Programmable RC filter

[0002] 1. Field of the Invention

[0003] This invention relates generally to an apparatus for controlling the frequency response of an electrical signal filter and more particularly to a programmable resistor-capacitor filter suitable for VLSI integrated circuits.

[0004] 2. Description of Prior Art

[0005] Electrical signal filters comprised of a combination of electrical impedances such as resistors(R) and capacitors(C) are well known and are used extensively in all types of electrical and electronics apparatus.

[0006] Typically RC filters fall into two classes, low-pass filters and high-pass filters. For example, FIG. 1 discloses a simple low-pass filter comprised of a single fixed resistor 10 and capacitor 12 connected between an input terminal 14 and an output terminal 16. The resistor 10 is connected in series between input and output terminals 14 and 16 while the capacitor is connected between a common junction 18 between the RC components and a point of reference potential shown as ground.

[0007] FIG. 2 on the other hand depicts a simple high-pass filter consisting of a resistor 10 and a capacitor 12 but now the resistor 10 is returned to ground from junction 18 while the capacitor 12 is connected in series between the input and output terminals 14 and 16. The filter circuits shown in FIGS. 1 and 2 have a single corner of frequency response depending upon the selected values of the resistor 10 and the capacitor 12.

[0008] Where there is a need to vary the corner of the frequency response low-pass and high-pass filters on demand, a plurality of resistors and capacitors have been used which are selectively switched in and out of a filter circuit. For example, FIG. 3 is illustrative of a switched resistor approach for implementing a high-pass filter and includes a single capacitor 12 and a plurality of fixed resistors 10₁, 10₂, . . . 10_n−1, and 10_nwhich are connectable between one side of the capacitor 12, as shown by reference numeral 22 and ground by means of a set of electrically controlled switch devices 24₁, 24₂, . . . 24_n. These switches are respectively closed or opened in response to control signals applied from a control source, not shown, to terminals 26₁, 26₂, . . . 26_n. The control source may be, for example, a multi-bit control register where each bit determines which resistor is to be connected into the circuit. In the circuit configuration shown in FIG. 3, one or all of the fixed resistors 10₁. . . 10_ncould be connected in parallel between 22 and ground. In such a configuration, a large coupling capacitor 28 is typically included between the output terminal 16 and the filter components to provide DC isolation due to the shunting effect of the high-pass filter resistors 10₁, . . . 10_n.

[0009] In a like manner, a plurality of capacitors can be utilized in a switched circuit configuration for implementing a high-pass filter. This is shown in FIG. 4. Now a plurality of fixed capacitors 12₁, 12₂, . . . 12_nand series connected switches 24₁, 24₂, . . . 24_nare connectable in parallel between the input terminal 14 and one side of the grounded resistor 10 as shown by reference numeral 23. As before, the controlled switches 24₁, 24₂, . . . 24_nare open and closed in response to control signals applied to terminals 30₁, 30₂, . . . 30_n. Thus, depending upon which capacitor, for example, 12₁or a combination of parallel capacitors 12₂, . . . 10_nis switched into the circuit a high-pass filter characteristic can be selectively programmed or changed as desired. Again, a large coupling capacitor 28 is connected between the filter and output terminal 16 to provide the required isolation between the low-pass filter and the circuitry 10 connected to the output terminal 16.

[0010] Where such circuitry is desired for use in connection with on-chip digital control of RC filters fabricated in a VLSI integrated circuit, both approaches as shown in FIGS. 3 and 4 suffer from an inherent disadvantage in that relatively large coupling capacitors are required to provide the necessary DC isolation due to the shunting effect of the resistors. Also, in the switched capacitor approach, as shown in FIG. 4, a relatively large area is required on the chip due to the need of multiple capacitors which by their very nature consumes an inordinate amount of space, i.e. area, on the structure.

[0011] Accordingly, it is an object of the present invention to provide improvement in electrical signal filters.

[0012] It is another object of the invention to provide improvement in programmable RC filters which are suitable for very large scale integrated circuits.

[0013] It is yet another object of the invention to provide a programmable RC filter which overcomes both size and isolation limitations of the prior art.

[0014] These and other objects are achieved by a modified switched resistor approach which overcomes the limitations of the prior art and involves the use of a resistive type voltage divider network where one of the resistors comprises one element of the filter and wherein the voltage divider also provides a DC bias to the circuitry coupled to the output of the filter.

[0015] In one aspect of the invention, there is provided a filter comprised of a resistive component and a capacitive component connected together in an electrical filter circuit configuration between an input terminal and an output terminal and having a predetermined frequency response and wherein the resistive component comprises one element of at least one voltage divider circuit connected across a DC voltage source so to provide a substantially fixed DC voltage of a predetermined magnitude at the output terminal

[0016] In another aspect of the invention, there is provided a filter comprised of a capacitor connected between an input terminal and an output terminal, and a plurality of switched voltage dividers being connected between a source of DC voltage and point of reference potential, each of said voltage dividers including at least two series connected circuit elements having an electrical resistance chracteristic and having a common circuit connection therebetween, said common circuit connection being commonly connected to said capacitor and said output terminal and thereby providing a DC bias voltage at said output terminal.

[0017] In yet another aspect of the invention, there is provided a filter comprised of a high-pass filter section including a first capacitor connected between an input terminal and an intermediate circuit junction, and a plurality of switched voltage dividers each including at least a pair of resistive circuit elements connected across a source of DC voltage and wherein said at least one resistive element is connected to a point of reference potential or ground, each of the voltage dividers having a series circuit connection commonly connected to the intermediate circuit junction, and wherein the voltage dividers are selectively switched in and out of circuit relationship such that one resistive element of said pair of resistive circuit elements and said first capacitor form a filter circuit having a programmable high-pass frequency response, and a low-pass filter section including a plurality of switched resistive circuit elements selectively connected in parallel between the intermediate circuit junctions and an output terminal, and a second capacitor connected between one side of said switched resistive elements common to the output terminal and said point of reference potential or ground, and wherein at least one resistive element of said switched resistive elements is switched into circuit relationship with the second capacitor so as to form a filter circuit having a programmable low-pass frequency response, and wherein a DC bias voltage is provided at said output terminal.

[0018] Where active semiconductor devices such as field effect transistors are utilized as the resistive elements of the filter, such an implementation utilizes very little area and consumes only a small amount of power on a semiconductor integrated circuit chip.

[0019] Further scope of applicability of the present invention will become apparent from the detailed description which follows. It should be noted however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are provided by way of illustration only, since various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

[0020] The present invention will become more fully understood when considered in conjunction with the accompanying drawings which are provided only for purposes of illustration and thus are not meant to be considered in a limiting sense, and wherein:

[0021] FIG. 1 is a simple electrical schematic diagram illustrative of a low-pass filter;

[0022] FIG. 2 is a simple electrical schematic diagram illustrative of a high-pass filter;

[0023] FIG. 3 is an electrical schematic diagram illustrative of a programmable high-pass filter utilizing switched resistors in the circuit;

[0024] FIG. 4 is an electrical schematic diagram of a programmable high-pass filter utilizing switch capacitors in the circuit;

[0025] FIG. 5 is a simplified electrical schematic diagram of a high-pass filter circuit which is also adapted to provide a DC bias voltage on the output;

[0026] FIG. 6 is an electrical schematic diagram illustrative of a switched resistor embodiment of the filter circuit shown in FIG. 5;

[0027] FIG. 7 is an electrical schematic diagram of an embodiment of the invention which utilizes active devices and operates as a programmable bandpass filter while additionally providing a DC bias voltage for circuitry connected to the output thereof;

[0028] FIG. 8 is a set of characteristic curves illustrative of the gain vs. frequency response of the bandpass filter shown in FIG. 7; and

[0029] FIG. 9 is a set of curves illustrative of the phase shift vs. frequency characteristic of the bandpass filter shown in FIG. 7.

[0030] Having discussed above what is considered to be the relevant prior art, the present invention is directed to an improved filter circuit which allows control of two desired parameters, a DC bias voltage which is provided at the output and the frequency response characteristic of a filter and one which requires very little space on a semiconductor chip and requires very little power.

[0031] In its simplest form, FIG. 5 discloses a schematic of a high-pass electrical signal filter the resistive element of which also comprises one element of a voltage divider. As shown, the filter circuit includes a fixed capacitor 32 and two fixed resistors 34 and 36, the latter components being coupled in series across a voltage V with a common connection therebetween 38 and also being connected to one side of the capacitor 32 and an output terminal 40. An input terminal 42 is connected to the other side of the capacitor 32.

[0032] As is well known, a low-pass filter such as shown in FIG. 1 includes a frequency response which has a high frequency corner (HFC) which can be defined by the equation:

[0033] HFC=½π R1C1  (1)

[0034] In a like manner, a high-pass filter such as shown in FIG. 2 includes a low frequency corner or LFC which can be defined as:

[0035] LFC=½πR2C2  (2)

[0036] With respect to the high-pass filter of FIG. 5, the value of R1 in equation (1) now becomes the parallel combination of R3 and R4 which can be expressed as:

[0037] R=R3R4/(R3+R4)  (3)

[0038] and therefore, the high frequency corner (HFC) will be:

[0039] HFC=½πRC3.  (4)

[0040] Also in FIG. 5, the DC bias potential V_outat the output terminal will be:

[0041] V_out=VR3/(R3+R4)  (5)

[0042] Equation 5 indicates that the value of the DC bias voltage can be set by selection of the suitable values of R3 and R4. Further, it can be seen from the above equations that an arbitrary pair of values for LFC and V_outcan be resolved simultaneously by solving equations (3) and (5) simultaneously.

[0043] This now leads to a consideration of a programmable high-pass RC filter which can be achieved by a switched resistor arrangement shown in FIG. 6. As shown, FIG. 6 includes a single filter capacitor 32 and a plurality of resistive voltage dividers consisting of resistors 34₁, . . . 34_nand 36₁, . . . 36_n, connectable to one side of capacitor 32 at 38 via electrically controlled switches 46₁, . . . 46_n, and 48₁. . . 48_nwhich are series connected to the individual resistors.

[0044] Further as shown in FIG. 6, the switches 46₁and 48₁are closed, while the switches 46₂, . . . 46_n, and 48₂. . . 48_nare open and in this regard resembles the switched resistor configuration in FIG. 3. Each pair of switches 46₁, 48₁, . . . 46_n, 48_nare controlled by the same control signal applied to terminals 50₁, 50₂, . . . 50_nfrom a control source such as a digital register. However, this filter circuit does not require a large coupling capacitor and provides a DC voltage component on the output voltage V_outwhich does not shift with the proper design and selection of the values for the individual voltage dividers and can thus be used as a DC bias voltage for a circuit coupled to the output terminal 40. Thus, what is provided is a programmable RC filter which provides three basic advantages of: small size, self-bias, and symmetry.

[0045] This concept can be employed in the implementation of either a low pass, a high pass, or a bandpass filter, a preferred embodiment which is shown, for example, in FIG. 7, and where active semiconductor devices, such as CMOS field effect transistors, are utilized which operate as both switches and resistor elements. It should be noted, however, that when desirable bipolar junction transistors may be utilized.

[0046] Referring now to FIG. 7, reference numeral 52 is directed to the high-pass portion of the bandpass filter, while reference numeral 54 is directed to the low-pass portion of the filter. The high-pass portion 52, as shown, includes a fixed capacitor 53 having one side connected to the input terminal 42, while its opposite side is connected to a circuit junction shown as a signal lead 56 which is connected to a common point between the sources and drains of four pairs of cascoded or series connected MOSFETs 60₁, . . . 60₄, and 62₁. . . 62₄. Further as shown, the drains of MOSFETs 60₁, 60₂, 60₃, and 60₄are commonly connected to a V_ccsupply source via a power bus 64. The sources of MOSFETs 62₁, 62₂, 62₃, and 62₄on the other hand, are commonly returned to ground via a pair of series connected MOSFETs 68 and 70, which have common gate-drain connections and act as a pair of diodes providing a voltage of V_cc/2 at circuit node VY.

[0047] The gate electrodes of the first pair of MOSFETs 60₁and 62₁are commonly connected to a control terminal 72. Likewise, the gates of MOSFET pairs 60₂, 62₂; 60₃, 62₃; and 60₄, 62₃, are connected to respective control terminals 74, 76 and 78. Control terminal 72, 74, 76 and 78 are adapted to be connected to a control signal source such as a four bit digital control register, not shown, for providing programmability of the low frequency corner (LFC) of the high-pass section 52 by activation of one or more pairs of the MOSFETs included in the high-pass portion 52 of the filter.

[0048] The low-pass portion 54 of the filter shown in FIG. 7 shows a single fixed capacitor 55 connected from the output terminal to ground. The output terminal 40 is also commonly connected to the emitters of four parallel MOSFETs 80, 82, 84 and 86 via circuit lead 88. The drains of MOSFETs 80, 82, 84 and 86 are commonly connected to circuit lead 90 which is connected to circuit node VX which is common to circuit junction lead 56 and thus provides an intermediate circuit connection between the filter portions 52 and 54.

[0049] The gate electrodes of the parallel MOSFETs 80, 82, 84 and 86 are respectively connected to control terminals 72, 74, 76 and 78 via circuit leads 92, 94, 96 and 98. Thus, for example, MOSFET 80 is rendered conductive and non-conductive simultaneously with MOSFETs 60₁, and 62₁. Likewise, MOSFETs 82, 84, and 86 of the low-pass section 54 are rendered simultaneously conductive and nonconductive with pairs of MOSFETs 60₂, 62₂; 60₃, 62₃; and 60₄, 62₄of the high-pass section 52.

[0050] The gain and phase responses for the circuit shown in FIG. 7 are shown in FIGS. 8 and 9. FIG. 8, for example, indicates that the low frequency corner (LFC) begins at 0.5 MHz and is doubled in each succeeding bit over the frequency band of 1×10⁵Hz to 1×10¹⁰Hz, while the high frequency corner (HFC) begins at 2 GHz and is halved in each succeeding bit, as evidenced by the curves 100, 102, 104, and 106. The phase shift of the bandpass filter over the frequency band of 1×10⁵Hz to 1×10¹⁰Hz is shown by curves 108, 110, 112 and 114.

[0051] The bandpass filter shown in FIG. 7, will be less susceptible to process tolerance due to its symmetrical construction, which indicates higher obtainable performance in comparison to conventional approaches. Whereas the voltage dividers in FIG. 6 are implemented with resistors, and the voltage dividers in FIG. 7 are implemented with active devices, it should be understood that, when desired, a mixed combination of resistors and active devices can be utilized.

[0052] Having thus shown what is considered to be the preferred embodiment of the invention, it should be noted that the same has been made by way of illustration and not limitation. Accordingly, all modifications, alterations and changes coming within the spirit and scope of the invention as set forth in the appended claims are herein meant to be included.