Abstract:
A bond wire transformer comprises a plurality of primary bond wires coupled in parallel; and a plurality of secondary bond wires coupled in parallel, each secondary bond wire being spaced apart from and oriented relative to a corresponding primary bond wire so as to achieve a desired mutual inductance between the corresponding primary and secondary bond wires, thereby providing magnetic coupling between the primary and secondary bond wires.

Description:
[0001]    This application claims the priority under 35 U.S.C. §119 of European patent application no. 10193629.2, filed on Dec. 3, 2010, and patent application no. 10194851.1, filed on Dec. 14, 2010, the contents of which are incorporated by reference herein. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a transformer in which the primary and the secondary windings are made from bond wires. 
       BACKGROUND OF THE INVENTION 
       [0003]    Many RF circuits and systems employ inductive components such as inductors and transformers. A few examples of these circuits are: voltage-controlled oscillators, low-noise amplifiers, matching networks, power combiners and power amplifiers. 
         [0004]    Integrated inductors and transformers are known to have low quality (given by figures of merit such as the quality factor or Q, and the self-resonance frequency) due to semiconductor process limitations. Still, the limited quality of integrated passives can be tolerated in low-power applications. In high-power applications (for example, a base station), the use of such integrated passives is very rarely employed in the high-power circuitry due to their inefficiency. 
         [0005]    Another approach to design inductors and transformers is by using standard bond wire fabrication techniques, the same techniques used for interconnections and package leads. Bond wire inductors and transformers avoid many of the technological limitations of standard integrated planar technologies. For example, they have reduced resistive and substrate losses (and thus, increased quality factors), reduced capacitive parasitics (and hence increased self-resonance frequencies), and can handle substantially higher currents. 
         [0006]    One of the major challenges and concerns for the use of inductive components fabricated from bond wires is the relatively low predictability of their specific values, due to magnetic interactions with the surrounding media and manufacturing shape variations. Experimentation can lead to a bond wire shape that yields the expected performance and this shape can be easily replicated, and the use of automatic wire bonding machines with loop control capability can control the manufacturing yield and tolerance of a designed bond wire shape. 
         [0007]    A common problem with state-of-the-art bond wire transformers is the complexity of arranging the many series connections required to fabricate the primary and secondary. The series connections are required to achieve the desired turns ratio, each loop of bond wire representing a turn. It can be inefficient in terms of space to provide sufficient bond pads for each series connection, and making each of the connections can be costly. Additionally, such series connection of the loops will increase the resistive losses of the structures, leading to lower power efficiency. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with a first aspect of the invention, there is provided a bond wire transformer comprising a plurality of primary bond wires coupled in parallel; and a plurality of secondary bond wires coupled in parallel, each secondary bond wire being spaced apart from and oriented relative to a corresponding primary bond wire so as to achieve a desired mutual inductance between the corresponding primary and secondary bond wires, thereby providing magnetic coupling between the primary and secondary bond wires. 
         [0009]    The invention exploits the fact that the coupling factor and transformer ratio, whilst being influenced by the number of turns or “loops” of the transformer windings, are defined ultimately by general geometrical properties such as the separation between the windings and their cross-sectional areas. This means that a good transformer does not necessarily require multi-loop windings; in fact, a single loop (or half-loop) can provide good magnetic coupling. 
         [0010]    Preferably, the number of primary bond wires is the same as the number of secondary bond wires. Alternatively, the number of primary bond wires may be approximately the same as the number of secondary bond wires, for example one fewer or one more. 
         [0011]    Typically, each of the primary bond wires is connected between first and second common bonding pads. 
         [0012]    Alternatively, the primary bond wires may be connected to nodes of a circuit such that the signals flowing in each of the primary bond wires are substantially the same. 
         [0013]    Thus, the primary bond wires are electrically connected in parallel either by a direct physical connection or by a “virtual” connection in which equivalent signals flow in each bond wire (that is the same current is caused to flow in each wire simultaneously). 
         [0014]    Similarly, each of the secondary bond wires may be connected between first and second common bonding pads to form a first section of the secondary bond wire. 
