JP3718000B2 - Band pass filter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a band-pass filter that selectively outputs only a signal in the vicinity of a predetermined frequency from input AC signals.
[0002]
[Prior art]
Conventionally, various band pass filters configured using active elements and passive elements are known. Particularly recently, a band-pass filter having an arbitrary frequency characteristic has been realized by a high-order active filter in which operational amplifiers (operational amplifiers) are connected in a plurality of stages.
[0003]
[Problems to be solved by the invention]
By the way, since various types of conventionally known active filters use operational amplifiers, there is a limit to increasing the frequency due to the limitation of the operating frequency. Therefore, it is difficult to construct a bandpass filter having a passband frequency of several tens of MHz or more, and there is a problem that an operational amplifier to be used becomes special and costs increase when attempting to obtain a frequency characteristic higher than this. . In addition, since the high-order active filter includes a plurality of operational amplifiers and passive elements, there is a problem that the configuration becomes complicated.
[0004]
Furthermore, the conventional band-pass filter realized by a high-order active filter requires changing the element constants of a plurality of elements included in each stage to change the frequency characteristics, and changing the frequency is not easy. .
[0005]
The present invention has been made in view of the above points, and an object of the present invention is to provide a band-pass filter that has a small number of parts, has a simple configuration, and can increase the frequency of the pass band. .
[0006]
Another object of the present invention is to provide a bandpass filter that can easily change the frequency in the passband.
[0007]
[Means for Solving the Problems]
The invention of each claim connects one inductor conductor of the LC element between the input and output of the first inverter circuit or the inverting amplifier, and connects the output terminal of the second inverter circuit or the inverting amplifier to the other of the LC element. By connecting to the inductor conductor, only a predetermined frequency determined by the inductor component and the capacitor component of the LC element can be passed. According to the present invention, a simple circuit configuration with a small number of parts can be realized.
[0008]
In particular, in the present invention, the pass bandwidth can be changed by changing the resistance value of the first resistor connected to the input side of the first inverter circuit or the like, and the first inverter circuit or the like is connected in parallel to the second inverter circuit or the like. The amplitude of the output signal can be changed by changing the resistance value of the second resistor. Further, by forming a pn junction layer between inductor conductors, an LC element which is a composite element in which electrostatic capacitance is distributed in a distributed constant manner between inductor conductors can be realized, and the reverse application to this pn junction layer is possible. By changing the bias voltage, the center frequency of the passband can be changed. Therefore, even when the entire components are integrally formed on the semiconductor substrate, the passband width, the center frequency of the passband, or the signal amplitude can be easily adjusted. In addition, when the component parts are integrally formed on the semiconductor substrate, the circuit can be reduced in size and cost can be reduced by integration.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a bandpass filter according to an embodiment to which the present invention is applied will be described in detail with reference to the drawings.
[0010]
FIG. 1 is a circuit diagram illustrating a configuration of a bandpass filter according to an embodiment. The band-pass filter 1 shown in FIG. 1 includes a variable resistor 12 serving as an input resistor, two inverter circuits 14 and 16 connected in series, a variable resistor 18 connected in parallel to the inverter circuit 16 at the subsequent stage, and an inverter And a distributed constant LC element 20 connected to each input / output terminal of the circuits 14 and 16.
[0011]
Each of the inverter circuits 14 and 16 normally receives a digital signal and inverts and outputs the logic of the input signal. In this embodiment, the inverter circuits 14 and 16 are used as analog elements. For example, an inverter circuit such as a commercially available CMOS 4000 series is used.
[0012]
The LC element 20 includes two inductor conductors, and is a composite element in which a capacitor is formed in a distributed constant manner between the two inductor conductors. The first or second input / output terminal is connected to the vicinity of both ends of one inductor conductor, and the first and second input / output terminals are respectively connected to the input / output terminals of the inverter circuit 14 in the previous stage. ing. Further, a third input / output terminal is connected in the vicinity of one end of the other inductor conductor, and this third input / output terminal is connected to the output terminal of the inverter circuit 16 at the subsequent stage.
[0013]
In the bandpass filter 1 having such a configuration, when an AC signal is input to one end of the variable resistor 12 serving as an input resistor, only a signal in the vicinity of a predetermined frequency is selected from the AC signal, and the inverter circuit 16 in the subsequent stage is selected. Is output from.
