Patent Publication Number: US-8981850-B2

Title: Multi-frequency multi-standard reconfigurable doherty amplifier

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to and claims priority to U.S. Provisional Application Ser. No. 61/678,870, filed Aug. 2, 2012, entitled “MULTI-FREQUENCY MULTI-STANDARD RECONFIGURABLE DOHERTY AMPLIFIER”, the entirety of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to signal amplifiers, and more particularly, to reconfigurable Doherty amplifiers. 
     BACKGROUND 
     Recent evolutions of wireless communications systems have resulted in a broad spectrum of standards, each standard specifying different frequencies and bandwidths for wireless communication. These multiple standards are expected to co-exist, and dynamic management of multiple wireless network standards has become essential. This calls for a shift from dedicated single carrier frequency radio systems to versatile and adaptive systems capable of efficiently handling wide spread frequency bands. In other words, future radio systems should be able to cope with dissimilar center frequencies and signal bandwidths imposed by diverse standards while maintaining competitive performance. In particular, power amplifiers of radio transceivers should be able to amplify signals for different standards and frequencies. 
     Referring to the drawing figures, there is shown in  FIG. 1  a block diagram of a known Doherty amplifier  10  used to amplify signals in a wireless communication system. For example, the Doherty amplifier  10  of  FIG. 1  may amplify signals to be transmitted by an antenna over an air interface. Also, the Doherty amplifier  10  of  FIG. 1  may amplify signals received by an antenna from the air interface. The Doherty technique was first introduced in 1936. It is an approach for actively modulating the load seen by a main transistor of a power amplifier to improve its efficiency in the back-off condition via a judiciously configured auxiliary transistor. 
     The Doherty amplifier  10  has a main amplifier  12 , an auxiliary amplifier  14 , and an input power divider  16 . The outputs of the main amplifier  12  and the auxiliary amplifier  14  are coupled to a combining network consisting of impedance inverters with characteristic impedance R T    30  and Z T    32 . The main power amplifier  12  includes an input matching network  18 , a power transistor  20  and an output matching network  22 . Similarly, the auxiliary amplifier  14  includes an input matching network  24 , a power transistor  26  and an output matching network  28 . 
     The main amplifier  12  is Class AB biased and matched to ensure peak efficiency at a predetermined p-dB back-off that corresponds to a peak to average power ratio, PAPR, of the input signal. The input signal voltage at the specific back-off is given by 
                 V   in     =       V     in   ,   max       p       ,         
where V in,max  is the maximum input voltage of the input signal. The auxiliary amplifier  14  is Class C biased and starts conducting at
 
     
       
         
           
             
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                 I   aux       I   main       =       p   ⁢           ⁢     α   ⁡     (     p   -   1     )           1   +       (     p   -   1     )     ⁢   α               
and the ratio of the two transistor currents is given by
 
                 V   in     ≥       V     in   ,   max       p       ,         
where α=0 at
 
               V   in     =       V     in   ,   max       p           
and increases slowly to 1 when V in =V in,max . Given the input signal PABR, p, the combining network element parameters are given by:
 
                 R   T     =     R   opt       ,     
     ⁢       Z     T   ⁢           ⁢   1       =         R   opt       p       =       R   T       p                 
Assuming that the transistors  20  and  26  are ideal, the impedances of the main amplifier  12  and the auxiliary amplifier  14  vary as a function of the input signal according to the following expressions:
 
     
       
         
           
             
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     To maintain good average efficiency, the load modulation of the Doherty amplifier  10  should be adjusted as a function of the PAPR. In addition, the auxiliary transistor  26  should supply a drain current (p−1) times larger than the current of the main transistor  20  at peak power. The resulting efficiency versus input power level varies as a function of frequency. As the frequency varies, the load modulation in the Doherty amplifier  10  is affected as a direct result of the variation of the electrical lengths of the impedance inverters with frequency. Hence, the impedance seen by the main transistor  20  deviates from the ideal case and consequently, the efficiency of the Doherty amplifier  10  deteriorates. Thus, the improper load modulation resulting from the frequency variation yields an accentuated degradation of efficiency versus frequency as the value of p increases. 
     Therefore, the conventional Doherty amplifier  10  does not perform well for multiple frequencies and multiple standards. 
     SUMMARY 
     The present invention advantageously provides a method for designing a reconfigurable Doherty amplifier system that includes determining, using a processor, a first set of ABCD transmission parameters of a first output compensation network in a main path of a Doherty amplifier for the case where an auxiliary amplifier of the Doherty amplifier is off. The method further includes determining, using a processor, a second set of ABCD transmission parameters of a second output compensation network in an auxiliary path of the Doherty amplifier based on the first set of ABCD transmission parameters. 
     According to this aspect, the first set of ABCD transmission parameters are determined based on a desired impedance transformation of a main amplifier in the main path of the Doherty amplifier. The first set of ABCD transmission parameters may be determined based on a product of a first matrix based on a transmission line of a first characteristic impedance, R T , and a second matrix based on a transmission line of a second characteristic impedance, Z T1 =R T /√2, the two transmission lines being coupled to an output of the first output compensation network. In particular, the first set of ABCD transmission parameters are determined based on the following equation: 
               [           -     p           0           0         -     1     p               ]     =           [         A       B           C       D         ]       OCD   main       [           ⁢                     cos   2     ⁢   θ     -                 2     ⁢           ⁢     sin   2     ⁢   θ                       j   ⁢           ⁢       R   T     ⁡     (     1   +     1     2         )                   sin   ⁢           ⁢   θ   ⁢           ⁢   cos   ⁢           ⁢   θ                             j   ⁢     1     R   T       ⁢     (     1   +     2       )       ⁢                       sin   ⁢           ⁢   θ   ⁢           ⁢   cos   ⁢           ⁢   θ                           cos   2     ⁢   θ     -                 1     2       ⁢     sin   2     ⁢   θ                 ]     ⁢           ⁢                 ⁢     [           -       p   2             0           0         -       2   p               ⁢           ]               
where θ is a frequency of operation of the Doherty amplifier, p is a desired peak to average power ratio, and R T  is a load resistance. The second set of ABCD transmission parameters may be determined based on a product of a first matrix based on a transmission line of a first characteristic impedance, R T , and a second matrix based on a transmission line of a second characteristic impedance, Z T1 =R T /√2, wherein the two transmission lines are coupled to an output of the second output compensation network. In particular, the second set of ABCD transmission parameters may be determined based on the following equation:
 
                     [           j   ⁢           ⁢   p           j   ⁢           ⁢     R   T                 j     R   T           0         ]     =         [         A       B           C       D         ]       OCD   main       ⁢             [                 cos   2     ⁢   θ     -       2     ⁢     sin   2     ⁢   θ     +       j   ⁡     (     1   +     1     2         )       ⁢   sin   ⁢           ⁢   θcos   ⁢           ⁢   θ           cos   ⁢           ⁢   θ     +     j   ⁢     1     2       ⁢   sin   ⁢           ⁢   θ               j   ⁢           ⁢     R   T     ⁢   sin   ⁢           ⁢   θ                 1     R   T       ⁢               ⁢         cos   2     ⁢   θ     -       1     2       ⁢     sin   2     ⁢   θ     +       j   ⁡     (     1   +     2       )       ⁢   sin   ⁢           ⁢   θ   ⁢           ⁢   cos   ⁢           ⁢   θ             cos   ⁢           ⁢   θ     +     j   ⁢     1     2       ⁢   sin   ⁢           ⁢   θ                 cos   ⁢           ⁢   θ           ]     ⁢               [         A       B           C       D         ]       OCD   aux       ⁡     [         1       0               p   -   2       R   T           1         ]                                       
The first output compensation network may be implemented to include at least one of micro-electro-mechanical systems, MEMS, semiconductor varactors and ferroelectric material. In some embodiments, the first output compensation network and the second output compensation network include a total of less than 9 tunable circuits to achieve the first set of ABCD transmission parameters and the second set of ABCD transmission parameters. In other embodiments, the first output compensation network and the second output compensation network include a total of less than 6 tunable circuits to achieve the first set of ABCD transmission parameters and the second set of ABCD transmission parameters.
 
     According to another aspect, the invention provides a reconfigurable power amplifier. The reconfigurable power amplifier includes a main amplifier and an auxiliary amplifier. The main amplifier is configured to operate using a specific impedance below a first predetermined power level. The auxiliary amplifier is configured to modulate the impedance of the main amplifier above the first predetermined power level. The power amplifier further includes a first reconfigurable output network coupled to an output of the main amplifier, and a second reconfigurable output network coupled to an output of the auxiliary amplifier. The power amplifier further includes a non-reconfigurable output impedance inverter coupled to the first reconfigurable output network and the second reconfigurable output network. 
     According to this aspect, in some embodiments, the first reconfigurable output network exhibits predetermined ABCD transmission parameters based on a desired load modulation. In some embodiments, the first reconfigurable output network and the second reconfigurable output network are configured to compensate for a frequency dependence of the non-reconfigurable output impedance inverter. Some embodiments further include a reconfigurable input matching network coupled to an input of the main amplifier. The reconfigurable input matching network may be configured to provide source matching and phase balancing at a frequency of operation of the power amplifier. In some embodiments, the first reconfigurable output network is configured to provide second harmonic impedance matching at a frequency of operation of the power amplifier. In some embodiments, the first reconfigurable output network is implemented using shunt discrete single-pole double-throw micro-electrical-mechanical systems, MEMS switch. In some embodiments, the first reconfigurable output network and the second reconfigurable output network are configured to provide a single standard multi-frequency Doherty amplifier using a total of not more than five tunable elements. In some embodiments, the first reconfigurable output network and the second reconfigurable output network are configured to provide a single-frequency multi-standard Doherty amplifier using a total of not more than four tunable elements. In some embodiments, the first reconfigurable output network and the second reconfigurable output network are configured to provide a multi-frequency multi-standard Doherty amplifier using a total of not more than eight tunable elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a diagram of a known Doherty amplifier; 
         FIG. 2  is a diagram of a multi-frequency multi-standard Doherty amplifier system designed and constructed according to principles of the present invention; 
         FIG. 3  is a diagram of a multi-frequency single standard Doherty amplifier system designed and constructed according to principles of the present invention; 
         FIG. 4  is a graph of drain current efficiency for the system of  FIG. 3 ; 
         FIG. 5  is a diagram of a single frequency multi-standard Doherty amplifier system designed and constructed according to principles of the present invention; 
         FIG. 6  is a graph of drain current efficiency for the system of  FIG. 5 ; 
         FIG. 7  is a diagram of another multi-frequency multi-standard Doherty amplifier system designed and constructed according to principles of the present invention; 
         FIGS. 8A ,  8 B and  8 C are graphs of drain current efficiencies at three different frequencies for the system of  FIG. 7 ; and 
         FIG. 9  is a flowchart of an exemplary process for designing a reconfigurable Doherty amplifier according to principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before describing in detail exemplary embodiments that are in accordance with the present invention, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to reconfigurable Doherty amplifiers. Accordingly, the system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. 
     To mitigate the effects of frequency variations and peak to average power ratio, PAPR, variations in a conventional Doherty amplifier, a reconfigurable Doherty architecture is described herein. The reconfigurable Doherty architecture includes two reconfigurable output compensation networks (OCN) that are added in the main and auxiliary paths of the Doherty architecture in addition to reconfigurable input and output matching networks. The reconfigurable OCNs are carefully designed to reduce the deviation of the impedance profiles from the ideal profiles and to adjust the impedance profiles as the value of p varies. The reconfigurable input and output matching networks (IMN, OMN) and the output compensation networks, OCN, are designed to obtain optimal source and load impedances to be seen by the transistors at different drive levels and operating frequencies, and to compensate for the transistors&#39; parasitics at different operating conditions. 
