# Patent

## Title
Transconductance filter circuit

## Abstract

Transconductance filter circuits. A transconductor includes two inputs for receiving two differential voltages, and a first output terminal and a second output terminal for outputting two differential signals. A first capacitor array includes at least one first switch capacitor unit controlled by a first signal, and a first equivalent capacitor coupled between the first output terminal and the second output terminal when the first signal is enabled. two second capacitor arrays each includes at least one second switch capacitor unit controlled by a second signal, two second equivalent capacitors respectively coupled between the first output terminal and a ground level, and between the second output terminal and the ground level when the second signal is enabled. The capacitance of the first equivalent capacitor exceeds that of the two second equivalent capacitors connected in serial.

## Background
BACKGROUND
The disclosure relates in general to transconductance filter circuits. In particular, the disclosure relates to a transconductance filter circuit for digital video disk (DVD) systems.
FIG. 1 is a circuit diagram of a conventional GM-C filter. A conventional GM-C filter comprises a transconductor GM and two capacitor arrays 10. In practice, frequency banks of the GM-C filter are switched using capacitor switches S1 ̃Sn. For example, read signals generated by a read circuit of a disc drive are in different frequency banks when the disc drive operates at different speeds. In FIG. 1, capacitor switch circuits S1 ̃Sn are arranged in an array, connected between output terminals Vo+ or Vo− of transconductor GM and a ground level. Switch transistors M of capacitor switch circuits S1 ̃Sn are respectively turned on or turned off according the logic level of bit control signals V1 ̃Vn. Capacitance of capacitors C1 ̃Cn are respectively c, 2c, 4c, . . . , 2nc. Capacitors with larger capacitance are larger. As capacitor switch circuits controlled by most significant bits (MSB) of a set of control signals are turned off, stray capacitance is larger due to W=Gm/C. The stray capacitance generated by switch transistors limits the range of the operating frequency bank of the conventional transconductance filter circuit.
FIG. 2 is a circuit diagram of another conventional GM-C filter. The difference between FIG. 2 and FIG. 1 is that the capacitor arrays 10′ are connected between output terminals Vo+ or Vo− of transconductor GM, capacitance of capacitors C1′ ̃Cn′ in capacitor switch circuits S1′ ̃Sn′ are respectively c/4, 2c/4, . . . , 2nc/4. As the capacitance of capacitors are 1⁄4 of that in FIG. 1, the total size of the capacitors in FIG. 4 is relativly smaller. In addition, the equivalent capacitance of each capacitor switch circuit S1′ ̃Sn′ is the same as that of corresponding capacitor switch circuits S1 ̃Sn. Using capacitor switch circuit S1′ as an example, the equivalent capacitance of two capacitors C1′ with capacitance c/4 connected in parallel is c/2, which is the same with that of two capacitors C1 with capacitance c connected in serial in FIG. 1. As the capacitor switch circuits controlled by least significant bits (LSB) of control signals is turned on, the output of capacitor switch circuits is influenced by the stray capacitance generated by bottom metal-dielectric-metal BMIM, deteriorating frequency response in high frequency operation.
SUMMARY
Transconductance filter circuits are provided. An embodiment of a transconductance filter circuit comprises: a transconductor comprising two inputs for receiving two differential voltages, and a first output terminal and a second output terminal for outputting two differential signals; a first capacitor array, comprising at least one first switch capacitor unit controlled by a first signal, a first equivalent capacitor coupled between the first output terminal and the second output terminal when the first signal is enabled; and two second capacitor arrays, each second capacitor array comprising at least one second switch capacitor unit controlled by a second signal, two second equivalent capacitors respectively coupled between the first output terminal and a ground level, and between the second output terminal and the ground level when the second signal is enabled, wherein the capacitance of the first equivalent capacitor exceeds that of the two second equivalent capacitors connected in serial.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will become more fully understood from the detailed description, given hereinbelow, and the accompanying drawings. The drawings and description are provided for purposes of illustration only and, thus, are not intended to be limiting of the present invention.
FIG. 1 is a circuit diagram of a conventional GM-C filter.
FIG. 2 is a circuit diagram of another conventional GM-C filter.
FIG. 3 is a circuit diagram of a transconductance filter circuit according to an embodiment of the invention.
FIG. 4 shows a circuit diagram of first switch capacitor unit 40-1 according to an embodiment of the invention.
FIG. 5 shows a circuit diagram of second switch capacitor unit 50-1 according to an embodiment of the invention.