         [0015]    The resulting bond wire transformer may further comprise a third common bonding pad, each of the secondary bond wires extending from the second common bonding pad to the third common bonding pad to form a second section of the secondary bond wire, whereby each of the first and second sections contribute to achieving the desired mutual inductance between the corresponding primary and secondary bond wires. 
         [0016]    Thus, whilst it is envisaged that each primary bond wire will preferably have only a single loop or turn, each secondary bond wire may comprise multiple loops or turns. Multiple loops or turns (formed still with paralleled bond wires) at the secondary will enhance the properties of the transformer due to an increase of the transformer turns ratio, which is beneficial from a bandwidth point of view for impedance transformation and other relevant applications. 
         [0017]    The secondary bond wires may be connected to nodes of a circuit such that the signals flowing in each of the secondary bond wires are substantially the same. 
         [0018]    In accordance with a second aspect of the invention, there is provided an outphasing amplifier system comprising first and second power amplifiers and first and second bond wire transformers according to the first aspect of the invention, each of the first and second bond wire transformers being connected to a respective one of the first and second power amplifiers such that together the first and second bond wire transformers combine the power generated by the first and second power amplifiers. 
         [0019]    In this outphasing amplifier, the secondary bond wires of the first and second bond wire transformers may be coupled together by a track that is adapted to have a desired capacitance to ground for use as a compensating element (for example, so-called Chireix compensation) in the outphasing amplifier system. 
         [0020]    In one embodiment, the first bond wire transformer has a magnetising inductance equal to the parallel combination of a dc-feed inductance coupling a power supply to the first and/or second power amplifier and a compensating inductance. This results in a different bond wire shapes in the first and second transformer because the magnetising inductance of the bond wires depends on the number of parallel bond wires and their specific shape. 
         [0021]    In another embodiment, the first and second bond wire transformers are identical and have their primary bond wires coupled in parallel, the primary bond wires of each bond wire transformer having a magnetising inductance equal to twice the DC-feed inductance required by each of the power amplifiers, the secondary bond wires of each bond wire transformer being coupled to a respective one of the first and second power amplifiers, and the outphasing system further comprises a compensating inductor coupling a power supply to the first power amplifier and having a value equal to the parallel combination of the DC-feed inductance required by the first power amplifier and an outphasing compensating inductance. 
         [0022]    In accordance with a third aspect of the invention, there is provided a balun comprising a bond wire transformer according to the first aspect of the invention, wherein the primary bond wires extend from a positive terminal to a common centre tap terminal and from the centre tap terminal to a negative terminal, and the secondary bond wires extend between common first and second terminals. 
         [0023]    In accordance with a fourth aspect of the invention, there is provided a power combiner comprising first and second bond wire transformers according to the first aspect of the invention, wherein each of the primary bond wires of the first transformer is connected between first and second bonding bars; each of the primary bond wires of the second transformer is connected between a third bonding bar and the second bonding bar; each of the secondary bond wires of the first transformer is connected between fourth and fifth bonding bars; and each of the secondary bond wires of the second transformer is connected between a sixth bonding bar and the fifth bonding bar. 
         [0024]    The primary and/or secondary bond wires of the first transformer ( 10   a ) and second transformer ( 11   a ) may be connected to nodes of a circuit such that the signals flowing in each of the primary and/or secondary bond wires are substantially the same. 
         [0025]    In accordance with a fifth aspect of the invention, there is provided a power amplifier system, such as the outphasing amplifier system according to the second aspect of the invention, comprising first and second transformers arranged to form a power combiner according to the fourth aspect of the invention. 
