Patent Publication Number: US-11381216-B2

Title: Wideband balun

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/380,212, filed Apr. 10, 2019, titled “WIDEBAND BALUN,” which claims the benefit of priority to U.S. Provisional Patent Application No. 62/789,789, filed Jan. 8, 2019, titled “WIDEBAND BALUN,” the disclosures of each of which are hereby incorporated in their entireties. 
    
    
     FIELD OF THE DISCLOSURE 
     Embodiments of the invention relate to electronic systems, and more particularly, to baluns. 
     BACKGROUND 
     A balun can operate between a balanced differential transmission path and an unbalanced, single-ended transmission path. Baluns can be used in a wide variety of electronic devices, including, for example, radio frequency and microwave electronics. Example applications for baluns include push-pull amplifiers, mixers, phase shifters, impedance transformers, and antenna feeds. 
     SUMMARY OF THE DISCLOSURE 
     Wideband baluns with enhanced amplitude and phase balance are provided. The wideband balun includes a first transmission line connected between a first port and a third port, and a second transmission line connected between a second port and a fourth port, and a third transmission line connected between the third port and a reference voltage, such as ground. To enhance phase and/or amplitude balance of the wideband balun, the wideband balun further includes a compensation structure operable to provide at least one of capacitive compensation or inductive compensation to balance the wideband balun. For example, in certain implementations, the compensation structure includes at least one of (i) a capacitor connected between the first port and the second port or (ii) a fourth transmission line connected between the first transmission line and the third port. 
     In one aspect, a wideband balun with enhanced amplitude and phase balance is provided. The wideband balun includes a plurality of ports including a first port, a second port, a third port, and a fourth port, a first transmission line electrically connected between the first port and the third port, a second transmission line electrically connected between the second port and the fourth port and magnetically coupled to the first transmission line, a third transmission line electrically connected between the third port and a reference voltage, and a compensation structure electrically connected to the first transmission line and operable to provide at least one of capacitive compensation or inductive compensation to balance the wideband balun. 
     In another aspect, a semiconductor die having an integrated wideband balun is provided. The semiconductor die includes two or more metallization layers, a first transmission line formed in the two or more metallization layers and electrically connected between a first port and a third port, a second transmission line formed in the two or more metallization layers and electrically connected between a second port and a fourth port, and a third transmission line formed in the two or more metallization layers and electrically connected between the third port and a reference voltage. The first transmission line and the second transmission line are magnetically coupled. The semiconductor die further includes a compensation structure operable to compensate for imbalance between the second port and the third port. 
     In another aspect, a wideband balun is provided. The wideband balun includes a plurality of ports including a first port, a second port, a third port, and a fourth port, a first transmission line electrically connected between the first port and the third port, a second transmission line electrically connected between the second port and the fourth port and magnetically coupled to the first transmission line, a third transmission line electrically connected between the third port and a reference voltage, and means for compensating for an imbalance between the second port and the third port. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic diagram of a wideband balun according to one embodiment. 
         FIG. 1B  is a schematic diagram of a wideband balun according to another embodiment. 
         FIG. 2  is a schematic diagram of a wideband balun according to another embodiment. 
         FIG. 3A  is a schematic diagram of a wideband balun according to another embodiment. 
         FIG. 3B  is a schematic diagram of a wideband balun according to another embodiment. 
         FIG. 3C  is a schematic diagram of a wideband balun according to another embodiment. 
         FIG. 4A  is a schematic diagram of a cross-section of portion of a semiconductor die according to one embodiment. 
         FIG. 4B  is a schematic diagram of a cross-section of portion of a semiconductor die according to another embodiment. 
         FIG. 5A  is a schematic diagram of a cross-section of portion of a semiconductor die according to another embodiment. 
         FIG. 5B  is a schematic diagram of a cross-section of portion of a semiconductor die according to another embodiment. 
         FIG. 6A  is a graph of amplitude versus frequency plots for two examples of baluns. 
         FIG. 6B  is a graph of phase versus frequency plots for two examples of baluns. 
         FIG. 7A  is a schematic diagram of a push-pull amplifier according to one embodiment. 
         FIG. 7B  is a schematic diagram of a single balanced mixer according to one embodiment. 
