Abstract:
The present invention is directed at an inductor which is capable of providing a variable inductance. The variable inductor is typically mounted/stored on an integrated circuit chip to provide continuous or multiple variable inductor values for wireless applications and the like.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to integrated circuits. More particularly, the present invention relates to a variable inductor for an integrated circuit. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the field of integrated circuits, technology is rapidly improving and new methods of implementing analog circuit blocks on integrated circuits are being realized. Furthermore, components of these analog circuit blocks are consistently being upgraded to improve their operational capacity. One such component is a variable inductor. Various examples of variable inductors are currently known and described in US Patent Application No. 2004/0140528 entitled “STACKED VARIABLE INDUCTOR” which was published on Jul. 22, 2004 and US Patent Application No. 2005/0068146 entitled “VARIABLE INDUCTOR FOR INTEGRATED CIRCUIT AND PRINTED CIRCUIT BOARD” which was published on Mar. 31, 2005, 
         [0003]      FIG. 1  shows a prior art embodiment of a variable inductor  10 . The variable inductor  10  is formed with a primary conductor  12 , a secondary conductor  14 , and a switch  16 . Primary conductor  12  implements a three-port inductor and is formed in a double spiral pattern. The primary conductor  12  is fabricated almost entirely on a low-loss metal layer (e.g. copper) except for one underpass  20  used to interconnect the two sections of the primary conductor  12 . Interconnects  22   a  and  22   b  of primary conductor  12  form two ports of the inductor and are not routed on an underlayer in order to achieve low-loss. A ‘tap’ pin forms the third port of the inductor and is provided with a power supply voltage, which is used by circuit components coupled to primary conductor  12 . 
         [0004]    Secondary conductor  14  is formed on the outside of, at a distance away from, the double spiral of primary conductor  12 . To attain low resistance, secondary conductor  14  is also fabricated almost entirely on the low-loss metal layer. Secondary conductor  14  is coupled in series with switch  16  and forms a loop  26  that is concentric with the double spiral for primary conductor  12 . The switch  16  functions to either open or close the loop and can be placed anywhere on the loop. However, since an underpass is needed to interconnect the two ends of secondary conductor  14 , switch  16  is fabricated on an underlayer and between the two interconnects  22   a  and  22   b  for primary conductor  12 , as shown. 
         [0005]    However, many current variable inductors, such as the one shown in  FIG. 1 , can not be used as a continuous variable inductor since they are not connected to the primary inductor and therefore the range of inductance values provided by the inductor is very narrow. 
         [0006]    Furthermore, some current variable inductors operate at a low quality (Q) factor which affects the overall operation of the analog circuit and ultimately the integrated circuit. 
         [0007]    It is, therefore, desirable to provide a variable inductor which overcomes some of the disadvantages of the prior art. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is directed at an inductor which is capable of providing a variable inductance. The variable inductor is typically mounted/stored on an integrated circuit chip to provide a continuous multiple-value variable inductor for wireless applications and the like. 
         [0009]    In one embodiment, the variable inductor includes a primary conductor which is surrounded by a secondary conductor. The conductors are operatively connected by a set of switches. When the switches are opened, the variable inductor provides a first inductance and when the switches are closed, the variable inductor provides a second inductance. By operatively connecting the primary and secondary conductors via the set of switches, the invention provides a higher inductor quality factor (Q) than other known variable inductors. Furthermore, the variable inductor may be used as a continuous variable inductor by using a transistor in triode for the set of switches. 
         [0010]    It is an object of the present invention to obviate or mitigate at least one disadvantage of previous variable inductors. 
         [0011]    In a first aspect, the invention provides an apparatus for providing a variable inductance comprising a primary conductor; a secondary conductor; and a set of switches operatively connecting the primary conductor and the secondary conductor; wherein the variable inductance is provided by an opening or closing of the set of switches or by changing the resistance of the switches. 
         [0012]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein: 
           [0014]      FIG. 1  is a schematic diagram of a prior art variable inductor; 
           [0015]      FIG. 2  is a schematic diagram of a variable inductor in accordance with the invention; 
           [0016]      FIG. 3   a  is a schematic diagram of a second embodiment of a variable inductor in accordance with the invention; 
           [0017]      FIG. 3   b  is a schematic diagram of a third embodiment of a variable inductor in accordance with the invention; 
           [0018]      FIG. 4  is a schematic diagram of a fourth embodiment of a variable inductor in accordance with the invention; 
           [0019]      FIG. 5   a  is a graph illustrating Q versus switch resistance; and 
           [0020]      FIG. 5   b  is a graph illustrating Inductance versus switch resistance. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Generally, the present invention provides a method and system for implementing a variable inductor for an integrated circuit. 
         [0022]    Turning to  FIG. 2 , a first embodiment of a variable inductor in accordance with the invention is shown. The variable inductor  30  comprises a primary conductor  32  and a secondary conductor  34 . The primary conductor  32  and secondary conductor  34  are preferably differential inductors. Ends  36  of the primary conductor  32  are operatively connected to ends  38  of the secondary conductor  34  via a pair of switches  40 . 
         [0023]    The primary conductor  32  comprises two sections  32   a  and  32   b  connected together by an underpass  42  while the secondary connector  34  comprises three sections  34   a ,  34   b  and  34   c  which are connected by a pair of underpasses  44  and  46 . In operation, if the pair of switches  40  are open, there is no current flowing through the secondary conductor  34 . As there is only current flowing through the primary conductor  32 , the variable inductor  30  provides a constant inductance value. 
         [0024]    However, when the set of switches  40  are closed, thereby operatively connecting the primary conductor  32  and the secondary conductor  34 , current flows through both of the conductors  32  and  34 . As will be understood, the direction of current flow in the primary conductor  32  is in the same direction of the current flow in the secondary conductor  34  causing positive mutual coupling on the primary conductor  32 . By changing the resistance of the set of switches  16 , the inductance may be varied, allowing the inductor to be a variable inductor  30 . The resistance levels of the switches  40  may be set at whatever level the implementer desires. In another embodiment, the set of switches  40  may be a transistor in triode. 
         [0025]    In this manner, the inductor  30  may be see as a continuous variable inductor  30  since the inductor is continuously operable whether the switches are opened or closed. Furthermore, the inductor is variable since the resistance of the set of switches may be changed while they are open so that when they are closed, the overall inductance value of the inductor is changed. Moreover, the inductance is also varied when the switches are transistors in triode. Variation is due to the switch resistance being able to change from a very low value to a very high value. The overall configuration including the primary and the secondary conductors are considered together to be a variable inductor. 
         [0026]    Turning to  FIG. 3 , another embodiment of a variable inductor in accordance with the invention is shown. The variable inductor  50  comprises a primary conductor  52  and a secondary conductor  54  operatively connected by a set of switches  56 . As with the variable inductor of  FIG. 2 , the primary conductor  52  comprises two sections  52   a  and  52   b  which are connected together by an underpass  58  while the secondary conductor  54  comprises three sections  54   a ,  54   b  and  54   c  which are connected together by a set of underpasses  60  and  61 . 
         [0027]    A tap  62  surrounds the secondary conductor  54  and is operatively connected via a second set of switches  64  to the primary conductor  52 . The tap  62  is connected to the secondary conductor  54  at the end away from the sets of switches  56  and  64 . The tap  62  allows for the inductance of the inductor  50  to be further varied. Therefore, different values of inductance are achieved when both sets of switches are opened (current flowing through the primary conductor  52 ), only the set of switches  56  are closed (current flowing through the primary and the secondary conductors  52  and  54 ), only the set of switches  64  are closed (current flowing through the primary conductor  52  and the tap  62 ) and both the sets of switches  56  and  64  are closed (current flowing through the primary and secondary conductors  52  and  54  and the tap  62 ). 
         [0028]    Turning to  FIG. 3   b , as schematically shown, the tap  62  may be connected at any location, as indicated by the dashed lines, to the secondary conductor  54  and does not have to be at an end as shown in  FIG. 3   a . It will be understood that the location of the contact between the tap  62  and the secondary conductor  54  allows for different inductance values. 
         [0029]    Turning to  FIG. 4 , yet a further embodiment of a variable inductor is shown. In this embodiment, the variable conductor  70  is similar to the inductor  50  of  FIG. 3  with the addition of multiple taps  72 . As will be understood, there is no limit to the number of taps  72 , however, this number is dependent on the amount of space available within the analog circuit block/integrated circuit. Each of the taps  72  are operatively connected to the primary conductor  52  via individual pairs of switches  76  and provide the functionality of varying the inductance of the variable inductor  70  in a manner similar to the one discussed above. The inductance is varied in accordance with the number of pairs of switches  76  that are opened or closed at a specific moment. The closed switches impart a resistance to the current flowing through the inductor to vary the inductance level in the inductor  70 . 
         [0030]    Turning to  FIGS. 5   a  and  5   b , graphs showing a variation of the inductance quality factor (Q) and the inductance with respect of the resistance to the set of switches is shown. As described above, the resistance of the set of switches may be varied in order to vary the inductance provided by the inductor. In comparison with one prior art inductor, at an inductance value of 604 pH, the prior art inductor provides a Q value of 9.6 while with the inductor of the invention, at an inductance value of 597 pH, the Q value is 14.2. 
         [0031]    A further advantage of the invention is that the quality factor (Q) is increased over other current variable inductors. Yet another advantage is that since the traces are active and connected to the inductor, Q does not drop. Furthermore since the inductors are in parallel, the resistance of the switches is in parallel as well, therefore the switch resistance has less effect on the Q of the inductor. Finally, when a large resistor is placed at the gate of the transistor, the gate floats. In order to decrease the parasitic capacitance of the switch transistors, a large resistor may be connected to the gate of the switch transistors. By using this resistor, the parasitic capacitance of the switch is reduced and therefore the dynamic range of the variable inductor is increased. 
         [0032]    In an alternative embodiment, the body of the switches (when they are transistors) may be connected to the source to reduce the body effect or to switch parasitic caps. In another embodiment, a resistor may be placed in series with the gate of a switch to decrease this effect. The resistor at the gate and the connection of the body to the source may be done together to reduce the switch resistance and also reduce the parasitic capacitance. 
         [0033]    The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto