Transformer with symmetric structure

This application claims the priority benefit of Taiwan application serial no. 97126432, filed on Jul. 11, 2008. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification.

1. Field of the Invention

The present invention generally relates to a transformer.

2. Description of Related Art

In a balanced-unbalanced band pass filter, an interactive magnetic coupling between two inductors constituting the transformer is used to generate a mutual induction, so as to deliver a differential signal on a single end. Under the miniaturization trend of electronic products, electronic components are integrated in a circuit substrate, for example, capacitors, inductors, resistors, and other passive components. In the transformer, two inductors formed by a coil may be achieved through a circuit layout of the circuit substrate.

FIG. 1 shows a conventional transformer. Referring to FIG. 1 , the conventional typical transformer includes a first inductor L 1 and a second inductor L 2 . Through an interactive electromagnetic induction between the first inductor L 1 and the second inductor L 2 , an input voltage between two ends of the first inductor L 1 is converted into an output voltage between two ends of the second inductor L 2 . Theoretically, a ratio of the input voltage to the output voltage is equal to a ratio of the first inductor L 1 value to the second inductor L 2 .

FIG. 2 shows another conventional transformer. Referring to FIG. 2 , different from the typical transformer of FIG. 1 , the transformer of FIG. 2 may convert a single signal into a differential signal pair. The transformer of FIG. 2 includes a first capacitor C 11 , a first inductor L 11 , a second capacitor C 12 , a second inductor L 12 , a third capacitor C 13 , and a third inductor L 13 . A first mutual inductor Lm 11 is generated by an interaction between the first inductor L 11 and the second inductor L 12 , and a second mutual inductor Lm 12 is generated by an interaction between the first inductor L 11 and the third inductor L 13 . Through the first mutual inductor Lm 11 and the second mutual inductor Lm 12 , a signal input to a first port P 11 is converted into a differential signal pair, and then output through a second port P 12 and a third port P 13 .

Accordingly, the present invention is directed to a transformer, for simplifying a layout design.

The present invention is directed to a transformer, applicable for a band pass filter, for generating a transmission zero to increase a stop band attenuation rate of the band pass filter.

A transformer is provided in the present invention, which includes four capacitors, four inductors, and a first additional capacitor. The first capacitor is electrically connected between a first port and ground in series. The first inductor is electrically connected to the first port in series. The second capacitor is electrically connected between the first inductor and ground in series. The second inductor is electrically connected between the first inductor and the second capacitor in series. The third capacitor is electrically connected between a second port and ground in series. The third inductor is electrically connected to the first port in series. The fourth capacitor is electrically connected between a third port and ground in series. The fourth inductor is electrically connected between the third inductor and the third port in series. The first additional capacitor satisfies a preset capacitance value. A first mutual inductor having a predetermined inductance value is generated by an interaction between the first inductor and the third inductor, and is electrically connected to the first additional capacitor in parallel.

In an embodiment of the present invention, the first additional capacitor is a parasitic capacitor and is generated by the interaction between the first inductor and the third inductor.

In an embodiment of the present invention, the transformer further includes a second additional capacitor. A second mutual inductor having a predetermined inductance value is generated by an interaction between the second inductor and the fourth inductor, and the second mutual inductor is electrically connected to the second additional capacitor in parallel.

In an embodiment of the present invention, inductance values of the first inductor, the second inductor, the third inductor, and the fourth inductor are substantially the same.

In an embodiment of the present invention, the second additional capacitor is a parasitic capacitor and is generated by the interaction between the second inductor and the fourth inductor.

In an embodiment of the present invention, the first inductor and the third inductor are symmetric to each other, and the second inductor and the fourth inductor are symmetric to each other.

In an embodiment of the present invention, the first capacitor and the third capacitor are symmetric to each other, and the second capacitor and the fourth capacitor are symmetric to each other.

In an embodiment of the present invention, the first inductor and the second inductor are formed by the same coil.

In an embodiment of the present invention, the third inductor and the fourth inductor are formed by the same coil.

When the transformer of the present invention is applied to a band pass filter, the transmission zero is generated by the parasitic capacitor and the mutual inductor generated by the interaction between the two inductors, thereby increasing the stop band attenuation rate of the band pass filter. In addition, the transformer of the present invention is changed from a conventional asymmetric structure into a symmetric structure, which is helpful for simplifying the layout design of the transformer.