         FIG. 7C  is a schematic diagram of a double balanced mixer according to one embodiment. 
         FIG. 7D  is a schematic diagram of a double balanced mixer according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The following detailed description of embodiments presents various descriptions of specific embodiments of the invention. However, the invention can be embodied in a multitude of different ways. In this description, reference is made to the drawings where like reference numerals may indicate identical or functionally similar elements. It will be understood that elements illustrated in the figures are not necessarily drawn to scale. Moreover, it will be understood that certain embodiments can include more elements than illustrated in a drawing and/or a subset of the elements illustrated in a drawing. Further, some embodiments can incorporate any suitable combination of features from two or more drawings. 
     To enhance integration of electronic systems operating in radio frequency (RF) and/or microwave frequency ranges, it is desirable to incorporate a balun in a semiconductor die, such as a monolithic microwave integrated circuit (MMIC). Advantages of integration include size reduction and large-scale production, which in turn results in lowered costs. Examples of fabrication processes suitable for MMICs include, but are not limited to, silicon processes and compound semiconductor processes, such as gallium arsenide (GaAs) or gallium nitride (GaN) technologies. 
     In certain configurations, a balun can receive a single-ended RF signal and output a differential RF signal, or vice versa. When outputting a differential RF signal, it is desirable for the non-inverted and inverted components of the differential RF signal to have equal amplitude, and a phase shift of 180 degrees. Examples of important performance parameters for broadband baluns include operating frequency range, insertion loss, amplitude balance, phase balance, and/or physical size. 
     Conventional coaxial-line or magnetic-core broadband baluns are limited in operation to relatively low frequencies. For example, as the operating frequency reaches 2 GHz or more, inductive and/or capacitive parasitics of the balun substantially impact insertion loss, phase balance, and/or amplitude balance of such baluns. 
     To extend bandwidth of operation, a balun can be fabricated using transmission lines rather than lumped inductors. In a first example, a coupled transmission line balun can be formed using coplanar striplines, also referred to as micro-striplines, of conductive material positioned over a substrate and magnetically coupled to one another. In a second example, a coupled transmission line balun can be formed using a pair of stacked conductors positioned over a substrate and magnetically coupled to one another. 
     In certain applications, the phase and/or amplitude balance of a balun can impact the performance of an electronic system that incorporates the balun. 
     In a first example, a push-pull amplifier has a distortion impacted by a phase and/or amplitude balance of a balun. For instance, a push-pull amplifier balances out even harmonics at the amplifier&#39;s output leaving the third harmonic as the principal source of distortion, and thus possesses inherent spurious signal rejection of even order distortion. Thus, phase/amplitude balance is important to achieve high output second-order intercept point (OIP2) and/or second-order harmonic distortion (HD2) for the push-pull amplifier. While narrow-band push-pull power amplifiers can improve OIP2 over single-ended topologies, broadband push-pull amplifiers have been infeasible due to an absence of a broadband balun with small phase/amplitude imbalance. 
     In a second example, mixers (for instance, a single balanced mixer or double balanced mixer) have performance impacted by phase and/or amplitude balance. For example, even with small amplitude and/or phase errors, linearity performance (for instance, input second-order intercept point and/or input third-order intercept point) and/or port-to-port isolation (for instance, LO to RF and LO to IF isolation) is degraded. Moreover, larger errors of amplitude and/or phase balance will further impact the conversion gain and noise figure. Accordingly, a bandwidth of a passive mixer is limited by the bandwidth of the balun. 
     Wideband baluns are provided herein. In certain embodiments, a wideband balun includes a first transmission line and a second transmission line magnetically coupled to one another. The first transmission line is connected between a first port and a third port, and the second transmission line is connected between a second port and a fourth port. The wideband balun further includes a third transmission line connected between the third port and a reference voltage, such as ground. To enhance phase and/or amplitude balance of the wideband balun, the wideband balun further includes at least one of (i) a capacitor connected between the first port and the second port or (ii) a fourth transmission line connected between the first transmission line and the third port. 
     By implementing the balun in this manner, phase and/or amplitude balance of the balun is enhanced over a broadband frequency range. Such baluns can be used in a wide range of applications, including, but not limited to, push-pull amplifiers and/or mixers to achieve superior linearity and/or wider bandwidth. 
