Patent Publication Number: US-2022216582-A1

Title: Tunable wilkinson splitter

Description:
TECHNICAL FIELD 
     The present teachings relate to radio frequency (RF) circuits. More particularly, the present teachings relate to methods and apparatuses for a tunable Wilkinson power splitter. 
     BACKGROUND 
       FIG. 1A  shows a prior art Wilkinson (power) splitter ( 100 A) that can be used to split an input RF signal at a (common) port, P 1 , into two (substantially) equal phase and (substantially) equal power RF signals at ports P 2  and P 3 . Conversely, the splitter ( 100 A) can be used to combine two equal phase RF signals at ports P 2  and P 3  into one RF signal at port P 1 . As can be seen in  FIG. 1A , the prior art splitter ( 100 A) is a three-port network, wherein ports P 1  and P 2  are coupled to respective ends of a first quarter wavelength (i.e., λ/4) transmission line ( 112   a ) and ports P 1  and P 3  are coupled to two respective ends of a second quarter wavelength (i.e., transmission line ( 113   a ). A termination resistor, R 23 , coupled between ports P 2  and P 3  provides output impedance matching at the two ports P 2  and P 3 , as well as isolation between ports P 2  and P 3  (i.e., essentially no current flows through the resistor). As known to a person skilled in the art, principle of operation of the splitter ( 100 A) is based on: i) a length of the two transmission lines ( 112   a ,  113   a ) to be equal to one fourth (¼) of a wavelength, λ, that is based on a frequency of operation (i.e., center frequency) of the splitter ( 100 A); and ii) in order to provide impedance matching between the port P 1  and respective ports P 2  and P 3 , at an impedance value Z O , a characteristic impedance (i.e., impedance at the frequency of operation) of the two transmission lines ( 112   a ,  113   a ) must be equal to √{square root over (2)}*Z O , and the impedance of the termination resistor, R 23 , must be 2*Z O . 
     Although the prior art splitter ( 100 A) can operate at high frequencies (e.g., center frequencies of 10 GHz and higher) thanks to its distributed elements (e.g., transmission lines) realization, its bulkiness may render it impractical for integration purposes. Accordingly, in many applications, a lumped elements approach may be used to realize a Wilkinson splitter as shown in  FIG. 1B . In such prior art approach, a Wilkinson splitter ( 100 B) implements functionality (e.g., at the center frequency) of the two branches between port P 1  and respective ports P 2  and P 3  via respective (Pi-) LC branches ( 112   b ) and ( 113   b ). As can be seen in  FIG. 1B , each of the two branches ( 112   b ) and ( 113   b ) includes a respective inductor L 12 , L 13 , coupled at each end to respective shunted capacitors (C 12 , C 21 ) and (C 13 , C 31 ). For each of the branches ( 112   b ) and ( 113   b ), values of the respective elements (L 12 , C 12 , C 21 ) and (L 13 , C 13 , C 31 ) are chosen so that at the frequency of operation (i.e., center frequency), an equivalent impedance of each branch is equal to √{square root over (2)} *Z O  and a phase shift through the branches is equal (e.g., −90 degrees in reference to port P 1 ). It should be noted that a person skilled in the art is well aware of design techniques and practices (e.g., matching of scattering parameters) for providing an equivalent lumped elements realization (e.g.,  112   b ,  113   b ) of a transmission line (e.g.,  112   a ,  113   a ) used in a distributed elements realization of a Wilkinson splitter, which are therefore beyond the scope of the present application. 
       FIG. 1C  shows graphs representative of performance of the prior art splitter ( 100 A,  100 B) about an exemplary center frequency (e.g., 1.0 GHz), the performance as provided by known in the art scattering parameters (e.g., S 11 , S 21 , S 22 , S 23  and S 33 ). For example, as shown in  FIG. 1C , an insertion loss performance provided by the parameter S 21  about the center frequency (e.g., 1.0 GHz) remains better than −3.1 dB for a relatively narrow band (labeled as BW). Similarly, for the same narrow band, an isolation performance between ports P 2  and P 3  provided by the parameter S 23  is better than −20 dB. As known to a person skilled in the art, an acceptable performance of the prior art splitter ( 100 A,  100 B) can be considered in view of the isolation between ports P 2  and P 3  (i.e., S 23  parameter) to be better than −20 dB. Accordingly, the relatively narrow band BW may be considered as a bandwidth of the splitter at the center frequency (e.g., 1.0 GHz). In other words, performance of the splitter can be considered as acceptable for an operating frequency that is located within the bandwidth, BW. It should be noted that the performance of the prior art splitter as reflected in the graphs of  FIG. 1C  is inherent to the design of the Wilkinson splitter as described above with reference to  FIG. 1A  and  FIG. 1B . Furthermore, it should be noted that the prior art splitter can be adapted to any center frequency, with a corresponding performance still reflected by the graphs shown in  FIG. 1C , wherein the bandwidth, BW, is adjusted according to the center frequency based on a fractional bandwidth of the splitter (i.e., ratio of bandwidth to center frequency is substantially constant). 
