Abstract:
A capacitor includes a first plate and a second plate parallel to the first plate. An RF source includes a first line and a second line through which RF is fed. The first line is electrically connected to the first plate. The second line is passed through the first and second plates and then looped around the first and second plates, and the pass and loop of the second line is repeated at least once. The second line is then passed through the first plate and electrically connected to the second plate to form a capacitor having negative capacitance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/144,914, filed Apr. 8, 2015, which application, including respective appendices, is hereby incorporated herein by reference, in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates generally to electrical circuit elements and, more particularly, to an electrical circuit element having negative capacitance. 
       BACKGROUND 
       [0003]    A capacitor comprises two insulated conductors, and the capacitance is characterized by the field distribution in the two-body system. If charge Q is placed on one conductor and −Q on the other conductor, the potential difference between the two conducting bodies is proportional to Q and the proportional constant is 1/C where C is called capacitance. 
         [0004]    The charge distribution produces an electric field, which in turn causes a potential difference V. The electric field is also affected by the material between the two conductors. When an external field is applied to a dielectric material, the molecules/atoms of the dielectric are polarized to become dipole moments which will produce a net electric field in the opposite direction to the external field, resulting in decreased potential difference. Consequently, the capacitance increases in the presence of dielectric material. The reduction of the electric field due to the presence of dielectric material is characterized by a dielectric constant ε r  (or relative permittivity), such that the resultant net electric field becomes 1/ε r  of the external electric field. 
       SUMMARY 
       [0005]    Normally, a dielectric constant is always larger than 1 (1 being free-space dielectric constant) because of the induced dipole moments within the dielectric. This invention introduces a novel device where an AC electric field is enhanced (i.e., the additional field is in the same direction as the external field) and the effective dielectric constant becomes less than 1, even less than zero, that is, a negative value. When the effective dielectric constant becomes negative, the capacitance also becomes negative. Both an inductor and a capacitor of negative capacitance provide positive reactance. However an advantage of a negative-capacitance capacitor is that it will be physically much smaller than an inductor having an equivalent positive (inductive) reactance. 
         [0006]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0008]      FIGS. 1-4  are schematic views exemplifying capacitors exhibiting capacitance that is smaller than conventional capacitors that do not incorporate features such as negative capacitance in accordance with principles of the present invention; 
           [0009]      FIG. 5  is a schematic view exemplifying a capacitor exhibiting capacitance larger than capacitors that do not incorporate extra features embodying principles of the present invention; and 
           [0010]      FIGS. 6-7  exemplify applications of capacitors having negative capacitance. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. Additionally, as used herein, the term “substantially” is to be construed as a term of approximation. 
         [0012]    The shape of the negative-capacitance capacitor is similar to those of other ordinary capacitors as exemplified in  FIG. 1 . The difference between a conventional capacitor and a capacitor having negative capacitance in accordance with principles of the present invention lies in the radio frequency (“RF”) feeding. As shown in  FIG. 1 , a capacitor designated by the reference numeral  100  embodying features of the present invention, comprises a ground line  101  from a radio frequency (“RF”) source  110  connected to a bottom ground plate  111 , and a line  103  from the center feeding pin  102  of a coaxial feed  104  that is connected to the RF source  110  passes through a small hole  105  in the top patch  112  and this line  103  feeds the capacitor cavity  106  again through a hole  107  at the bottom ground plate, thus having the current sources in the space between the plates point in the same direction. Note that at a low frequency, these current sources produce larger fields by constructive interference, assuming the inductive effect is negligible at such low frequency. There can be a number of current sources in the capacitor cavity  106  to increase the field strength by repeating the above process of line  103  looping around the plates  111  and  112  inside the plates in one direction and outside the plates in the other direction. The end of the line  103  is connected to the top patch  112  at  109 , thus making the device a capacitor. In the present case, the current line  103  before being connected to the top patch  112  provides a field similar to the external field in a conventional capacitor. The other feeding elements act as additional field sources to produce extra fields beyond the external field similar to polarization (dipole moment density) in a dielectric medium. In an insulator, the induced dipoles produce an opposing field to the external field, thus making the electric susceptibility χ e  positive where the dielectric constant ε r =1+χ e . Now in the present invention, since the additional field points in the same direction as the effective external field, the equivalent electric susceptibility becomes negative. When there is sufficient current flowing in the same direction with the original current element to have a negative effective electric susceptibility with a magnitude larger than 1, then the effective dielectric constant becomes negative. Under this circumstance, the capacitance of the two-parallel plate capacitor becomes negative as well. 
