Patent Application
20110260819
The present invention relates to the field of integrated inductors, and particularly towards integrated tunable inductors.
An inductor is an electrical device that introduces inductance into a circuit or functions by inductance within a circuit. In some applications, it is useful for inductors to be tunable. For example, circuits designed for RF applications may benefit by using tunable inductors. In particular, tuned circuits that include LC tanks used for loads, filters, impedance matching, or the like may use tunable inductors for tuning center frequencies.
The inductance value, L, of an inductor is dependent upon (among other factors) the number of windings in the coil between two electrical contact points, and one may adjust the number of windings between end points. Such a variable inductor, however, is not available in integrated circuit technology, where mechanically adjustable armatures are not practical. Some known devices use the eddy current to vary the inductance of an inductor. Eddy current is formed when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor, or due to variations of the field with time.
An example of a device that uses eddy current to vary the inductance of an inductor is shown in the paper, M. Rais-Zadeh, P. A. Kohl, and F. Ayazi, A Packaged Micromachined Switched Tunable Inductor, Proc. 20th, IEEE Micro Electro Mechanical Systems Conf. (MEMS 2007), Kobe, Japan, January 2007, pp. 799-802 (“Rais-Zadeh”). Rais-Zadeh describes the implementation of tunable inductors using micromachined electrostatically-actuated switches. The tunable inductor of Rais-Zadeh is limited in that it can only be tuned in discrete increments and not across a continuous range of values. A further disadvantage of Rais-Zadeh's tunable inductor is that the micromachined switches are not easily integrated into system-on-a-chip (SOC) designs.
Another example of a device that makes use of eddy current to vary the inductance of an inductor is shown in U.S. Pat. No. 7,202,768, issued to Harvey et al. (“the '768 patent”). The tunable inductor of the '768 patent has an inductor in proximity to one or more sets of eddy current coils. Each eddy current coil is coupled to a corresponding switch that controls whether the eddy current coil is grounded or floating. By selectively coupling and decoupling one or more eddy current coils to ground, the inductance of the inductor can be selectively tuned. As with Rais-Zadeh, the tunable inductor of the '768 patent can only be tuned in discrete increments.
Another example of a device that makes use of eddy current to vary the inductance of an inductor is shown in U.S. Pat. No. 7,598,838, issued to Hargrove et al. (“the '838 patent”). A variable inductor of the '838 patent includes a second closed-loop inductor placed immediately above or below a primary inductor. A current applied to the primary inductor induces an eddy current in the second inductor by inductive coupling. The second current in the second inductor then alters the impedance of the primary inductor by mutual inductance. To produce a variable inductor, each of the closed loop inductors may have its closed loop, i.e. closed current path, selectively broken. There are several disadvantages of the '838 patent. As with the art discussed above, the application is limited to inductance tuning in discrete increments. Also, the presence of switches in a series connection with the spiral inductor can significantly degrade the performance of the inductor due to the high series resistance of the switches.
An integrated tunable inductor includes a primary inductor having a plurality of inductor turns, at least one closed loop eddy current coil proximate the primary inductor, and at least one variable resistor integrated in series with the eddy current coil.
The above and other features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention that is provided in connection with the accompanying drawings.
The accompanying drawings illustrate preferred embodiments of the invention, as well as other information pertinent to the disclosure, in which:
This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description, relative terms such as “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms concerning attachments, coupling (whether physical or electrical) and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to, or communicate with, one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise.
An improved tunable inductor is described below in connection with the drawings. In embodiments, the tunable inductor is configured to allow for continuous tuning of the inductance value of the tunable inductor across a range of values, as opposed to only in discrete increments. In other embodiments, the inductor can be tuned in discrete increments but without the need for high resistance switches in series with the primary inductor, which can cause performance problems as discussed above.
The tunable inductor employs the eddy current effect to tune the inductance of a primary inductor. The tunable inductor includes a, primary inductor, such as a helical or spiral inductor, formed on a semiconductor substrate. The primary inductor can have any number of shapes, such as circular, rectangular, hexagonal, octagonal, etc. A closed loop secondary inductor is magnetically coupled to the primary inductor. The secondary inductor includes one or more eddy current coils and is disposed proximate the primary inductor. One or more variable resistors is placed in series with the secondary inductor to control eddy current in the closed loop secondary inductor. A controller may be provided to adjust the resistance of the variable resistor. The variable resistor may be a voltage variable resistor (MOS transistor), a switch resistor array, or the like.
The tunable inductor, which has an inductance and parasitic capacitance, can provide an optimal inductance-capacitance (LC) tank for high frequency applications. The tunable inductor is relatively simple to implement in a complementary metal-oxide semiconductor (CMOS) processes, such as those used for wireless circuit applications. The tunable inductor described herein can be used in any number of applications, such as wideband CS LNA circuits with a low noise amplifier, phase tuning circuits, high performance LC tanks having high frequency voltage controlled oscillators (VCOs), impedance matching networks, or various filter circuits.
As shown in the figures, secondary inductor 3 is a closed-loop having one or more electrical connections 13. The closed-loop configuration of secondary inductor 3 may be broken on-chip in several ways to include one or more variable resistors 7. Each variable resistor 7 is integrated in series with the closed-loop of the secondary inductor 3.
In an alternative embodiment of secondary inductor 3, the secondary inductor 3 may include two or more closed-loop coils each having a variable resistor 7 integrated in series with a respective closed loop. This configuration allows various tuning ranges as described in, for example, U.S. Pat. No. 7,202,768, the entirety of which is hereby incorporated by reference herein.
In operation, a first time-varying current is coupled to the primary inductor 5 and induces a first magnetic field that in turn induces a time-varying voltage in the eddy current coil 3. For example, the first time-varying current in inductor 5 may flow in the clockwise direction. The current induces a magnetic field in a direction normal to the major plane of primary inductor 5. If the eddy current coil of the secondary inductor 3 is opened or in series with a high resistance, no eddy current flows through the eddy current coil and the inductance of the primary inductor 5 remains unchanged. However, if the eddy current coil is not opened, e.g., is floating or a closed-loop, an eddy current flows through the eddy current coil. The eddy current, which flows in the opposite direction of the first time-varying current, induces a second magnetic field. The second magnetic field, which opposes the first magnetic field, reduces the inductance of the primary inductor 5.
The variable resistor(s) 7 provided in series with the eddy current coil of the secondary inductor 3 provide a means for controlling the eddy current in the secondary inductor 3. By varying the resistance of the variable resistors 7, the eddy current can be increased or decreased, which changes the inductance of the primary inductor 5. That is, if the resistance is increased, then the eddy current in the eddy current coil of secondary inductor 3 reduces, which reduces the strength of the secondary magnetic field opposing the first magnetic field. With increased resistance, the inductance of the primary inductor 5 approaches the standard inductance of the primary inductor. Of course, if the resistance is decreased, then the eddy current in the eddy current coil of the secondary inductor 3 increases, which increases the strength of the secondary magnetic field opposing the first magnetic field. This reduces the inductance of the primary inductor 5.
If the resistance of the variable resistor is itself continuously variable across a range of resistances, then the inductance of the primary inductor 5 can also be continuously tuned across a range of inductances. In one embodiment, variable resistors 7 may be a MOS transistor biased to act as a resistor.
In an alternative embodiment, the variable resistor can be any kind of switch resistor array, or the like. In this embodiment, the level of granularity of the tuning of the inductance of the primary inductor 5 is limited only by the discrete resistance changes available from the switch resistor array.
Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly to include other variants and embodiments of the invention that may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.