     For applications that require a wider bandwidth than one provided by the prior art splitter (e.g.,  100 A,  100 B), prior art implementations of a power divider may use a plurality of cascaded stages as shown in  FIG. 2A . Such applications may include wide band support of cellular standards that may include a plurality of different frequency bands spanning over an extended (wide) range of frequencies (e.g., 1.2 GHz or larger), each band having a different center frequency and a relatively narrow bandwidth of operation (e.g., 0.5 GHz or smaller). One such example being the U-NII-5/6/7/8 radio band that is part of the radio frequency spectrum used, for example, by IEEE 802.11ax devices for wireless communication at frequency bands having center frequencies in the range of 5 to 7 GHz, or even to 8 GHz. 
     The prior art cascaded (power) splitter ( 200 A) of  FIG. 2A , includes plurality of stages ( 112   b   1 ,  113   b   1 , R 231 ), . . . , ( 112   bn ,  113   bn , R 23n ), each such stage equivalent to the splitter ( 100 B) described above with reference to  FIG. 1B , or alternatively (not shown), equivalent to the splitter ( 100 A) described above with reference to  FIG. 1A . In particular, respective branches ( 112   b   1 ,  113   b   1 ) of the first stage ( 112   b   1 ,  113   b   1 , R 23 ) are coupled at one end to the (common) port P 1 , and at the other end, to one end of respective branches ( 112   b   2 ,  113   b   2 ) of the next stage. Likewise, as can be clearly seen in the detail at the bottom of  FIG. 2A , for any stage other than the first stage ( 112   b   1 ,  113   b   1 , R 23 ), respective ends of the respective branches (e.g.,  112   bp ,  113   bp ) are coupled to ends of branches of adjacent stages ( 112   b (p−1),  113   b (p−1), R 23(p-1) ) and ( 112   b (p+1),  113   b (p+1), R 23(p+1) ). 
     As shown in  FIG. 2B , the cascaded configuration ( 200 A) of  FIG. 2A  provides a relatively larger (e.g., about 3 times larger) bandwidth of operation, BW, as compared to the narrower bandwidth of the single stage configurations of  FIG. 1A  and  FIG. 1B . It should be noted that the bandwidth, BW, shown in  FIG. 2A  is an “instantaneous” bandwidth, or in other words, such wider bandwidth is provided at all time during operation of the cascaded splitter ( 200 A). 
     Another prior art approach for providing a wider bandwidth is shown in the configuration ( 200 C) of  FIG. 2C . In this approach, the wider bandwidth is provided by a plurality of single stage splitters ( 100 B 1 , . . . ,  100 Bn), wherein at any given time of operation, only one of the plurality of single stage splitters is selected for processing of an RF signal. Selection is performed by a combination of an input switch SW IN  and an output switch SW OUT , wherein the input switch, SW IN , couples a port P 1   IN  of the configuration ( 200 C) to a port P 1  of a selected splitter of the plurality of single stage splitters ( 100 B 1 , . . . ,  100 Bn) and the output switch, SW OUT , couples ports P 2   OUT  and P 3   OUT  of the configuration ( 200 C) to respective ports P 2  and P 3  of the selected splitter. In other words, the switches SW IN  and SW OUT  selectively couple ports (P 1 , P 2 , P 3 ) of one of the plurality of single stage splitters ( 1001 , . . . ,  100 Bn) to respective ports (P 1   IN , P 2   OUT , P 3   OUT ). Accordingly, the configuration ( 200 C) of  FIG. 2C  provides a relatively narrow instantaneous bandwidth that is based on the relatively narrow bandwidth of a selected single stage splitter (e.g., per  FIG. 1C  described above). In other words, in the prior art configuration ( 200 C), wide band support is provided by a plurality of relatively narrow instantaneous bandwidths, each having different center frequencies. 
