JPH11186873A - Filter circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stable band-pass filter without the change of a center frequency suitable for being made into an IC by feeding back the output signals of an addition-subtraction means for adding or subtracting two input signals through a phase transition means to one of input terminals. SOLUTION: The addition-subtraction means 12 is provided with the two input terminals and the output is fed back through the phase transition means 13 to the input of the addition-subtraction means 12 and subtracted from the signal inputted to the input terminal 11. Attenuation is maximum for the signal whose phase transition is 0 degree, the attenuation is reduced as the phase transition is increased, amplification is performed when the phase transition is advanced further and the amplification becomes maximum when the phase transition is 180 degrees. Then, when the phase transition is advanced further from 180 degrees, the amplification is reduced and the attenuation is performed. The attenuation becomes maximum again when the phase transition is 360 degrees. That is, band-pass filter characteristics in which the signal whose phase transition is 180 degrees is a passing center frequency are indicated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter circuit which can be constituted by an electronic circuit and can be made into a semiconductor.
In particular, it is intended to be applied to a channel selection filter of a receiver such as a mobile phone or a wireless data terminal.

[0002]

2. Description of the Related Art A receiver for a wireless device such as a portable telephone or a wireless data terminal requires a channel selection filter called an IF filter in order to remove adjacent channels and extract only a desired signal. Conventionally, a ceramic filter has been used as an IF filter. However, there is a demand for a semiconductor device for the IF filter due to demands for miniaturization of the wireless device and easiness in manufacturing the wireless device. An active filter including a capacitor, a resistor, and an amplifier is used as a filter suitable for semiconductor conversion.

FIG. 8 shows a configuration of a conventional active filter. In FIG. 8, 1 is an input terminal, 2 and 3 are resistors, 4 and 5 are capacitors, 7 is an operational amplifier, and 8 is an output terminal. The circuit in FIG. 8 forms a bandpass filter.

[0004]

However, in the above-described conventional filter circuit, a change in the capacitance value or the resistance value greatly affects a change in the center frequency of the band-pass filter. IF
When a conventional filter circuit is used as a filter, a change in the center frequency due to a change in the capacitance value or the resistance value and a variation in components is a nonnegligible level because the IF frequency is as high as 455 kHz. In particular, when a capacitance or a resistor is incorporated in an IC, it is difficult to construct an IF filter having a stable characteristic due to a large variation in the capacitance and the resistance due to the parts and a change in the temperature. For this reason, complicated processing such as correcting the capacitance value or resistance value by trimming, providing a means for detecting a shift in the center frequency of the band-pass filter, and providing means for performing feedback so as to correct the capacitance value or the resistance value is performed. There are cases. However, there is a problem that a circuit for performing such complicated processing is required, and the cost increases due to an increase in power consumption and an increase in circuit scale.

[0005]

According to the present invention, there is provided an add / subtract means for adding or subtracting two input signals and a phase shift means for shifting a phase of an output signal of the add / subtract means by a predetermined amount. A band-pass filter having a configuration in which the output signal of the phase shifting means is fed back to one input terminal of the addition / subtraction means. Since the phase shifting means for determining the center frequency can be constituted by a delay element or the like which does not require a capacitance value and a resistance value, a stable band-pass filter with little change in the center frequency is realized.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to an add / subtract means having two input terminals for adding or subtracting two input signals inputted to the respective input terminals, and changing a phase of an output signal of the add / subtract means by a predetermined amount. And a phase transition unit for causing the output signal of the phase transition unit to be fed back to one input terminal of the addition / subtraction unit. The other input of the addition / subtraction unit is used as an input terminal to output the output of the addition / subtraction unit or the phase. The output of the transition means is used as an output terminal. In addition, a stable band-pass filter suitable for IC integration and having no change in center frequency can be realized with a simple configuration.

Also, at least two of the frequencies to be passed
A low-pass filter or a band-pass filter for attenuating a double frequency, an addition / subtraction unit having two input terminals for adding or subtracting two input signals input to the respective input terminals, and a phase of an output signal of the addition / subtraction unit. Phase shifting means for shifting by a predetermined amount, wherein the output of the low-pass filter or the band-pass filter is connected to one input terminal of the adding / subtracting means, and the output signal of the phase shifting means is fed back to the other input terminal. The input of the low-pass filter or the band-pass filter is used as an input terminal, and the output of the adding / subtracting means or the output of the phase shifting means is used as an output terminal. Then, it is possible to suppress generation of a spurious signal occurring at a frequency that is an integral multiple of the center frequency.