     Design of the compensation networks may follow a procedure that employs a set of closed form equations. These equations enable determination of ABCD parameters of the compensation networks given the values of the frequency (represented by θ), and the input signal PAPR (represented by p). 
     Referring to the drawing figures, there is shown in  FIG. 2  a generalized multi-standard multi-frequency Doherty amplifier system  36  constructed in accordance with principles of the present invention. The Doherty amplifier system  36  includes a main power amplifier  38 , an auxiliary power amplifier  40  and an input power divider  42 . The main power amplifier  38  has a reconfigurable input matching network  44 , a transistor  46  and a reconfigurable output matching network  48 . The auxiliary power amplifier  40  has an input matching network  50 , a transistor  52  and a reconfigurable output matching network  54 . Note that, in one embodiment, the transistor  52  of the auxiliary power amplifier  40  may have a power rating of 2.5 times the power rating of the transistor  46  of the main power amplifier  38 . For example the transistor  52  may be rated at 10 Watts and the transistor  46  may be rated at 25 Watts. 
     The output path of the main power amplifier  38  includes a reconfigurable output compensation network  56 . The output path of the auxiliary power amplifier  40  includes a reconfigurable output compensation network  58 . The output compensation networks  56  and  58  are electrically coupled to a fixed combining network  60  that includes the impedances  30 ,  32  and  34 . In this embodiment, the fixed combining network  60  is unchanged from the known combining network of  FIG. 1 . 
     The fixed combining network  60  may also be referred to herein as a non-reconfigurable output impedance inverter. The output networks that include the output matching network  48  and the output compensation network  56 , as well as the output matching network  54  and the output compensation network  58 , combine to compensate for a frequency dependence of the non-reconfigurable output impedance inverter  60  and/or to compensate for a PAPR dependence of the non-reconfigurable impedance inverter  60 . Also, the output network that includes output matching network  48  and output compensation network  56  may be configured to provide second harmonic impedance matching at an operating frequency of the Doherty power amplifier  36 . The input matching networks compensate for transistor parasitics and variation of the source impedance with frequency. 
     The synthesis of the reconfigurable Doherty amplifier system  36  may be carried out in two steps. First, the ABCD parameters of the output compensation network  56  are determined by analyzing the circuit that results from turning off the auxiliary power amplifier  40  at low power levels. Second, the knowledge of the output compensation network  56  is used for determining the ABCD parameters of the output compensation network  58 . In both cases, the ideal ABCD parameters are chosen to satisfy the conditions for proper load modulation. 
     The first set of ABCD parameters may be determined based on a desired impedance transformation of the main amplifier  38  in the main path of the Doherty amplifier system  36 . For example, the first set of ABCD transmission parameters may be determined based on a product of a first matrix based on a transmission line of the first characteristic impedance, R T , and a second matrix based on a transmission line of the second characteristic impedance, Z T1 =R T /√2. The two transmission lines are coupled to an output of the first output compensation network  56 . More particularly, the first set of ABCD parameters for the output compensation network  56  may be given by: 
               [           -     p           0           0         -     1     p               ]     =           [         A       B           C       D         ]       OCD   main       [           ⁢                     cos   2     ⁢   θ     -                 2     ⁢           ⁢     sin   2     ⁢   θ                       j   ⁢           ⁢       R   T     ⁡     (     1   +     1     2         )                   sin   ⁢           ⁢   θ   ⁢           ⁢   cos   ⁢           ⁢   θ                             j   ⁢     1     R   T       ⁢     (     1   +     2       )       ⁢                       sin   ⁢           ⁢   θ   ⁢           ⁢   cos   ⁢           ⁢   θ                           cos   2     ⁢   θ     -                 1     2       ⁢     sin   2     ⁢   θ                 ]     [           ⁢           -       p   2             0           0         -       2   p               ⁢           ]           
where θ is a frequency of operation of the Doherty amplifier, p is a desired peak to average power ratio, and R T  is a load resistance.
 