         [0013]      FIGS. 2-4  exemplify in capacitors  200 ,  300 , and  400  a few of many alternate embodiments of the invention that yield various dielectric constants that are less than 1. The number of current elements in the capacitor cavity can be adjusted to vary the effective susceptibility and dielectric constant. It is also possible to control the effect of the current source by changing the separation distance d 1  or d 2  of the two plates at the source point ( 201  or  202 ) as shown in  FIG. 2 . 
         [0014]    For fabrication convenience, we can also consider making the device with fewer holes or even no holes in the conducting plates ( FIGS. 3 and 4 ). In  FIG. 3 , for example, there is only one hole  301  through which a feed line  302  passes. The feed line  302   a  enters the capacitor cavity  303  again near bottom plate  304  and bends sharply to produce a vertical current element  306 , after which the feed line  302   b  travels along the top plate  305  and exits the capacitor cavity  303 . This process may continue to put as many vertical current elements (such as  306 ) as needed. At the end, the feed line  302  is connected to the top patch  305  at  307 . 
         [0015]    In  FIG. 4 , there are no holes in the capacitor plates  402  and  403 . The RF source  401  is located outside the capacitor where the ground  408  of the RF source  409  is connected to the bottom plate  402 . The feed line  404  enters the capacitor cavity  405  near the bottom plate  402  and makes a sharp turn within the capacitor cavity to have a vertical current source  406 , after which the feed line makes another sharp turn near the top patch  403  and travels along the top patch  403  before exiting the capacitor cavity  405 . This process may continue to put as many vertical current elements (such as  406 ) as needed within the capacitor cavity  405 . The feed line is eventually connected to the top patch  403  at  407 . 
         [0016]      FIG. 5  exemplifies in a capacitor  500  how, by changing the direction of the each vertical current element, it is possible to have a variable susceptibility that is positive, resulting in a large dielectric constant. The feed line  502  from the RF source  501  produces an upward vertical current source  503  within the capacitor cavity  504 , and comes out of the capacitor cavity through a hole  505  located at the upper plate  512 . The ground line  515  of the RF source  501  is connected to the bottom plate  511  of the capacitor  500 . The feed line  502  enters the capacitor cavity  504  again through a hole  506  at the upper plate  512  and produces a downward current source  508  within the capacitor cavity  504  before coming out of the capacitor cavity through a hole  507  at the bottom plate  511 . The feed line  502  goes around the capacitor  500  and enters again through a hole at the top plate to produce a downward current source within the cavity. This process may continue to produce as many downward current sources as needed before the feed line  502  is connected to a point  513  at the upper plate  512  at the end. In some applications, a very large dielectric constant is desirable and such a device can be produced artificially using the technique in this invention. 
         [0017]    Wireless Charger Application: 
         [0018]    In  FIG. 6 , an RF source  601  is connected to a capacitor  600  that comprises an upper conducting plate  603 , a bottom conducting plate  602 , and capacitor space  604  between the two conductors  602  and  603  that is filled with insulating dielectric material(s) (not shown). Such a capacitor source is usually available in most areas having electric power provided at an electric outlet. For example, the RF source  601  can be a source for conventional alternating current (“AC”) that provides electric power to households. The capacitor  600  may be modeled as comprising many small sub-capacitors, each of which has a pair of small conducting surface patches on the two conductors that are connected by electric field lines. Here the capacitance of the capacitor is the sum of all capacitances of the sub-capacitors. Each sub-capacitor consists of a small patch of conducting surface at the bottom plate  602  and that at the upper plate  603  that are connected by electric field lines. For example, in  FIG. 6 , a small conducting surface  605  at the bottom plate  602  is connected to another small conducting surface  606  at the upper plate  603  by electric line  607 . Those two conducting patches and the insulating space between those two conducting patches filled with electric field lines behave as a sub-capacitor among other many sub-capacitors. Another example of a sub-capacitor is a pair of conducting surfaces of  608  and  609  at the lower and upper conductors of  602  and  603 , respectively. These two conducting patches are connected by electric field lines  610  that are outside of the capacitor space  604 , which may be used as a source capacitor for wireless power collection. 