DETAILED DESCRIPTION
FIG. 3 is a circuit diagram of a transconductance filter circuit according to an embodiment of the invention, comprising a transconductor GM, a first capacitor array 4 and two second capacitor arrays 5.
Transconductor GM comprises two inputs for receiving differential voltages Vi+ and Vi−, and a first output terminal O1 and a second output terminal O2 for outputting two differential signals. Currents in the transconductor GM generate differential voltages Vo+ and Vo− by capacitor coupling.
First capacitor array 4 comprises at least one first switch capacitor unit 40-1 controlled by a first signal Vn, MSB of a set of bit control signal as an example. A first equivalent capacitor is coupled between the first output terminal O1 and the second output terminal O2 when the first signal Vn is enabled.
Second capacitor array 5 comprises at least one second switch capacitor unit 50-1 controlled by a second signal V1, LSB of a set of a set of bit control signal as an example. The second equivalent capacitors respectively coupled between the first output terminal O1 and a ground level, and between the second output terminal O2 and the ground level when the second signal V1 is enabled.
The capacitance of the first equivalent capacitor of first switch capacitor units 40-1 are respectively 2n c/2, where n=1,2, . . . . , n. The capacitance of the second equivalent capacitor of second switch capacitor units 50-1 connected in serial from output terminals O1 and O2 is c/2. Thus, the capacitance of the first equivalent capacitor is 2n times that of the two second equivalent capacitors connected in serial, where n is a natural number.
FIG. 4 shows a circuit diagram of first switch capacitor unit 40-1 according to an embodiment of the invention. First switch capacitor unit 40-1 comprises two capacitors Cn′ and the switches M. One of the switches M and capacitor Cn′ are connected in serial between output terminals O1 and O2. The other switches M and capacitor Cn′ are connected in serial between output terminals O2 and O1.
FIG. 4 only shows switch M, which is an n-type transistor, switch M can be, however, an n-type transistor or a p-type transistor. Switch M comprises a gate coupled to first signal Vn (MSB of a set of bit control signals as an example), a source/drain coupled to capacitor Cn′, and a drain/source coupled to output terminals O1 or O2. Switch M is turned on or turned off according the logic level of the first signal Vn.
FIG. 5 shows a circuit diagram of second switch capacitor unit 50-1 according an embodiment of the invention. Second switch capacitor unit 50-1 comprises capacitor C1 and the switch M. Switch M and capacitor C1 are connected in serial between output terminal O1 and ground level, and between output terminal O2 and ground level.
FIG. 5 only shows switch M, which is an ntype transistor, switch M can be, however, an n-type transistor or a p-type transistor. Switch M comprises a gate coupled to second signal V1 (LSB of a set of bit control signal as an example), a source/drain coupled to capacitor Cn′, and a drain/source coupled to output terminals O1 or O2. Switch M is turned on or turned off according the logic level of the second signal V1.
According to the transconductance filter circuit of the invention, when the LSB of the set of bit control signal is asserted, transconductance filter circuit operates at high frequency. Thus, operating second capacitor arrays 5 in FIG. 3 are not influenced by the stray capacitor of switches, improving frequency response in high frequency operation. When the MSB of the set of bit control signals is asserted, the transconductance filter circuit operates at low frequency. Thus, the size of the transistors in operating first capacitor arrays 4 in FIG. 3 is relatively small, with less stray capacitance, improving frequency response in high frequency operation. Thus, the transconductance filter circuit according to the embodiments of the invention operates in wide frequency banks.
The foregoing description of several embodiments have been presented for the purpose of illustration and description. Obvious modifications or variations are possible in light of the above teaching. The embodiments were chosen and described to provide the best illustration of the principles of this invention and its practical application to thereby enable those skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.

## Claims

1. A transconductance filter circuit, comprising:
a transconductor comprising two inputs for receiving two differential voltages, and a first output terminal and a second output terminal for outputting two differential signals;
a first capacitor array comprising at least one first switch capacitor unit controlled by a first signal comprising a most significant bit and omitting a least significant bit of a control signal, and a plurality of first capacitors coupled between the first output terminal and the second output terminal and have a first equivalent capacitance when the first signal is enabled; and
two second capacitor arrays, each second capacitor array comprising at least one second switch capacitor unit controlled by a second signal comprising the least significant bit and omitting the most significant bit of the control signal, and a plurality of second capacitors, wherein the second capacitors of the two second capacitor arrays are respectively coupled between the first output terminal and a ground level, and between the second output terminal and the ground level, and have a second equivalent capacitance when the second capacitors of the two second capacitor arrays are coupled in serial and the second signal is enabled, wherein the first equivalent capacitance exceeds that of the second equivalent capacitance.

2. The transconductance filter circuit as claimed in claim 1, wherein the first switch capacitor unit comprises:
a first capacitor coupled to the first output terminal;
a first switch coupled to the first capacitor in serial between the first output terminal and the second output terminal;
a second switch coupled to the first output terminal; and
a second capacitor coupled to the second switch in serial between the first output terminal and the second output terminal.

3. The transconductance filter circuit as claimed in claim 2, wherein the first switch and the second switch are n-type transistors respectively comprising a gate coupled to the first signal, a first source/drain coupled to the first capacitor or the second capacitor, and a second source/drain coupled to the first output terminal or the second output terminal.

4. The transconductance filter circuit as claimed in claim 2, wherein the first switch and the second switch are p-type transistors respectively comprising a gate coupled to the first signal, a first source/drain coupled to the first capacitor or the second capacitor, and a second source/drain coupled to the first output terminal or the second output terminal.

5. The transconductance filter circuit as claimed in claim 1, wherein the second switch capacitor unit comprises a third capacitor and a third switch connected in serial between the ground level and the first output terminal or the second output terminal.

6. The transconductance filter circuit as claimed in claim 5, wherein the third switch is an n-type transistor comprising a gate coupled to the second signal, a third source/drain coupled to the third capacitor, and a fourth source/drain coupled to the ground level.

7. The transconductance filter circuit as claimed in claim 5, wherein the third switch is a p-type transistor comprising a gate coupled to the second signal, a third source/drain coupled to the third capacitor, and a fourth source/drain coupled to the ground level.

8. The transconductance filter circuit as claimed in claim 1, wherein the first equivalent capacitance 2n times of the second equivalent capacitance, where n is a natural number.