# Patent

## Title
Varactor device

## Abstract

A varactor device includes a capacitance circuit having a capacitor set and a first transistor connected across the capacitor set; a first variable resistor; and a second transistor coupled to the first transistor and connected in series to the first variable resistor for feeding an output signal generated by applying voltage to the capacitance circuit back to the first transistor, thereby controlling a gain of the first transistor by tuning the first variable resistor.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a varactor and, more particularly, to a varactor device capable of successively tuning its capacitance magnitude in a large range.
2. Description of Related Art
Generally, a varactor is completed by a MOS process. As shown in FIG. 1, the MOS varactor has a capacitance tuning ratio which is lower than three and needs complicated masks in production, thus it cannot meet the practical requirement. FIG. 2 is a view of a typical diode varactor produced by a CMOS process, which has a capacitance tuning which is ratio lower than two, thus it can tune the capacitance only in a small range and cannot meet the practical requirement as well.
In addition, a digital varactor is designed, as shown in FIG. 3. The digital varactor consists of capacitors CF, CD0 ̃N, and switch transistors S0 ̃N The switch transistors S0  ̃N control the respective capacitors CD0 ̃N to be connected in parallel with the capacitor CF or not. Accordingly, a number of capacitors connected in parallel are changed by respectively controlling the switch transistors S0 ̃N on and off. Thus, the capacitance tuning effect is obtained. In addition, the capacitance tuning ratio can relatively increase with the increased number of the capacitors CD0 ̃N However, such a tuning can tune the capacitance only to a specific value rather than tuning the capacitance successively.
Therefore, it is desirable to provide an improved varactor device to mitigate and/or obviate the aforementioned problems.
SUMMARY OF THE INVENTION
An object of the invention is to provide a varactor device, which can successively tune its capacitance magnitude in a large range.
Another object of the invention is to provide a differential varactor device, which can successively tune its capacitance magnitude in a large range.
In accordance with one aspect of the invention, there is provided a varactor device including a capacitance circuit having a capacitor set and a first transistor connected across the capacitor set; a variable resistor; and a second transistor connected to the first transistor and cascaded the variable resistor for feeding an output signal generated by applying a voltage to the capacitance circuit back to the first transistor, thereby controlling a gain of the first transistor by tuning the variable resistor.
In accordance with another aspect of the invention, there is provided a differential varactor device including: a first capacitance circuit, which has a first capacitor set and a first transistor connected across the first capacitor set; a variable resistor; a second transistor, which is coupled to the first transistor and connected in series to the variable resistor for feeding an output signal generated by applying voltage to the first capacitance circuit back to the first transistor; a second capacitance circuit, which has a second capacitor set and a third transistor connected across the second capacitor set; and a fourth transistor, which is coupled to the third transistor and connected in series to the variable resistor for feeding an output signal generated by applying voltage to the second capacitance circuit back to the third transistor, thereby controlling gains of the first and the third transistors by tuning the variable resistor.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a typical varactor produced by a MOS process;
FIG. 2 is a cross-section view of a typical diode varactor produced by a CMOS process;
FIG. 3 is a diagram of a typical digital varactor device;
FIG. 4 is a diagram of a varactor device according to the invention;
FIG. 5 is a diagram of an equivalent small signal circuit of FIG. 4 in accordance with the present invention; and
FIG. 6 is a diagram of a differential varactor device in accordance with the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 4 is a diagram of a varactor device in accordance with the invention. As shown 4, the varactor device includes a capacitance circuit 41 and a feedback circuit 42. The capacitance circuit 41 has a capacitor set 411 and a transistor 412, a current source 413 and a resistance device 414. The feedback circuit 42 has a transistor 415, a variable resistor 416 and a current source 417. In this embodiment, the transistors 412 and 415 are MOS transistors.
In the capacitance circuit 41, the transistor 412 has a source S and a drain D respectively connected to two ends of the capacitor set 411. The capacitor set 411 is a capacitor 4111 or a plurality of capacitors 4111 connected in parallel. The current source 413 is connected in series to the source S of the transistor 412 for providing a bias current to the transistor 412 such that the transistor 412 can operate in its working region. The resistance device 414 is connected in series to the capacitor set 411 and the transistor 412. The resistance device 414 can be a resistor or variable resistor. In this case, the resistance device 414 is a variable resistor, and preferably a voltage controlled resistor. When a voltage Vn is applied to, for example, the capacitance circuit 41, a capacitance effect is generated and an output signal is generated by the source S of the transistor 412.
In the feedback circuit 42, the variable resistor 416 is preferably a voltage controlled resistor, and the transistor 415 has a gate G connected to the source S of the transistor 412 and a drain D connected to both the variable resistor 416 and a gate G of the transistor 412. The current source 417 is connected in series to the drain D of the transistor 415 for providing the bias current to the transistor 415 such that the transistor 415 can operate in its working region. The feedback circuit 42 feeds the output signal generated by applying the voltage to the capacitance circuit 41 back to the transistor 412 for controlling a gain gm of the transistor 412 by tuning the variable resistor 416 and a gain gm of the transistor 415 by tuning the resistance device 414.
To further analyze the varactor device, FIG. 5 illustrates its equivalent small signal circuit, where C represents a capacitance of the capacitor set 411, Rv1 and Rv2 respectively represent resistance of the resistance device 414 and the variable resistor 416, gm1 and gm2 respectively represent gains of the transistors 412 and 415, Vgs1 and Vgs2 respectively represent gate-source voltages of the transistors 412 and 415, Cgs1 and Cgs2 respectively represent gate-source equivalent capacitance of the transistors 412 and 415. Accordingly, an equivalent resistance on a node N of the circuit can be obtained by an equation:

Zeff=(1/j•C)[1+gm1Rv1+gm1gm2Rv1Rv2+jωRv1C]. 

When gm1gm2Rv1Rv2>>ωRv1C, gm1gm2Rv1Rv2>>gm1Rv1 and gm1gm2Rv1Rv2>>1, by selecting appropriate device dimensions, capacitance and resistance, the equation can be simplified as:

Zeff=(gm1gm2Rv1Rv2/jωCeff)],

where Ceff indicates an equivalent capacitance. Accordingly, equivalent capacitance Ceff=C/gm1gm2Rv1Rv2 is obtained. As such, it is known that the resistance Rv2 of the variable resistor 416 is employed to tune the gain gm1 (as gm1Rv2) while the resistance Rv1 of the resistance device 414 us employed to tune the gain gm2 (as gm2Rv2). Therefore, the capacitance can be tuned successively in a large range by tuning the resistance of the variable resistor 416 or both resistance of the variable resistor 416 and the resistance device 414. In addition, the capacitance C of the capacitor set 411 is further changed by increasing or reducing a number of capacitors 4111 connected in parallel. Thus, the tuning range of the varactor circuit is changed.

FIG. 6 is a circuit diagram of another embodiment in accordance with the invention, which is a differential varactor device. The differential varactor circuit is composed of two identical varactor devices 61, 62, each having a circuit configuration the same as that of FIG. 4, except that the varactor devices 61, 62 share a variable resistor 63 and a resistance device 64. The variable resistor 63 is connected between gates of transistors 612 and 622 while the resistance device 64 is connected between gates of transistors 615 and 625. Since the circuit configurations of the varactor devices 61, 62 respectively are the same as that of FIG. 4, successively tuning capacitance and changing capacitance tuning range can also be obtained in the differential varactor device by the same reason as the previous embodiment, and thus a detailed description is deemed unnecessary.
In view of the foregoing, it is known that the invention uses the circuit technique to implement the varactor device and all circuit components of which can be made by the typical CMOS process. In addition, the invention can simply use an analog manner to control and tune the capacitance, and also greatly increase the capacitance tuning ratio by appropriately determining a number of capacitors of the capacitor set.
Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.

## Claims

1. A varactor device, comprising:
a capacitance circuit, which has a capacitor set and a first transistor connected across the capacitor set;
a variable resistor; and
a second transistor, which is coupled to the first transistor and connected in series to the variable resistor, for feeding an output signal generated by applying voltage to the capacitance circuit back to the first transistor, thereby controlling a gain of the first transistor by tuning the variable resistor,
wherein the first transistor and the second transistor are metal oxide semiconductor (MOS) transistors, and the second transistor has a drain connected to the variable resistor and a gate of the first transistor.

2. The varactor device as claimed in claim 1, wherein the capacitor set having at least one parallel capacitor.

3. The varactor device as claimed in claim 1, wherein the second transistor has a gate connected to a source of the first transistor.

4. The varactor device as claimed in claim 1, further comprising a resistance device connected in series to the capacitor set and the first transistor.

5. The varactor device as claimed in claim 4, wherein the resistance device is a resistor.

6. The varactor device as claimed in claim 4, wherein the resistance device is a voltage controlled resistor.

7. The varactor device as claimed in claim 1, further comprising a first current source and a second current source respectively connected in series to the first and the second transistors for providing a bias current to the first transistor and the second transistor.

8. The varactor device as claimed in claim 1, wherein the variable resistor is a voltage controlled resistor.

9. A differential varactor device, comprising:
a first capacitance circuit, which has a first capacitor set and a first transistor connected across the first capacitor set;
a variable resistor;
a second transistor, which is coupled to the first transistor and connected in series to the variable resistor, for feeding an output signal generated by applying voltage to the first capacitance circuit back to the first transistor;
a second capacitance circuit, which has a second capacitor set and a third transistor connected across the second capacitor set; and
a fourth transistor, which is coupled to the third transistor and connected in series to the variable resistor for feeding an output signal generated by applying voltage to the second capacitance circuit back to the third transistor, thereby controlling gains of the first and the third transistors by tuning the variable resistor.

10. The differential varactor device as claimed in claim 9, wherein the first and the second capacitor sets are composed of at least one parallel capacitor respectively.

11. The differential varactor device as claimed in claim 9, wherein the first, the second, the third and the fourth transistors are metal oxide semiconductor (MOS) transistors.

12. The differential varactor device as claimed in claim 11, wherein the second transistor has a drain connected to the variable resistor and the gate of the first transistor, and the fourth transistor has a drain connected to the variable resistor and the gate of the third transistor.

13. The differential varactor device as claimed in claim 12, wherein the second transistor has a gate connected to a source of the first transistor, and the fourth transistor has a gate connected to a source of the third transistor.

14. The differential varactor device as claimed in claim 9, further comprising a resistance device connected between the first and the third transistors.

15. The differential varactor device as claimed in claim 14, wherein the resistance device is a resistor.

16. The differential varactor device as claimed in claim 14, wherein the resistance device is a voltage controlled resistor.

17. The differential varactor device as claimed in claim 9, further comprising a first current source, a second current source, a third current source and a fourth current source respectively connected in series to the first, the second, the third and the fourth transistors for providing a bias current to the first, the second, the third and the fourth transistors.

18. The differential varactor device as claimed in claim 9, wherein the variable resistor is a voltage controlled resistor.

19. A varactor device, comprising:
a capacitance circuit, which has a capacitor set and a first transistor connected across the capacitor set;
a variable resistor;
a second transistor, which is coupled to the first transistor and connected in series to the variable resistor, for feeding an output signal generated by applying voltage to the capacitance circuit back to the first transistor, thereby controlling a gain of the first transistor by tuning the variable resistor; and
a first current source and a second current source respectively connected in series to the first and the second transistors for providing a bias current to the first transistor and the second transistor.

20. The varactor device as claimed in claim 19, wherein the capacitor set having at least one parallel capacitor.

21. The varactor device as claimed in claim 19, wherein the first transistor and the second transistor are metal oxide semiconductor (MOS) transistors.

22. The varactor device as claimed in claim 21, wherein the second transistor has a drain connected to the variable resistor and a gate of the first transistor.

23. The varactor device as claimed in claim 22, wherein the second transistor has a gate connected to a source of the first transistor.

24. The varactor device as claimed in claim 19, further comprising a resistance device connected in series to the capacitor set and the first transistor.

25. The varactor device as claimed in claim 24, wherein the resistance device is a resistor.

26. The varactor device as claimed in claim 24, wherein the resistance device is a voltage controlled resistor.

27. The varactor device as claimed in claim 19, wherein the variable resistor is a voltage controlled resistor.