JPH09148880A - Electronic circuit, filter device using it and radio equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform an operation without distortion even if the signal of a high peak level is inputted at the time of the low power source voltage by the adjustment of the distribution of the capacity values in an FDNR(frequency- dependent negative resistance), to make the high resistance generated by the input of a signal into a small value by an equivalent conversion and to reduce parasitic capacity accompanying the resistance. SOLUTION: In the FDNR type active filter composed on an integrated circuit, an FDNR circuit 10 is realized by connecting the inversion input 16 of an operational amplifier 14 with each one end of first capacity 11 and an impedance element 6, connecting the output terminal 17 of the operational amplifier 14 with each one end of the impedance element 6 and the second capacity 12 and connecting the other ends with each other of the first capacity 11 and the second capacity 12. In the impedance element 6, a resistance 7 and a resistance 8 are serially connected, the connection terminals of the resistances 7 and 8 are connected with the one end of a resistance 9 and the other end of the resitance 9 is alternatingly grounded. The size of the value of the second capacity 12 is larger than that of the first capacity 11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic circuit formed in an analog integrated circuit and a filter device using the same, and particularly to a frequency-dependent negative resistance (hereinafter referred to as FDNR-).
The present invention relates to an electronic circuit such as a frequency-dependent negative resistance −) circuit and a filter device using the same.

[0002]

2. Description of the Related Art When constructing a filter on an integrated circuit,
By using the active element, it is possible to realize a filter circuit that does not use an inductor. As a method for realizing this, an example using a feedback circuit using an operational amplifier or G
Various examples using an m-amp have been proposed.

As one of the designing procedures of an active filter, there is a method of performing equivalent conversion from a ladder type filter circuit composed of an inductance, a capacitance and a resistance by using the capacitance, the active element and the resistance. For example, as shown in FIG. 13, it is called a Ford / Girling circuit (FDNR (Frequency Dependent Negative Res).
istor) and R, C are equivalent conversion methods (IEEE PRO
C. Vol. 128, Pt. G. No. 4, pp. 195-pp. 197).

FIG. 13 is a circuit diagram for explaining the FDNR conversion. Reference numeral 3 is a connection point of the capacitors C1 and C2,
11 is the first capacitance (C1), 12 is the second capacitance (C1)
2) and 13 are first resistors (R0). First and second
A first resistor 13 and an operational amplifier 14 are connected in parallel between the other ends of the connection points 3 of the capacitors 11 and 12 opposite to the one end. The operational amplifier 14 has a positive side input connected to the ground terminal 15, a negative side input to which the inverting input 16 is supplied, and an output 17.

FIG. 14 shows an example of a third-order low-pass filter realized by using this equivalent conversion. In FIG. 14, resistors 25 and 26 obtained by equivalent conversion are inserted in series between the input terminal 1 and the output terminal 2, and the resistor 2
A capacitor 29 obtained by equivalent conversion is connected in parallel between 6 and the output terminal 2. The FDNR circuit 10 is connected in parallel between the resistors 25 and 26.
The DNR circuit 10 includes first and second capacitors 11 and 12 connected in parallel between the input terminal 1 and the output terminal 2, respectively.
And a first resistor 13 inserted into one end of each of the first and second capacitors 11 and 12, and the first resistor 1
3 and an operational amplifier 14 connected in parallel. This operational amplifier 14 includes a positive side input connected to a ground terminal 15, an inverting input 16 connected to one ends of the first capacitor 11 and the first resistor 13, one end of the second capacitor 12 and Calculation output 1 connected to the other end of the first resistor 13
7 and. The conversion procedure to the filter circuit of FIG. 14 will be described below.

This is an LCR ladder filter as a model of a filter to be realized as shown in FIG. 15 (a), in which L is not used as shown in FIG. 15 (b), that is, the original ladder filter is s. Convert to a divided form. As a result, inductance becomes resistance, resistance becomes capacitance, and capacitance becomes FD.
It is converted to NR (termed by s−2 when impedance is used). Although the FDNR itself does not actually exist in the passive element, the FDNR can be equivalently realized by using an active circuit as shown in FIG.

