JPH06101662B2 - Filter integrated circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Use of the Invention] The present invention relates to a monolithic IC formed on silicon or the like.
The present invention relates to a filter integrated circuit suitable for integrating a filter therein.

[Background of the Invention]

Conventionally, in an electronic circuit, an external block filter composed of an inductor, a capacitor, and a resistor is often used as a low pass wave filter, a high pass wave filter, a band pass wave filter, and a phase equalizer to obtain a desired signal. There is. For example, in a VHS type magnetic recording / reproducing apparatus (hereinafter abbreviated as VTR), a luminance signal a separated from a video signal is FM-modulated by an integrated FM modulator 1 as shown in FIG. In order to prevent the band from overlapping with the band of the chroma signal b converted into the low band by the color signal processing circuit (not shown), the band is limited by the external high band wave filter 2, and the mixer 3 Low frequency conversion chroma signal b
After mixing with, it is recorded on tape at head 4. In recent years, the integration of active element circuits has been increasing more and more. In this way, filters are external parts of ICs, which is a factor to prevent cost reduction, size reduction and weight reduction of devices using electronic circuits. I'm running. In particular, for portable VTRs that are desired to be small and lightweight, it is absolutely necessary to integrate filters.

Inductance is not suitable for integration, which causes the inability to integrate filters.
Some of them can be composed only of capacitance and resistance, and such a kind is suitable for integration. For example, as a trap filter, as shown in FIG. 5, a Twin-T circuit including only a capacitor and a resistor is known. In Fig. 5, resistance and capacitance are respectively Then, the trap frequency _T is Becomes

On the other hand, when integrating such trap filters, there arises a problem of characteristic variation. That is, the capacitance value in the IC,
The resistance value is affected by variations due to the impurity concentration in the semiconductor and mask shift, and as a result, large fluctuations occur, such as absolute capacitance ± 15% and absolute resistance ± 10%. Therefore, the trap frequency _{T of} this trap filter is also a as shown in FIG.
It fluctuates in the range from 1 to b, and fluctuates up to ± 25% in the above example, which is extremely difficult to put into practical use. As a countermeasure against this, Japanese Patent Publication No. 57-58083 discloses that the resistance value is changed by laser trimming or the like on the IC chip to absorb the variation, and it is implemented. So I still have many problems.

Further, Japanese Patent Publication No. 52-36813 and U.S. Pat. No. 3,761,741 disclose a variable attenuator circuit that utilizes the fact that the emitter resistance of a transistor changes depending on the emitter current. It is conceivable to adjust the variation of the filter characteristics due to the variation of However, in this technique, for example, the resistance as shown in FIG.
It is difficult to apply to a trap filter composed of R ₁ , R ₂ and R ₃ and also difficult to apply to all filters. In addition, adjustment means must be provided to adjust from the outside to absorb variations in the IC elements, which increases the number of IC pins and requires adjustment work.

[Object of the Invention]

The object of the present invention is to solve the above-mentioned problems of the prior art, to automatically absorb the variation in the filter characteristics caused by the variation in the capacitance value and the resistance value in the IC, and to reduce the number of IC pins and peripheral components. A filter integrated circuit having a structure is provided.

[Outline of Invention]

In order to achieve the above-mentioned object, the present invention uses a reference level generating circuit composed of a plurality of IC internal resistances (or external resistances may be used) with sufficient specific accuracy, an IC internal resistance Ra, and an IC internal junction capacitance Ca. To the pseudo filter circuit, the output level of the reference level generation circuit and the output level of the pseudo filter circuit are compared by the comparison circuit, and the comparison circuit output is output so that both output levels match. Va is negatively fed back to one end of the junction capacitance Ca in the IC (a fixed voltage Vb is applied to the other terminal). The capacitance value of the junction capacitance Ca in the IC changes according to the change in the voltage applied across the IC, and the variation in the filter characteristics of the pseudo filter circuit is absorbed. Further, the desired filter circuit is configured by the IC internal resistance Ra of the pseudo filter circuit, the IC internal junction capacitance Ca, and the IC internal resistance Rb and the IC internal junction capacitance Cb, each of which is sufficiently accurate, and the IC internal junction capacitance Cb By supplying the above-mentioned comparison circuit output Va, the variation in the filter characteristics of the desired filter circuit is automatically absorbed with high accuracy and the adjustment is eliminated.

Further, a differential amplifier is configured at the input stage of the desired filter circuit, and a voltage corresponding to the fixed voltage Vb is applied to one end of the output load of the differential amplifier on the power supply side to correspond to the fixed voltage Vb for the filter input signal. The applied voltage is superimposed. A voltage corresponding to the output Va of the comparison circuit is supplied to one end of the junction capacitance C _{2 in} the IC, and the same applied voltage across both ends of the junction capacitances Ca and Cb in the IC is supplied. As a result, the DC voltage level can be reapplied at the filter input terminal without increasing the number of IC pins and peripheral components.