     The second set of ABCD transmission parameters are determined based on a product of a first matrix based on the transmission line of the first characteristic impedance, R T , and a second matrix based on the transmission line of the second characteristic impedance, Z T1 =R T /√2. The two transmission lines are coupled to an output of the second output compensation network  58  and the output of the first output compensation network  56 . More particularly, the second set of ABCD transmission parameters may be determined based on the following equation: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     These closed form equations enable design of the output compensation networks  56 ,  58  to maintain proper load modulation given the operating frequencies and the desired PAPR. When these ABCD parameters are realized by tunable circuits  62 , such as micro-electromechanical systems, MEMS, switches, the output network formed by the combination of output matching network  48  and output compensation network  56  of the Doherty amplifier system  36  accounts for the frequency dependency of the fixed combining network  60 . The input matching network  44  accounts for the effects of transistor parasitics and the variation-of the source impedance with frequency by performing source matching and phase balancing. These variations may be mitigated using the MEMS switches, or, in an alternative embodiment, using semiconductor varactors or ferroelectric material. A minimum number of tunable elements  62 , such as single pole-double throw MEMS switches, may be determined by performing a sensitivity analysis of a multi-step transmission line based matching network to identify the most effective segment of transmission lines that maximize the tuning range. 
       FIG. 3  is a diagram of a multi-frequency single standard Doherty amplifier  64  designed and constructed in accordance with principles of the present invention. In particular, the Doherty amplifier system  64  exhibits high efficiency for multiple frequencies when driven by a constant PAPR signal. The Doherty amplifier system  64  of  FIG. 3  may be designed to operate at 1.9, 2.14 and 2.6 Gigahertz, GHz, for a PAPR of 6 dB (p=2). In particular, the combination of the output matching network  48  and the output compensation network  56  may achieve optimal second harmonic termination for efficiency enhancement. Simulations of the configuration of  FIG. 3  indicate that good performance can be achieved using five tunable elements  62 . 
     As shown in  FIG. 3  the input matching network  44  includes four transmission lines  44   a - 44   d  with two tunable elements  62  attached to two of the transmission lines. The output matching network  48  includes three transmission lines  48   a - 48   c . The input matching network  50  also includes three transmission lines  50   a - 50   c . The output matching network  54  includes three transmission lines  54   a - 54   c . The output compensation network  56  includes four transmission lines  56   a - 56   d  and one tunable element  62 . The output compensation network  58  also includes three transmission lines  58   a - 58   c  and one tunable element  62 . The length and width of the transmission lines can be determined by computer analysis using known techniques based on the calculated ABCD parameters. 
       FIG. 4  is a graph of drain current efficiencies of the Doherty amplifier system  64  for each of the three frequencies for which the Doherty amplifier system  64  is tuned. The graph of  FIG. 4  shows that drain efficiency of about 70% is achieved at peak power levels of 40 dB. In addition, a drain efficiency of about 60% is maintained at a 6 dB output back-off over the three frequencies. 
       FIG. 5  is a diagram of a single frequency multi-standard Doherty amplifier system  68  designed and constructed in accordance with principles of the present invention. In particular, the Doherty amplifier system  68  achieves high average efficiency when the amplifier is operated at a single frequency but driven with signals having varying PAPR. In some embodiments, the Doherty amplifier system  66  operates at a center frequency of 2.6 GHz and for 6, 9 and 12 dB back-off levels. Note that the back-off levels are directly related to a signal&#39;s PABR and probability density function. Simulations show that four tunable elements  62  may be sufficient for circuit realization, when used in conjunction with a fixed harmonic tuning stub  66 . 
     A difference between the embodiments of  FIGS. 3 and 5  is the number of tunable elements in each of the matching and compensation networks. For example, the embodiment of  FIG. 3  has two tunable elements  62  in the input matching network  44 , whereas the embodiment of  FIG. 5  has only one tunable element  62  in the input matching network  44 . In the embodiment of  FIG. 5 , the output matching network has one tunable element  62 , whereas in  FIG. 3 , the output matching network  48  has no tunable element. Also, the embodiment of  FIG. 5  includes a tunable element  62  in the output matching network  54 , whereas the output matching network  54  of  FIG. 3  has no tunable element. Also, the dimensions of the tunable elements may be different for the different embodiments. As mentioned, the length and width of the transmission lines can be determined by computer analysis using known techniques based on the calculated ABCD parameters. 
       FIG. 6  is a graph of drain current efficiency for the three different back-off levels. The graph shows a peak efficiency of about 71% while back-off efficiency exceeds 65% for the 6, 9 and 12 dB back-off levels. According to  FIG. 6 , the peak power is about 27 dBm. Hence, the 6, 9 and 12 dB back-off levels correspond to an input power of 21, 18, and 15 dBm, respectively. 
       FIG. 7  is a diagram of a multi-frequency multi-standard Doherty amplifier system  70  designed and constructed in accordance with principles of the present invention. In particular, the Doherty amplifier system  70  achieves high average efficiency when the amplifier is operated at multiple frequencies and driven with signals having varying PAPR. In some embodiments, the Doherty amplifier system  70  is tunable with eight tunable elements  62  to operate at center frequencies of 1.9, 2.14 and 2.6 GHz and for back-off levels of 6, 9 and 12 dB. 
     Thus, a difference between the embodiments of  FIGS. 3 ,  5 , and  7  is the number of tunable elements  62  in each of the matching and compensation networks. For example, the number of tunable elements of the input matching network  44  of  FIG. 7  is two, the number of tunable elements of the output matching network  48  of  FIG. 7  is one, the number of tunable elements of the output compensation network  56  of  FIG. 7  is 2, the number of tunable elements of the input matching network  50  of  FIG. 7  is zero, the number of tunable elements of the output matching network  54  of  FIG. 7  is one, and finally, the number of tunable elements of the output compensation network  58  is one. Also, the dimensions of the various transmission lines included in the matching and compensation networks may be different for the different embodiments. 
       FIGS. 8A ,  8 B, and  8 C are graphs of drain current efficiency for the embodiment of  FIG. 7  for the three different frequencies, 2.6 Ghz ( 8 A), 2.14 Ghz ( 8 B) and 1.9 Ghz ( 8 C) for the three back-off levels 6 dB  72 , 9 dB  74  and 12 dB  76 , demonstrating peak drain efficiency of more than 69% while the back-off efficiency exceeds 62%. 
       FIG. 9  is a flowchart of an exemplary process for designing a reconfigurable Doherty amplifier system  64 ,  68 ,  70  according to principles of the present invention. First ABCD parameters are computed for a first output compensation network  56  in a path of the main amplifier  46  with the auxiliary amplifier  47  turned off (block S 100 ). Second ABCD parameters are computed for a second output compensation network  58  in the path of the auxiliary amplifier  47  based on the first ABCD parameters (block S 102 ). A sensitivity analysis is performed to determine locations of tunable elements  62  to realize the first and second output compensation networks  56 ,  58  (block S 104 ). 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.