         [0026]    Thus, in the outphasing amplifier system according to the second aspect of the invention, the first and second transformers may be arranged to form a power combiner according to the fourth aspect of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Examples of the invention will now be described in detail with reference to the accompanying drawings, in which: 
           [0028]      FIG. 1  shows a perspective view of one possible implementation of a bond wire transformer according to the invention; 
           [0029]      FIGS. 2   a ,  2   b  and  2   c  show, schematically, plan and side views of possible implementations of bond wire transformers according to the invention (including that of  FIG. 1 ); 
           [0030]      FIGS. 3   a  and  3   b  show effective equivalent circuits for the bond wire transformers according to the invention; 
           [0031]      FIG. 4  shows a circuit diagram of part of an outphasing amplifier system using a transformer-based combiner according to the invention; 
           [0032]      FIG. 5  shows how the transformer-based combiner and the so-called Chireix compensation elements (a compensating capacitor and a compensating inductor) may be integrated altogether; 
           [0033]      FIG. 6  shows a circuit diagram of part of another outphasing amplifier system using a transformer according to the invention; 
           [0034]      FIG. 7  shows a balun fabricated using a transformer according to the invention; and 
           [0035]      FIG. 8  shows a power combiner comprising two bond wire transformers according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0036]      FIG. 1  shows a perspective view of a transformer fabricated from bond wires. A first set of primary bond wires  1  is connected in parallel between metal bonding bars  2  and  3 . Similarly, a second set of secondary bond wires  4  is connected in parallel between metal bonding bars  5  and  6 . The primary  1  and secondary  4  bond wires interdigitate as they span the gaps between their respective bonding bars. Thus, each individual pair of adjacent primary and secondary bond wires form a transformer due to the magnetic coupling between them. However, since the bond wires  1  and  4  are electrically connected in parallel through the bonding bars  2 ,  3 ,  5  and  6 , the transformer effect of each pair of bond wires is combined. It should be understood that the use of metal bonding bars  2 ,  3 ,  5  and  6  is just one possible way to create the parallel connection of the primary  1  and secondary  4  bond wires. There are other possibilities such as, but not limited to, bonding pads on transistor and capacitor dies, PCB metal traces, and the like. 
         [0037]      FIG. 2   a  shows schematically a plan view and side view of the arrangement of  FIG. 1 . Also shown is a second side view where, instead of the bond wires  1  and  4  interdigitating as in  FIG. 1 , the primary bond wires  1  span over the secondary bond wires  4 . As can be seen from the second side view, bond wires  1  can lie in the same plane as bond wires  4 , leading to a smaller pitch and hence increased magnetic coupling factor. 
         [0038]      FIG. 2   b  shows a variant of this scheme in which the secondary bond wires are made up of several sections connected in series. The primary bond wires  1  still extend between bonding bars  2  and  3 . However, the secondary bond wires comprise a first section  4   a  extending from bonding bar  5  to bonding bar  7   a ; a second section  4   b  extending from bonding bar  7   a  to bonding bar  7   b ; and a third section  4   c  extending from bonding bar  7   b  to bonding bar  6 . As can be seen from the side view, bond wire  1  can lie in the same plane as bond wire section  4   a  and bond wire section  4   b  can lie in the same plane as bond wire section  4   c . The bond wire sections  4   a ,  4   b  and  4   c  are designed to maximize the cross-sectional area of the secondary bond wires in order to enhance the magnetic flux and hence provide better magnetic coupling. 
         [0039]      FIG. 2   c  shows another variant, similar to the one of  FIG. 2   b , in which the parallel secondary loops are made up of several bond wire sections and conductive traces on a substrate connected in series. The primary bond wires  1  still extend between bonding bars  2  and  3 . However, the secondary bond loops comprise a first bond wire section  4   a  extending from bonding bar  5  to one edge of conductive trace  4   b ; a second section is the conductive trace  4   b  which is on top of substrate  8 ; and a third section is the bond wire  4   c  extending from another edge of conductive trace  4   b  to bonding bar  6 . As can be seen from the side view, bond wire  1  can lie in the same plane as bond wire section  4   a . The secondary winding sections  4   a ,  4   b  and  4   c  are designed to maximize the cross-sectional area of the secondary loop in order to enhance the magnetic flux and hence provide better magnetic coupling. Also, the conductive traces  4   b  can all be separated as in  FIG. 2   c , or joined together in a single conductive plate. 