     The wideband baluns herein can handle signals of a variety of frequencies, including not only RF signals between 100 MHz and 7 GHz, but also signals of higher frequencies, such as those in the X band (about 7 GHz to 12 GHz), the K u  band (about 12 GHz to 18 GHz), the K band (about 18 GHz to 27 GHz), the K a  band (about 27 GHz to 40 GHz), the V band (about 40 GHz to 75 GHz), and/or the W band (about 75 GHz to 110 GHz). Accordingly, the teachings herein are applicable to a wide variety of RF systems, including microwave systems. 
     In contrast, conventional transformer baluns have a relatively narrow operating frequency range. For example, to achieve efficient magnetic coupling, a conventional transform balun can implement the self-impedances of the primary and secondary inductors to be relatively higher than the terminal impedance. Thus, the lowest frequency of operation of such baluns is limited by the amount of inductance in the balun, and the upper end of the operating frequency is limited by parasitic capacitance (for instance, due to resonance with the inductors). Such limitations hinder the ability of the conventional transformer balun to operate over a broad frequency range since the lowest frequency is based on sizing with large dimensions to achieve high inductance while the highest frequency is based on sizing with small dimensions to achieve low parasitic capacitance. 
     Moreover, ideal amplitude and phase balance between the plus and minus terminals of a conventional transformer balun requires perfect magnetic coupling between the primary and secondary windings. It is not even possible to achieve perfect magnetic coupling in the best environments, let alone the integrated circuit environment. Furthermore, the parasitic capacitance of the balanced winding can be unsymmetrical due to coupling of segments of the balanced windings to the grounded end of the unbalanced winding. 
       FIG. 1A  is a schematic diagram of a wideband balun  20  according to one embodiment. The wideband balun  20  includes a first port  1 , a second port  2 , a third port  3 , a fourth port  4 , a first transmission line  11 , a second transmission line  12 , a third transmission line  13 , and a fourth transmission line  14 . 
     As shown in  FIG. 1A , the first transmission line  11  is connected in series with the fourth transmission line  14  between the first port  1  and the third port  3 . Additionally, the second transmission line  12  is connected between the fourth port  4  and the second port  2 . In this embodiment, the fourth port  4  is electrically connected to a reference voltage, such as ground. Due to the inclusion of the fourth transmission line  14 , the total length of transmission line between the first port  1  and the third port  3  is greater than the total length of transmission line between the fourth port  4  and the second port  2 . 
     The first transmission line  11  and the second transmission line  12  are coupled to one another, for instance, negatively coupled magnetically. The first transmission line  11  and the second transmission line  12  can be magnetically coupled in a wide variety of ways, including, but not limited to, using the transmission line configurations of  FIGS. 4A-5B . In certain implementations, the first transmission line  11  and the second transmission line  12  are tightly spaced to provide electromagnetic coupling of both magnetic and electric fields. 
     With continuing reference to the  FIG. 1A , the third transmission line  13  is electrically connected in shunt with respect to the signal path through the first transmission line  11  and the fourth transmission line  14 . For example, the third transmission line  13  is electrically connected between the third port  3  and a reference voltage, such as ground, in this example. In certain implementations, the third transmission line  13  is magnetically isolated from the first transmission line  11 , the second transmission line  12 , and/or the fourth transmission line  14 . 
     When an unbalanced or single-ended signal is applied at the first port  1 , a balanced or differential signal is generated between the third port  3  and the second port  2 . Since the applied signal at the first port  1  is voltage divided by the impedances of the first transmission line  11  and the fourth transmission line  14 , the third port  3  has about the same phase (non-inverted) as the signal at the first port  1 . Furthermore, since the first transmission line  11  and the second transmission line  12  are negatively coupled magnetically, a phase inversion is provided at the second port  2 . 
     Accordingly, when a single-ended signal is applied to the first port  1 , the signal phase at the second port  2  is about 180 degrees different from the signal phase at the third port  3 . Conversely, when a differential signal is applied between the third port  3  and the second port  2 , the components of the differential signal combine to generate a single-ended signal as an output at the first port  1 . 