         [0019]    When an object, especially conducting metal, approaches near a source capacitor, the field lines are distorted, and consequently the capacitance of the source capacitor will change due to mutual coupling between the source, or active, capacitor and the passive object. Normally, change of the capacitance is relatively small due to very weak fields around the capacitor outside the space between the two conducting plates. 
         [0020]      FIG. 7  shows a system  700  having a passive capacitor  701  of negative capacitance that is near a source capacitor  702 . There is a sub-capacitor of a pair of conducting patches  703  and  704  at the source capacitor  702  that are connected to the passive capacitor  701  by electric field lines. In this case, the sub-capacitor of  703  and  704  is connected to the passive capacitor  701  of negative capacitance in series. The total capacitance Cs of the pair of the sub-capacitor and the passive negative-capacitance capacitor can be obtained from 1/C=1/C1+1/C2 where C1 is the capacitance of the sub-capacitor of  703  and  704  and C2 is the capacitance of the passive capacitor  701 . Since the sub-capacitor is split into two after introduction of the passive capacitor, 1/C1=1/C1u+1/C1d where C1u is the capacitor of the sub-capacitor above the passive capacitor and C1d is that below. Normally C1 is very small, which gives a small value of Cs regardless of the value of C2. However when C2 is negative, the situation changes drastically. In fact, making C2 equal to C1 in magnitude but negative, the effective capacitance becomes very large, essentially producing a conduction channel of current along the electric field lines that connect those two capacitors. The capacitance of the source capacitor  702  is significantly altered with the presence of the passive capacitor  701 . Especially when the resonance occurs with C2 close to −C1, the total capacitance of the source capacitor  902  is almost equal to Cs, which is very large. At this point, a substantial amount of current flows from the RF source  710  to the sub-capacitor of  703  and  704  and then to the passive capacitor  701 . 
         [0021]    In order to collect power wirelessly from the active capacitor, the load resistance RL  711  is connected to the passive capacitor in series as shown in  FIG. 7 . Also in the figure is shown a capacitor of variable capacitance  712  in series for fine tuning. In the design process, the magnitude of the negative capacitance is preferably made slightly smaller than the sub-capacitor capacitance C1 so that resonance is obtained by adjusting the capacitance of the added capacitor Ca  712 . At resonance, the current through the resistor is Vs/RL where Vs is the voltage across the active source capacitor. When a resonance occurs, then the impedance of the resultant active (source) device is determined mainly by that of the pair of the sub-capacitor and the passive charging system that are connected by electric field lines. 
         [0022]    All inductors may be replaced with more compact and efficient capacitors having negative capacitance as described herein, such devices including wireless-charging devices as described in patent application Ser. No. 13/476,850, filed May 21, 2012, now U.S. Pat. No. 9,030,053, issued May 12, 2015, and patent application Ser. No. 14/210,740 filed Mar. 14, 2014, both of which patent applications and patent are incorporated by reference herein in their entireties, and are included as Appendices A and B to U.S. Provisional Application No. 62/144,914, filed Apr. 8, 2015, the benefit of which is being claimed herein. 
         [0023]    Energy Harvesting 
         [0024]    It is well known that there is substantial electromagnetic power in our environment, especially at the low-frequency spectrum. There are various sources of such power at low frequencies, such as lightning and other natural events. In most cases, it is difficult to collect such power because of improper impedance matching to a power collection device. Thus as soon as a reasonable load resistance RL is connected, the driver voltage of the device vanishes. However with this new capacitor of negative capacitance, it is now realizable that the very large reactance (magnitude) of the device can be maintained while the load resistor of small resistance is connected to draw the power from such environment. 
         [0025]    Charger Orientation and Shape: 
         [0026]    In order to have efficient reception, the charger is preferably oriented to maximize coupling with the sub-capacitor that is connected to the wireless charger by electric field lines. For example, for a two-parallel-plate capacitor, the electric field before placement of the charger is preferred to have its direction perpendicular to the plate surfaces as closely as possible. Also, a larger charger size in the direction perpendicular to the electric field will increase coupling between the source and charger capacitors. 
         [0027]    Still further, the charger shape can influence C1, the capacitance between the source capacitor and charger. An enlarged shape of the charger in the direction of the electric field line will increase C1. In most cases, C1 is very small and its increased value will be helpful in designing an efficient charger. 
         [0028]    Preferably, any values of C1 can be accommodated by introducing C2 that is close to −C1 to have a resonant condition. 
         [0029]    Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered obvious and desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.