Also, at least two of the frequencies to be passed
A low-pass filter or a band-pass filter for attenuating a double frequency, an addition / subtraction unit having two input terminals for adding or subtracting two input signals input to the respective input terminals, and a phase of an output signal of the addition / subtraction unit. Phase shifting means for shifting by a predetermined amount, feeding back the output signal of the phase shifting means to one input terminal of the adding / subtracting means, and outputting the output of the adding / subtracting means or the output of the phase shifting means to the low-pass filter or band. It is connected to the input of a pass filter, and the other input of the addition / subtraction means is used as an input terminal, and the output of the low-pass filter or the band-pass filter is used as an output terminal. In addition, it is possible to remove spurious signals generated at a frequency that is an integral multiple of the center frequency, and to remove noise of high-frequency components generated by phase shifting means and the like.

Further, at least two of the frequencies to be passed
A first low-pass filter or a band-pass filter for attenuating double the frequency, a second low-pass filter or a band-pass filter for attenuating at least twice the frequency to be passed, and two input terminals. An adder / subtractor for adding or subtracting two input signals input to the input terminal; and a phase shifter for shifting a phase of an output signal of the adder / subtractor by a predetermined amount, wherein the first low-pass filter or the band-pass filter The output is connected to one input terminal of the addition / subtraction means, and the output signal of the phase transition means is fed back to the other input terminal. The output of the addition / subtraction means or the output of the phase transition means is supplied to the second low-pass filter or band. Connect to the input of the pass filter and input the input of the first low pass filter or band pass filter In which the output of said second low-pass filter or a band-pass filter as a child and an output terminal. Further, it is possible to suppress the generation of a spurious signal generated at a frequency that is an integral multiple of the center frequency, and even if a spurious signal is generated, the spurious signal is removed by a subsequent-stage filter and the high-frequency wave generated by a phase shifting unit or the like is removed. Component noise can be removed.

The phase shifting means is constituted by a delay element. Further, it is possible to configure a phase shift unit that is not affected by changes in the capacitance value and the resistance value.

The phase shifting means includes an A / D converter for converting an input signal into a digital signal, and a D / D converter for converting the digital signal into an analog signal after a predetermined time has elapsed and outputting the analog signal.
It is composed of an A converter. Further, it is possible to configure a phase shift unit that is not affected by changes in the capacitance value and the resistance value.

Further, the phase shifting means has inverting means for inverting an input signal, connects the input signal and the inverted output of the inverting means by a series circuit of a capacitor and a resistor, and connects a connection point of the capacitor and the resistor. A 180-degree phase transition circuit to be output is connected in two stages in series. Then, the phase shifting means can be configured with a simple configuration.

Further, the adding / subtracting means has a level adjusting means so that the amount of feedback can be adjusted. Then, the pass band width can be adjusted by changing the Q value of the band-pass filter.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

Embodiment 1 FIG. 1 is a block diagram of a filter circuit according to Embodiment 1 of the present invention.

In FIG. 1, reference numeral 11 denotes an input terminal, 12 denotes an addition / subtraction unit, 13 denotes a phase shift unit, and 14 denotes an output terminal. The addition / subtraction means has two input terminals. Then, the output of the addition / subtraction unit 12 is fed back to the input of the addition / subtraction unit 12 via the phase shift unit 13 and is subtracted from the signal input to the input terminal 11. That is, the output of the addition / subtraction means 12 =
(Signal input to input terminal 11−k × phase shifting means 13
(1) is performed. Here, k is a constant, and is set to 0 <k <1.

Now, consider the case where the phase shift means 13 shifts the phase by 180 degrees. Then, the signal input to the input terminal 11 and the output signal of the phase shifting means 13 are in opposite phases, and are added in phase by subtraction. Therefore, the output of the adding / subtracting means 12 is given by equation (1) = (1 + k) ×
(A signal input to the input terminal 11). That is, the level of the signal input to the input terminal 11 is amplified by (1 + k) times.