The characteristic of this filter circuit is that the DC path between the signal input / output path and the input / output of the operational amplifier constituting the FDNR is completely cut off. By constructing a filter using this FDNR, the DC offset voltage generated in the operational amplifier does not appear in the filter output, so that a design independent of the DC offset generated in the operational amplifier can be performed when designing the filter. This is a very desirable property for a low pass filter because the DC offset becomes a pseudo input and degrades the S / N of the filter.
Further, the DC voltage of the operational amplifier and the DC potential of the signal path can be treated independently, which greatly increases the degree of freedom in circuit design.

Although the circuit has many advantages as described above, it has the following problems. The gain itself in the pass band of this filter is 0 dB, but the output of the operational amplifier forming the FDNR has a signal gain of, for example, several dB to several tens of dB. Therefore, when handling a signal of a relatively large level with a low power supply voltage, the signal level at the output of the operational amplifier exceeds the operation output range of the operational amplifier and the signal is distorted.

On the other hand, when this filter is used in a low frequency band (several tens to several hundreds of kHz) such as the base band of a radio device, the element value forming the filter becomes large, which is particularly inconvenient when considering integration. is there.

That is, the condition for equivalent conversion of the FDNR value D is expressed as D = C1.C2.R0 (1). In the case of integration, the capacitance values C1 and C2 used as a result of equivalent conversion must be capacitance values that can be realized on the integrated circuit, and cannot be increased so much because of the occupied area. Assuming that the value of D is constant, the values of C1 and C2 are calculated from the equation (1) (this is also a value that can be realized on an integrated circuit: several tens of pF).
When divided by, the value of R0 has a high resistance of several hundreds kΩ or more. If this resistance is realized on an integrated circuit, the occupied area becomes very large. Further, as shown in FIG. 16, a parasitic capacitance of about several pF to several tens of pF is distributed over the IC substrate or the like in the resistance formed in the integrated circuit. Since this resistance constitutes a negative feedback loop of the operational amplifier, this parasitic capacitance causes a phase delay in the feedback loop, so that desired negative feedback cannot be performed. As a result, there has been a problem that the operation with respect to the AC signal becomes unstable and, in some cases, oscillation occurs.

[0011]

DISCLOSURE OF THE INVENTION According to the present invention, in the FDNR used in an active filter circuit, even if a signal with a large amplitude is input even under a low power supply voltage, the distortion of the circuit constituting the FDNR is small and the FDNR operates stably. It is to provide an FDNR circuit.

[0012]

In the semiconductor integrated circuit of the present invention, one end of the first capacitor and the first resistor is connected to the inverting input of the first operational amplifier, and the second end is connected to the output end of the operational amplifier.
Of the first resistor and the second capacitor are connected, the other ends of the first capacitor and the second capacitor are connected, and the other ends of the first resistor and the second resistor are also AC connected on one side. An FDNR circuit realized by connecting to a grounded third resistor is characterized in that the magnitude of the second capacitance mentioned above is larger than that of the first capacitance.

As another aspect of the present invention, at least two or more resistors are connected in series instead of the first resistor, and one terminal is AC-grounded at one end of the resistor string. The other end is connected with at least one or more.

As in the present invention, by largely selecting the second capacitance by the first capacitance, it is possible to reduce the amplitude at the output of the operational amplifier and operate with low distortion even with a low voltage power supply. Also, by changing the configuration of the resistors forming the negative feedback circuit, the total sum of the resistance values is reduced to reduce the area occupied by the integrated circuit,
At the same time, the stability can be improved by reducing the parasitic capacitance.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The voltage gain of an output terminal of an operational amplifier used in the FDNR circuit shown in FIG. 2A with respect to a node 3 to which C1 and C2 are commonly connected is G = -A.R ₀ / [{(1 + A) / sC1} + R ₀ ] (2) where A is the DC gain of the operational amplifier. Especially A >>
In the case of 1, G = −sC1 · R ₀ . That is, the gain decreases as the size of the capacitor C1 decreases.