     As described above, the prior art approach for providing a wider bandwidth relies on a plurality of single stage splitters (similar to splitters shown in  FIGS. 1A and 1B ) that are either cascaded to provide a wider instantaneous bandwidth or selected individually via switches to provide a plurality of narrow instantaneous bandwidths. Not only can such approaches be inefficient in terms of die area and component count which may negatively affect cost and integration of the splitter, but also can degrade performance due for example to signal losses incurred through the multiple cascaded stages (e.g.,  FIG. 2A ) or through the switches (e.g.,  FIG. 2C ). 
     Teaching according to the present disclosure describe a tunable Wilkinson power splitter for wider bandwidth support without the drawback of the prior art configurations. 
     SUMMARY 
     According to a first aspect of the present disclosure, a tunable Wilkinson splitter (TWS) is presented, the TWS comprising: a first tunable LC branch coupled between a first port and a second port; a second tunable LC branch coupled between the first port and a third port; and a termination resistor coupled between the second port and the third port, wherein the first and the second tunable LC branches are configured to be tuned to provide operation of the TWS according to a plurality of different center frequencies. 
     According to second aspect of the present disclosure, a multi-stage tunable splitter is presented, the multi-stage tunable filter comprising: a plurality of cascaded stages, each stage comprising first and second branches according to branches of a Wilkinson splitter that are coupled to one another at one end via a termination resistor, wherein at least one stage of the plurality of stages comprises first and second branches that are tunable LC branches to provide operation according to a plurality of different center frequencies. 
     According to a third aspect of the present disclosure, a tunable splitter is presented, the tunable splitter comprising: a number N of LC branches, each of the N LC branches coupled between a first port and a respective port of N ports, N being an integer number equal to or larger than three; and N termination resistors, each coupled between the respective port and a common node, wherein at least one of the N LC branches is a tunable LC branch that is configured to be tuned to provide operation of the tunable splitter according to a plurality of different center frequencies. 
     According to a fourth aspect of the present disclosure, a method for providing a tunable Wilkinson splitter (TWS) is presented, the method comprising: providing a first tunable LC branch; providing a second tunable LC branch; coupling the first tunable LC branch between a first port and a second port; coupling the second tunable LC branch between the first port and a third port; coupling a termination resistor between the second port and the third port, adjusting reactive elements of the first and the second tunable LC branches to tune the TWS for operation according to a Wilkinson splitter at a first center frequency; and adjusting reactive elements of the first and the second tunable LC branches to tune the TWS for operation according to a Wilkinson splitter at a second center frequency. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present disclosure and, together with the description of example embodiments, serve to explain the principles and implementations of the disclosure. 
         FIG. 1A  shows a layout of a prior art Wilkinson splitter realized with distributed elements. 
         FIG. 1B  shows a circuit of a prior art Wilkinson splitter realized with lumped elements. 
         FIG. 1C  shows graphs representative of performance of a prior art Wilkinson splitter. 
         FIG. 2A  shows a circuit of an exemplary prior art splitter that uses a plurality of cascaded stages for operation over a wider band. 
         FIG. 2B  shows graphs representative of performance of the prior art splitter of  FIG. 2A . 
         FIG. 2C  shows a circuit of another exemplary prior art splitter that uses a plurality of selectable narrow band splitters for operation over a wider band. 
         FIG. 3A  shows a circuit of a tunable Wilkinson splitter according to an embodiment of the present disclosure. 
       FIG.  3 A 1 , FIG.  3 A 2 , and FIG.  3 A 3  show various LC network topologies. 
         FIG. 3B  shows graphs representative of an isolation-between-ports performance of the tunable Wilkinson splitter of  FIG. 3A . 
         FIG. 3C  shows graphs representative of an insertion loss performance of the tunable Wilkinson splitter of  FIG. 3A . 
         FIG. 3D  shows a circuit of a multi-port tunable splitter according to an embodiment of the present disclosure including three separate branches. 
         FIG. 3E  shows a circuit of a multi-port tunable splitter according to an embodiment of the present disclosure including k separate branches. 
         FIG. 4A  shows a circuit of a tunable inductor according to an exemplary embodiment. 
         FIG. 4B  shows a circuit of a tunable inductor according to another exemplary embodiment. 
         FIG. 4C  shows a circuit of a tunable reactive element according to another exemplary embodiment. 
         FIG. 5A  shows a circuit of an exemplary splitter according to the present disclosure that uses a plurality of cascaded stages for operation over a wider band, at least one of the cascaded stages based on the tunable Wilkinson splitter of  FIG. 3A . 