         [0040]    In  FIGS. 1 ,  2   a ,  2   b  and  2   c  the primary and secondary bond wires  1  and  4  (including the conductive trace  4   b  of  FIG. 2   c ) are connected electrically in parallel by direct connection through the metal bonding bars  2 ,  3 ,  5 ,  6 ,  7   a  and  7   b . This is illustrated in  FIG. 3   a , along with the equivalent model of the resulting transformer, having a primary winding with an inductance L p  and a secondary winding with an inductance L s . The coupling between the windings is shown by the coupling factor k m . By coupling many bond wires in parallel, the currents are distributed uniformly and the resistive losses are greatly reduced. The overall coupling factor k m  is increased and the inductance L p , L s  of each winding reduced. 
         [0041]    In  FIG. 3   b , a second way of connecting the bond wires  1  in parallel is shown along with the equivalent circuit. In this way, one end of each bond wire is connected to a ground terminal (which can be an AC- or DC-ground) and the other is connected to a respective transistor. If the transistors are identical and fed with the same signal then the same signal will be present in the bond wires  1 . It is equivalent to physically connecting them in parallel. 
         [0042]    The number of parallel bond wires is arbitrary, but the preference is for a number close to the maximum number of possible structures for a given application. For example, if transistor dies are connected to one of the transformer terminals, then the maximum number of bond wire pairs is determined by the number of transistor die bond pads available (spaced by the same pitch as the primary bond wires  1 ). If transmission line-like metal structures are used, then this maximum number will be determined by the transmission line&#39;s physical width. The number of parallel bond wires as well as their shape will determine the overall self inductance of each winding of the transformer, and those values can be designed according to the application needs. 
         [0043]    An example of how the parallel bond wire transformer may be set out for use with large transistor dies (as in high RF power transistors) is as follows. If the minimum bond wire pitch is 150 μm (determined by the bonding machines) then the minimum pitch for each winding of the transformer is 300 μm. Assuming the use of a transistor die with 2.4 mm of drain bond pads (continuous or not), the primary winding can contain a maximum of 2.4 mm/300 μm=8 parallel bond wire pairs. The secondary winding can have a number close to 8 parallel bondwire sets (for example, 7 or 9), where each of the bond wire pairs would go in parallel planes spaced 150 μm from the primary bond wire planes. 
         [0044]    The bond wire transformers described above enable the implementation of high-frequency, high-quality and high-power lumped transformers. The quality of these transformers is much greater (for example, quality factors in excess of 100 at 2.14 GHz) than the quality of standard integrated planar transformers (which typically have quality factors in the order of 10-15 at the same frequency range) or dedicated IPD transformers (with typical quality factors in the order of 25). Also, the bond wire transformer is a low cost solution which is compatible with standard manufacturing of current RF systems. 
         [0045]    One particularly useful application of the bond wire transformer described above is in outphasing amplifiers. An outphasing amplifier produces an amplitude modulated signal by combining the outputs of two power amplifiers driven with signals of different time-varying phases. The phase modulation causes the instantaneous vector sum of the two power amplifier outputs to follow the desired input signal amplitude, allowing amplitude and phase information to be amplified. The outputs of such power amplifiers are equal in amplitude (i.e., constant envelope), and hence high efficiency amplifiers can be employed. This eliminates the linearity requirements of the branch amplifiers, which can operate in a high-efficiency mode (e.g., in a non-linear power amplifier class, such as a switch-mode class). 
         [0046]    A key innovation in outphasing amplifiers was to realize that time-varying loads were seen by the individual amplifiers instead of a constant load. Each of the equivalent power amplifier loads was a function of the outphasing angle (which is a function of the time-varying input signal). This time-dependency in the power amplifier loads results in a reduction of the “power factor” (or, equivalently, the “instantaneous drain/collector efficiency”) as the output power is reduced. This effect could be represented electrically by shunt reactive lumped elements in front of each power amplifier: an inductive element for one of the power amplifiers and a capacitive element for the other. It is possible to compensate for this undesired effect by placing opposite susceptances in shunt with the output of each amplifier branch. Those compensating susceptances are the so-called Chireix compensating elements. Ideally, this compensation can effectively maintain each-branch-PA efficiency high for as much as 10 dB below peak power, approximately, which is still reasonably good for the modern communication standards. 
         [0047]    Most modern implementations of outphasing amplifier employ quarter wavelength transmission lines for the implementation of the output combiner. This limits the frequency performance of an outphasing amplifier. By employing the bond wire transformer presented above, the lack of high-frequency, high-power and high-efficiency transformers is overcome. Thus, high-power broadband outphasing amplifiers can now be implemented using these transformers. 
         [0048]    Such an outphasing amplifier can be implemented as shown in  FIGS. 4 and 5  using the bond wire transformers discussed above. Two transformers  10 ,  11  of the type shown in  FIG. 1  implement the core of the output combiner. The magnetizing inductance of the transformers  10 ,  11  is initially designed such as to implement the DC-feed inductance of the power amplifiers. Thus, the two primary windings  10   a ,  11   a  of the transformers  10 ,  11  provide a DC path to couple the supply voltage V DD  to each of the two amplifiers M 1 , M 2 . The power amplifiers M 1 , M 2  are Class-E power amplifiers. For Class-E power amplifiers, the use of finite DC-feed inductances (provided by the primary windings  10   a ,  11   a ) to the power amplifiers M 1 , M 2  has advantages in terms of output power, maximum frequency of operation and results in a higher load resistance than the classical RF-choke class-E power amplifier configuration. 
         [0049]    In the outphasing system of  FIGS. 4 and 5 , the compensating elements referred to above can be placed at either the primary or secondary side of each transformer  10 ,  11 . In  FIG. 4 , it can be observed that the capacitive compensating element  12  is placed at the secondary side of the transformer  11 , while the inductive element  13  is placed at the primary side of transformer  10 . By doing this, both compensating elements  12 ,  13  can be absorbed by the bond wire transformer-based combiner implementation. This is shown in  FIG. 5 . In this, the transformer windings  10   a ,  10   b ,  11   a  and  11   b  are clearly visible as parallel bond wires, as in  FIG. 1 . Transformer  10  is finally designed to have a magnetizing inductance equal to the parallel combination of the DC-feed inductance (L E ) and the compensating inductance  13  (L θ2 ). In practice, this represents a small variation in shape of transformer  10  with respect to the other transformer  11 . The capacitive compensating element  12  can be implemented by the parasitic capacitance to ground of the metal track  14  that connects both transformers  10  and  11 . The parasitic inductive elements of the transformers  10 ,  11  (i.e., the leakage inductances) and the metal track  14  are absorbed into the output matching network. 
         [0050]    The outphasing system described here is power scalable due to the bond wire transformer concept. Since the high-power RF transistors used in an outphasing amplifier are manufactured as folded transistors, the number of transistors (and bond pads) increases linearly with the rating power of the transistor. The magnetizing inductance of the bond wire transformer reduces with an increased number of parallel bond wire sets (whose number is proportional to the bond pads available, and hence it is proportional to the power rating of the transistors). To maintain a given Class-E operation, if the transistor&#39;s output parasitic capacitance increases for higher power systems, then the DC-feed inductance must reduce proportionately. 
         [0051]    An important advantage of the outphasing system of  FIGS. 4 and 5  is that the transformers  10  and  11  do not require a unitary transformer ratio (i.e., compensating the non-unitary coupling factor with the primary to secondary inductance ratio is not required). The higher the magnetic coupling factor, the more broadband the performance of the combiner will be. Additionally, by controlling the duty-cycle of the driving signal of the output stage (by any means) the performance of the outphasing system can be enhanced at several frequencies (i.e., the outphasing performance can be optimized as a function of frequency band) and at the specific operating frequency (i.e., by fine tuning of the performance due to possible manufacturing deviations of the bond wire transformer combiner). 
         [0052]    Another version of outphasing system is shown in  FIG. 6 . The class-E inductance L E  in the first branch is set by the magnetizing inductances of both transformers  20 ,  21  in parallel. The capacitive compensating element  22  (C θ ) for the first branch is set by the capacitance that is provided by a track connecting the transformers  20 ,  21  as in the versions of  FIGS. 4 and 5 . The class-E DC-feed inductance (L E ) in the second branch is an additional inductance element. The inductive compensating element  23  (L θ ) for the second branch is connected in parallel to the second L E , giving a single effective inductance. The output consists of a multi-stage LC matching network that incorporates the transformers&#39;  20 ,  21  leakage inductances. The additional inductance element  23  could be shifted to the output side. 
         [0053]    The outphasing systems described above offer enhanced power efficiency with respect to state-of-the-art outphasing systems that are specially targeted for base-station applications. Also, it overcomes the problem of power scalability, enabling the design of above-100 W implementations in a reduced space. Because of the use of lumped transformer elements in its implementation, the frequency bandwidth is also increased which is useful for multi-band amplifiers. Furthermore, the magnetising inductance of the two transformers in the combiner enable a straightforward and easy way of implementing the Class-E tuning condition for both power amplifiers in the outphasing systems. 
         [0054]    The bond wire transformers described above make use of conductive pads in the substrate on which they are built for connection of the bond wires. These pads can be arranged in different ways depending on the specific application, providing means to have the winding terminals at any side of the structure if required. As an example of how this can be employed,  FIG. 7  shows a balun/power combiner which can be used for differential driven power amplifiers for wideband operation. 
         [0055]    In this, the primary bond wires  30   a ,  30   b  are connected between a positive terminal  31   a  and a centre tap  31   c  and between a negative terminal  31   b  and the centre tap  31   c  respectively. The secondary bond wires  32  are connected between terminals  33   a  and  33   b.    
         [0056]    The bond wire transformer described above can be implemented using standard bond wire processes, techniques and equipment. This makes it a low cost solution, compatible with current fabrication of RF circuit and systems. Simulation results have shown that such transformers can achieve reasonably good magnetic coupling with a broad range of possible inductance values. Furthermore, a quality factor in excess of 100 at 2.14 GHz can be achieved. 
         [0057]    In applications where power transfer is the main concern, the transformer efficiency can be defined as the ratio of the power delivered to the load to the input power. For a passive structure, this figure is always less than 1 and is ideally 1 for a lossless passive two-port structure. It has been found that transformers according to this invention can achieve a figure of around 0.97, which indicates a very good power efficiency. 
         [0058]    Applications of the invention are (reconfigurable) transmitters for connectivity and cellular applications, where the modulation standards with high peak-to-average ration (PAR) require the power amplifier to be efficient over a large dynamic range and frequency range. These transmitters are useful for systems in which wide-band complex envelope signals are processed, such as: multi-carrier GSM/EDGE, WCDMA, HSxPA, WiMAX (OFDM) and 3G-LTE (OFDM). 
         [0059]      FIG. 8  shows a power combiner (both schematically and in perspective view) formed from two bond wire transformers of the type shown in  FIG. 1 . In this, the first transformer  10  comprises a set of primary bond wires  10   a , which are connected between a first bonding pad  40   a  and a second bonding pad  41 . The primary bond wires  11   a  of the second transformer  11  are connected from the second bonding pad  41  to a third bonding pad  40   b . The secondary bond wires  10   b  of the first transformer  10  are connected between a fourth bonding pad  42   a  and a fifth bonding pad  43 , and the secondary bond wires  11   b  of the second transformer  11  area connected between the fifth bonding pad  43  and a sixth bonding pad  42   b.    
         [0060]    As is shown in the perspective view, the second and fifth bonding bars  41 ,  43  are provided by metallic tracks to which the bond wires can be connected. This is similar to the track  14  shown in  FIG. 5 . The tracks  41 ,  43  may be designed to provide a desired capacitance to ground, although they do not necessarily need to be. 
         [0061]    The power combiner of  FIG. 8  may be used to provide the two transformers required in the outphasing amplifier system described above with reference to  FIGS. 4 to 6 . It may also be used more generally as a power combiner with other types of power amplifier systems (i.e. not necessarily outphasing amplifiers). 
         [0062]    Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.