On the other hand, the FDNR value D and the equivalent capacitance C F
The condition for DNR equivalent conversion is expressed as D = C1 · C2 · R0 (3) C = C1 + C2 (4). In order to set the FDNR value D and the equivalent capacitance C to desired design values, C1 and C2 have an inversely proportional relationship. From the above two conditions, in order to suppress the signal gain, by making C1 as small as possible and making C2 large, the amplitude at the output of the operational amplifier can be made small, and thus the signal distortion at the output of the operational amplifier can be reduced.

FIG. 1 shows an F according to a first embodiment of the present invention.
It is a circuit diagram of an active low-pass filter using DNR. In the figure, a third-order low-pass filter is shown. This is realized by the active element and RC by equivalent conversion from the LCR ladder filter as described above with reference to FIG. Since the details have been described with reference to FIG. 14, only the differences will be described here.

In FIG. 1, the point different from the circuit shown in FIG. 14 is the detailed configuration of the FDNR circuit 10. That is, the FDNR circuit 10 of FIG. 14 includes the first resistor 13 between one ends of the first and second capacitors 11 and 12, but the FDNR circuit 10 of FIG. 1 uses the impedance element 6 instead. I am inserting it. The impedance element 6 includes a series body of resistors 7 and 8 inserted between one ends of the first and second capacitors 11 and 12, respectively, and one end of which is connected to a connection point of the resistors 7 and 8 and the other end of which is connected. Is grounded to the resistor 9.

FIG. 3 shows the frequency characteristics of the output voltage of the output terminal 2 of the third-order Butterworth type low-pass filter and the output terminal 17 of the operational amplifier constructed according to the first embodiment shown in FIG. This characteristic has a cutoff frequency of 200.
It is designed at kHz (-3 dB down), and the capacitance ratio of C1 and C2 is C1: C2 = 1: 9 in the example of the present invention. The signal output at the output end of the operational amplifier becomes maximum near the cutoff frequency, but it is about 6 dB lower than that of the capacitance ratio of C1 and C2 of 5: 5, that is, about 1/2 in amplitude. I understand.

As shown in FIG. 2B, a resistor 18 having resistance values R1 and R2 smaller than R0, for example, about one tenth, is provided at both ends of the resistor (input / output terminals of the operational amplifier).
And 19 are inserted. The inverting input 16 of the operational amplifier 14 is a virtual ground in terms of alternating current, and the output of the operational amplifier 14 has resistors 18 and 19 (R1, R) as long as it has sufficient driving capability.
Even if 2) is inserted, the conditions hardly change in terms of AC. Then, the newly inserted resistors 18 and 19 (R1, R
2) and the feedback resistor 13 (R0) are considered as a Δ type resistance network, and are replaced with a T type network using Δ-T conversion.

The relationship of equivalent conversion is expressed by using the element numbers of FIG. 2C. R3 = R1.R0 / (R0 + R1 + R2) (5) R4 = R1.R2 / (R0 + R1 + R2) (6) R5 = R2 .. R0 / (R0 + R1 + R2) (7) As a result of these conversions, the newly obtained resistance 7
And the resistance values R3 and R5 of 8 are the resistance value R0 of the feedback resistor 13.
Of the resistance value R4 of the resistor 9
Also, the resistance value R1 or R2 of the resistor 18 or 19 can be reduced to about one tenth. As a result, the parasitic capacitance of the resistor can be reduced to about 1/10, which contributes to stable operation.

FIG. 4 shows a second embodiment using the FDNR circuit of the present invention.
FIG. In addition to the configuration of the feedback resistors of the operational amplifier 14 which is formed by three resistor networks as shown in FIG. 1, the total number of resistors can be further reduced by using a ladder-shaped configuration having a plurality of stages. It is also possible to adjust the DC loop gain of the FDNR circuit by making the ladder configuration multistage.

FIG. 5 shows a third embodiment using the FDNR circuit of the present invention. In FIG. 1, the immittance in which one side is grounded is realized, whereas in the embodiment in which the immittance in which both terminals are floated is realized. By increasing the capacitance value C1 of the capacitor 11 with respect to the capacitance value C2 of the capacitor 12, the signal level at the output terminal of the operational amplifier can be suppressed. The relationship of conversion from immittance to FDNR is exactly the same as that of the circuit according to the first embodiment of FIG.

FIG. 6 shows a fourth embodiment in which the resistor 13 used in the FDNR circuit and the filter circuit using the same is replaced with a switched capacitor 41. In FIG. 6, a switched capacitor 42 is provided between the input terminal 1 and the FDNR circuit 10, and F
A switched capacitor 43 is provided between the DNR circuit 10 and the output terminal 2.

As a result, a large resistance can be realized with a small capacitance and a switching element, so that the area occupied by the integrated circuit can be reduced. Further, since the accuracy is determined by the capacity ratio, it is possible to realize a highly accurate transfer characteristic as compared with the case where the resistance and the capacity are used. Further, by realizing the feedback resistance that constitutes the FDNR circuit by using a switched capacitor, the parasitic capacitance can be reduced, so that a highly stable one can be realized.

FIG. 7 shows an F according to the fifth embodiment of the present invention.
It is a circuit diagram of a DNR circuit. In the configuration of the filter device according to the first embodiment of FIG. 1, the ground terminal 15 needs to apply a bias suitable for the operation of the operational amplifier when the operational amplifier 14 is operated by a single power supply. Therefore, it is necessary to maintain a state in which the terminal of the non-inverting input 16 of the operational amplifier or the ground terminal 15 of the feedback resistor net is grounded in an AC manner while applying a constant DC voltage. As a method for realizing this, an external voltage source 55 having a sufficiently low output impedance is generally prepared.

However, when this is realized by a passive element due to restrictions such as current consumption and circuit scale, the ground terminal should be the power source 5.
It is also possible to realize it by a circuit 56 in which 5 is divided by a resistor. In order to lower the AC impedance with respect to the frequency band to be handled, the connection end of the voltage dividing resistor 56 may be grounded via a capacitor. Further, in the network of the feedback resistors 7 and 8 of the FDNR circuit, one of the resistors 57 and 58 realizes a resistor whose one side is grounded and which is one of the resistors 57 and 58.
By connecting one end to the power supply side, the other end to the connection point of the resistors 7 and 8, and the other end of the resistor 58 to the connection point of the resistors 7 and 8, the DC potential of the resistor network can be maintained without changing the loop gain. Can be set.

FIG. 8 shows a sixth embodiment according to the present invention. The resistance connected between the connection point of the series-connected resistors in FIG. 4 and the AC ground point is the same as in FIG.
Resistors 57 and 5 each having one end connected to the power supply side and the GND side
It is realized by No. 8 and does not require an external power source for realizing AC grounding.

FIG. 9 shows a seventh embodiment according to the present invention. In FIG. 9, a passive element circuit 50 is inserted in parallel with the FDNR circuit 10 between the input terminal 1 and the output terminal 2. The passive element circuit 50 has an input terminal 1 at one end.
Is connected between the output terminal 2 and the output terminal 2, and the other end is connected to the first capacitor 11, and the resistor 51 is connected in parallel to the resistor 51 so that one end is connected between the terminals 1 and 2 and the other end is The resistor 52 is connected to the second capacitor 12, and the resistor 53 is inserted in series between two connection points on one end side of each of the resistors 51 and 52. When sufficient attenuation cannot be obtained in the cutoff region of the filter, a notch may be inserted at a certain frequency as necessary to secure the required amount of attenuation.

FIG. 10 shows the structure of a passive element model in which a transfer characteristic obtained by adding a notch to a filter circuit having an LPF characteristic is obtained. FIG. 11 shows the frequency characteristic of the LPF with a notch. By inserting the inductance 45 in series with the capacitance 22 whose one side is grounded as shown in FIG.
Series resonance frequency f of this capacitance 22 and inductance 45
At o, the impedance to ground becomes extremely small. Therefore, the pass characteristic near the resonance frequency fo becomes extremely close to zero. As a result, when the transfer characteristic between the input and output of this filter circuit is viewed on the frequency axis as shown in FIG. The characteristic 201 with the notch 203 close to zero added is observed. LPF
When a large amount of attenuation is required at a frequency where sufficient attenuation is not yet obtained in the monotonic attenuation region of
By adjusting o, it is possible to secure the required attenuation without increasing the order of the filter.
From this model, each impedance is converted by dividing by s (FIG. 10 (b)).

In FIG. 10B, the passive element circuit 46 is provided between the input terminal 1 and the output terminal 2 in parallel with the FDNR circuit.
Is inserted. This passive element circuit 46 has a FDNR
A resistor 47 inserted in series with the immittance 27 obtained by equivalent conversion of the circuit, and resistors 25 and 26 are respectively interposed between the connection point of the resistor 47 and the input terminal 1 and the output terminal 2. There is. Note that FIG. 10C shows the FDNR circuit 10 in FIG. 9 which is equivalently converted and is represented by the immittance 27 and the capacitor 28.

In the case of the LPF, the frequency fo of the notch inserted in the stop band is usually the cutoff frequency fc of the LPF.
The resistance value of the resistance connected in series to the FDNR circuit is very small compared to the resistance value of the resistance inserted in the signal line. For example, when the resistance that enters the signal line is on the order of several kΩ, the resistance value in the FDNR circuit is often converted to a very small value of about several to several tens of Ω. When such a low resistance is realized in an LSI, the notch frequency cannot be accurately set to a predetermined value due to the influence of the wiring resistance and the resistance of the contact connecting the wiring and the element. .
Therefore, by equivalently converting the T-type resistance network into the Δ-type, it is possible to convert all of them into resistance values of the same order, and the problem that the notch frequency due to the wiring resistance, contact resistance, etc. cannot be set to a predetermined value. Solvable. The conversion method can be obtained by reversing the relationships of the expressions (5) to (7) described above.

In the above-described embodiment, an example in which one FDNR circuit is used is shown. However, a higher order filter circuit can be realized by preparing a plurality of FDNR circuits and connecting them in cascade. is there.

FIG. 12 is a block diagram showing an eighth embodiment in which a low pass circuit (LPF) using the electronic circuit (FDNR circuit) of the present invention is applied to a radio device.

In FIG. 12, a radio device 60 is inputted via an antenna 61 for transmitting or receiving a radio frequency (RF-Radio Frequency-) signal, a changeover switch 62 for switching transmission or reception of the signal, and an antenna 61. RF
A frequency conversion means 63 for directly converting a signal into a baseband (base frequency) signal, and a low pass filter (LP) for channel-selecting the baseband signal obtained by the direct conversion.
F-Low Pass Filter-) 64, a digital signal processing unit 65 for analog / digital (A / D) converting the received RF signal and a digital / analog (D / A) conversion for the baseband signal, and D / An LPF 66 for removing the quantization noise of the A-converted baseband signal and a frequency conversion means 67 for converting the baseband signal into an (RF) signal are provided.

As a block having an effect of application in the radio as shown in FIG. 12, one is channel selection of a baseband signal obtained by directly frequency converting an RF signal input from an antenna in a receiver (RX). It is a filter that does. The other is a filter that removes quantization noise after D / A conversion of the baseband modulated signal immediately before up-converting the baseband signal output from the digital signal processing unit in the transmitter (TX) to an RF signal. . When a DC offset occurs in these filters, in the case of a signal including frequency components up to DC, such as a QPSK modulated signal, the receiver receives this modulated signal and DC.
It cannot be distinguished from the offset component and cannot be received. Further, in the transmitter, this DC component becomes carrier leak after frequency conversion. LP using the FDNR circuit of the present invention
Since F does not generate a DC offset, by using this LPF in a radio device, the problem due to the DC offset of the above filter can be solved.

[0037]

As described above, according to the present invention, F
The high resistance that accompanies the realization of the DNR circuit can be converted into a small value by equivalent conversion, and the parasitic capacitance, which has been a problem in the layout area and the operational stability of the FDNR, can be reduced. Further, by devising the distribution of the capacitance values forming the FDNR, it is possible to operate with less distortion even under a low power supply voltage or when a high peak level signal is input.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a filter device including a frequency-dependent negative resistance (FDNR) circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing an equivalent conversion of the filter device according to the present invention.