     By including the fourth transmission line  14 , the amplitude and/or phase balance of the wideband balun  20  is improved. For example, the fourth transmission line  14  can provide inductive compensation that enhances the amplitude and/or phase balance of the wideband balun  20  relatively to an implementation in which the fourth transmission line  14  is omitted. In certain implementations, the fourth transmission line  14  is magnetically isolated from the first transmission line  11 , the second transmission line  12 , and/or the third transmission line  13 . 
     The wideband balun  20  can be implemented on a semiconductor die with a planar structure, rather than using a wire-wound magnetic core structure. Furthermore, the wideband balun  20  can be implemented on two or more metal layers during backend processing of a semiconductor die fabricated using a standard semiconductor integrated circuit (IC) process. Furthermore, the wideband balun  20  can be implemented with a relatively compact balun and without a magnetic core, which results in broader bandwidth and better amplitude and phase balance compared to the traditional on-chip balun structures. 
     An electronic system can incorporate one or more of the wideband baluns  20  to achieve high performance. For example, push-pull amplifiers, mixers, phase shifters, impedance transformers, and antenna feeds can have a performance impacted by a balun&#39;s operating characteristics. By using one or more instantiations of the wideband balun  20  in such systems, enhanced performance can be achieved. 
       FIG. 1B  is a schematic diagram of a wideband balun  25  according to another embodiment. The wideband balun  25  of  FIG. 1B  is similar to the wideband balun  20  of  FIG. 1A , except that the wideband balun  25  further includes a series capacitor  19 , which is in series with the first transmission line  11  and the fourth transmission line  14  between the first port  1  and the third port  3 . 
     Including the series capacitor  19  extends a low frequency bandwidth of the wideband balun  25 . For example, a capacitance of the series capacitor  19  (along with a parasitic capacitance of the first transmission line  11  and/or a parasitic capacitance of the fourth transmission line  14 ) operates with an inductance at the first port  1  to generate a resonance that improves the return loss of the wideband balun  25 . Accordingly, low frequency bandwidth can be extended. 
       FIG. 2  is a schematic diagram of a wideband balun  30  according to another embodiment. The wideband balun  30  includes a first port  1 , a second port  2 , a third port  3 , a fourth port  4 , a first transmission line  11 , a second transmission line  12 , a third transmission line  13 , and a compensation capacitor  21  (also referred to herein as capacitor C 1 ). 
     As shown in  FIG. 2 , the first transmission line  11  is connected between the first port  1  and the third port  3 , and the second transmission line  12  is connected between the fourth port  4  and the second port  2 . In this embodiment, the fourth port  4  is electrically connected to a reference voltage, such as ground. The first transmission line  11  and the second transmission line  12  are coupled to one another, for instance, negatively coupled magnetically. The first transmission line  11  and the second transmission line  12  can be magnetically coupled in a wide variety of ways, including, but not limited to, using the transmission line configurations of  FIGS. 4A-5B . 
     With continuing reference to the  FIG. 2 , the third transmission line  13  is electrically connected in shunt with respect to the signal path from the first port  1  to the third port  3  through the first transmission line  11 . For example, the third transmission line  13  is electrically connected between the third port  3  and a reference voltage, such as ground, in this example. In certain implementations, the third transmission line  13  is magnetically isolated from the first transmission line  11  and the second transmission line  12 . 
     As shown in  FIG. 2 , the compensation capacitor  21  is electrically connected between the second port  2  and the first port  1 , and operates to provide capacitance compensation to the wideband balun  30  to thereby enhance amplitude balance and/or phase balance. For example, capacitive compensation enhances the amplitude and/or phase balance of the wideband balun  30  relatively to an implementation in which the compensation capacitor  21  is omitted. 
     The wideband balun  30  can be implemented on a semiconductor die with a planar structure, rather than using a wire-wound magnetic core structure. Furthermore, the wideband balun  30  can be implemented on two or more metal layers during backend processing of a semiconductor die fabricated using a standard semiconductor IC process. One or more instantiations of the wideband balun  30  can be used in a push-pull amplifier, a balanced mixer, or other electronic system. 
       FIG. 3A  is a schematic diagram of a wideband balun  40  according to another embodiment. The wideband balun  40  a first port  1 , a second port  2 , a third port  3 , a fourth port  4 , a first transmission line  11 , a second transmission line  12 , a third transmission line  13 , a fourth transmission line  14 , and a compensation capacitor  21 . 
     By implementing the wideband balun  40  with both the fourth transmission line  14  and the compensation capacitor  21 , further enhancements in phase and/or amplitude balance can be achieved. 
       FIG. 3B  is a schematic diagram of a wideband balun  50  according to another embodiment. The wideband balun  50  of  FIG. 3B  is similar to the wideband balun  40  of  FIG. 3A , except that the wideband balun  50  further includes a decoupling capacitor  22  (also referred to herein as capacitor C 2 ). 
     As shown in  FIG. 3B , the decoupling capacitor  22  includes a first end electrically connected to one end of the third transmission line  13  and to the fourth port  4 , and a second end electrically connected to a reference voltage, such as ground. By including the decoupling capacitor  22 , the first port  1  and the second port  2  are decoupled from ground DC potential, thereby enhancing flexibility of and/or simplifying DC biasing. 
       FIG. 3C  is a schematic diagram of a wideband balun  60  according to another embodiment. The wideband balun  60  of  FIG. 3C  is similar to the wideband balun  50  of  FIG. 3B , except that the wideband balun  60  further includes the series capacitor  19  (also referred to herein as capacitor C 3 ), which was described above with respect to  FIG. 1B . Including the series capacitor  19  extends a low frequency bandwidth of the wideband balun  60 . 
       FIG. 4A  is a schematic diagram of a cross-section  130  of portion of a semiconductor die according to one embodiment. The cross-section  130  depicts a portion of the die&#39;s metallization layers, including a first conductive layer  111 , a second conductive layer  112 , and a first dielectric layer  115  that is between the first conductive layer  111  and the second conductive layer  112 . The metallization layers are formed over a die substrate  110 , and separated therefrom by another dielectric layer  117 , in this embodiment. 
     As shown in  FIG. 4A , the first conductive layer  111  includes a ground plane  120 , while the second conductive layer  112  has been patterned to include a first transmission line  121  and a second transmission line  122 . 
     Although  FIG. 4A  depicts a cross-section, persons of ordinary skill will appreciate that the first transmission line  121  and the second transmission line  122  have lengths extending at least in part in a direction perpendicular to the cross-section  130  (for instance, in a directional perpendicular to the figure sheet). For example, the first transmission line  121  and the second transmission line  122  can have coil or spiral shapes when viewed from above. 
     The first transmission line  121  and the second transmission line  122  illustrate one implementation of the first transmission line  11  and the second transmission line  12 , respectively (or vice versa), of the wideband baluns of  FIGS. 1A-3C . In the illustrated embodiment, the first transmission line  121  and the second transmission line  122  are edge-coupled. 
     Although not illustrated in  FIG. 4A , one or more additional transmission lines (for instance, the third transmission line  13  and/or the fourth transmission line  14 ) can be implemented using the metallization layers of the semiconductor die. Furthermore, one or more capacitors can be implemented on-chip with the transmission lines, thereby providing an integrated and compact design. 
     In certain implementations, the cross-section  130  corresponds to that of a semiconductor die fabricated using a silicon process, such as a complementary metal oxide semiconductor (CMOS) process, a silicon-on-insulator process, or a silicon germanium process. However, other implementations are possible. 
       FIG. 4B  is a schematic diagram of a cross-section  140  of portion of a semiconductor die according to another embodiment. The cross-section  140  depicts a portion of the die&#39;s metallization layers, including a first conductive layer  111 , a second conductive layer  112 , a third metallization layer  113 , a first dielectric layer  115 , and a second dielectric layer  116 . The first dielectric layer  115  is between the first conductive layer  111  and the second conductive layer  112 , and the second dielectric layer  116  is between the second conductive layer  112  and the third conductive layer  113 . The metallization layers are formed over a die substrate  110 , and separated therefrom by another dielectric layer  117 . 
     As shown in  FIG. 4B , the first conductive layer  111  includes a ground plane  120 , the second conductive layer  112  has been patterned to include a first transmission line  121 , and the third conductive layer  113  has been patterned to include a second transmission line  122 . 
     Although  FIG. 4B  depicts a cross-section, persons of ordinary skill will appreciate that the first transmission line  121  and the second transmission line  122  have lengths extending at least in part in a direction perpendicular to the cross-section  140 . For example, the first transmission line  121  and the second transmission line  122  can have coil or spiral shapes when viewed from above. 
     The first transmission line  121  and the second transmission line  122  illustrate one implementation of the first transmission line  11  and the second transmission line  12 , respectively (or vice versa), of the wideband baluns of  FIGS. 1A-3C . In the illustrated embodiment, the first transmission line  121  and the second transmission line  122  are broadside-coupled. Although not illustrated in  FIG. 4B , one or more additional transmission lines (for instance, the third transmission line  13  and/or the fourth transmission line  14 ) can be implemented using the metallization layers. 
     In certain implementations, the cross-section  140  corresponds to that of a semiconductor die fabricated using a silicon process, such as a CMOS process, a silicon-on-insulator process, or a silicon germanium process. However, other implementations are possible. 
       FIG. 5A  is a schematic diagram of a cross-section  150  of portion of a semiconductor die according to another embodiment. The cross-section  150  depicts a portion of the die&#39;s metallization layers, including a first conductive layer  111 , a second conductive layer  112 , a backside metallization layer  114 , and a dielectric layer  115 . The dielectric layer  115  is between the first conductive layer  111  and the second conductive layer  112 . The first and second metallization layers are formed over a first side of a die substrate  110 , while the backside metallization layer  114  is formed over a second side of the die substrate  110 . 
     In the illustrated embodiment, the backside metallization layer  114  includes a ground plane  120 . Additionally, the first conductive layer  111  has been patterned to include a first transmission line  121 , and the second conductive layer  112  has been patterned to include a second transmission line  122 . Additionally dielectric  118  is included where portions of the first conductive layer  111  were patterned. 
     Although  FIG. 5A  depicts a cross-section, persons of ordinary skill will appreciate that the first transmission line  121  and the second transmission line  122  have lengths extending at least in part in a direction perpendicular to the cross-section  150 . For example, the first transmission line  121  and the second transmission line  122  can have coil or spiral shapes when viewed from above. 
     The first transmission line  121  and the second transmission line  122  illustrate one implementation of the first transmission line  11  and the second transmission line  12 , respectively (or vice versa), of the wideband baluns of  FIGS. 1A-3C . In the illustrated embodiment, the first transmission line  121  and the second transmission line  122  are broadside-coupled. Although not illustrated in  FIG. 5A , one or more additional transmission lines (for instance, the third transmission line  13  and/or the fourth transmission line  14 ) can be implemented using the metallization layers. 
     In certain implementations, the cross-section  150  corresponds to that of a semiconductor die fabricated using a compound semiconductor process, such as gallium arsenide. However, other implementations are possible. 
       FIG. 5B  is a schematic diagram of a cross-section  160  of portion of a semiconductor die according to another embodiment. The cross-section  160  depicts a portion of the die&#39;s metallization layers, including a first conductive layer  111 , a second conductive layer  112 , a backside metallization layer  114 , and a dielectric layer  115 . The dielectric layer  115  is between the first conductive layer  111  and the second conductive layer  112 . The first and second metallization layers are formed over a first side of a die substrate  110 , while the backside metallization layer  114  is formed over a second side of the die substrate  110 . 
     As shown in  FIG. 5B , the backside metallization layer  114  includes a ground plane  120 . Additionally, a first portion of the first conductive layer  111  and a first portion of the second conductive layer  112  have been patterned and interconnected with a first via  125   a  to form a first transmission line  121 . Furthermore, a second portion of the first conductive layer  111  and a second portion of the second conductive layer  112  have been patterned and interconnected with a second via  125   b  to form a second transmission line  122 . Furthermore, dielectric  118  is included where portions of the first conductive layer  111  were patterned. 
     Although  FIG. 5B  depicts a cross-section, persons of ordinary skill will appreciate that the first transmission line  121  and the second transmission line  122  have lengths extending at least in part in a direction perpendicular to the cross-section  160 . For example, the first transmission line  121  and the second transmission line  122  can have coil or spiral shapes when viewed from above. 
     The first transmission line  121  and the second transmission line  122  illustrate one implementation of the first transmission line  11  and the second transmission line  12 , respectively (or vice versa), of the wideband baluns of  FIGS. 1A-3C . In the illustrated embodiment, the first transmission line  121  and the second transmission line  122  are edge-coupled on two metallization layers. Although not illustrated in  FIG. 5B , one or more additional transmission lines (for instance, the third transmission line  13  and/or the fourth transmission line  14 ) can be implemented using the metallization layers. 
     In certain implementations, the cross-section  160  corresponds to that of a semiconductor die fabricated using a compound semiconductor process, such as gallium arsenide. However, other implementations are possible. 
       FIG. 6A  is a graph of amplitude versus frequency plots for two examples of baluns. The graph includes a first plot  201  for one implementation of the wideband balun  20  of  FIG. 1A  in which the first transmission line  11  and the second transmission line  12  are edge-coupled. The graph further includes a second plot  202  for another balun implementation in which the fourth transmission line  14  of  FIG. 1A  is omitted. 
     As shown in  FIG. 6A , including the fourth transmission line  14  of  FIG. 1A  improves amplitude balance. 
       FIG. 6B  is a graph of phase versus frequency plots for two examples of baluns. The graph includes a first plot  203  for one implementation of the wideband balun  20  of  FIG. 1A  in which the first transmission line  11  and the second transmission line  12  are edge-coupled. The graph further includes a second plot  204  for another implementation in which the fourth transmission line  14  of  FIG. 1A  is omitted. 
     As shown in  FIG. 6B , including the fourth transmission line  14  of  FIG. 1A  improves phase balance. 
       FIG. 7A  is a schematic diagram of a push-pull amplifier  310  according to one embodiment. The push-pull amplifier  310  includes a first balun  301 , a second balun  302 , a first amplification circuit  305 , and a second amplification circuit  306 . 
     As shown in  FIG. 7A , the first port of the first balun  301  receives a single-ended input signal, while the fourth port of the first balun  301  is grounded. Additionally, the first balun  301  operates to generate a differential input signal across the balun&#39;s third port and second port. The first amplifier  305  and second amplifier  306  amplify the differential input signal to generate a differential output signal that is applied across the third port and the second port of the second balun  302 . Additionally, the fourth port of the second balun  302  is grounded, while the first port of the second balun  302  outputs a single-ended output signal. 
     As persons of ordinary skill in the art will appreciate, the first amplifier  305  and the second amplifier  306  operate in a push-pull configuration. Thus, even-order harmonics are rejected and distortion is lowered. 
     The push-pull amplifier  310  illustrates one embodiment of an electronic system that can include one or more baluns implemented in accordance with the teachings herein. For example, the first balun  301  and/or the second balun  302  can be implemented in accordance with any of the wideband baluns herein. By using wideband baluns in the push-pull amplifier  310 , phase/amplitude balance is enhanced, thereby providing high OIP2 and/or HD2. 
       FIG. 7B  is a schematic diagram of a single balanced mixer  320  according to one embodiment. The single balanced mixer  320  includes a balun  301  and a mixer  315 . 
     As shown in  FIG. 7B , the first port of the balun  301  receives a single-ended local oscillator (LO) signal, while the fourth port of the balun  301  is grounded. Additionally, the balun  301  generates a differential LO signal between the third port and the second port. The differential LO signal is provided to a differential input of the mixer  315 . 
     The mixer  315  further receives an intermediate frequency (IF) signal and generates a radio frequency (RF) signal, or vice versa. For example, the IF signal can be received and the RF signal generated when the single balanced mixer  320  is providing frequency upconversion, while the RF signal can be received and the IF signal generated when the single balanced mixer  320  is providing frequency downconversion. 
     The single balanced mixer  320  illustrates another embodiment of an electronic system implemented in accordance with the teachings herein. For example, the balun  301  can be implemented in accordance with any of the wideband baluns of  FIGS. 1A-3C . Although  FIG. 7B  illustrates another embodiment of an electronic system, the wideband baluns disclosed herein can be used in a wide range of electronic systems. 
       FIG. 7C  is a schematic diagram of a double balanced mixer  330  according to one embodiment. The double balanced mixer  330  includes a first balun  301 , a second balun  302 , and a mixer  325 . 
     As shown in  FIG. 7C , the first port of the first balun  301  receives a single-ended LO signal, while the fourth port of the first balun  301  is grounded. Additionally, the first balun  301  generates a differential LO signal between the balun&#39;s third port and second port, and provides the differential LO signal to a differential input of the mixer  325 . 
     When providing frequency upconversion, the mixer  325  receives an IF signal, and generates a differential RF signal that is provided across the third port and the second port of the second balun  302 . Additionally, the fourth port of the second balun  302  is grounded, and the first port of the second balun  302  outputs a single-ended RF signal. 
     When providing frequency downconversion, the second balun  302  receives the single-ended RF signal at the first port, and generates the differential RF signal across the balun&#39;s third port and second port. Additionally, the mixer  325  receives the differential RF signal, and generates the IF signal. 
     The double balanced mixer  330  illustrates another embodiment of an electronic system implemented in accordance with the teachings herein. For example, the first balun  301  and/or the balun  302  can be implemented in accordance with any of the wideband baluns of  FIGS. 1A-3C . Although  FIG. 7C  illustrates another embodiment of an electronic system, the wideband baluns disclosed herein can be used in a wide range of electronic systems. 
       FIG. 7D  is a schematic diagram of a double balanced mixer  340  according to another embodiment. The double balanced mixer  340  includes a first balun  301 , a second balun  302 , a third balun  303 , and a mixer  335 . 
     The double balanced mixer  340  of  FIG. 7D  is similar to the double balanced mixer  330  of  FIG. 7C , except that the double balanced mixer  340  of  FIG. 7D  further includes the third balun  303  to provide single-ended to differential signal conversion of the IF signal. Thus, when providing frequency upconversion, a single-ended IF signal is provided to the first port of the third balun  303  while the balun&#39;s fourth port is grounded, and a differential IF signal (IF+/IF−) is generated across the balun&#39;s third port and second port. Additionally, when providing frequency downconversion, the mixer  335  provides the differential IF signal across the third port and the second port of the third balun  303 , and the first port of the third balun  303  outputs the signal-ended IF signal. 
     The double balanced mixer  340  illustrates another embodiment of an electronic system implemented in accordance with the teachings herein. For example, the first balun  301 , the balun  302 , and/or the third balun  303  can be implemented in accordance with any of the wideband baluns of  FIGS. 1A-3C . Although  FIG. 7D  illustrates another embodiment of an electronic system, the wideband baluns disclosed herein can be used in a wide range of electronic systems. 
     Applications 
     Devices employing the above described schemes can be implemented into various electronic systems. Examples of the electronic systems can include, but are not limited to, consumer electronic products, parts of the consumer electronic products, electronic test equipment, communication infrastructure applications, etc. Further, the electronic systems can include unfinished products, including those for communication, industrial, medical and automotive applications. 
     Conclusion 
     The foregoing description may refer to elements or features as being “connected” or “coupled” together. As used herein, unless expressly stated otherwise, “connected” means that one element/feature is directly or indirectly connected to another element/feature, and not necessarily mechanically. Likewise, unless expressly stated otherwise, “coupled” means that one element/feature is directly or indirectly coupled to another element/feature, and not necessarily mechanically. Thus, although the various schematics shown in the figures depict example arrangements of elements and components, additional intervening elements, devices, features, or components may be present in an actual embodiment (assuming that the functionality of the depicted circuits is not adversely affected). 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosure. Indeed, the novel apparatus, methods, and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the disclosure. For example, while the disclosed embodiments are presented in a given arrangement, alternative embodiments may perform similar functionalities with different components and/or circuit topologies, and some elements may be deleted, moved, added, subdivided, combined, and/or modified. Each of these elements may be implemented in a variety of different ways. Any suitable combination of the elements and acts of the various embodiments described above can be combined to provide further embodiments. Accordingly, the scope of the present invention is defined only by reference to the appended claims. 
     Although the claims presented here are in single dependency format for filing at the USPTO, it is to be understood that any claim may depend on any preceding claim of the same type except when that is clearly not technically feasible.