[0014]
When the applicant actually manufactured a circuit in which the resistance values of the two variable resistors 12 and 18 of the bandpass filter 1 shown in FIG. To make sure it works. The oscillation frequency is determined by the inductance of the inductor conductor of the LC element 20 and the capacitance appearing between the two inductor conductors, and it is confirmed that changing these values also changes the oscillation frequency.
[0015]
Further, in this sine wave oscillator, when the resistance value of the variable resistor 18 connected in parallel with the inverter circuit 16 in the subsequent stage is decreased, the oscillation stops when the amplitude of the oscillation output is gradually decreased below a certain value. .
[0016]
FIG. 2 is a diagram showing the relationship between the resistance value R2 of the variable resistor 18 and the signal amplitude of the oscillation output at this time. As shown in the figure, the oscillation stops when the resistance value R2 of the variable resistor 18 is smaller than A, the amplitude also changes according to the resistance value change between A and B, and the output amplitude is almost equal to or higher than B. Saturated.
[0017]
(Bandpass filter usage example 1)
In the band pass filter 1 shown in FIG. 1, the resistance value R2 of the variable resistor 18 connected in parallel to the inverter circuit 16 in the subsequent stage is set to a value a slightly smaller than A shown in FIG. The resistance value of the provided variable resistor 12 is set to a predetermined value (finite value). By setting the resistance value of each variable resistor in this way, the bandpass filter 1 can only output a signal in the vicinity of the oscillation frequency of the sine wave oscillator described above from the AC signal input to one end of the variable resistor 12. Since it is pulled out and output, it operates as a filter that allows only signals near the oscillation frequency to pass.
[0018]
FIG. 3 is a diagram illustrating frequency characteristics of the bandpass filter 1 described above. In the figure, the horizontal axis represents the frequency of the input signal, and the vertical axis represents the gain, that is, the ratio of the signal amplitude between the input and output signals in dB.
[0019]
As shown in the figure, only a signal near a certain frequency passes, an output signal having substantially the same amplitude as the input signal is output at the center frequency, and attenuated at other frequencies.
[0020]
Further, by changing the resistance value R1 of the variable resistor 12 provided in the previous stage of the inverter circuit 14, the Q of the bandpass filter 1, that is, the passband width of the signal can be changed. As shown in FIG. 3, when the resistance value R1 of the variable resistor 12 is large, only a signal having an extremely narrow frequency near the oscillation frequency of the sine wave oscillator described above is drawn, so that Q is large and the passband width is narrowed. On the other hand, when the resistance value R1 of the variable resistor 12 is reduced, a relatively wide range of signals is drawn, so that Q is small and the passband width is widened.
[0021]
(Bandpass filter usage example 2)
In the above description, the resistance value R2 of the variable resistor 18 connected in parallel to the inverter circuit 16 is set to a slightly smaller than A shown in FIG. The case of setting b between A and B shown in FIG. 2 is a state in which sine wave oscillation is performed in a state where no AC signal is input, and even when an AC signal is input in such a state, oscillation occurs. It has been confirmed that only signals near the frequency are drawn and only signals near the frequency pass.
[0022]
FIG. 4 is a diagram illustrating frequency characteristics of a band-pass filter in which a resistance value R2 is set so as to oscillate when no AC signal is input. In the figure, the horizontal axis represents the frequency of the input signal, and the vertical axis represents the gain, that is, the ratio of the signal amplitude between the input and output signals in dB.
[0023]
The frequency characteristic shown in the figure is basically similar to the frequency characteristic shown in FIG. 3, and only a signal in the vicinity of a certain frequency from the AC signal input to the bandpass filter 1 passes, and the others. It attenuates at the frequency of.
[0024]
In addition, it has been confirmed that the gain is larger than 0 near the center frequency of the pass band, and the input signal is amplified. Therefore, when the band-pass filter 1 is used so as to sine-wave oscillate in the absence of an input signal, it can be operated as a tuning amplifier that passes only signals near the oscillation frequency and amplifies the signals. .
[0025]
When the resistance value R1 of the variable resistor 12 serving as the input resistor is varied, the Q of the bandpass filter, that is, the passband width can be changed, similarly to the characteristics shown in FIG.
[0026]
As described above, the band-pass filter 1 described above is configured by components such as an inverter circuit, an LC element, or a variable resistor, and can be realized with a simple circuit configuration with a small number of components, thereby achieving cost reduction. Further, the pass band width of the band pass filter 1 can be changed only by changing the resistance value R1 of the variable resistor 12, and the characteristic can be easily changed.
[0027]
Further, the oscillation frequency, that is, the center frequency of the pass band of the band-pass filter 1 is determined by the inductance and capacitance of the LC element 20, and from another viewpoint, only the element constant of the LC element 20 is changed. The center frequency of the pass band can be changed with.
[0028]
In addition, the product name “Filmac” (manufactured by Niigata Seimitsu Co., Ltd.) is actually used as the distributed constant LC element 20 and the circuit of FIG. 1 is configured using the CMOS 4000 series inverter circuits 14 and 16. It has been confirmed that the center frequency of the passband is about 30 MHz. This frequency far exceeds the operating frequency when the CMOS 4000 series inverter circuits 14 and 16 are used as digital circuits. In other words, even if the bandpass filter 1 is configured using an inexpensive inverter circuit whose maximum operating frequency on the catalog is not so high, since the operating frequency can be set to several tens of MHz or more, it is easy to increase the frequency. You can plan. Therefore, for example, even when the center frequency of the passband is set to about 100 MHz, all or most of the components of the bandpass filter 1 can be integrated on the chip by using a widely used process. Become.
[0029]
(Specific example of LC element)
Next, a specific example of the LC element 20 will be described in detail. As described above, the LC element 20 can be configured using the trade name “Filmac”, but an LC element that is suitable for integration and that can change the element constant will be described below.
[0030]
FIG. 5 is a plan view of the LC element formed on the semiconductor substrate. FIG. 6 is an enlarged cross-sectional view taken along line AA shown in FIG.
[0031]
The LC element 20 shown in these drawings includes a spiral n ⁺ region 202 formed near the surface of a p-type silicon substrate (p-Si substrate) 200, which is a semiconductor substrate, and a spiral formed in a part thereof. A p ⁺ region 204 having a shape is included, and a pn junction layer 206 is formed by the n ⁺ region 202 and the p ⁺ region 204. In addition, a reverse bias voltage is applied between the p-Si substrate 200 and the n ⁺ region 202, and the p-Si substrate 200 functions as an isolation region between the adjacent n ⁺ regions 202 that circulate. are doing.
[0032]
Further, in the LC element 20, a spiral first electrode 210 is formed on the surface of the n ⁺ region 202 described above at a position along the n ⁺ region 202. Similarly, a surface of the p ⁺ region 204, the second electrode 212 of the spiral shape is formed at a position along the p ⁺ region 204. In addition, three input / output electrodes 214, 216, and 218 are connected to both ends of the first electrode 210 and one end (for example, the outer peripheral side) of the second electrode 212, respectively. The three input / output electrodes 214, 216, and 218 are attached outside the active region so as not to damage the thin n ⁺ region 202 or p ⁺ region 204 as shown in FIG.
[0033]
In the LC element 20 having such a structure, each of the first and second electrodes 210 and 212 having a spiral shape functions as an inductor conductor. When the pn junction layer 206 electrically connected to each of the first and second electrodes 210 and 212 is used in a reverse bias state, it functions as a spiral capacitor. Therefore, a composite element is formed in which the inductor formed by the first and second electrodes 210 and 212 and the capacitor formed by the pn junction layer 206 exist in a distributed constant manner.
[0034]
In addition to the LC element 20 having the above-described structure, the p-Si substrate 200 is integrally formed with other components such as the inverter circuit 14 shown in FIG. It is integrated on one chip.
[0035]
FIG. 7 is a diagram showing an equivalent circuit of the LC element whose structure is shown in FIGS. As shown in FIG. 5A, the first electrode 210 functions as an inductor having an inductance L1, and the second electrode 212 functions as an inductor having an inductance L2. In addition, a pn junction layer 206 is formed between the first and second electrodes 210 and 212 along the spiral-shaped circumferential direction. By using the pn junction layer 206 with a reverse bias, static electricity is generated. A distributed constant capacitor having a capacitance C is formed. Note that the LC element 20 included in the circuit shown in FIG. 1 is a simplified version of the equivalent circuit shown in FIG.
[0036]
FIG. 7B shows a configuration for applying a reverse bias voltage to the pn junction layer 206 included in the LC element 20. Specifically, a bias power source 220 for applying a predetermined reverse bias voltage is connected between the first input / output electrode 214 and the third input / output electrode 218, and the first input / output electrode 214. Capacitor 222 is inserted in series. The capacitor 222 is for blocking direct current, and has an extremely small impedance, that is, a large capacitance so as not to affect the operation.
[0037]
Further, as shown in FIG. 7C, by replacing the above-described bias power source 220 with a reverse bias power source 224 that can arbitrarily change the value of the reverse bias voltage, a spiral pn junction layer 206 is used. The capacitance of the formed capacitor can be arbitrarily changed within a certain range.
[0038]
Therefore, the frequency characteristic can be arbitrarily changed within a certain range by configuring the band-pass filter 1 shown in FIG. 1 using the LC element 20 capable of arbitrarily changing the capacitance of the distributed constant capacitor. can do. For example, in a general variable capacitance diode, the capacitance can be changed by about 50% by changing the reverse bias voltage. Therefore, by using the LC element 20 having the structure shown in FIG. It can be seen that it can be changed by about 10%.
[0039]
FIG. 8 is a diagram illustrating an example of a manufacturing process of the LC element 20 illustrated in FIG. 5 and the like. The state for every manufacturing process of the BB expanded cross section of FIG. 5 is shown.
[0040]
(1) Growth of epitaxial layer:
First, after removing the oxide film on the surface of the p-Si substrate 200 (wafer), an n ⁺ -type epitaxial layer 226 is grown on the entire surface of the p-Si substrate 200 (FIG. 8A).
[0041]
(2) Formation of isolation region:
Next, in order to make the region excluding the n ⁺ region 202 and the p ⁺ region 204 shown in FIG. 5 an isolation region, p-type impurity diffusion or ion implantation is performed.
[0042]
Specifically, first, the surface of the epitaxial layer 226 is thermally oxidized to form an oxide film 228. Then, after removing the oxide film 228 at the position where the p region is to be formed by photolithography, the p region is selectively formed by selectively adding a p-type impurity by thermal diffusion or ion implantation. The p region thus formed becomes a part of the p-Si substrate 200 to form an isolation region ((B) in the figure).
[0043]
As a result of the formation of the isolation region in this way, a spiral n ⁺ region 202 is formed by the remaining epitaxial layer 226.
[0044]
(3) Formation of pn junction layer:
Next, a p-type impurity is introduced into a part of the n ⁺ region 202 formed in a spiral shape by thermal diffusion or ion implantation, thereby forming a spiral p ⁺ region 204 ((C) in the figure).
[0045]
Specifically, first, the oxide film 230 is formed by thermally oxidizing the surface of the p-Si substrate 200 including the n ⁺ region 202. Then, after removing the oxide film 230 at the position where the p ⁺ region 204 should be formed by photolithography, the p ⁺ region 204 is selectively formed by selectively adding p-type impurities by thermal diffusion or ion implantation. Is done.
[0046]
Since this p ⁺ region 204 needs to be formed in the previously formed n ⁺ region 202, the p ⁺ region 204 is added by adding p-type impurities that are more than the amount of n-type impurities already introduced. Is formed.
[0047]
In this manner, a spiral pn junction layer 206 composed of the n ⁺ region 202 and the p ⁺ region 204 is formed.
[0048]
(4) Formation of electrode:
Next, after an oxide film 232 is formed on the surface by thermal oxidation, spiral holes are formed in the respective surfaces of the n ⁺ region 202 and the p ⁺ region 204 by photolithography, and then the portions that are formed in this spiral shape are formed. In addition, the first and second electrodes 210 and 212 are formed by evaporating aluminum, for example (FIG. 4D). Then, each of the three input / output electrodes 214, 216, 218 is formed by vapor deposition of aluminum.
[0049]
The process of manufacturing the LC element 20 described above is basically similar to the process of manufacturing a normal bipolar transistor or diode, and is different in the shape of the pn junction layer 206 and the isolation region therebetween. Therefore, it can be dealt with by changing the shape of the photomask in the process of manufacturing a general bipolar transistor, which is easy to manufacture and suitable for miniaturization.
[0050]
Further, in the manufacturing process of the LC element 20 described above, the case where isolation is performed after the n ⁺ region is first formed on the entire surface by epitaxial growth has been described as an example. However, an oxide film is formed on the surface of the p-Si substrate 200. After the formation, holes corresponding to the spiral n ⁺ region 202 are formed by photolithography, and the n ⁺ region 202 is formed by introducing an n-type impurity into this portion by thermal diffusion or ion implantation. Alternatively, the p ⁺ region 204 may be formed directly. Moreover, about the method of forming a pn junction layer, other general semiconductor manufacturing techniques can be used.
[0051]
In addition, this invention is not limited to said embodiment, A various deformation | transformation implementation is possible within the range of the summary of this invention.
[0052]
For example, although the band-pass filter 1 described above is configured using the two inverter circuits 14 and 16, at least one may be configured by an inverting amplifier such as a source grounded circuit.
[0053]
In the LC element 20 shown in FIG. 5, the pn junction layer 206 is formed so as to correspond to almost the entire length of the first and second electrodes 210 and 212. However, as shown in FIG. The LC element 20a may be replaced.
[0054]
The band-pass filter 1 described above includes two variable resistors 12 and 18, which can be specifically realized by using a junction type or MOS type FET. That is, when the channel formed between the source and drain of the FET is used as a resistor and used as the variable resistors 12 and 18, the gate voltage is variably controlled to change the channel resistance arbitrarily within a certain range. Can be made.
[0055]
The variable resistor 12 or 18 may be configured by connecting a p-channel FET and an n-channel FET in parallel. The resistance value can be varied by changing the magnitude of the gate voltage. As described above, by configuring the variable resistor by combining the two FETs, it is possible to improve the nonlinear region of the FETs, so that the distortion of the output signal of the bandpass filter 1 can be reduced.
[0056]
Further, at least one of the two variable resistors 12 and 18 described above may be replaced with a resistor having a fixed resistance value. When the resistance value of the variable resistor 12 is fixed, the bandpass filter 1 having a fixed pass bandwidth can be configured. When the resistance value of the variable resistor 18 is fixed, the bandpass filter having a fixed output amplitude. The filter 1 can be configured.
[0057]
In the description of the LC element 20 described above, the stray capacitance generated between the p-Si substrate 200 and the pn junction layer 206 is ignored. However, when each component shown in FIG. 1 is actually formed on the semiconductor substrate. The effect of this stray capacitance cannot be ignored. This point has also been confirmed by simulation, and the characteristics are much gentler than those shown in FIG. Further, since there is a stray capacitance, the degree of change in the passing center frequency when the capacitance of the pn junction layer 206 is varied is reduced, which is not practical. Since such a number of inconveniences occur when the stray capacitance occurs, it is convenient if the generation of unnecessary capacitance itself can be avoided.
[0058]
FIG. 10 is a plan view of the LC element 20b in which the generation of stray capacitance is suppressed. FIG. 11 is an enlarged cross-sectional view taken along the line CC shown in FIG.
[0059]
The LC element 20b shown in these drawings is different from the LC element 20 shown in FIG. 5 in that a pn junction layer 206 is formed in the vicinity of the surface of a p-well 302 formed on a part of the n-Si substrate 300. ing. The p-well 302 is provided with an electrode 310 for applying a predetermined voltage.
[0060]
In the LC element 20 b having such a configuration, when a reverse bias voltage is applied to the pn junction layer 206, the potential of the p well 302 and the potential of the n ⁺ region 202 in contact with the p well 302 are substantially the same. In addition, a predetermined voltage is applied to the electrode 310. If the potentials of the p-well 302 and the n ⁺ region 202 become substantially the same in this way, the generation of stray capacitance in the vicinity of the boundary adjacent to them can be suppressed. Similarly, FIG. 12 is a plan view of the LC element 20c in which the generation of stray capacitance is suppressed, and a configuration corresponding to the LC element 20a shown in FIG. 9 is shown.
[0061]
【The invention's effect】
As described above, according to the present invention, a band-pass filter that passes only a predetermined frequency can be realized with a simple circuit configuration with a small number of components.
[0062]
In particular, according to the present invention, the pass bandwidth can be changed by changing the resistance value of the first resistor, and the amplitude of the output signal can be changed by changing the resistance value of the second resistor. Further, by forming a pn junction layer between inductor conductors, an LC element which is a composite element in which electrostatic capacitance is distributed in a distributed constant manner between inductor conductors can be realized, and the reverse application to this pn junction layer is possible. By changing the bias voltage, the center frequency of the passband can be changed. Therefore, even when the entire components are integrally formed on the semiconductor substrate, the passband width, the center frequency of the passband, or the signal amplitude can be easily adjusted. In addition, when the component parts are integrally formed on the semiconductor substrate, the circuit can be reduced in size and cost can be reduced by integration.
[Brief description of the drawings]
FIG. 1 is a circuit diagram of a band-pass filter according to an embodiment to which the present invention is applied.
FIG. 2 is a diagram illustrating a relationship between a resistance value of a variable resistor connected in parallel to a second-stage inverter circuit and a signal amplitude of an oscillation output.
FIG. 3 is a diagram showing frequency characteristics of the bandpass filter shown in FIG. 1;
4 is a diagram showing frequency characteristics of the bandpass filter shown in FIG. 1. FIG.
FIG. 5 is a plan view of an LC element formed on a semiconductor substrate.
6 is an enlarged cross-sectional view taken along line AA shown in FIG.
7 is a diagram showing an equivalent circuit of the LC element shown in FIG.
FIG. 8 is a diagram showing an example of a manufacturing process of an LC element.
FIG. 9 is a diagram showing a modification of the LC element.
FIG. 10 is a diagram showing another modification of the LC element.
11 is an enlarged cross-sectional view taken along the line CC shown in FIG.
FIG. 12 is a diagram showing another modification of the LC element.
[Explanation of symbols]
1 Band-pass filters 12 and 18 Variable resistors 14 and 16 Inverter circuit 20 LC element 200 p-Si substrate 202 n ⁺ region 204 p ⁺ region 206 pn junction layer 210 first electrode 212 second electrode

Claims (10)

A first inverter circuit to which an AC signal is input via a first resistor;
A second resistor connected in parallel and connected to the output side of the first inverter circuit;
There are two inductor conductors formed in parallel on the semiconductor substrate and a capacitor formed in a distributed constant between the two inductor conductors, and one of the two inductor conductors. An LC element in which the output of the first inverter circuit is fed back to the input side via one of the two inductor conductors and the other end of the two inductor conductors is connected to the output side of the second inverter circuit;
A band-pass filter that passes a signal in the vicinity of a predetermined frequency from the AC signal input to the first resistor and outputs the signal from the second inverter circuit. A first inverting amplifier to which an AC signal is input via a first resistor;
A second resistor connected in parallel and connected to the output side of the first inverting amplifier;
There are two inductor conductors formed in parallel on the semiconductor substrate and a capacitor formed in a distributed constant between the two inductor conductors, and one of the two inductor conductors. An LC element in which the output of the first inverting amplifier is fed back to the input side via one of the two inductor conductors and the other end of the two inductor conductors is connected to the output side of the second inverting amplifier;
A band-pass filter that passes a signal in the vicinity of a predetermined frequency from the AC signal input to the first resistor and outputs the signal from the second inverting amplifier. In claim 1 or 2,
The first resistor is a variable resistor, and a pass band width is changed by changing a resistance value. In claim 1 or 2,
The band-pass filter according to claim 1, wherein the second resistor is a variable resistor, and the amplitude of the output signal is changed by changing a resistance value. In claim 3 or 4,
A bandpass filter characterized in that the variable resistance is formed by a channel of an FET, and the channel resistance is changed by changing a gate voltage. In claim 3 or 4,
A bandpass filter characterized in that the variable resistor is formed by connecting a p-channel FET and an n-channel FET in parallel, and the channel resistance is changed by changing the gate voltage of each FET. In any one of Claims 1-6,
The LC element is
Two concentric adjacent electrodes disposed on the semiconductor substrate and functioning as the two inductor conductors;
Formed near the surface of the semiconductor substrate and along the two electrodes, a p-region is electrically connected to one of the two electrodes, and an n-region is electrically connected to the other, and a reverse bias voltage A spiral pn junction layer that functions as the capacitor by applying
A band-pass filter comprising: In any one of Claims 1-6,
The LC element is
A well formed on the semiconductor substrate;
Two spiral electrodes arranged concentrically adjacent to the well and functioning as the two inductor conductors;
It is formed near the surface of the well and along the two electrodes, the p region is electrically connected to one of the two electrodes, and the n region is electrically connected to the other, and a reverse bias voltage is applied. A spiral pn junction layer that functions as the capacitor when applied;
The band-pass filter is characterized in that the potential of the well and the potential of the p region or the n region in contact with the well are substantially the same. In claim 7 or 8,
A band pass filter, wherein a center frequency of a pass band is changed by changing a reverse bias voltage applied to the pn junction layer to change a capacitance of the pn junction layer. In any one of Claims 1-9,
A band-pass filter, wherein component parts are integrally formed on the semiconductor substrate.

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