Patent Abstract:
An amplifier circuit includes a first amplifier operable to turn on at a first power level, a second amplifier operable to turn on at a second power level below the first power level and a third amplifier operable to turn on at all power levels. A first power combiner is operable to combine an output of the third amplifier with an output of the second amplifier at a first power combining node to form a first combined amplifier output. A second power combiner is operable to combine the first combined amplifier output with an output of the first amplifier at a second power combining node to form a second combined amplifier output. An impedance transformer is operable to transform a load impedance of the amplifier circuit to a transformed impedance at the second power combining node, the transformed impedance matching an impedance of the first and second power combiners.

Full Description:
PRIORITY CLAIM 
     This application is a Divisional of U.S. application Ser. No. 13/163,388, filed on Jun. 17, 2011, the content of said application incorporated herein by reference in its entirety. 
    
    
     FIELD OF TECHNOLOGY 
     The present application relates to Doherty amplifiers, in particular Doherty amplifiers designed for a wideband frequency range of operation. 
     BACKGROUND 
     RF (radio frequency) power architectures within the telecommunications field focus on achieving high DC-to-RF efficiency at significant power back off from Psat (the average output power when the amplifier is driven deep into saturation). This is due to the high peak to average ratio (PAR) of the transmitted digital signals such as W-CDMA (wideband code division multiple access), LTE (long term evolution) and WiMAX (worldwide interoperability for microwave access). The most popular power amplifier architecture currently employed is the Doherty amplifier. The Doherty amplifier employs a class AB main amplifier and a class C peaking amplifier, and efficiency is enhanced through load modulation of the main amplifier from the peaking amplifier. However, if high efficiency at a high output backoff (OBO) is required, a highly asymmetric ratio between the main and peaking amplifiers is required. 
     The Doherty architecture has an inherent degradation in the efficiency between the peak OBO point and the peak power point. To overcome this, a three way Doherty architecture can be used, in which the main class AB amplifier is replaced with a Doherty amplifier and load modulation is provided to the first peaking amplifier between the peak OBO point and the peak power point. However, the main amplifier is connected to the external load impedance (typically 50 Ohms) through a series of three ¼λ (quarter wavelength) transmission lines prior to any device impedance matching. This can lead to the amplifier being narrow band in nature due to the band-limiting characteristics of the ¼λ transmission lines. As such, three way Doherty amplifiers are typically designed for a specific band of operation used for wireless communication applications like WCDMA, LTE, WiMAX, etc. Such bands of operation are 1805-1880 MHz, 1930-1990 MHz, etc. 
     SUMMARY 
     Embodiments described herein employ a constant impedance combiner having a characteristic impedance equal to the required load modulated high impedance state of the main amplifier in a three way wideband Doherty amplifier circuit when the first and second peaking amplifiers are turned off. In this most narrow band case there is minimal band limiting presented to the main amplifier. A significant amount of band limiting is removed from the main amplifier path when running in the back off power region, yielding more constant power versus frequency and more constant efficiency versus frequency at a fixed back off power level. The amplifier embodiments described herein are well suited for wider band applications such that one amplifier circuit can simultaneously cover two or more adjacent bands of operation or be more consistent across a complete band of operation than currently existing architectures. 
     According to one embodiment of an amplifier circuit, the amplifier circuit includes a main amplifier biased at Class B or AB mode, a first peaking amplifier biased at Class C mode, a second peaking amplifier biased at Class C mode and a constant impedance combiner. The constant impedance combiner has a first node connected to an output of the main amplifier, a second node connected to an output of the first peaking amplifier, a third node connected to an output of the second peaking amplifier and a fourth node connected to a load. The constant impedance combiner is operable to transform a load impedance at the fourth node to a transformed impedance at the third node, and maintain the same transformed impedance at the first, second and third nodes. 
     According to one embodiment of a method of operating an amplifier circuit, the method includes: biasing a main amplifier at Class B or AB mode; biasing a first peaking amplifier at Class C mode; biasing a second peaking amplifier at Class C mode; connecting a first node of a constant impedance combiner to an output of the main amplifier, a second node of the constant impedance combiner to an output of the first peaking amplifier, a third node of the constant impedance combiner to an output of the second peaking amplifier, and a fourth node to a load; and transforming a load impedance at the fourth node to a transformed impedance at the third node so that the same transformed impedance is maintained at the first, second and third nodes. 
     According to another embodiment of an amplifier circuit, the amplifier circuit includes a first amplifier operable to turn on at a first power level, a second amplifier operable to turn on at a second power level below the first power level and a third amplifier operable to remain on at all power levels. A first power combiner is operable to combine an output of the third amplifier with an output of the second amplifier at a first power combining node to form a first combined amplifier output. A second power combiner is operable to combine the first combined amplifier output with an output of the first amplifier at a second power combining node to form a second combined amplifier output. An impedance transformer is operable to transform a load impedance of the amplifier circuit to a transformed impedance at the second power combining node, the transformed impedance matching an impedance of the first and second power combiners. 