         FIG. 5B  shows a circuit of another exemplary splitter according to the present disclosure that uses a plurality of cascaded stages for operation over a wider band, at least one of the cascaded stages based on the tunable Wilkinson splitter of  FIG. 3A . 
         FIG. 5C  shows a circuit of an exemplary splitter according to the present disclosure that uses a plurality of cascaded stages for operation over a wider band, each of the plurality of cascaded stages based on the tunable Wilkinson splitter of  FIG. 3A . 
         FIG. 6  is a process chart showing various steps of a method according to an embodiment of the present disclosure for providing a tunable Wilkinson splitter. 
     
    
    
     DETAILED DESCRIPTION 
     Throughout this description, embodiments and variations are described for the purpose of illustrating uses and implementations of the inventive concept. The illustrative description should be understood as presenting examples of the inventive concept, rather than as limiting the scope of the concept as disclosed herein. 
     The present disclosure describes electrical circuits in electronic devices (e.g., cell phones, radios, base stations, etc.) having a plurality of devices, such as for example, transistors (e.g., MOSFETs). Persons skilled in the art will appreciate that such electrical circuits comprising transistors can be arranged as amplifiers. 
     As used herein, the expression “operating frequency” can refer to a frequency of a signal being input to a device (such as an amplifier). 
     As used herein, the expression “center frequency” can refer to a reference frequency about which the operating frequency varies. The center frequency may be, for example, associated to a band or channel of operation of an RF communication system, and the operating frequency may be associated to a bandwidth of the band or channel of operation. 
     The present teachings overcome prior art shortcoming of wide band power splitter implementations described above by implementing a tunable Wilkinson (power) splitter as shown in  FIG. 3A . Such tunable Wilkinson splitter ( 300 A) includes two tunable branches ( 312 ) and ( 313 ) coupled at their respective ends to (common) port P 1  and respective ports P 2  and P 3 . Tuning of each of the branches is provided by adjustable/variable/configurable/tunable capacitive and inductive elements. For example, the tunable branch ( 312 ) of the splitter ( 300 A) shown in  FIG. 3A  is coupled to port P 1  and port P 2 , and includes adjustable elements (C′ 12 , L′ 12 , C′ 21 ) configured according to an exemplary (Pi-) LC network similar to the network ( 112   b ) of  FIG. 1B . Likewise, the tunable branch ( 313 ) of the splitter ( 300 A) shown in  FIG. 3A  is coupled to port P 1  and port P 3 , and includes adjustable elements (C′ 13 , L′ 13 , C′ 31 ) configured according to an exemplary (Pi-) LC network similar to the network ( 113   b ) of  FIG. 1B . Accordingly, the tunable branches ( 312 ) and ( 313 ) may be refereed herein as tunable LC branches, each comprising adjustable L and C elements. 
     With reference to the exemplary (Pi-) LC networks used in the tunable branches ( 312 ,  313 ) shown in  FIG. 3A , it should be noted, that as known to a person skilled in the art, other LC network topologies different from the shown Pi networks may be used to control phase and amplitude through each of such tunable branches. Non limiting examples of such other topologies may include, for example, i) a known in the art tunable L-C-L (Tee-) network. (e.g.,  31   a ) as shown in FIG.  3 A 1 ; ii) a combination (e.g., cascade) of a tunable Pi-LC network and a tunable Tee-LC network, (e.g.,  31   b ) as shown in FIG.  3 A 2 ; or iii) a plurality of series-connected adjustable L elements combined with a plurality of shunted adjustable C elements, (e.g.,  31   c ) as shown in FIG.  3 A 3 . In other words, each of the tunable branches (e.g.,  312 ,  313 ) may include an LC network of adjustable L and C elements that in combination can provide a desired phase and amplitude of a signal conducted though the branch. Furthermore, although for reasons of practicality a same topology may be used for both of the tunable branches, alternative implementations with different topologies are also envisioned and possible. 
     With continued reference to the tunable splitter ( 300 A) of  FIG. 3A , according to an embodiment of the present disclosure, at any time during operation of such splitter, relationship between values of the adjustable elements of the two branches ( 312 ) and ( 313 ) is configured to maintain the principle of operation of a Wilkinson splitter as described above with reference to the prior art Wilkinson splitter (e.g.,  100 A,  100 B of  FIGS. 1A, 1B ). In other words, at any time during operation of the tunable splitter ( 300 A), for each of the branches ( 312 ) and ( 313 ), values of the respective adjustable elements (C′ 12 , L′ 12 , C′ 21 ) and (C′ 13 , L′ 13 , C′ 31 ) are chosen so that at a frequency of operation (i.e., center frequency), an equivalent impedance of each branch is equal to √{square root over (2)}*Z O  and a phase shift through the branches is equal (e.g., −90 degrees in reference to port P 1 ). Furthermore, similar to the above described prior art implementations, the termination resistor, R 23 , having a value of 2*Z O  and coupled between ports P 2  and P 3 , provides output impedance matching at the two ports P 2  and P 3 , as well as isolation between ports P 2  and P 3  (i.e., essentially no current flows through the resistor). A person skilled in the art would clearly understand that the impedance Z O  is based on a target system impedance, such as, for example, Z O =50 Ohms. 
     With continued reference to the tunable splitter ( 300 A) of  FIG. 3A , according to an embodiment of the present disclosure, tuning of the adjustable elements (C′ 12 , L′ 12 , C′ 21 ) and (C′ 13 , L′ 13 , C′ 31 ) may be based on a desired frequency of operation of the tunable splitter ( 300 A). In other words, a center frequency is selected, and then values of the adjustable elements (C′ 12 , L′ 12 , C′ 21 ) and (C′ 13 , L′ 13 , C′ 31 ) are set for operation according to a Wilkinson power splitter. In other words, the tunable splitter ( 300 A) according to the present teachings can be configured (controlled) in real time for operation according to different center frequencies. For example, when implemented for support of multiple frequency bands of operation of a wireless communication system (e.g., U-NII-5/6/7/8 radio band), a signal-aware controller (e.g., a transceiver) may control the adjustable elements (C′ 12 , L′ 12 , C′ 21 ) and (C′ 13 , L′ 13 , C′ 31 ) in real time for operation according to a desired frequency band of the multiple frequency bands at one time, and for operation according to a different frequency band of the multiple frequency bands at another time. 
       FIG. 3B  and  FIG. 3C  respectively show graphs representative of an isolation-between-ports (P 2  and P 3  represented by parameter S 23 ) performance and an insertion loss performance (represented by parameter S 21 ) of the tunable Wilkinson splitter ( 300 A) of  FIG. 3A  during operation according to, for example, three different center frequencies (e.g., 0.8 GHz, 1.0 GHz and 1.2 GHz). As can be seen in the graphs of  FIG. 3B  and  FIG. 3C , for each of the exemplary three center frequencies, a corresponding bandwidth (BW 1 , BW 2 , BW 3 ) and performance as provided by a shape of the graph, is derived from the bandwidth and graph of the prior art Wilkinson splitter shown in  FIG. 1C , adjusted for a different center frequency (e.g., per fractional bandwidth). As can be clearly understood by a person skilled in the art, the tunable Wilkinson splitter ( 300 A) provides support for a wider band of operation (e.g., bandwidth BW) via a plurality of instantaneous relatively narrower bands of operation (e.g., BW 1 , BW 2 , BW 3 ), each such relatively narrower band of operation provided by a performance of the prior art Wilkinson splitter described above with reference to  FIGS. 1A-1C . As can be clearly seen in  FIG. 3B , the bandwidths BW 1 , BW 2  and BW 3  overlap at frequency regions such as to provide a contiguous frequency region (e.g., BW) over which the isolation between ports P 2  and P 3  is better (greater) than, for example, −20 dB. Likewise, as can be clearly seen in  FIG. 3C , the bandwidths BW 1 , BW 2  and BW 3  overlap at frequency regions such as to provide the contiguous frequency region (e.g., BW, same as one shown in  FIG. 3B ) over which the insertion loss is better (greater) than, for example, −3.1 dB. It should be noted that more or less width of the effective bandwidth, BW, can be obtained by supporting more of less center frequencies of operation, each such center frequency of operation being supported by a different set of values of the adjustable elements (C′ 12 , L′ 12 , C′ 21 ) and (C′ 13 , L′ 13 , C′ 31 ) to provide performance of a corresponding Wilkinson splitter. 
     With continued reference to  FIG. 3C , a person skilled in the art would appreciate the improved out of band rejection provided by the tunable Wilkinson splitter ( 300 A) of  FIG. 3A  when compared to the prior art cascaded splitter ( 200 A) whose performance is shown in  FIG. 2B . Increased out of band rejection is provided by the relatively narrower instantaneous bandwidth of the tunable Wilkinson splitter ( 300 A). Furthermore, as described above, reduced number of components of the tunable splitter ( 300 A) according to the present disclosure can allow for a reduced cost and better integration when compared to the prior art implementations of a wider band splitter (e.g.,  FIGS. 2A and 2C ). 
     As clearly understood to a person skilled in the art, teachings according to the present disclosure can equally apply to a multi-port (multi-branch) power splitter (and combiner) having a plurality of branches (split ports) beyond the exemplary two described above with reference to  FIG. 3A . This is shown in  FIG. 3D , wherein a tunable splitter ( 300 D) may include three branches ( 312 ,  313 ,  314 ) coupled between the port P 1  and respective ports P 2 , P 3  and P 4 . Each such branch configured for adjustment via respective adjustable L and C elements in a similar manner as operation of the branches ( 312 ) and ( 313 ) described above with reference to  FIG. 3A . Because of the higher number of branches (e.g., 3), adjustment of each branch can be configured to provide an equivalent impedance at a frequency of operation (e.g., center frequency) that is equal to √{square root over (3)}*Z O  and a phase shift through the branches that is equal (e.g., −90 degrees in reference to port P 1 ). As shown in  FIG. 3E , termination between any two ports (P 2 , P 3 ), (P 2 , P 4 ) and (P 3 , P 4 ), is provided by respective termination resistors (R 234 , R 324 , R 423 ) coupled from respective ports (P 2 , P 3 , P 4 ) to common node, N C . Values of such resistors are configured such that the respective port sees a termination resistance that is equal to √{square root over (3)}*Z O  at the frequency of operation. 
     As shown in  FIG. 3E , the configuration of  FIG. 3D  can be extended to any configuration having a plurality of k branches, k&gt;1, such as for example, 2, 3, 4, 10, 20, etc . . . , each branch configured to provide an equivalent impedance at a frequency of operation (e.g., center frequency) that is equal to √{square root over (k)}*Z O  and a phase shift through the branches that is equal (e.g., −90 degrees in reference to port P 1 ). Termination between any two ports of the ports (P 2 , P 3 , . . . , P(k−1)) may be provided by respective termination resistors (R 234 , R 324 , . . . , R (k-1) . . .  ) coupled from respective ports (P 2 , P 3 , . . . , P(k−1)) to the common node, N C . Values of such resistors are configured such that the respective port sees a termination resistance that is equal to √{square root over (k)}*Z O  at the frequency of operation. Furthermore, it should be noted that in such configurations shown in  FIGS. 3D and 3E , not all of the branches need to be adjustable, or including adjustable L and C elements. According to some exemplary embodiments of the present disclosure, at least one of the branches may be adjustable, and the other branches fixed (e.g., include fix value L and C elements). 
     With reference back to  FIG. 3A , each of the adjustable elements of the branches ( 312 ) and ( 313 ) may be any adjustable or variable or configurable or tunable reactive element known to a person skilled in the art, such as, for example, a digitally tunable reactive element, such as a digitally tunable capacitor or a digitally tunable inductor.  FIG. 4A  and  FIG. 4B  show two different exemplary circuits (L′ 13a , L′ 13b ) for implementation of a digitally tunable inductor, such as, for example, the inductor L′ 12  or L′ 13  shown in  FIG. 3A . 
     As can be seen in  FIG. 4A , the circuit L′ 13a  provides different values inductors between ports P 1  and P 3 , based on a selection, by a switch SW 140 , of a branch of a plurality of branches between ports P 1  and P 3  having different number of series connected inductors (L 131 , L 132 , . . . , L 13n ). For example, a first branch (lower branch) includes only one inductor L 131  that can be switched between ports P 1  and P 3 , a second branch includes two inductors L 131  and L 132  in series connection that can be switched between ports P 1  and P 3 , and a last branch includes n inductors L 131 , L 132 , . . . , L 13n  in series connection that can be switched between ports P 1  and P 3 . On the other hand, as can be seen in  FIG. 4B , the circuit L′ 13b  provides different value inductors that can be switched between ports P 1  and P 3 , based on a selection, by a switch SW 140 , of a branch of a plurality of branches between ports P 1  and P 3 , each branch having a different value inductor L 131 , L 132 , . . . , or L 13n . For example, a first branch (lower branch) includes one inductor L 131  that can be switched between ports P 1  and P 3 , a second branch includes one inductor L 132  that can be switched between ports P 1  and P 3 , and a last branch includes one inductor L 13n  that can be switched between ports P 1  and P 3 , wherein values (inductance) of the inductors L 131 , L 132 , . . . , and L 13n  are different from one another. 
     A person skilled in the art would clearly understand that any of the two architectures shown in  FIG. 4A  and  FIG. 4B  can be used to implement any of the adjustable elements (C′ 12 , L′ 12 , C′ 21 ) and (C′ 13 , L′ 13 , C′ 31 ) of the branches ( 312 ) and ( 313 ). For example, the architecture shown in  FIG. 4B  can be used for implementation of any adjustable reactive (capacitor or inductor) element Z′ 14  as shown in  FIG. 4C , by substituting reactances (Z 131 , Z 132 , . . . , Z 13n ) with capacitances (e.g., C 131 , C 132 , . . . , C 13n ) to implement adjustable capacitors, and with inductances (L 131 , L 132 , . . . , L 13n ) to implement adjustable inductors. It should be noted that effective values of the fixed capacitances and/or inductances used to implement the tunable capacitors and inductors may be in view of parasitic/stray elements inherent to a physical implementation/layout of a corresponding circuit such that in combination, desired performance of the adjustable branches (e.g.,  312 ,  313  of  FIG. 3A ) at a given center frequency is obtained. 
       FIG. 5A  shows a circuit of an exemplary (power) splitter ( 500 A) according to the present disclosure that uses a plurality of cascaded stages ( 112   a   1 ,  113   a   1 , R 231 ), . . . , ( 112   an ,  113   an , R 23n ), for operation over a wider band, at least one stage ( 312 ,  313 , R 232 ) of the cascaded stages based on the tunable Wilkinson splitter of  FIG. 3A . A person skilled in the art would clearly understand that the configuration ( 500 A) is based on the prior art configuration ( 200 A) described above with reference to  FIG. 2A , wherein at least one stage is a tunable stage based on the tunable splitter ( 300 A) of  FIG. 3A . Accordingly, a wider instantaneous bandwidth can be obtained while allowing further tuning of a center frequency of operation via the tunable branches ( 312 ) and ( 313 ). As shown in  FIG. 5A , the other stages can include transmission lines ( 112   a   1 ,  113   a   1 , etc.) branches based on the distributed elements model described above with reference to  FIG. 1A , or, as shown in  FIG. 5B , can include LC branches ( 112   b   1 ,  113   b   1 , etc.) based on lumped elements model described above with reference to  FIG. 1B . It should be noted that although impractical, the stages that are not according to the tunable Wilkinson splitter of  FIG. 3A , may include a combination of lumped elements stages and distributed elements stages. 
     It should be noted that the cascaded stages of the configurations ( 500 A) or ( 500 B) may include one or more stages that are based on the tunable Wilkinson splitter of  FIG. 3A , which therefore include branches (e.g.,  312 ,  313 ) having adjustable elements per  FIG. 3A . A person skilled in the art will appreciate increased tuning of a center frequency of operation based on an increased number of tunable stages. Furthermore, bandwidth may be controlled not only with respect to its center frequency, but also its spread (width). As shown in  FIG. 5C , according to an exemplary embodiment of the present disclosure, all of the cascaded stages ( 3121 ,  3131 , R 231 ), . . . , ( 312   n ,  313   n , R 23n ), are tunable stages according to  FIG. 3A . Such configuration can allow complete control of bandwidth (spread) and center frequency of the configuration. 
       FIG. 6  is a process chart ( 600 ) showing various steps of a method for providing a tunable Wilkinson splitter (TWS). As can be seen in  FIG. 6 , such steps comprise: providing a first tunable LC branch, per step ( 610 ); providing a second tunable LC branch, per step ( 620 ); coupling the first tunable LC branch between a first port and a second port, per step ( 630 ); coupling the second tunable LC branch between the first port and a third port, per step ( 640 ); coupling a termination resistor between the second port and the third port, per step ( 650 ); adjusting reactive elements of the first and the second tunable LC branches to tune the TWS for operation according to a Wilkinson splitter at a first center frequency, per step ( 660 ); and adjusting reactive elements of the first and the second tunable LC branches to tune the TWS for operation according to a Wilkinson splitter at a second center frequency, per step ( 670 ). 
     It should be noted that the various embodiments of the tunable Wilkinson splitter according to the present disclosure, may be implemented as a monolithically integrated circuit (IC) according to any fabrication technology and process known to a person skilled in the art. 
     Applications that may include the novel apparatus and systems of various embodiments include electronic circuitry used in high-speed computers, communication and signal processing circuitry, modems, single or multi-processor modules, single or multiple embedded processors, data switches, and application-specific modules, including multilayer, multi-chip modules. Such apparatus and systems may further be included as sub-components within a variety of electronic systems, such as televisions, cellular telephones, personal computers (e.g., laptop computers, desktop computers, handheld computers, tablet computers, etc.), workstations, radios, video players, audio players (e.g., mp3 players), vehicles, medical devices (e.g., heart monitor, blood pressure monitor, etc.) and others. Some embodiments may include a number of methods. 
     The term “amplifier” as used in the present disclosure is intended to refer to amplifiers comprising single or stacked transistors configured as amplifiers, and can be used, for example, as power amplifiers (PAs) and/or low noise amplifiers (LNAs). An amplifier can refer to a device that is configured to amplify a signal input to the device to produce an output signal of greater magnitude than the magnitude of the input signal. Stacked transistor amplifiers are described for example in U.S. Pat. No. 7,248,120, issued on Jul. 24, 2007, entitled “Stacked Transistor Method and Apparatus”, U.S. Pat. No. 7,123,898, issued on Oct. 17, 2006, entitled “Switch Circuit and Method of Switching Radio Frequency Signals”, U.S. Pat. No. 7,890,891, issued on Feb. 15, 2011, entitled “Method and Apparatus Improving Gate Oxide Reliability by Controlling Accumulated Charge”, and U.S. Pat. No. 8,742,502, issued on Jun. 3, 2014, entitled “Method and Apparatus for use in Improving Linearity of MOSFETs Using an Accumulated Charge Sink—Harmonic Wrinkle Reduction”, the disclosures of which are incorporated herein by reference in their entirety. As used herein, the term “amplifier” can also be applicable to amplifier modules and/or power amplifier modules having any number of stages (e.g., pre-driver, driver, final), as known to those skilled in the art. 
     The term “MOSFET”, as used in this disclosure, means any field effect transistor (FET) with an insulated gate and comprising a metal or metal-like, insulator, and semiconductor structure. The terms “metal” or “metal-like” include at least one electrically conductive material (such as aluminum, copper, or other metal, or highly doped polysilicon, graphene, or other electrical conductor), “insulator” includes at least one insulating material (such as silicon oxide or other dielectric material), and “semiconductor” includes at least one semiconductor material. 
     As should be readily apparent to one of ordinary skill in the art, various embodiments of the invention can be implemented to meet a wide variety of specifications. Unless otherwise noted above, selection of suitable component values is a matter of design choice and various embodiments of the invention may be implemented in any suitable IC technology (including but not limited to MOSFET structures), or in hybrid or discrete circuit forms. Integrated circuit embodiments may be fabricated using any suitable substrates and processes, including but not limited to standard bulk silicon, silicon-on-insulator (SOI), and silicon-on-sapphire (SOS). Unless otherwise noted above, the invention may be implemented in other transistor technologies such as bipolar, GaAs HBT, GaN HEMT, GaAs pHEMT, and MESFET technologies. However, the inventive concepts described above are particularly useful with an SOI-based fabrication process (including SOS), and with fabrication processes having similar characteristics. Fabrication in CMOS on SOI or SOS enables low power consumption, the ability to withstand high power signals during operation due to FET stacking, good linearity, and high frequency operation (i.e., radio frequencies up to and exceeding 50 GHz). Monolithic IC implementation is particularly useful since parasitic capacitances generally can be kept low (or at a minimum, kept uniform across all units, permitting them to be compensated) by careful design. 
     Voltage levels may be adjusted or voltage and/or logic signal polarities reversed depending on a particular specification and/or implementing technology (e.g., NMOS, PMOS, or CMOS, and enhancement mode or depletion mode transistor devices). Component voltage, current, and power handling capabilities may be adapted as needed, for example, by adjusting device sizes, serially “stacking” components (particularly FETs) to withstand greater voltages, and/or using multiple components in parallel to handle greater currents. Additional circuit components may be added to enhance the capabilities of the disclosed circuits and/or to provide additional functionality without significantly altering the functionality of the disclosed circuits. 
     A number of embodiments according to the present disclosure have been described. It is to be understood that various modifications may be made without departing from the spirit and scope of such embodiments. For example, some of the steps described above may be order independent, and thus can be performed in an order different from that described. Further, some of the steps described above may be optional. Various activities described with respect to the methods identified above can be executed in repetitive, serial, or parallel fashion. 
     It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the disclosure, which is defined by the scope of the following claims, and that other embodiments are within the scope of the claims. (Note that the parenthetical labels for claim elements are for ease of referring to such elements, and do not in themselves indicate a particular required ordering or enumeration of elements; further, such labels may be reused in dependent claims as references to additional elements without being regarded as starting a conflicting labeling sequence).