Example of Invention

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

FIG. 1 is a block diagram showing an embodiment of a filter integrated circuit according to the present invention, in which reference numeral 5 is a reference signal source, 6 is a reference level generating circuit consisting of an internal resistor ReRf having a sufficient relative accuracy, and 7 Is a pseudo filter circuit consisting of the resistance Ra in the IC and the junction capacitance Ca in the IC, 8 and 9 are level detectors that detect the level of the input signal, 10 is a comparison circuit, 11 is an external capacitor, and 12 is a reference voltage Vb. A constant voltage source, 13 is a differential amplifier consisting of transistors 14 and 15, a load resistor 16 and a constant current source 17, 18 is a DC shift circuit consisting of a resistor 19 and a constant current source 20, 21 is transistors 22 to 26, constant current Source 27-31, resistance in IC Rb ₁ , Rb ₂ , Rb ₃ , Rb ₄ and junction capacitance Cb ₁ , Cb ₂ , Cb ₃ , Cb ₄ in IC, 32
Is an FM modulator, and 33 and 34 are IC pins.

In the figure, the reference signal input from the IC pin 33 is supplied to the reference level generation circuit 6 and the pseudo filter circuit 7. From the reference level generation circuit 6, a signal of a certain level indicated by the straight line D in FIG.
Is supplied to. On the other hand, a signal attenuated according to the filter characteristic is taken out from the pseudo filter circuit 7 and supplied to the level detector 9. The output signals of the level detectors 8 and 9 are respectively input to the comparison circuit 10, and the capacitor 11 obtains a comparison output which is the DC voltage control signal Va. This comparison output is fed back to one end of the in-IC junction capacitance Ca of the pseudo filter circuit 7 as a control signal to change the voltage applied across the in-IC junction capacitance Ca. By this feedback control, the capacitance value of the junction capacitance Ca in the IC is changed so that the output of the pseudo filter circuit 3 becomes the same level as the output of the reference level generating circuit 6. That is, with respect to the variations in the filter characteristics as shown by the curves A and B in FIG. 2 caused by the variations in the resistance Ra in the IC and the junction capacitance Ca in the IC, the frequency of the reference signal source 5 in The filter characteristic C shown in FIG. 2 is controlled so as to match the output D. That is, resistance Ra and capacitance Ca
The product of (time constant) is always constant. Furthermore, the resistance in the IC that constitutes the high-frequency wave filter 21, which is the desired integrated filter,
Rb _{1 to} Rb ₄ , the IC junction capacitances Cb _{1 to} Cb ₄ and the IC internal resistance Ra, the IC junction capacitance Ca are sufficiently accurate, and both ends of the IC junction capacitances Ca and Cb _{1 to} Cb ₄ are By making the applied voltages equal to each other, the capacitance values of the junction capacitances in the respective ICs similarly change, and variations in the filter characteristics in the high pass filter 21 are automatically absorbed.

Here, the output signal of the FM modulator 32 is input to the differential amplifier 13. The output signal of the differential amplifier 13 is the reference voltage of the constant voltage source 12.
Equivalent to the product of the resistance value R 16 of the resistor ₁₆ and the current value I ₁₇ of the constant current source _{17 which} is about half of Vb The resulting voltage is a signal whose DC component is the reduced voltage. next,
This signal is the base-emitter voltage V of transistor 22.
And only DC shifts _BE is applied to the anode side of the junction capacitance Cb ₁ to CB ₃ the IC. Furthermore, it is supplied to the anode side of the junction capacitance Cb _{4 in} the IC via the transistors 24 and 25. That is, from the voltage Vb applied to the anode side of the junction capacitance Ca in the IC An FM signal whose DC component is a voltage shifted by only DC is supplied to the anode side of the junction capacitances Cb _{1 to} Cb _{4 in} the IC configuring the high pass filter 21.

On the other hand, the DC voltage Va is supplied to the cathode side of the junction capacitance Ca in the IC by the above feedback control. This voltage Va
Is the resistance 19 Product of the resistance value R ₁₉ of the constant current source 20 resistor value a current value I ₂₀ of (constant current source 17 and the high specific accuracy possible) is R ₁₆ resistor 19 (take sufficiently a resistor 16 relative accuracy) is Of the base-emitter voltage by the transistor 23.
It is DC-shifted by V _BE and supplied to the cathode side of the junction capacitances Cb _{1 to} Cb _{4 in} the IC. That is, applied to the cathode side of the junction capacitance Ca in the IC Only the DC shifted voltage is supplied to the cathode side of the IC in the junction capacitance Cb ₁ ~Cb _4.

Therefore, the same voltage as the applied voltage across the junction capacitance Ca in the IC can be applied to the junction capacitances Cb _{1 to} Cb ₄ in the IC, and the output signal of the FM modulator 32 can be applied to the output signal of the FM modulator 32 in the IC with a simple configuration in the IC. A DC control signal applied across the junction capacitance can be applied. Further, in order to give the DC control signal applied to both ends, the output signal of the FM modulator 32 is once output outside the IC, and the DC cutting is performed by the external capacitance having a large capacitance value. The effect is that an unadjusted filter integrated circuit can be realized without increasing the number of IC pins, and the IC pins that are not required can achieve higher integration. Note that in the high-frequency wave 21, the junction in the IC capacitance Cb ₁ to CB ₃ and IC in resistance Rb ₁ ~Rb ₃
The trap filter characteristic is obtained by the above, and the high pass first order filter characteristic is obtained by the junction capacitance Cb ₄ in the IC and the resistance Rb _{4 in the} IC, and the low range is sharply reduced as a whole.

FIG. 3 is a block diagram showing another embodiment of the filter integrated circuit according to the present invention, in which a DC shift circuit 35 supplies a voltage corresponding to the voltage Vb of the constant voltage source 12 to the power source side of the differential amplifier 13. , 36 to 43 are transistors, 42 and 43 are resistors, 44 is a constant voltage source, 4
Reference numerals 5 and 46 denote constant current sources, and the same or equivalent parts as those in FIG. 1 are designated by the same reference numerals.

In FIG. 3, the constant current sources 20 and 17 are formed by the transistors 40 and 41, the resistors 42 and 43, and the constant voltage source 44, each of which has a sufficient ratio accuracy. And the transistors 36 and 37 form a current mirror circuit. At one end of the resistor 19 (the side connected to the transistor 38), the voltage Vb of the constant voltage source 12 is applied to the resistor 19
(As in Fig. 1, the resistance value is the same as that of resistor 16 and is R ₁₆ ) In addition, the transistor 3 with sufficient ratio accuracy was obtained.
One end of the resistor 16 via 8,39 (power supply side of the differential amplifier 13)
To Will be supplied. Therefore, it occurs at the other end of the resistor 16.
The average DC output of the FM modulator 32 may or may not flow through the resistor 16 with the current source 17. Is a signal with a DC component.

Next, the signal of the same DC component (voltage Vb) is passed through the transistors 42 and 43 with sufficient relative accuracy to the junction capacitance Cb _{1 in the} IC.
~ Is supplied to the anode side of Cb ₄ . On the other hand, IC junction capacitance C
_On the cathode side of b _{1 to} Cb _4, the same applied voltage across the voltage Va is always applied to the junction capacitances Cb _{1 to} Cb ₄ in the IC. Here, if there is a difference between the terminal applied voltage of the junction capacitance Ca in the IC and the terminal applied voltage of each of the junction capacitances Cb _{1 to} Cb _{4 in} the IC, for example, in FIG. However, in this case, the control range of the junction capacitance in the IC is narrowed by that amount for a given current voltage. In the embodiment of FIG. 3, since there is no difference as described above, there is an effect that the maximum absorption range can be obtained with respect to variations in filter characteristics.

In each of the above embodiments, the FM modulator output is shown as the input signal to the filter integrated circuit, but it is obvious that the same effect can be obtained with the video signal and the audio signal.

〔The invention's effect〕

As described above, according to the present invention, variations in filter characteristics can be automatically absorbed, and a simple circuit configuration inside the IC can be used.
The absorption control range can be improved without increasing the number of IC pins, and large block filters, which were conventionally external components, can be integrated without the need for external adjustment means, resulting in low cost. It is possible to provide a filter integrated circuit that can be made smaller, smaller and lighter.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a filter integrated circuit according to the present invention, FIG. 2 is an operation explanatory view of this embodiment, and FIG. 3 is a block diagram showing another embodiment of a filter integrated circuit according to the present invention. 4 is a block diagram of a magnetic recording / reproducing apparatus using a filter, FIG. 5 is a circuit diagram showing an example of a filter having a trap characteristic, and FIG. 6 is a characteristic diagram showing its frequency characteristic. 5 ... Reference signal source, 6 ... Reference level generation circuit, 7 ...
Pseudo filter circuit, 8, 9 ... Level detector, 10 ... Comparison circuit, 13 ... Differential amplifier, 18 ... DC shift circuit, 21 ...
… High-frequency wave filters, 33,34 …… IC pins, 35 …… DC shift circuit

Claims (1)

[Claims] 1. A reference level generating circuit for attenuating an input reference signal by a plurality of first resistor groups having high relative accuracy, and the first resistor using the reference signal as an input signal. A pseudo filter circuit composed of a second resistor and a first junction capacitance having high ratio accuracy with the group; a reference voltage source for supplying a DC voltage signal to one end of the first junction capacitance; A first level detecting circuit for detecting an output level of the reference level generating circuit; a second level detecting circuit for detecting an output level of the pseudo filter circuit;
A high ratio precision is obtained between the voltage comparator circuit that uses both outputs of the second and second level detection circuits as input signals and the output is fed back to the other end of the first junction capacitance as a control signal, and the second resistor. A filter circuit including a third resistor group and a second junction capacitor group having a high ratio accuracy with the first junction capacitor is provided, and a differential amplifier is provided in an input stage of the filter circuit. , A voltage supplied from the reference voltage source as a power source of the differential amplifier, and a voltage that is the same as the reference voltage source or increased or decreased by a certain voltage is supplied to one end of the second junction capacitor, and the control is performed. A filter integrated circuit, wherein the same voltage as the signal or a voltage increased / decreased by a certain constant voltage is supplied to the other end of the second junction capacitor.