GHz, CPL 2.2 GHz and CPL 2.3 GHz) demonstrates a significantly higher efficiency over the entire frequency band compared with the conventional combiner (illustrated by lines marked according to the legends Ch 2 GHz, Ch 2.1 GHz, Ch 2.2 GHz and Ch 2.3 GHz), with the sole exception of the centre frequency 2 GHz where the performance is roughly equal. 
     The results illustrated in  FIG. 11  show the case when the combiner  903  is tuned for a flat frequency response. This illustrates that the combiner  903  can handle a 10% bandwidth centred around 2 GHz with a minimal deterioration in efficiency. If some small loss of efficiency can be tolerated at small outphasing angles (which, according to the signal statistics, may occur at rare instances), the combiner can be successfully implemented using a relative bandwidth of more than 25%, resulting in a significant improvement in power amplifier performance. 
     Two further alternative exemplary embodiments of a three-way Doherty amplifier implementation are illustrated in  FIGS. 12 and 13 , these embodiments being equivalent to three-way Doherty amplifiers described in further detail in WO 2009/081341, but with the addition of coupled transmission lines connected between the outputs of further amplifier stages and the output load. According to a first alternative embodiment, shown in  FIG. 12 , the power amplifier  1200  comprises three amplifier stages including a first main amplifier stage  1201  and two further peak amplifier stages  1202   a ,  1202   b . A coupled pair of first and second transmission lines  1205   a ,  1205   b  is connected between the output of the first amplifier  1201  and the output of the first peak amplifying stage  1202   a  and a second coupled pair of third and fourth transmission lines  1205   c ,  1205   d  is connected between the output of the second amplifier  1202   a  and the load  1204 . As with the other embodiments described above, the second transmission line  1205   b  and the fourth transmission line  1205   d  are connected to the output load  1204  via fifth and sixth transmission lines  1205   e ,  1205   f , each of which is at least a quarter wavelength. The first and second transmission lines  1205   a ,  1205   b  are connected to the output load  1204  via the third transmission line  1205   c.    
     In the alternative embodiment shown in  FIG. 13 , the power amplifier  1300  also comprises three amplifier stages including a first main amplifier stage  1301  and two further peak amplifier stages  1302   a ,  1302   b . A coupled pair of first and second transmission lines  1305   a ,  1305   b  is connected between the output of the first amplifier  1301  and the output load  1304 , a second coupled pair of third and fourth transmission lines  1305   c ,  1305   d  is connected between the output of the second amplifier  1302   a  and the output of the third amplifier  1302   b , and in addition a further coupled pair of fifth and sixth transmission lines  1305   e ,  1305   f  is connected between the output of the third amplifier stage  1302   b  and the load  1304 . The second and sixth transmission lines  1305   b , and  1305   f  are connected to the output load  1304  via seventh, and ninth transmission lines  1305   g  and  1305   i , and the fourth transmission line  1305   d  is connected to the output of the third amplifying stage  1302   b  via the eight transmission line  1305   h . Each of the second, sixth and ninth transmission lines is at least a quarter wavelength. The third and fourth transmission lines  1305   c ,  1305   d  are connected to the output load  1304  also via the fifth transmission line  1305   e . The first to sixth transmission lines  1305   a - f  are preferably each a quarter wavelength in length. 
     Power amplifiers according to embodiments of the invention may be used to improve performance of contemporary high efficiency power amplifiers for use in telecommunication base stations using load modulation techniques. Doherty and Chireix outphasing power amplifier implementations may be particularly relevant where important power amplifier parameters such as gain, efficiency, linearity and output power need to be maintained over the entire operational frequency band, since the operational bandwidth of such amplifiers can be increased according to the embodiments disclosed herein. 
     Other embodiments are also within the scope of the invention, which is to be defined by the following claims.