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 shows a conventional transformer.

FIG. 2 shows another conventional transformer.

FIG. 3 shows a transformer according to an embodiment of the present invention.

Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

FIG. 3 shows a transformer according to an embodiment of the present invention. Referring to FIG. 3 , a transformer of this embodiment includes four capacitors C 21 -C 24 and four inductors L 21 -L 24 . A first capacitor C 21 is electrically connected between a first port P 21 and ground in series. A first inductor L 21 is electrically connected to the first port P 21 in series. A second capacitor C 22 is electrically connected between the first inductor L 21 and ground in series. A second inductor L 22 is electrically connected between the first inductor L 21 and the second capacitor C 22 in series. A third capacitor C 23 is electrically connected between a second port P 22 and ground in series. A third inductor L 23 is electrically connected to the second port P 22 in series. A fourth capacitor C 24 is electrically connected between a third port P 23 and ground in series. A fourth inductor L 24 is electrically connected between the third inductor L 23 and the third port P 23 in series.

In addition, the transformer further includes a first additional capacitor Cp 21 . A first mutual inductor Lm 21 is generated by an interaction between the first inductor L 21 and the third inductor L 23 , and the first additional capacitor Cp 21 is electrically connected to the first mutual inductor Lm 21 in parallel, such that a high-frequency transmission zero characteristic is generated, thereby enhancing a capability of inhibiting the high-frequency noise interference.

A capacitance value of the first additional capacitor Cp 21 and an inductance value of the first mutual inductor Lm 21 are both preset values. The process of generating the first additional capacitor Cp 21 and the first mutual inductor Lm 21 and controlling the capacitance value and the inductance value thereof is to adjust the inductance values of the first inductor L 21 and the third inductor L 23 and distance between the first inductor L 21 and the third inductor L 23 .

In this embodiment, the first additional capacitor Cp 21 may be a parasitic capacitor and is generated by the interaction between the first inductor L 21 and the third inductor L 23 .

In this embodiment, a second additional capacitor Cp 22 and a second mutual inductor Lm 22 are generated by an interaction between the second inductor L 22 and the fourth inductor L 24 , in which the second additional capacitor Cp 22 is electrically connected to the second mutual inductor Lm 22 in parallel, such that the high-frequency transmission zero characteristic is generated, thereby enhancing the capability of inhibiting the high-frequency noise interference.

A capacitance value of the second additional capacitor Cp 22 and an inductance value of the second mutual inductor Lm 22 are both preset values. The process of generating the second additional capacitor Cp 22 and the second mutual inductor Lm 22 and controlling the capacitance value and the inductance value thereof is to adjust the inductance values of the second inductor L 22 and the fourth inductor L 24 and the positions between the second inductor L 22 and the fourth inductor L 24 .

In this embodiment, the second additional capacitor Cp 22 may be a parasitic capacitor and is generated by the interaction between the second inductor L 22 and the fourth inductor L 24 . In this embodiment, the inductance values of the first inductor L 21 , the second inductor L 22 , the third inductor L 23 , and the fourth inductor L 24 are substantially the same.

In this embodiment, the first inductor L 21 and the third inductor L 23 may be symmetric to each other, and the second inductor L 22 and the fourth inductor L 24 may be symmetric to each other. In addition, the first capacitor C 21 and the third capacitor C 23 may be symmetric to each other, and the second capacitor C 22 and the fourth capacitor C 24 may be symmetric to each other. Furthermore, the first inductor L 21 and the second inductor L 22 may be formed by the same coil, and the third inductor L 23 and the fourth inductor L 24 may be formed by the same coil. The first inductor L 21 and the second inductor L 22 , the third inductor L 23 and the fourth inductor L 24 are only the names for the convenience of differentiation. In terms of the actual circuit layout, the first inductor L 21 and the second inductor L 22 are respectively two parts of one inductor coil, and the third inductor L 23 and the fourth inductor L 24 are respectively two parts of another inductor coil. Therefore, the layout design of the transformer is simplified.

To sum up, when the transformer of the present invention is applied to a band pass filter, the parasitic capacitor and the mutual inductor generated by the interaction between the two inductors are used for achieving a transmission zero, thereby enhancing the stop band attenuation rate of the band pass filter. In addition, the transformer of the present invention is changed from a conventional asymmetric structure into a symmetric structure, which is helpful for simplifying the layout design of the transformer.

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.