     According to another embodiment of a method of operating an amplifier circuit, the method includes: turning on a first amplifier at a first power level; turning on a second amplifier at a second power level below the first power level; turning on a third amplifier at all power levels; combining an output of the third amplifier with an output of the second amplifier at a first power combining node to form a first combined amplifier output; combining the first combined amplifier output with an output of the first amplifier at a second power combining node to form a second combined amplifier output; and transforming a load impedance of the amplifier circuit to a transformed impedance at the second power combining node with no impedance transformation occurring from the second power combining node to the first power combining node. 
     According to an embodiment of a three way wideband Doherty amplifier circuit, the circuit includes a first peaking amplifier operable to turn on at a first power level, a second peaking amplifier operable to turn on at a second power level below the first power level and a main power amplifier operable to turn on at all power levels. The main power amplifier has a high impedance load modulated state when the first and second peaking amplifiers are turned off. The Doherty amplifier circuit further includes a constant impedance combiner connected to an output of each amplifier. The constant impedance combiner has a characteristic impedance which matches the impedance of the main amplifier in the high impedance load modulated state with or without an output matching device connecting the main amplifier output to the constant impedance combiner, as viewed from the output of the main amplifier. 
     Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. The features of the various illustrated embodiments can be combined unless they exclude each other. Embodiments are depicted in the drawings and are detailed in the description which follows. 
         FIG. 1  illustrates a schematic circuit diagram of a three way wideband Doherty amplifier circuit having a constant impedance combiner according to an embodiment. 
         FIG. 2  illustrates a schematic circuit diagram of a three way wideband Doherty amplifier circuit having a constant impedance combiner according to another embodiment. 
         FIG. 3  illustrates a schematic circuit diagram of a three way wideband Doherty amplifier circuit having a constant impedance combiner according to yet another embodiment. 
         FIG. 4  illustrates a schematic circuit diagram of a three way wideband Doherty amplifier circuit having a constant impedance combiner according to still another embodiment. 
         FIG. 5  illustrates a schematic circuit diagram of a three way wideband Doherty amplifier circuit having a constant impedance combiner which provides no impedance transformation according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an embodiment of a three way wideband Doherty amplifier circuit which includes a main amplifier  100  biased at Class B or AB mode, a first peaking amplifier  110  biased at Class C mode, a second peaking amplifier  120  biased at Class C mode, and a constant impedance combiner  130 . Each amplifier  100 ,  110 ,  120  includes one or more active devices and may also include a respective output matching device  102 ,  112 ,  122 . At low power levels, only the main amplifier  100  is operational. The efficiency of the main amplifier  100  increases as the power level increases. The main amplifier  100  eventually reaches a maximum efficiency point as the power level continues to rise. At this power level, the first peaking amplifier  110  turns on. The efficiency of the first peaking amplifier  110  similarly increases for power levels above this point. A second maximum efficiency point is attained when the first peaking amplifier  110  reaches its maximum efficiency point, at which point the second peaking amplifier  120  turns on. The second peaking amplifier  120  reaches a third maximum efficiency at a yet higher power level (full power). As such, the three way wideband Doherty amplifier circuit has three maximum efficiency points. 
     Load modulation is implemented by the fundamental current ratio between the main amplifier  100  and the peaking amplifiers  110 ,  120 . In addition, the constant impedance combiner  130  combines or sums the load currents of the amplifiers  100 ,  110 ,  120  so that the output voltage of the wideband Doherty amplifier circuit is determined by the summation of the load currents multiplied by the load impedance. In more detail, the constant impedance combiner  130  has a first node  132  connected to the output of the main amplifier  100 . In one embodiment, the first node  132  of the constant impedance combiner  130  is connected to the main amplifier output via an output matching device  102 . Input matching devices  104 ,  114 ,  124  may similarly be provided at the respective inputs of the amplifiers  100 ,  110 ,  120 . In another embodiment, the first node  132  of the constant impedance combiner  130  is directly connected to the main amplifier output and no impedance match device is provided for the main amplifier  100  as described in more detail later herein. In each case, the constant impedance combiner  130  also has a second node  134  connected to the output of the first peaking amplifier  110  and a third node  136  connected to the output of the second peaking amplifier  120  either of which may or may not also include a respective output matching device  112 ,  122 . 
     The impedance of the main amplifier  100  in a high impedance load modulated state when the peaking amplifiers  110 ,  120  are turned off is a function of the load cu rent delivered by the peaking amplifiers  110 ,  120 . The main amplifier  100  operates in the high impedance state when the peaking amplifiers  110 ,  120  are turned off and in a low impedance state when the peaking amplifiers  110 ,  120  are turned on. For example, if the input power is relatively small so that neither peaking amplifier  110 ,  120  is turned on, the impedance presented to the main amplifier  100  will increase (2× is indicative, but the impedance could increase from 1× to 4× or more based on the implementation) compared to its low impedance state. At low power level the output impedance of the peaking amplifiers  110 ,  120  is theoretically infinite or very large when looking into the amplifier device on the output side after the corresponding output match network  112 ,  122  from the second node  134  and the third node  134  of the constant impedance combiner  130  respectively, because the peaking amplifiers  110 ,  120  are turned off and contribute zero load current. 
     The constant impedance combiner  130  has a characteristic impedance which matches the impedance of the main amplifier  100  in the high impedance load modulated state with or without the output matching network  102  connecting the main amplifier output to the constant impedance combiner  130 , as viewed from the output of the main amplifier  100 . In doing so, the output back off of the amplifier circuit has minimal bandwidth limitations which enables the amplifier circuit to be used in wideband applications. 
     In the embodiment illustrated in  FIG. 1 , the constant impedance combiner  130  also has a first power combiner  138  with a first terminal  140  connected to the first node  132  of the constant impedance combiner  130  and a second terminal  142  connected to the second node  134  of the constant impedance combiner  130 . A second power combiner  144  has a first terminal  146  connected to the second node  134  of the constant impedance combiner  130  and a second terminal  148  connected to the third node  136  of the constant impedance combiner  130 . The first and second power combiners  138 ,  144  have the same (i.e. identical or nearly identical) impedance. 
     The constant impedance combiner  130  further has an impedance transformer  150  having a first terminal  152  connected to the load  160  of the amplifier circuit and a second terminal  154  connected to the third node  136  of the constant impedance combiner  130 . The impedance transformer  150  is a wideband impedance transformer according to this embodiment in that the transformer  150  has a wider end and a narrower end. The narrower end may be connected to the load  160  and the wider end connected to the third node  136  of the constant impedance combiner  130  as shown in  FIG. 1 . Alternatively, the wider end of the transformer  150  may be connected to the load  160  and the narrower end connected to the third node  136  of the constant impedance combiner  130  in other embodiments. 
     During operation, the wideband impedance transformer  150  transforms the load impedance at its first terminal  152  to a transformed impedance at its second terminal  154  which matches (i.e. identically or nearly identically) the impedance of the first and second power combiners  138 ,  144 . As such, no additional appreciable impedance transformation occurs between the third and second nodes  136 ,  134  (via the second power combiner  144 ) and between the second and first nodes  134 ,  132  (via the first power combiner  138 ) of the constant impedance combiner  130  because the reactance of the power combiners  138 ,  144  is minimal when the transformed impedance matches the power combiner impedances. As such, a constant impedance is maintained at the first, second and third nodes  132 ,  134 ,  136 . 
     For example, the load impedance may be 50 Ohms and the wideband impedance transformer  150  may be shaped to transform the 50 Ohm load impedance to 20 Ohms at the third node  136  of the constant impedance combiner  130 . Each of the power combiners  138 ,  144  may be 20 Ohm ¼λ (quarter wavelength) transmission lines. As such, the power combiners  138 ,  144  provide no further impedance transformation and the same 20 Ohm transformed impedance is maintained at the third, second and first nodes  136 ,  134 ,  132  of the constant impedance combiner  130 . Hence the term ‘constant impedance combiner’. A single impedance transformation occurs between the load  160  and the third node  136  of the constant impedance combiner  130 , and no further impedance transformation occurs between nodes  136 ,  134  and  132 , and the amplifier circuit  100  has an optimized impedance for correct load modulation at back off 2 (i.e. the main amplifier is on, and the first and second peaking amplifiers are off), back off 1 (i.e. the main and peaking 1 amplifier are on, and the second peaking amplifier is off) and full power (i.e. all amplifiers are on). 
     As shown in  FIG. 1 , the output of each amplifier  100 ,  110 ,  120  may be connected to the respective node  132 ,  134 ,  136  of the constant impedance combiner  130  via a corresponding output matching device  102 ,  112 ,  122 . Each output matching device  102 ,  112 ,  122  provides further impedance transformation between the specific transformed impedance presented at the three nodes  132 ,  134 ,  136  of the constant impedance combiner  130  and the corresponding amplifier output. Returning to the 50 Ohm load impedance example given above, the main amplifier  100  has an 11.1 Ohm load impedance at its output and the first peaking amplifier  110  may have an 18 Ohm output impedance after its output matching device  112  and the second peaking amplifier  120  may have a 50 Ohm output impedance after its output matching device  122 . The respective output matching devices  102 ,  112 ,  122  provide the desired impedance transformation between the transformed impedance presented at the three nodes  132 ,  134 ,  136  of the constant impedance combiner  130  and the corresponding amplifier output impedance. 
       FIG. 2  illustrates another embodiment of a three way wideband Doherty amplifier circuit. The circuit of  FIG. 2  is similar to the circuit of  FIG. 1 , except there is no output matching device ( 102 ) connecting the output of the main amplifier  100  to the first node  132  of the constant impedance combiner  130 . Instead, the output of the main amplifier  100  is directly connected to the first node  132 . According to this embodiment, the main amplifier  100  has the same impedance as that of the first power combiner  138  when the main amplifier  100  operates in the high impedance state (i.e. when the peaking amplifiers are turned off). Again returning to the 50 Ohm load impedance example given above, the 11.1 Ohm output impedance of the main amplifier  100  transforms to 20 Ohms (1.8× transformation factor) in the high impedance state which matches the 20 Ohm impedance present at the first node  132  of the constant impedance combiner  130 . Accordingly, no further impedance transformation is needed between the first node  132  of the constant impedance combiner  130  and the main amplifier output in the high impedance (load modulated) state. 
       FIG. 3  illustrates yet another embodiment of a three way wideband Doherty amplifier circuit. The circuit of  FIG. 3  is similar to the circuit of  FIG. 2 , except the parasitic output capacitance which for a MOS device is drain-to-source capacitance (schematically illustrated as capacitor Cds in  FIG. 3 ) of the main amplifier  100  and the parasitic inductance (schematically illustrated as inductor Lseries in  FIG. 3 ) associated with connecting the main amplifier output to the first node  132  of the constant impedance combiner  130  are absorbed into the impedance of the first power combiner  138  of the constant impedance combiner  130 . According to this embodiment, the main amplifier impedance matches that of the first power combiner  138  as viewed from the first node  132  of the constant impedance combiner  130  when the main amplifier  100  operates in the high impedance load modulated state (i.e. when the peaking amplifiers are turned off). 
       FIG. 4  illustrates still another embodiment of a three way wideband Doherty amplifier circuit. The circuit of  FIG. 4  is similar to the circuit of  FIG. 1 , except the impedance transformer  170  of the constant impedance combiner  130  is a ¼λ transmission line instead of a wideband (tapered) impedance transformer ( 150 ). According to this embodiment, the ¼λ transmission line  170  connects the load  160  to the third node  136  of the constant impedance combiner  130  and transforms the load impedance at its first terminal  172  to a transformed impedance at its second terminal  174  so that the transformed impedance at the third node  136  matches the impedance of the first and second power combiners  138 ,  144 . Once again returning to the 50 Ohm load impedance example given above, both power combiners  138 ,  144  may have a 20 Ohm impedance and the ¼λ transmission line  170  which connects the 50 Ohm load  160  to the third node  136  of the constant impedance combiner  130  may have an impedance of 31 Ohms. Accordingly, only a single impedance transformation is performed by the constant impedance combiner  130 . In yet another embodiment, the impedance transformer  170  can be two transmission lines connected in series or a lumped L (inductive) or a lumped C (capacitive) structure. Yet other types of impedance transformers may be used. 
       FIG. 5  illustrates an embodiment of a three way wideband Doherty amplifier circuit where the constant impedance combiner  130  has no impedance transformer. Instead, each power combiner  138 ,  144  has an impedance with matches the load impedance. According to this embodiment, the constant impedance combiner  130  provides no impedance transformation and the impedance at nodes  132 ,  134  and  136  of the combiner  130  corresponds to the load impedance in each case. 
     Specific exemplary impedance values for the different amplifier circuit components have been described herein for illustrative purposes only. These specific examples are not intended to limit the scope of the claims in any way unless explicitly claimed. For example, load impedances other than 50 Ohms may also be considered such as 75 Ohms, etc. The components of the constant impedance combiner can be sized accordingly to ensure only a single impedance transformation is performed by the constant impedance combiner. This also applies for the specific amplifier impedance values expressed herein. 
     Terms such as “same”, “match” and “matches” as used herein are intended to mean identical, nearly identical or approximately so that some reasonable amount of variation is contemplated without departing from the spirit of the invention. The term “constant” means not changing or varying, or changing or varying slightly again so that some reasonable amount of variation is contemplated without departing from the spirit of the invention. Further, terms such as “first”, “second”, and the like, are used to describe various elements, regions, sections, etc. and are also not intended to be limiting. Like terms refer to like elements throughout the description. 
     As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise. 
     It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.

Technology Classification (CPC): 7