FIG. 3 is a characteristic diagram illustrating equivalent conversion of the filter device according to the present invention.

FIG. 4 is a circuit diagram showing an FDNR circuit according to a second embodiment of the present invention.

FIG. 5 is a circuit diagram showing equivalent conversion of an FDNR circuit according to a third embodiment of the present invention.

FIG. 6 is a circuit diagram showing a filter device according to a fourth embodiment of the present invention.

FIG. 7 is a circuit diagram showing an FDNR circuit according to a fifth embodiment of the present invention.

FIG. 8 is a circuit diagram showing an FDNR circuit according to a sixth embodiment of the present invention.

FIG. 9 is a circuit diagram showing a filter device according to a seventh embodiment of the present invention.

FIG. 10 is a circuit diagram showing a configuration of a passive element model in the filter device according to the present invention.

11 is a characteristic diagram showing transfer characteristics of the filter device according to the embodiment of FIG.

FIG. 12 is a block diagram showing a wireless device to which a filter device according to an eighth embodiment of the present invention is applied.

FIG. 13 is a circuit diagram illustrating general FDNR conversion.

FIG. 14 is a circuit diagram showing a filter device using a conventional FDNR circuit.

FIG. 15 is a circuit diagram illustrating equivalent conversion of a third-order low-pass filter.

FIG. 16 is a circuit diagram illustrating parasitic capacitance distributed in a resistor.

[Explanation of symbols]

 3 Connection point of C1 and C2 6 Impedance element 10 FDNR circuit 11 1st capacity element 12 2nd capacity element 14 Operational amplifier 15 Inversion input 16 Ground terminal

Claims (10)

[Claims] 1. An inverting input terminal of a differential amplifier is connected to one end of a first capacitive element and one end of an impedance element including a resistance component, and an output terminal of the differential amplifier is connected to the other end of the impedance element and the first impedance element. One end of a second capacitance element having a capacitance value larger than that of the first capacitance element is connected, and the other end of the first capacitance element and the other end of the second capacitance element are connected to each other. Electronic circuit. 2. A first capacitor and one end of a first resistor are connected to at least an inverting input of an operational amplifier, and one ends of the first resistor and a second capacitor are connected to an output end of the operational amplifier, An electronic circuit comprising a circuit in which the other ends of the first capacitor and the second capacitor are connected to each other, wherein the magnitude of the second capacitance value is larger than that of the first capacitance value. 3. The first resistor comprises a third capacitor and the third capacitor.
The electronic circuit according to claim 2, wherein the electronic circuit is configured by a switched capacitor circuit using a switching element that charges and discharges the capacitance of. 4. An inverting input of an operational amplifier is connected to one end of a first capacitor, an output terminal of the operational amplifier is connected to one end of a second capacitor, and the one end of the first capacitor and the second capacitor is connected. The other ends are connected to each other, a plurality of first to n-th (n ≧ 2) resistors are connected in series to the inverting input and the output end of the operational amplifier, and from the connection point of the plurality of resistors connected in series, An electronic circuit comprising one or more resistors that are at least AC-grounded. 5. The electronic circuit according to claim 4, wherein the value of the second capacitance is larger than the value of the first capacitance. 6. A filter device comprising the electronic circuit according to claim 1, a plurality of first impedance elements including a resistance component, and a second impedance element including a capacitance component. 7. A switched capacitor circuit using a switch element for controlling charge and discharge of the capacitance instead of the resistance component.
3. The filter device according to 1.). 8. A filter device comprising the electronic circuit according to claim 4, a plurality of resistance elements, and a capacitance element. 9. A filter device according to claim 6, frequency conversion means for converting a frequency between a radio frequency (RF) signal and a baseband signal, and a signal processing unit for digitally converting the baseband signal for processing. A wireless device comprising at least: 10. The filter device according to claim 8, frequency conversion means for converting a frequency between a radio frequency (RF) signal and a baseband signal, and a signal processing section for digitally converting the baseband signal for processing. A wireless device comprising at least: