JP2008219792A - Filter and semiconductor integrated circuit device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a filter and semiconductor integrated circuit device in which circuit design can be easily performed in a small circuit scale, regarding the filter and the semiconductor integrated circuit device capable of switching frequency characteristics. <P>SOLUTION: The present invention relates to a filter (100:200) comprising a plurality of stages of filter circuits (111, AMP 11/112, AMP 12;113, AMP 13:211, 212 and AMP 21) and capable of switching required frequency characteristics. In the plurality of stages of filter circuits (111, AMP 11/112, AMP 12;113 and AMP 13), filter circuits (111, AMP 11/112 and AMP 12) of which the quality factor Q is greater than a fixed value are provided as many as a number corresponding to required frequency bands (fc1, fc2), and connections of the filter circuits (111, AMP11/112, AMP 12:211, 212, AMP 21) are switched in accordance with the required frequency bands (fc1, fc2). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a filter and a semiconductor integrated circuit device, and more particularly to a filter and a semiconductor integrated circuit device whose frequency characteristics can be switched.

  In recent years, it has been desired to incorporate a filter in a semiconductor integrated circuit in accordance with demands for downsizing and integration of electronic devices.

  As a filter circuit built in a semiconductor integrated circuit device or the like, an active filter that can be miniaturized is generally used. The active filter is a filter circuit configured by combining an active element such as an operational amplifier or a transistor and a passive element such as a resistor or a capacitor.

  Further, it is desired that the filter circuit can switch frequency characteristics such as a cut-off frequency by a switching signal. Switching of frequency characteristics such as a cut-off frequency is performed, for example, by increasing or decreasing the capacitance component of the built-in capacitor.

  FIG. 8 is a block diagram showing an example of a conventional filter circuit.

  The filter circuit 10 shown in FIG. 8 is an active filter circuit with variable secondary frequency characteristics, and includes resistors R1, R2, capacitors C1a, C1b, C2a, C2b, switch circuits SW1, SW2, and active element circuit unit AMP1. It is composed of

  In the filter circuit 10, by turning on and off the switches SW1 and SW2, the capacitor C1b is connected / disconnected in parallel with the capacitor C1a, and the capacitor C2b is connected / disconnected in parallel with the capacitor C2a. The filter circuit 10 is configured to switch the frequency characteristics such as the cut-off frequency by increasing / decreasing the capacitance component by connecting / disconnecting the capacitors C1b, C2b.

At this time, the cutoff frequency fc of the active filter circuit 10 is
fc = 1 / (2π · Cf · Rf) (1)
Is required.

Therefore, the capacitance component Cf for obtaining the cut-off frequency fc is given by the following equation (1): Cf = 1 / (2π · fc · Rf) (2)
Is required.

here,
Rf = R1 = R2 (3)
C1 = 2Q · Cf (4)
C2 = Cf / 2Q (5)
Q = 1 / {√ (C2 / C1) + √ (C2 / C1)} (6)
Determined by. In equations (4) to (6), C1 is a combined capacity of the capacitor C1a and the capacitor C1b, C2 is a combined capacity of the capacitor C1a and the capacitor C1b, and Q is a quality factor of frequency characteristics.

  When the filter circuit 10 is mounted on a semiconductor integrated circuit device, the resistors R1, R2, capacitors C1a, C1b, C2a, C2b, and switch circuits SW1, SW2 have parasitic capacitance.

  First, the parasitic capacitance Ce1 of the resistors R1 and R2 will be described.

  FIG. 9 shows a cross-sectional view of the resistors R1 and R2.

  As shown in FIG. 9, the resistors R 1 and R 2 are constituted by a p-type diffusion layer 23 formed on an n-type epitaxial layer 22 formed on a semiconductor substrate 21. At this time, a parasitic capacitance Ce1 was formed between the n-type epitaxial layer 22 and the p-type diffusion layer 23.

  Next, the parasitic capacitance Ce2 of the capacitors C1a, C1b, C2a, C2b will be described.

  FIG. 10 shows a cross-sectional view of the capacitors C1a, C1b, C2a, C2b.

  As shown in FIG. 10, capacitors C 1 a, C 1 b, C 2 a, C 2 b form an n-type epitaxial layer 31 on a semiconductor substrate 21 and form a conductor layer 33 on the n-type epitaxial layer 31 via a dielectric layer 32. The capacitance component is formed by the dielectric layer 32 interposed between the n-type epitaxial layer 31 and the conductor layer 33. At this time, a parasitic capacitance Ce2 was formed between the semiconductor substrate 21 and the n-type epitaxial layer 31.

  Next, the parasitic capacitance Ce3 of the switch circuits SW1 and SW2 will be described.

  FIG. 11 is a circuit configuration diagram of the switch circuits SW1 and SW2, and FIG. 12 is a cross-sectional view of the switch circuits SW1 and SW2.

  The switch circuits SW1 and SW2 have a configuration in which the emitters and collectors of NPN transistors Q1 and Q2 are complementarily connected. A switching signal cnt is supplied to the base, and the transistors Q1 and Q2 are turned on / off by the switching signal cnt. Thus, the capacitors C1b and C2b are connected / disconnected in parallel with the capacitors C1a and C2a.

  The transistors Q1 and Q2 have an n-type epitaxial layer 41 formed on the semiconductor substrate 21 as an emitter, a p-type diffusion layer 42 formed on the n-type epitaxial layer 41, a base, and a p-type diffusion layer 42. An n-type diffusion layer 43 is formed thereon to form a collector. At this time, a parasitic capacitance Ce3 was formed between the p-type semiconductor substrate 21 and the n-type epitaxial layer 41.

  FIG. 13 shows an equivalent circuit diagram of an example of a conventional filter circuit.

  13 is a circuit configuration diagram in which the filter circuit 10 shown in FIG. 8 includes resistors R1, R2, capacitors C1a, C1b, C1a, C2b, switch circuits SW1, SW2, parasitic capacitances Ce1, Ce2, Ce3, and a resistance component r. Is shown.

  As shown in FIG. 13, the filter circuit 10 shown in FIG. 8 includes a resistor element, a capacitor element, and parasitic capacitances Ce1, Ce2, and Ce3 of the selector terminal on each part of the filter on the actual chip. In consideration of the parasitic capacitances Ce1, Ce2, and Ce3, in the conventional configuration, the series combined capacitance of the capacitor C1b and the capacitor C1b grounded to GND loses the capacitance value of the capacitor C1a. The loss is adjusted by subtracting from the capacitor C2a.

  However, when a filter having a quality factor Q> 1 is to be constructed, C1 / C2> 4, the combined capacitance C2 is small, and the combined capacitance C1 is large. When the cut-off frequency is switched by the conventional method, both the capacitors C1a and C1b have very large values, the above-described combined capacitance is very large, and the loss of the capacitor C1a increases.

  If the capacitor C1a is increased in order to compensate for the loss due to the combined capacitance, the value of the capacitor C1b when the switch SW1 is connected is inevitably adjusted. If the adjustment is made, the combined capacitance also fluctuates. It was very difficult to find the optimum value of the capacitor C1b, and the design was not easy.

Also, when switching and using frequency characteristics such as the filter cutoff frequency, prepare a plurality of filter circuits set to the cutoff frequency to be obtained, and select the filter circuit to be used as necessary Circuits have been proposed (see, for example, Patent Documents 1 to 5).
JP 57-159114 A JP-A-4-342372 JP-A-5-300371 Japanese Patent Laid-Open No. 5-161159 JP 2004-247935 A

  However, the filter circuit as shown in FIG. 8 has a problem that circuit design is difficult. Also, as shown in Patent Documents 1-5, a plurality of filter circuits having different frequency characteristics are provided, and a filter circuit is selected according to a switching signal, and a filter circuit is provided for each frequency characteristic to be obtained. Therefore, there is a problem that the circuit scale becomes large.

  The present invention has been made in view of the above points, and an object of the present invention is to provide a filter and a semiconductor integrated circuit device capable of easily designing a circuit with a small circuit scale.

  The present invention is a filter (100: 200) comprising a plurality of stages of filter circuits (111, AMP11 / 112, AMP12; 113, AMP13: 211, 212, AMP21) and capable of switching the required frequency characteristics. Of the plurality of stages of the filter circuits (111, AMP11 / 112, AMP12; 113, AMP13), the frequency band (111, AMP11 / 112, AMP12) in which the quality factor Q is larger than a certain value is required. fc1, fc2) are provided according to the number, and the connection of the filter circuits (111, AMP11 / 112, AMP12: 211, 212, AMP21) is switched according to the required frequency band (fc1, fc2). .

  The filter circuit (111, AMP11 / 112, AMP12; 113, AMP13) is composed of an active filter circuit having an active element circuit (AMP11, AMP12, AMP13) and a passive element circuit (111, 112, 113). It is characterized by.

  The filter circuit (211, 212, AMP21) whose connection is switched according to the required frequency band (fc1, fc2) requires a common active element circuit (AMP21) and a passive element circuit (211, 212). The number is set according to the frequency band (fc1, fc2) to be switched, and is switched according to the required frequency band (fc1, fc2).

  In addition, the present invention is composed of a plurality of stages of filter circuits (111, AMP11 / 112, AMP12; 113, AMP13: 211, 212, AMP21), and a filter (100: 200) in which a required frequency characteristic can be switched. ), A filter circuit (111, AMP11 / 112, AMP12) having a quality factor Q greater than a certain value among the plurality of stages of filter circuits (111, AMP11 / 112, AMP12; 113, AMP13). As many as the required frequency bands (fc1, fc2) are provided, and the filter circuits (111, AMP11 / 112, AMP12: 211, 212, AMP21) are connected according to the required frequency bands (fc1, fc2). It is characterized by switching.

  In addition, the said reference code is a reference to the last, and description of a claim is not limited by this.

  According to the present invention, a filter circuit (111, AMP11 / 112, AMP12) having a quality factor Q larger than a certain value among the plurality of stages of filter circuits (111, AMP11 / 112, AMP12; 113, AMP13) is required. By providing the number corresponding to the frequency band (fc1, fc2), and switching the connection of the filter circuits (111, AMP11 / 112, AMP12: 211, 212, AMP21) according to the required frequency band (fc1, fc2) Since the parasitic capacitance of the filter circuit (111, AMP11 / 112, AMP12) whose quality factor Q is larger than a certain value can be substantially fixed, and the filter can be designed in consideration of the parasitic capacitance, the filter design is easy. Yes.

  FIG. 1 is a circuit diagram showing an embodiment of the present invention.

  The filter circuit 100 of this embodiment is mounted on a one-chip semiconductor integrated circuit device, and constitutes a low-pass filter by connecting positive feedback active filter circuits in two stages in cascade. The filter circuit 100 includes passive element circuit units 111, 112, and 113, active element circuit units AMP11, AMP12, AMP13, and a switch SW11. By switching the switch SW11, the cut-off frequency is changed to the first cut-off frequency. To the second cutoff frequency.

  The passive element circuit unit 111 includes resistors R11 and R12 and capacitors C11 and C12, and is selected when the cutoff frequency of the frequency characteristic of the low-pass filter is set to the first cutoff frequency.

  The passive element circuit unit 112 includes resistors R21 and R22 and capacitors C21 and C22, and is selected when the cut-off frequency of the frequency characteristic of the cut-off frequency of the low-pass filter is set to the second cut-off frequency.

  Input signals are supplied to the passive element circuit units 111 and 112.

  The passive element circuit unit 113 includes resistors R31 and R32, capacitors C31a, C31b, C32a and C32b, and switches SW21 and SW22. When the cut-off frequency of the frequency characteristic of the low-pass filter is set to the first cut-off frequency. The switches SW21 and SW22 are turned off by the control signal cnt, only the capacitors C31a and C31b are connected, and the switch SW21 and SW22 are turned on by the control signal cnt when the cutoff frequency of the frequency characteristic of the low-pass filter is set to the second cutoff frequency. Then, the combined capacitance of the capacitor C31a, the capacitor C32a, and the parasitic capacitance C41 of the switch SW21 is added, and the combined capacitance of the capacitor C31b, the capacitor C32b, and the parasitic capacitance C42 of the switch SW22 is added.

  The active element circuit unit AMP11 is an amplifier circuit and constitutes an active filter circuit together with the passive element circuit unit 111. The active element circuit unit AMP12 is an amplifier circuit and constitutes an active filter circuit together with the passive element circuit unit 112. The active element circuit unit AMP13 is an amplifier circuit and constitutes an active filter circuit together with the passive element circuit unit 113.

  The switch SW11 is supplied with the output signal of the active element circuit unit AMP11 and the output signal of the active element circuit unit AMP12, and the output signal of the active element circuit unit AMP11 or the output of the active element circuit unit AMP12 according to the control signal cnt. One of the signals is selectively supplied to the passive element circuit unit 113.

  FIG. 2 is a circuit configuration diagram of the active element circuit unit AMP11.

  The active element circuit unit AMP11 includes NPN transistors Q51 to Q53, a resistor R51, and current sources 121, 122, and 123, and constitutes a 1 × non-inverting amplifier circuit. The output of the active element circuit unit AMP11 is fed back to the connection point between the resistor R11 and the resistor R12 via the capacitor C11.

  The active element circuit unit AMP11 and the passive element circuit unit 111 constitute a positive feedback active filter having a fixed frequency characteristic. The active element circuit unit AMP21 has the same configuration as the active element circuit unit AMP11 shown in FIG.

  The passive element circuit units 111 and 112 and the active element circuit units AMP11 and AMP12 constitute primary and secondary filter circuits having a high quality factor Q, respectively, and have frequency characteristics such as a cutoff frequency. Are set to resistors R11, R12; R21, R22 and capacitors C11, C12; C21, C22.

  The switch SW11 selects the output signal of the active element circuit unit AMP11 or the output signal of the active element circuit unit AMP12 according to the switching signal cnt and supplies the selected signal to the passive element circuit unit 113.

  FIG. 3 shows a circuit configuration diagram of the switch SW11.

  The switch SW11 includes NPN transistors Q61 to Q68, resistors R61 to R66, an inverter INV, and constant current sources 131 and 132.

  Transistors Q61 and Q62, resistors R61 and R62, inverter INV, transistors Q64 and Q65, and resistors R63 and R64 constitute a switch circuit. A switch circuit including transistors Q61 and Q62, resistors R61 and R62, and an inverter INV turns on / off the supply of the signal that has passed through the filter circuit unit 111 in accordance with the switching signal cnt. The switch circuit including the transistors Q64 and Q65 and the resistors R63 and R64 turns on / off the supply of the signal that has passed through the filter circuit unit 112 according to the switching signal cnt.

  When the switch circuit composed of the transistors Q61 and Q62, the resistors R61 and R62, and the inverter INV is on, the switch circuit composed of the transistors Q64 and Q65 and the resistors R63 and R64 is turned off, and the transistors Q61 and Q62, the resistor When the switch circuit composed of R61 and R62 and the inverter INV is off, the switch circuit composed of the transistors Q64 and Q65 and the resistors R63 and R64 is controlled to be turned on.

  Transistors Q63, Q66, and Q67, resistors R65 to R66, and constant current sources 131 and 133 constitute a differential amplifier circuit. The transistor Q63 and the resistor R65 constitute a non-inverting input stage to the differential amplifier circuit for the signal that has passed through the switch circuit composed of the transistors Q61 and Q62, the resistors R61 and R62, and the inverter INV. The resistor R66 constitutes a non-inverting input stage to the differential amplifier circuit for the signal that has passed through the switch circuit composed of the transistors Q64 and Q65 and the resistors R63 and R64. The transistor Q67 and the constant current source 133 constitute an inverting input stage to the differential amplifier circuit.

  The transistor Q68 and the constant current source 132 are output circuits, which invert and output the output of the differential amplifier circuit composed of the transistors Q63, Q66, and Q67, resistors R65 to R67, and the constant current source 131.

  With the above configuration, the switch SW11 outputs the output signal of the active element circuit unit AMP11 when the switching signal cnt is high level, and outputs the output signal of the active element circuit unit AMP12 when the switching signal cnt is low level. To do. The signal selected by the switch SW11 is supplied to the passive element circuit unit 113.

  The passive element circuit unit 113 includes resistors R31 and R32, capacitors C31a, C31b, C32a and C32b, and switches SW21 and SW22, and forms a positive feedback active filter whose frequency characteristics change together with the active element circuit unit AMP13. In response to the switching signal cnt, the switches SW21 and SW22 are turned on / off, and the frequency characteristics are varied. The passive element circuit 113 filter circuit 113 constitutes a third-order and fourth-order filter circuit. The active element circuit unit AMP13 has the same configuration as the active element circuit unit AMP11 shown in FIG.

  Next, the operation of the filter 100 of this embodiment will be described.

  FIG. 4 shows a frequency characteristic diagram of one embodiment of the present invention. In FIG. 4, “♦” indicates that the switching signal cnt is at a high level, that is, when the cutoff frequency is switched to a low frequency fc1, and “▲” indicates that the switching signal cnt is at a low level, that is, the cutoff frequency is switched to a high frequency fc2. The frequency characteristics are shown.

  When the switching signal cnt is at a high level, the output of the filter circuit unit 111 is selected by the switch SW11 and supplied to the filter circuit unit 113. At this time, in the filter circuit unit 113, the switches SW31 and SW32 are turned on, the capacitor C31b is connected in parallel to the capacitor C31a, and the capacitor C32b is connected in parallel to the capacitor C32a, so that the capacitance component increases. As a result, the cut-off frequency can be set to a low frequency fc1.

  When the switching signal cnt is at a low level, the output of the filter circuit unit 112 is selected by the switch SW11 and supplied to the filter circuit unit 113. At this time, in the filter circuit unit 113, the switches SW21 and SW22 are turned off, the capacitance components are determined by the capacitors C31a and C32a, and the capacitance components are reduced. As a result, the cutoff frequency can be set to the high frequency fc2.

In FIG. 4, the positive feedback active filter composed of the primary and secondary passive element circuit units 111 and 112 and the active element circuit units AMP11 and AMP12 constitutes a primary and secondary filter. The quality factor Q value is
Q = 1.93
It is.

The positive feedback type active filter composed of the passive element circuit unit 113 and the active element circuit unit AMP13 constitutes a third-order and fourth-order filter, and the quality factor Q value is
Q = 0.52
It is.

  By switching the switching signal cnt to the high level or the low level, the cutoff frequency can be switched to fc1 or fc2.

  According to the present embodiment, the resistances R11 and R12 of the passive element circuit units 111 and 112 constituting the first and second order filter circuits having a high quality factor Q value, for example, the quality factor Q value is “1” or more; R21, R22 and capacitors C11, C12; C21, C22 were fixed, and the switch for switching the capacitors was deleted. As a result, the parasitic capacitances of the resistors R11, R12; R21, R22 and the capacitors C11, C12; C21, C22 are fixed, and the capacitance component can be set in consideration of these parasitic capacitances. In particular, it is possible to easily design a high-frequency filter that is adjusted with a low capacity.

  Note that the arrangement of the filter circuit units 111 and 112 and the filter circuit unit 113 is not limited to the arrangement shown in FIG. 1, but in this embodiment, the first and second high quality factor Q values are initially high. The following filter circuit is arranged, and these output signals are selected by the switch SW11 and supplied to the low-order third- and fourth-order filter circuits. As a result, it is possible to prevent the input signal from being deteriorated by the third, fourth and fourth order filter circuits having a low quality factor Q value and thereafter being supplied to the first and second order filter circuits having a high quality factor Q value. The primary and secondary filter circuits having a high quality factor Q value can be effectively operated.

  In the present embodiment, the passive element circuit unit 113 constituting the third-order and fourth-order filter circuits connects / disconnects the capacitor C31b to / from the capacitor C31a by turning on / off the switch SW21 according to the control signal cnt, Further, the frequency characteristics are switched by connecting / disconnecting the capacitor C32b to / from the capacitor C32a by turning on / off the switch SW22 in accordance with the control signal cnt. By switching the capacitance component of the passive element circuit unit 113 constituting the third-order and fourth-order filter circuits by the switches SW21; SW22, the parasitic capacitances C41a, C41b; C42a, C42b of the switches SW21; Although it is given, since the circuit has a low quality factor Q value, the influence on the frequency characteristic is small, and the deterioration of the frequency characteristic is small.

  In this embodiment, the positive feedback active filters themselves constituting the first and second order filter circuits are switched. However, the active element circuit portions of the positive feedback active filters are shared, and the passive element circuit portions 111 and 112 are used. May be switched.

  FIG. 5 is a block diagram showing a first modification of the embodiment of the present invention. In the figure, the same components as in FIG.

  The filter 200 according to the present modification includes the passive element circuit unit 211, the passive element circuit unit 212, and the active element circuit unit 113. The switch SW31 is provided between the passive element circuit unit 211 and the active element circuit unit 113. The active element circuit unit AMP21 is commonly used by the unit 212.

  The passive element circuit unit 211 has a switch SW41 that switches in response to the control signal cnt, and turns on / off the connection between the output of the active element circuit unit AMP21 and the capacitor C11. The passive element circuit unit 211 has a parasitic capacitance C51 of the capacitor C11 and parasitic capacitances C52 and C53 of the switch SW41.

The passive element circuit unit 212 includes an inverter INV that inverts the control signal cnt, and a switch SW42 that switches according to the output of the inverter INV, and turns on / off the connection between the output of the active element circuit unit AMP21 and the capacitor C21. ing. The switch SW42 is turned off when the switch SW41 is on, and is turned on when the switch SW41 is off.
The passive element circuit unit 212 includes a parasitic capacitance C61 of the capacitor C21 and parasitic capacitances C62 and C63 of the switch SW42.

  When the switch SW31 is switched so that the output of the passive element circuit unit 211 and the active element circuit unit 113 are connected, the switch SW41 of the passive element circuit unit 211 is turned on and the switch SW42 of the passive element circuit unit 212 is turned off.

  When the switch SW41 of the passive element circuit unit 211 is turned on, the passive element circuit unit 211 and the active element circuit unit AMP21 function as a positive feedback active filter. At this time, since the switch SW42 of the passive element circuit unit 212 is turned off, the parasitic capacitance C61 of the capacitor C21 and the parasitic capacitor C62 of the switch SW42 are disconnected from the active element circuit unit AMP21, and therefore the parasitic capacitance C61 of the capacitor C21 and the switch SW42. The design can be performed without considering the parasitic capacitance C62. However, although the parasitic capacitance C63 on the active element circuit unit AMP21 side of the switch SW42 remains, the influence is small because the output impedance of the active element circuit unit AMP21 is small.

  When the switch SW31 is switched so that the output of the passive element circuit unit 212 and the active element circuit unit 113 are connected, the switch SW41 of the passive element circuit unit 211 is turned off and the switch SW42 of the passive element circuit unit 212 is turned on.

  When the switch SW42 of the passive element circuit unit 212 is turned on, the passive element circuit unit 212 and the active element circuit unit AMP21 function as a positive feedback active filter. At this time, since the switch SW41 of the passive element circuit unit 211 is turned off, the parasitic capacitance C51 of the capacitor C11 and the parasitic capacitance C52 of the switch SW41 are disconnected from the active element circuit unit AMP21, and therefore the parasitic capacitance C51 of the capacitor C11 and the switch SW41. The design can be performed without considering the parasitic capacitance C52. However, although the parasitic capacitance C53 on the active element circuit unit AMP21 side of the switch SW41 remains, since the output impedance of the active element circuit unit AMP21 is small and fixed, the design is made in consideration of the parasitic capacitances C53 and C63. Design is easy.

  As described above, according to this modification, even when the active element circuit unit AMP21 is shared by the passive element circuit unit 211 and the passive element circuit unit 212, the passive element circuit unit 212 can be used when the passive element circuit unit 211 is used. The influence of the parasitic capacitances C61 and C62 can be eliminated, and the influence of the parasitic capacitances C51 and C52 of the passive element circuit unit 211 can be eliminated when the passive element circuit unit 212 is used. Therefore, the parasitic capacitance to be considered can be reduced and the design can be easily performed.

  In the present embodiment, the configuration in which the present invention is applied to the fourth-order active filter circuit has been described. However, the present invention is not limited to this, and the present invention can also be applied to an active filter having a sixth or higher order. Needless to say.

  FIG. 6 is a block diagram showing a second modification of the embodiment of the present invention. In the figure, the same components as in FIG.

  The filter circuit 300 of the present modification example is composed of an n-th order active filter circuit, and can switch k cut-off frequencies. The filter circuit 300 includes first to m-th order filter circuit units 311-1 to 311-M... 31k-1 to 31k-M, (m + 1) th to n-th order filter circuit units 321-1 to 321-N. , And a selection circuit 331.

  The first to m-th order filter circuit units 311-1 to 311-M... 31k-1 to 31k-M have resistances R11 and R12; Capacitors C11 and C12; C21 and C22 are fixed and set to values corresponding to the respective orders. Note that m, n (> m), k, M, and N are natural numbers, and N is (nm).

  FIG. 7 shows a circuit configuration diagram of the filter circuit section 321-1.

  The filter circuit unit 321-1 includes resistors R311 and R312, active element circuit unit AMP311, capacitors C310, C311 to 31k; C320, C321 to C32k, switch circuits SW311 to SW31k; SW321 to SW32k, and a switching signal CNT. The capacitors C310 and C311 to 31k; C320 and C321 to C32k are connected / disconnected by turning on / off the switch circuits SW311 to SW31k; SW321 to SW32k in accordance with the frequency characteristics.

  The switch circuits SW311 to SW31n; SW321 to SW32n have the same configuration as the switch circuits SW1 and SW2 shown in FIG.

  A switching signal CNT is supplied to the selection circuit 331, and filters of the first to m-th order filter circuit units 311-1 to 311-M... 31k-1 to 31k-M according to the switching signal CNT. The signals that have passed through the circuit units 31i-1 to 31i-M are selected and supplied to the filter circuit unit 321-1. Note that i is one of 1 to k.

  The filter circuit units 321-2 to 321-n have the same configuration as the filter circuit unit 321-1 and the resistors R311 and R312, capacitors C310 and C311 to 31k; C320 and C321 to C32k have values corresponding to the respective orders. Is set.

  Note that k may be set to a value corresponding to the order of the filter circuit 300.

  In the present embodiment, the low-pass filter is described as an example of the filter characteristics. However, the present invention is not limited to the low-pass filter, and can be applied to a high-pass filter and a band-pass filter using an active filter circuit. Needless to say, you can.

  In addition, this invention is not limited to the said Example, It cannot be overemphasized that a various modified example can be considered in the range which does not deviate from the summary of this invention.

It is a circuit block diagram of one Example of this invention. It is a circuit block diagram of the active element circuit part AMP11. 2 is a circuit configuration diagram of a switching circuit 115. FIG. It is a frequency characteristic figure of one Example of this invention. It is a circuit block diagram of the 1st modification of one Example of this invention. It is a block block diagram of the 2nd modification of one Example of this invention. It is a circuit block diagram of the filter circuit part 321-1. It is a block block diagram of an example of the conventional filter circuit. It is sectional drawing of resistance R1, R2. It is sectional drawing of capacitor | condenser C1-C4. It is a circuit block diagram of switch circuit SW1 and SW2. It is sectional drawing of switch circuit SW1, SW2. It is an equivalent circuit diagram of an example of a conventional filter circuit.

Explanation of symbols

100, 200, 300 Filters 111, 112, 113, 211, 212 Passive element circuit units R11, R12, R21, R22, R31, R32 Resistors C11, C12, C21, C22, C31a, C31b, C32a, C32b Capacitors C41a, C41b , C41c, C42a, C42b, C42c,
C51, C52, C53, C61, C62, C63 Parasitic capacitance AMP11, AMP12, AMP13, AMP21 Active element circuit part SW11, SW21, SW22, SW31 switch

Claims (6)

A filter composed of a plurality of stages of filter circuits, the required frequency characteristics being switchable,
Of the plurality of stages of filter circuits, a number of filter circuits having a quality factor Q larger than a certain value are provided according to the required frequency band,
A filter that switches the connection of the filter circuit according to the required frequency band.   The filter according to claim 1, wherein the filter circuit includes an active filter circuit having an active element circuit and a passive element circuit. In the filter circuit whose connection is switched according to the required frequency band, the active element circuit is shared,
3. The filter according to claim 2, wherein a number corresponding to a frequency band required by the passive element circuit is provided, and switching is performed according to a required frequency band. A semiconductor integrated circuit having a filter composed of a plurality of stages of filter circuits and capable of switching a required frequency characteristic,
A plurality of filter circuits of a plurality of stages are provided according to the frequency band requiring a filter circuit having a large quality factor Q,
A semiconductor integrated circuit that obtains a required frequency band by switching connection of a plurality of filter circuits constituting a filter circuit having a quality factor Q larger than a certain value in accordance with the required frequency band.   The semiconductor integrated circuit according to claim 4, wherein the filter circuit includes an active filter circuit having an active element circuit and a passive element circuit. In the filter circuit whose connection is switched according to the required frequency band, the active element circuit is shared,
6. The semiconductor integrated circuit according to claim 5, wherein a number corresponding to the frequency band required by the passive element circuit is provided, and switching is performed according to the frequency band required.

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