Next, 0 degree or 3 degree
Consider a case where the phase shifts by 60 degrees. Then input terminal 1
1 and the output signal of the phase shifting means 13 have the same phase, so the output of the adding / subtracting means 12 = (1--
k) × (signal input to input terminal 11). That is, the level of the signal input to the input terminal 11 is attenuated by (1-k) times. As described above, in the output of the addition / subtraction means 12, the frequency signal whose phase shifts by 180 degrees is amplified by the phase shift means 13, and the frequency signal whose phase shifts by 0 degrees or 360 degrees is attenuated. Furthermore, a signal having a phase transition of 0 degree has the maximum attenuation, and the attenuation decreases as the phase transition increases, and the signal is amplified as the phase transition further proceeds. The amplification becomes maximum at a phase transition of 180 degrees. When the phase shift further progresses from 180 degrees, the amplification decreases and attenuates. Then, when the phase shift is 360 degrees, the attenuation becomes maximum again. That is, the band-pass filter characteristic shows a signal having a phase shift of 180 degrees as a pass center frequency. The constant k is a constant for setting the sharpness Q, and the larger the k, the higher the Q. However, if k is larger than 1, oscillation occurs, so that k <1 is set. The phase shifting means 13 can be constituted by delay means. As the delay means, for example, there is a delay element BBD (bucket brigade device) used in the MN3300 series of Matsushita IC. Since the delay time of the BBD is determined by the clock frequency, a stable band-pass filter characteristic free from temperature changes and element variations can be easily realized using the BBD.

(Embodiment 2) FIG. 2 is a block diagram of a filter circuit according to Embodiment 2 of the present invention.

In FIG. 2, the same functional blocks as in FIG. 1 are given the same numbers. The difference from FIG. 1 is that a low-pass filter 15 is provided in a stage preceding the adding / subtracting means 12. The output of the adding / subtracting means 12 amplifies a signal having a phase transition of an odd multiple of 180 degrees by the phase transition means 13. That is, a plurality of peaks of the band-pass filter occur. Therefore, 180 × 3 = 540 by the low-pass filter 15 in order to take out only the peak whose phase shift is 180 degrees.
Eliminate frequency signals that produce a phase shift of more than degrees. A frequency signal that causes a phase shift of 180 × 3 = 540 degrees or more is not input to the input of the addition / subtraction means 12, and only one peak of the band-pass filter is provided. The phase shifting means 13 can be constituted by delay means. As the delay means, for example, there is a delay element BBD (bucket brigade device) used in the MN3300 series of Matsushita IC. Since the delay time of the BBD is determined by the clock frequency, a stable band-pass filter characteristic free from temperature changes and element variations can be easily realized using the BBD.

(Embodiment 3) FIG. 3 is a block diagram of a filter circuit according to Embodiment 3 of the present invention.

In FIG. 3, the same functional blocks as those in FIG. 1 are given the same numbers. The difference from FIG. 1 is that a low-pass filter 16 is provided downstream of the addition / subtraction means 12. The output of the adding / subtracting means 12 amplifies a signal having a phase transition of an odd multiple of 180 degrees by the phase transition means 13. That is, a plurality of peaks of the band-pass filter occur. Therefore, in order to extract only a peak having a phase shift of 180 degrees, 180 × 3 = 540 by the low-pass filter 16.
Eliminate frequency signals that produce a phase shift of more than degrees. The frequency signal having a phase shift of 180 × 3 = 540 degrees or more at the output of the addition / subtraction means 12 is removed by the low-pass filter 16, and only one band-pass filter is output to the output terminal 14. The phase shifting means 13 can be constituted by delay means. As the delay means, for example, there is a delay element BBD (bucket brigade device) used in the MN3300 series of Matsushita IC. Since the delay time of the BBD is determined by the clock frequency, a stable band-pass filter characteristic free from temperature changes and element variations can be easily realized using the BBD.

(Embodiment 4) FIG. 4 is a block diagram of a filter circuit according to Embodiment 4 of the present invention.

In FIG. 4, the same functional blocks as in FIGS. 1 to 3 are given the same numbers. The difference from FIG. 1 is that a low-pass filter 15 and a low-pass filter 16 are provided at both the front and rear stages of the addition / subtraction unit 12, respectively. The low pass filter 15 removes a frequency signal that causes a phase shift of 180 × 3 = 540 degrees or more. A frequency signal that causes a phase shift of 180 × 3 = 540 degrees or more is not input to the input of the addition / subtraction means 12, and only one peak of the band-pass filter is provided. However, addition / subtraction means 1
When distortion occurs in the phase shifter 2 or the phase shifter 13, the addition / subtraction means 1
The output of 2 has a plurality of peaks due to spurious. The low pass filter 16 removes the spurious signal.
By using both the low-pass filter 15 and the low-pass filter 16 as in the present embodiment, the performance of the band-pass filter can be accurately extracted. Phase transition means 1
3 can be constituted by delay means. As the delay means, for example, there is a delay element BBD (bucket brigade device) used in the MN3300 series of Matsushita IC. Since the delay time of the BBD is determined by the clock frequency, a stable band-pass filter characteristic free from temperature changes and element variations can be easily realized using the BBD.

(Embodiment 5) FIG. 5 is a block diagram showing the configuration of the phase shifting means 13 of the present invention. In FIG. 5, 1
7 is an A / D converter, 18 is a memory, and 19 is a D / A converter. The A / D converter 17 converts an input analog signal into a digital signal according to a predetermined sampling clock. Then, the converted digital signal is stored in the memory 18. After a lapse of a predetermined time, the memory 1
The digital signal stored in 8 is converted into an original analog signal by a D / A converter 19 at a predetermined sampling clock and output. As shown above, the circuit of FIG. 5 functions as a delay unit. That is, it can be used as a phase shifting means. Here, the same clock can be used as the sampling clock of the A / D converter 17 and the D / A converter 19. If the sampling clock is selected to be about twice the pass center frequency of the band-pass filter, the data stored in the memory 18 may be about one or two data.

(Embodiment 6) FIG. 6 is a block diagram showing another configuration of the phase shifting means 13 of the present invention. In FIG. 6, reference numeral 20 denotes a forward amplifier having a gain of 1, 21 denotes an inverting amplifier having a gain of 1, 22 denotes a capacitor, 23 denotes a resistor, 24 denotes a first 180-degree phase shift circuit, and 25 denotes a second phase shift circuit. 18
When the input frequency to the 0-degree phase shift circuit 24 is low, the output of the inverting amplifier 21 becomes the output of the 180-degree phase shift circuit 24. When the input frequency to the 180-degree phase shift circuit 24 is high, the output of the non-inverting amplifier 20 becomes the output of the 180-degree phase shift circuit 24. Therefore, the 180 degree phase shift circuit 24
Output undergoes a phase transition in the range of 180 degrees to 0 degrees depending on the frequency. Therefore, when the frequency is changed from low to high by connecting two stages of phase shift circuits of the same configuration in cascade, the phase can be changed from 360 degrees to 0 degrees. The pass center frequency of the band-pass filter is a frequency at which the phase shift becomes 180 degrees.

(Embodiment 7) FIG. 7 shows the addition / subtraction means 1 of the present invention.
2 is a block diagram showing a configuration of FIG. In FIG. 7, 26,
27 and 29 are resistors, 28 is a variable resistor, and 30 is an operational amplifier. The resistance 26 and the resistance 27 have the same resistance value. The variable resistor 28 is adjusted to a value smaller than the value of the resistor 29. When the variable resistor 28 is set to the same value as the resistor 29, both the non-inverting input and the inverting input have a gain of 1 and are subtracted at the same level. By setting the variable resistor 28 to a value larger than the resistor 29, the gain of the inverting input can be made smaller than the gain of the inverting input, and the subtraction ratio between the two inputs can be adjusted. The sharpness Q can be arbitrarily adjusted by changing the resistance value of the variable resistor 28. The resistance value of the variable resistor 28 can also be adjusted at a factory by a method such as laser trimming. Although the resistor 28 is a variable resistor, the other resistors 26, 2
Which of 7, 29 may be a variable resistor.

Although this embodiment has been described using all the subtraction circuits as the addition / subtraction means 12, an addition circuit may be used. When an adder circuit is used, a bandpass filter having a pass center frequency when the phase shift by the phase shift means is 0 degrees or 360 degrees can be configured. Further, since the phase transition becomes a pass frequency with a transition of 0 degrees, it can be used as a low-pass filter.

Further, as the output terminal 14, the addition / subtraction means 12
However, the output of the phase shifting means 13 may be used.

[0030]

As apparent from the above description, the filter circuit of the present invention has the following effects.

An addition / subtraction unit having two input terminals for adding or subtracting the two input signals input to each of the input terminals, and a phase transition unit for shifting the phase of the output signal of the addition / subtraction unit by a predetermined amount. The output signal of the phase shifting means is fed back to one input terminal of the addition / subtraction means, and the other input of the addition / subtraction means is used as an input terminal, and the output of the addition / subtraction means or the output of the phase shifting means is used as an output terminal. Therefore, it is possible to realize a stable band-pass filter having a simple configuration and suitable for integration into an IC and having no change in the center frequency.

At least two of the frequencies to be passed
A low-pass filter or a band-pass filter for attenuating a double frequency, an addition / subtraction unit having two input terminals for adding or subtracting two input signals input to the respective input terminals, and a phase of an output signal of the addition / subtraction unit. Phase shifting means for shifting by a predetermined amount, wherein the output of the low-pass filter or the band-pass filter is connected to one input terminal of the adding / subtracting means, and the output signal of the phase shifting means is fed back to the other input terminal. Since the input of the low-pass filter or the band-pass filter is used as an input terminal and the output of the adding / subtracting means or the output of the phase shifting means is used as an output terminal, the generation of spurious signals occurring at a frequency that is an integral multiple of the center frequency can be suppressed. it can.

At least two of the frequencies to be passed
A low-pass filter or a band-pass filter for attenuating a double frequency, an addition / subtraction unit having two input terminals for adding or subtracting two input signals input to the respective input terminals, and a phase of an output signal of the addition / subtraction unit. Phase shifting means for shifting by a predetermined amount, feeding back the output signal of the phase shifting means to one input terminal of the adding / subtracting means, and outputting the output of the adding / subtracting means or the output of the phase shifting means to the low-pass filter or band. Connected to the input of a pass filter, the other input of the addition / subtraction means is used as an input terminal and the output of the low-pass filter or band-pass filter is used as an output terminal, so that spurious signals generated at a frequency that is an integral multiple of the center frequency can be removed. High frequency component noise generated by the phase shifting means or the like can be removed.

Also, at least two of the frequencies to be passed
A first low-pass filter or a band-pass filter for attenuating double the frequency, a second low-pass filter or a band-pass filter for attenuating at least twice the frequency to be passed, and two input terminals. An adder / subtractor for adding or subtracting two input signals input to the input terminal; and a phase shifter for shifting a phase of an output signal of the adder / subtractor by a predetermined amount, wherein the first low-pass filter or the band-pass filter The output is connected to one input terminal of the addition / subtraction means, and the output signal of the phase transition means is fed back to the other input terminal. The output of the addition / subtraction means or the output of the phase transition means is supplied to the second low-pass filter or band. Connect to the input of the pass filter and input the input of the first low pass filter or band pass filter Since the output of the second low-pass filter or the band-pass filter is used as an output terminal, the generation of a spurious signal occurring at a frequency that is an integral multiple of the center frequency can be suppressed, and even if a spurious signal is generated. The spurious signal can be removed by a filter at the subsequent stage, and high frequency component noise generated by the phase shifting means or the like can be removed.

Further, since the phase shifting means is constituted by a delay element, it is possible to constitute a phase shifting means which is not affected by a change in the capacitance value or the resistance value, and to realize a stable bandpass filter having no change in the center frequency of passing. .

The phase shifting means includes an A / D converter for converting an input signal into a digital signal and a D / D converter for converting the digital signal into an analog signal after a predetermined time has elapsed and outputting the analog signal.
Since it is composed of the A-converter, it is possible to constitute a phase shifter which is not affected by the change of the capacitance value or the resistance value, and it is possible to realize a stable bandpass filter having no change of the pass center frequency.

The phase shifting means has an inverting means for inverting an input signal, connects the input signal and the inverted output of the inverting means with a series circuit of a capacitor and a resistor, and connects a connection point of the capacitor and the resistor. Since a 180-degree phase shift circuit as an output is connected in two stages in series, the phase shift means can be configured with a simple configuration.

Further, since the addition / subtraction means has a level adjusting means and is configured to be able to adjust the amount of feedback, it is possible to change the Q value of the band-pass filter and adjust the pass bandwidth.

[Brief description of the drawings]

FIG. 1 is a block diagram of a filter circuit according to a first embodiment of the present invention.

FIG. 2 is a block diagram of a filter circuit according to a second embodiment of the present invention.

FIG. 3 is a block diagram of a filter circuit according to a third embodiment of the present invention.

FIG. 4 is a block diagram of a filter circuit according to a fourth embodiment of the present invention.

FIG. 5 is a block diagram of a phase shift unit according to a fifth embodiment of the present invention.

FIG. 6 is a block diagram of a phase shift unit according to a sixth embodiment of the present invention.

FIG. 7 is a block diagram of an addition / subtraction unit according to a seventh embodiment of the present invention.

FIG. 8 is a block diagram of a conventional filter circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Input terminal 12 Addition / subtraction means 13 Phase transition means 14 Output terminal 15 Low-pass filter 16 Low-pass filter 17 A / D converter 18 Memory 19 D / A converter 24 180-degree phase transition circuit 25 180-degree phase transition circuit 28 Variable resistance

Claims (8)

[Claims] An adder / subtractor having two input terminals for adding or subtracting two input signals input to the respective input terminals, and a phase shifter for shifting a phase of an output signal of the adder / subtractor by a predetermined amount. The output signal of the phase shift means is fed back to one input terminal of the addition / subtraction means, and the other input of the addition / subtraction means is used as an input terminal to output the output of the addition / subtraction means or the output of the phase shift means. Filter circuit used as a terminal. 2. A low-pass filter or a band-pass filter for attenuating at least twice the frequency to be passed, and an addition / subtraction unit having two input terminals for adding or subtracting two input signals input to the respective input terminals. Means, and a phase shifting means for shifting the phase of the output signal of the adding / subtracting means by a predetermined amount, and connecting the output of the low-pass filter or the band-pass filter to one input terminal of the adding / subtracting means and to the other input terminal. A filter circuit configured to feed back an output signal of the phase shifting means, and having an input of the low-pass filter or the band-pass filter as an input terminal and an output of the adding / subtracting means or an output of the phase shifting means as an output terminal. 3. A low-pass filter or a band-pass filter for attenuating at least twice the frequency to be passed, and an addition / subtraction unit having two input terminals for adding or subtracting two input signals input to the respective input terminals. Means, and a phase shifting means for shifting the phase of the output signal of the adding / subtracting means by a predetermined amount, feeding back the output signal of the phase shifting means to one input terminal of the adding / subtracting means, and outputting the output of the adding / subtracting means or A filter circuit in which the output of the phase shifting means is connected to the input of the low-pass filter or the band-pass filter, and the other input of the adding / subtracting means is used as an input terminal and the output of the low-pass filter or the band-pass filter is used as an output terminal. 4. A first low-pass filter or band-pass filter for attenuating at least twice the frequency to be passed, and a second low-pass filter or band-pass for attenuating at least twice the frequency to be passed. A filter, adding / subtracting means having two input terminals for adding or subtracting the two input signals input to the respective input terminals, and phase shifting means for shifting the phase of the output signal of the adding / subtracting means by a predetermined amount. The output of the first low-pass filter or the band-pass filter is connected to one input terminal of the addition / subtraction means, the output signal of the phase transition means is fed back to the other input terminal, and the output of the addition / subtraction means or the phase transition Connecting the output of the means to the input of the second low-pass filter or the band-pass filter; Filter or input of a bandpass filter as an input terminal said second low-pass filter or a band-pass filter circuit having an output terminal an output of the filter. 5. The filter circuit according to claim 1, wherein said phase shift means comprises a delay element. 6. A phase shift means comprising an A / D converter for converting an input signal into a digital signal and a D / A converter for converting the digital signal into an analog signal after a predetermined time has elapsed and outputting the analog signal. The filter circuit according to any one of claims 1 to 4. 7. The phase shifting means includes an inverting means for inverting an input signal, connecting the input signal and an inverted output of the inverting means by a series circuit of a capacitor and a resistor, and connecting a connection point of the capacitor and the resistor. The filter circuit according to any one of claims 1 to 4, wherein two 180-degree phase shift circuits serving as outputs are connected in series. 8. The filter circuit according to claim 1, wherein said adding / subtracting means has a level adjusting means and is capable of adjusting a feedback amount to be fed back.