Patent Publication Number: US-7902939-B2

Title: Stripline balun

Description:
BACKGROUND 
     Baluns convert between balanced and unbalanced electrical signals and can also provide impedance transformation. Baluns are widely used to couple power transistors such as push-pull or switched power transistors to a single-ended (i.e., unbalanced) 50Ω environment such as a coaxial cable. The balun converts between the balanced output of the power transistor and the unbalanced output line and matches the relatively low drain impedance of the power transistor to the relatively high impedance of the single-ended load. A greater impedance transformation ratio can be realized by coupling two transformers together. Typically, one or both of the transformers include a discrete wire-wound structure such as a coaxial cable wound around a guide or a conductive microstrip structure printed onto a single layer of a PCB (printed circuit board). One transformer is coupled to a single-ended output line while the other transformer is coupled to the power transistor drain. The transformers are conventionally capacitively coupled to the drain of the device by one or more DC blocking capacitors. A similar balun arrangement is used at the input (gate) side of the power transistor. As such, the input and output of the power transistor are capacitively coupled to respective single-ended input and output lines through multistage baluns. The DC blocking capacitors of each balun tend to be small in size. At high power levels (e.g., 300 W or greater), significant heating occurs. Excessively high temperatures destroy DC blocking capacitors, limiting the usefulness of conventional multistage baluns to power applications of about 300 W or less. 
     Most circuits using conventional multistage baluns also typically have a single-sided DC feed path to the drain of the power transistor. In many applications, the drain of a power transistor has a relatively wide trace so that the drain is low impedance (e.g., 10Ω or less). Providing DC power to the drain of a power transistor through a single-sided DC feed path causes both sides of the drain to be terminated at different electrical lengths, e.g., ¼ at the DC feed path side and ½ at the other side. Single-sided DC feed structures cause unequal terminating impedances and/or high inductance feeding, both of which adversely affect transistor operation. A high inductance feed path to the drain of a power transistor is particularly problematic for high bandwidth applications such as COFDM (coded orthogonal frequency-division multiplexing) video where signal power levels rapidly rise and fall. Under these signal switching conditions, a high inductance feed can cause repetitive L di/dt avalanche breakdown conditions to occur in the power transistor. 
     It is known to use a single broadside-coupled stripline structure as a transformer in a power amplifier device. A broadside-coupled stripline structure typically includes two ground planes between which one stripline conductor is spaced apart and electromagnetically coupled to a second stripline conductor. However, the single broadside-coupled stripline transformer is still capacitively coupled to a wire-wound transformer or a transformer microstrip structure to complete the impedance matching and balun structure. This type of structure is still prone to excessive DC blocking capacitor heating at high power conditions as explained above, and thus is limited to lower power applications. This type of multistage balun also uses a single-sided path to feed DC power to the drain of a power transistor, causing unequal terminating impedances and/or high inductance feeding. 
     SUMMARY 
     According to an embodiment, a balun includes one or more transformers configured to block DC power between a line and a device at microwave frequencies. The one or more transformers block DC power between the line and the device by electromagnetically coupling the device to the line. 
     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 DRAWINGS 
         FIG. 1  is a multi-layer view of an embodiment of a multistage balun with broadside-coupled stripline transformers. 
         FIG. 2  is an equivalent circuit diagram of the multistage balun of  FIG. 1 . 
         FIG. 3  is a plan view of upper stripline regions of the broadside-coupled stripline transformers of  FIG. 1 . 
         FIG. 4  is a plan view of lower stripline regions of the broadside-coupled stripline transformers of  FIG. 1 . 
         FIG. 5  is a circuit schematic of an embodiment of a multistage balun with broadside-coupled stripline transformers. 
         FIG. 6  is a plan view of an embodiment of an assembly including a power transistor device coupled to at least one multistage balun. 
         FIG. 7  is a flow diagram of an embodiment of a method for connecting a multistage balun to a device. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a three-dimensional view of an embodiment of a balun  100 . The equivalent circuit diagram of the balun  100  is shown in  FIG. 2 . In one embodiment, the balun  100  includes at least two transformers  102 ,  104 . In another embodiment, the balun  100  includes just the second transformer  104  which has a center tap region  152  for providing a central DC feed path, impedance shuffling and signal splitting as described in more detail later herein. Returning to the multistage embodiment, the first transformer  102  includes a broadside-coupled stripline structure having an upper conductive stripline  106  spaced apart from a lower conductive stripline  108 . The upper and lower striplines  106 ,  108  are electromagnetically coupled together during operation of the balun  100 . The second transformer  104  also includes a broadside-coupled stripline structure having upper and lower spaced-apart conductive striplines  110 ,  112  electromagnetically coupled together during operation of the balun  100 . The striplines  106 - 112  comprise relatively flat strips of metal which can be arranged between two ground planes (not shown), e.g., a bottom ground plane and a top ground plane.  FIG. 3  shows the upper striplines  106 ,  110  of both transformers  102 ,  104  formed in one plane and  FIG. 4  shows the lower striplines  108 ,  112  of both transformers  102 ,  104  formed in a different plane. In one embodiment, the upper and lower striplines  106 - 112  are formed in two or more different planes of a multi-layer PCB (not shown). Conductive vias  114  can be used to connect the upper and lower striplines  106 ,  108  of the first transformer  102  in a particular configuration as described in more detail later. Additional conductive vias  116  can be provided for coupling one or more non-DC blocking capacitors (not shown) to the balun  100 . In another embodiment, the upper and lower striplines  106 - 112  of the balun  100  are formed in different single-layer PCBs (not shown) which are connected together. 
     The balun  100  connects an unbalanced (i.e., single-ended) line  118  to a power transistor device  120  having a balanced output as schematically shown in  FIG. 2 . Particularly, the upper stripline  106  of the first transformer  102  is coupled to the unbalanced line  118 . In an embodiment, the upper stripline  106  of the first transformer  102  has two branches  122 ,  124  coupled in series. Both branches  122 ,  124  of the upper stripline  106  taken together represent the high impedance side of the first transformer  102  and have a total electrical length of approximately ½ λ. The first branch  122  couples the unbalanced line  118  to the second branch  124  which is tied to ground as shown in  FIG. 2 . The end of the second branch  124  tied to ground is also directly coupled to a center tap region  126  of the lower stripline  108  of the first transformer  102  meaning that the second upper branch  124  provides both AC signal information and DC bias to the center tap region  126  of the lower stripling  108 . According to this embodiment, the lower stripline  108  of the first transformer  102  also has two branches  128 ,  130 . The branches  128 ,  130  of the lower stripline  108  are relatively symmetric and extend from the center tap region  126  to opposing end regions  132 ,  134 . Each branch  128 ,  130  of the lower stripline  108  has an electrical length of approximately ¼λ and taken together represent the low impedance side of the first transformer  102 . 
     Connecting the grounded end of the upper stripline  106  of the first transformer  102  to the center tap region  126  of the underlying lower stripline  108  enables the first transformer  102  to convert a single-ended (unbalanced) signal carried by the upper stripline  106  to a differential (balanced) signal in the lower stripline  108  or vice-versa. Each branch  128 ,  130  of the lower stripline  108  carries a signal approximately 180° out of phase with the signal carried by the other symmetric branch. Each branch  128 ,  130  of the lower stripline  108  of the first transformer  102  is directly coupled to a corresponding branch  136 ,  138  of the lower stripline  112  of the second transformer  104 . Accordingly, no DC blocking capacitors are used to connect the transformers  102 ,  104  of the balun  100 . 
     In one embodiment, the lower striplines  108 ,  112  of the transformers  102 ,  104  have first ends  132 ,  140  directly coupled to each other by a first conductive stripline  144  and second ends  134 ,  142  directly coupled to each other by a second conductive stripline  146 . The lower stripline  112  of the second transformer  104  represents the high impedance side of the second transformer  104  and the upper stripline  110  of the second transformer  104  represents the low impedance side. The lower stripline  112  of the second transformer  104  has two branches  136 ,  138  which together have a total electrical length of approximately ½ λ. During operation, a differential signal carried by the lower stripline  112  of the second transformer  104  is electromagnetically coupled to the upper stripline  110  of the second transformer  104  or vice-versa. 
     In one embodiment, the upper stripline  110  of the second transformer  104  is generally omega shaped as shown in  FIGS. 1 and 3 . According to this embodiment, two conductive and generally symmetric stripline branches  148 ,  150  extend from a center tap region  152  of the upper stripline  110  to respective spaced-apart end regions  154 ,  156 . In one embodiment, each end region  154 ,  156  of the omega-shaped upper stripline  100  is connected to a different drain (D) of the power transistor device  120  as shown in  FIG. 2 . According to this embodiment, the power device includes a pair of power transistors  158 ,  160 . The drain (D) of each power transistor  158 ,  160  is coupled to a respective end  154 ,  156  of the upper stripline  110  of the second transformer  104 . The power transistor sources (S) are tied to ground and gates (G) to respective inputs. 
     Coupling the power transistor device  120  to the unbalanced line  118  using the balun  100  eliminates the need for DC blocking capacitors. Instead, the lower striplines  108 ,  112  of the transformers  102 ,  104  are directly coupled to each other as described above. Accordingly, the power transistor device  120  is electromagnetically coupled to the unbalanced line  118 . The power device  120  can be used in relatively high power applications (e.g., 300 W and above) because there are no DC blocking capacitors subject to excessive heating. Moreover, the broadside-coupled stripline transformers  102 ,  104  reliably operate in the microwave frequency range (300 MHz and above). Simulation has shown balun operating frequencies in excess of 2 GHz. In addition, the broadside-coupled stripline transformers  102 ,  104  provide an impedance transformation between the power transistor device  120  and the unbalanced line  118  of approximately 30:1 or greater at microwave frequencies. The balun  100  also has a bandwidth of approximately 60% or better at microwave operating frequencies (e.g., a bandwidth of approximately 400 MHz or greater). Accordingly, the balun  100  is well suited for applications having high frequency, bandwidth and power requirements such as COFDM video. The balun  100  can be used in other applications as well. 
     Non-DC blocking capacitors can be added at different sections of the balun  100  to improve the operating characteristics of the balun  100 . In one embodiment, tuning capacitors (not shown) are coupled to the common connection point between the lower striplines  108 ,  112  of the transformers  102 ,  104 . Particularly, one or more conductive vias  116  can extend from the end  132 ,  134  of each respective branch  128 ,  130  of the lower stripline  108  to a capacitor connection region  162  as shown in  FIGS. 1 and 4 . Connecting tuning capacitors to the capacitor connection region  162  extends the length of the low impedance side of the first transformer  102  for tuning and impedance matching. 
     In another embodiment, a capacitor  164  is coupled between ground and the center tap region  152  of the upper stripline  110  of the second transformer  104  as shown in  FIG. 2 . This capacitor  164  RF grounds the center tap region  152  of the upper stripline  110  of the second transformer  104 . RF grounding the center tap region  152  in this way enables baseband filtering with a very high cutoff frequency. RF grounding the center tap region  152  also allows for DC power to be centrally fed to the power transistor device  120  through the center tap region  152  instead of a single-sided feed path. DC power can be applied to the drain of each power transistor  158 ,  160  through the respective branches  148 ,  150  of the upper stripline  110  of the second transformer  104  when the center tap region  152  of the upper stripline  110  is capacitively coupled to ground. The DC power applied to the RF grounded center tap region  152  is fed to the drains of the power transistor  158 ,  160  via the symmetric branches  148 ,  150  of the upper stripline  100  of the second transformer  104  which are each approximately ¼ λ wavelength. Thus, both sides of the power transistor drain are terminated approximately at the same wavelength. Moreover, both sides of the power transistor drain are relatively evenly matched when the upper stripline  110  of the second transformer  104  is generally omega-shaped as described above because each point on one drain terminal has approximately the same distance to the center tap region  152  as the same point on the other drain terminal as will be described in more detail later herein. According to one embodiment, the balun  100  includes only the second generally omega-shaped transformer  104  for providing a central DC feed path to the power transistor device  120  or any other type of suitable device. The second broadside-coupled stripline transformer  104  can be of any suitable configuration, shape and/or dimension. For example, the vertical orientation of the striplines  110 ,  112  of the second transformer  104  can be flipped depending on the type of application. 
       FIG. 5  illustrates a circuit schematic of a balun  500  with two broadside-coupled stripline transformers  502 ,  504  directly coupled together. However, any number of transformers can be used depending on the type of application. An upper stripline of the first transformer  502  is formed by first and second conductive branches  506 ,  508  coupled in series by a conductor  510 . The first branch  506  of the upper stripline is directly coupled to a single-ended (unbalanced) line  512  through a conductor  514  which can be capacitively coupled to ground via one or more chip capacitors  516 ,  518 . The second branch  508  of the upper stripline is tied to ground and directly coupled to a lower stripline of the first transformer  502 . The lower stripline of the first transformer  502  is formed by first and second conductive branches  520 ,  522  joined together at a center tap region  524 . The center tap region  524  is where the second branch  508  of the upper stripline connects to the lower stripline. This arrangement allows for single-ended to differential signal conversion as previously described herein. Each branch  520 ,  522  of the lower stripline of the first transformer  502  is directly coupled to a corresponding branch  526 ,  528  of a lower stripline of the second transformer  504 . In one embodiment, the lower stripline branches  520 / 528 ,  522 / 526  are directly coupled together through respective conductors  530 ,  532 . A tuning capacitor  534  can also be coupled between the ends of the branches  520 ,  522  of the lower stripline of the first transformer  502 . 
     The lower stripline branches  526 ,  528  of the second transformer  504  are directly coupled together at a center tap region  536 . Each lower stripline branch  526 ,  528  of the second transformer  504  is electromagnetically coupled to a corresponding branch  538 ,  540  of an upper stripline of the second transformer  504  during operation of the balun  500 . The upper stripline branches  538 ,  540  of the second transformer  504  are also directly coupled together at a center tap region  542  and extend to respective conductive signal lines  544 ,  546 . The center tap region  542  of the upper stripline of the second transformer  504  can be coupled to ground by a capacitor  548 , RF grounding the center tap region  542 . The RF grounded center tap region  542  provides a common DC bias feed point. The ends of the upper stripline branches  538 ,  540  of the second transformer  504  can be coupled together by a tuning capacitor  550 . Additional non-DC blocking capacitors (not shown) can be coupled to the balun  500  depending on the type of application. Also, the broadside-coupled stripline transformers  502 ,  504  can be of any suitable configuration, shape and/or dimension. For example, the respective upper and lower striplines  106 / 108 ,  110 / 112  discussed previously herein can be flipped in orientation and/or be of a different shape, size, dimension, etc. Broadly, the balun  500  with the broadside-coupled stripline transformers  502 ,  504  can be used to electromagnetically couple a power transistor device to an unbalanced line  512  without using DC blocking capacitors. 
       FIG. 6  illustrates an embodiment of a subassembly  600  including a balun  602  with two broadside-coupled stripline transformers  604 ,  606  coupled to the output of a power transistor device  608 . Again, any number of transformers can be used depending on the type of application. The broadside-coupled stripline transformers  604 ,  606  are directly coupled together as previously explained herein.  FIG. 6  is a plan view of the subassembly, so only the upper stripline regions  610 ,  612  of the transformers  604 ,  606  are visible. The balun  602  electromagnetically couples the drain of the power transistor device  608  to an unbalanced line  614  without using DC blocking capacitors, e.g., as illustrated by Step  700  of  FIG. 7 . The balun  602  also transforms the impedance between the drain of the power transistor device  608  and the unbalanced line  614 , e.g., as illustrated, e.g., as illustrated by Step  710  of  FIG. 7 . 
     In more detail, the unbalanced line  614  is coupled to the upper stripline  610  of the first transformer  604 . The other end of the upper stripline  610  is coupled to an underlying stripline (out of view) at a center tap region of the lower stripline by one or more conductive vias  616 . The lower stripline of the first transformer  604  is directly connected to a lower stripline (out of view) of the second transformer  606 . The ends of the lower stripline branches can be coupled to one or more tuning capacitors (not shown) at a capacitor contact region  618 . The lower stripline of the second transformer  606  is electromagnetically coupled to the overlying stripline  612  of the second transformer  606 . Branches  620 ,  622  of the upper stripline  612  of the second transformer  606  extend from a center tap region  624  to different drain terminals  626 ,  628  of the power transistor device  608 . In one embodiment, the upper stripline  612  of the second transformer  606  is generally omega-shaped as shown in  FIG. 6  so that each point on one drain terminal  626 / 628  is approximately the same distance to the center tap region  624  as the same point on the other drain terminal  628 / 626  as indicated by the dashed lines in  FIG. 6 . 
     In one embodiment, the center tap region  624  of the upper stripline  612  of the second transformer  606  is capacitively coupled to ground so that a DC power feed can be evenly applied to the power transistor device  608  through the center tap region  624  while the center tap  624  is RF grounded. Moreover, the branches  620 ,  622  of the upper stripline  612  of the second transformer  606  are generally symmetric. Accordingly, the DC feed path to the drain terminals  626 ,  628  of the power transistor device  608  has near equal distribution across the drain terminals  626 ,  628 . This in turn provides relatively even impedance matching and termination across the drain terminals  626 ,  628  at fundamental, 2 nd  harmonic and baseband frequencies. The upper stripline  612  of the second transformer  606  can be made relatively wide as shown in  FIG. 6  so that the inductance between the DC feed point at the center tap region  624  and the drain terminals  626 ,  628  is low, reducing L di/dt induced voltage peaks which occur in certain applications such as COFDM video. The low inductance at the drain terminals  626 ,  628  also increases operating bandwidth which is important for certain applications such as video. Bandwidth increases because the cutoff frequency of the baseband termination is substantially increased which is ideal for certain push-pull applications. In some simulations, a bandwidth of 60% or greater have been achieved at microwave frequencies. This is in addition to an impedance transformation ratio of 30:1 or greater at microwave frequencies. Electromagnetically coupling the power transistor device  608  to the unbalanced line  614  using the balun  602  also decreases low-frequency parasitic gain spikes which can be problematic unless filtered or otherwise attenuated. 
     The input (gate) side of the power transistor device  608  can be similarly coupled to an unbalanced input line  630  using a second balun  632 . The balun  632  on the input side of the power device  608  also includes at least two broadside-coupled stripline transformers  634 ,  636  directly coupled together. Again, because  FIG. 6  is a plan view of the subassembly, only the upper stripline regions  638 ,  640  of the second balun  632  are shown. In more detail, the third broadside-coupled stripline transformer  634  includes an upper stripline  638  coupled to different gate terminals  642 ,  644  of the power transistor device  608  and a lower stripline (out of view) spaced apart from and underlying the upper stripline  638 . The fourth broadside-coupled stripline transformer  636  also has an upper stripline  640  spaced apart from and overlying a lower stripline (out of view). The upper stripline  640  of the fourth transformer  636  is coupled to the unbalanced input line  630  and to a center tap region (out of view) of the underlying lower stripline by one or more conductive vias  646 . The lower striplines of the third and fourth transformers  634 ,  636  are directly coupled to each other as described herein so that DC blocking capacitors are not needed at the input side of the power transistor device  608 . One or more tuning capacitors (not shown) can be coupled to the connection point between the lower striplines of the third and fourth transformers  634 ,  636  at a capacitor contact region  648 . In one embodiment, the upper stripline  638  of the third transformer  634  includes two physically separate branches  650 ,  652  which do not share a common center tap region so that the gate terminals  642 ,  644  can be DC isolated from each other. Common RLC components have been excluded from  FIG. 6  for ease of illustration and explanation only. However, those skilled in the art will readily recognize that different RLC components can be added to the subassembly  600  depending on the application under consideration. 
     With the above range of variations and applications in mind, it should be understood that the present invention is not limited by the foregoing description, nor is it limited by the accompanying drawings. Instead, the present invention is limited only by the following claims and their legal equivalents.