JP2007221264A - Filter circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a filter circuit capable of outputting an output voltage with its offset voltage decreased. <P>SOLUTION: A compensation circuit 40 is attached to the filter circuit 10, in order to increase a discharging current I11 by a current equivalent to an increment I3/hfe of a charging current I12, due to a current I3 flowing from a comparator 20 to an output terminal OUT 1 of the filter circuit 10. Since the balance between the discharging current I11 and the charging current I12 can be maintained independently of the current I3 supplied from the comparator 20, the offset voltage contained in a filter processing voltage outputted from the filter circuit 10 can be decreased. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a filter circuit that is provided in a preceding stage of a comparator and performs low-pass filter processing on a voltage to be input to the comparator to stabilize an input voltage to the comparator.

  For example, when the power supply voltage is input to the comparator and the level of the power supply voltage is determined, even if noise is superimposed on the power supply voltage, the comparator voltage is stabilized to stabilize the power supply voltage input to the comparator. A filter circuit may be provided in the previous stage.

  FIG. 4 is a circuit diagram showing a configuration including a filter circuit in such an application. As shown in FIG. 4, the power supply voltage VCC1 whose level is to be determined is divided by a resistor R100 and a resistor R200 connected in series, and input to the filter circuit 10. In the filter circuit 10, even when spike noise or the like is superimposed on the power supply voltage VCC1, low-pass filter processing for reducing high-frequency components such as spike noise is performed in order to stabilize the divided voltage of the power supply voltage VCC1. Execute.

  The output voltage of the filter circuit 10 is supplied to the comparator 20, and the comparator 20 compares it with the reference voltage. The comparator 20 outputs a signal corresponding to the comparison result obtained by comparing the magnitude relationship between the output voltage of the filter circuit 10 and the reference voltage. In this way, the level of the power supply voltage VCC1 can be determined. The constant voltage circuit 30 supplies a constant voltage to the filter circuit 10 and the comparator 20.

  An example of detailed circuits of the filter circuit 10 and the comparator 20 is shown in FIG. As shown in FIG. 5, the filter circuit 10 includes bipolar transistors T1 to T19, resistors R1 to R12 that limit a current passed through each bipolar transistor, and a smoothing capacitor C1.

  An input terminal of the filter circuit 10 to which the divided power supply voltage VCC1 is input is connected to the mutually connected bases of the PNP transistors T1 and T2. On the other hand, the output terminal of the filter circuit 10 is connected to the mutually connected bases of the PNP transistors T6 and T9. These transistors T1, T2 and T6, T9 are all connected to the collector of the PNP transistor T5 via resistors R2, R4, respectively. Since a constant current from the constant current circuit 30 is applied to the base of the transistor T5, the transistor T5 flows a constant collector current I1.

  With such a configuration, when the input voltage applied to the input terminal IN is smaller than the output voltage (filter processing voltage) output from the output terminal OUT1 of the filter circuit 10, the transistors T1 and T2 have a size relationship. Turns on. Conversely, when the input voltage is greater than the output voltage, the transistors T6 and T9 are turned on. The transistors T1 and T9 may be omitted.

  The collector of the transistor T2 is connected to a mirror circuit having a mirror ratio of 1: 1 / n composed of NPN transistors T3 and T4. Therefore, when the collector current I1 flows through the transistor T2 by turning on the transistor T2, the transistors T3 and T4 are also turned on. At this time, the collector current of approximately I1 flows through the transistor T3, while the collector current of the transistor T4 is attenuated according to the mirror ratio (1: 1 / n), and thus is approximately I1 / n. Similarly, a mirror circuit having a mirror ratio of 1: 1 / n ′ composed of NPN transistors T7 and T8 is connected to the collector of the transistor T6. Therefore, when the transistor T6 is turned on, a collector current of approximately I1 / n ′ flows through the transistor T8.

  The collector of the transistor T4 is connected to a mirror circuit having a mirror ratio of 1: 1 / m, which is composed of PNP transistors T10 and T11. Therefore, when the collector current I1 / n flows through the transistor T4, a collector current of I1 / n flows through the transistor T10, and a collector current attenuated to I1 / nm flows through the transistor T11. Similarly, the collector of the transistor T8 is connected to a mirror circuit having a mirror ratio of 1: 1 / m ′ composed of PNP transistors T16 and T17. Therefore, when the collector current I1 / n ′ flows through the transistor T8, the collector current attenuated to I1 / n′m ′ flows through the transistor T17.

  A series circuit composed of PNP transistors T14 and T15 is connected between the high potential side power supply VCC and the low potential side power supply GND. A constant voltage from the constant voltage circuit 30 is applied to the base of the transistor T14 in this series circuit. Since the transistor T14 has characteristics equivalent to those of the transistor T5, the collector current of the transistor T14 is I1 like the transistor T5. By supplying the collector current I1 to the emitter of the transistor T15, the base current Ib1 of the transistor T15 becomes I1 / hfe. Note that hfe is the current amplification factor of the transistor 15.

  Since the base of the transistor T15 is connected to the collector of the transistor T11, the base current Ib1 of the transistor T15 is added to the collector current I1 / nm of the transistor T11. This added current becomes the discharge current I11 of the smoothing capacitor C1.

  The collectors of NPN transistors T12 and T13 are connected to both ends of the capacitor C1, and the bases thereof are connected so that a discharge current I11 is applied. Therefore, when the discharge current I11 flows and the charging voltage of the smoothing capacitor C1 by the discharge current I11 reaches the turn-on voltage of the transistors T12 and T13, the transistors T12 and T13 are turned on. Thereby, the charging voltage of the smoothing capacitor C1 is discharged to the low potential side power supply GND through the transistors T12 and T13.

  In the series circuit including the transistors T14 and T15 described above, since the base current Ib1 ′ from the output circuit composed of PNP transistors T18 and T19, which will be described later, is included in the charging current I12, the discharging current I11 is smaller than the charging current I12. It is provided to maintain balance.

  When the collector current I1 / n 'flows through the transistor T8 and the attenuated collector current I1 / n'm' flows through the transistor T17, the charging current I12 for charging the smoothing capacitor C1 flows. This charging current I12 is obtained by adding the collector current I1 / n'm 'of the transistor T17 and the base current Ib1' of the transistor T18 constituting the output circuit.

  The output circuit is composed of PNP transistors T18 and T19 connected in series, and outputs a voltage corresponding to the charging voltage of the smoothing capacitor C1 from the output terminal OUT1 as a filter processing voltage. Hereinafter, the operation of the output circuit will be described.

  The transistor T19 has characteristics equivalent to those of the transistors T5 and T14, and the constant voltage from the constant voltage circuit 30 is applied to the base thereof, so that the collector current I1 flows as in the transistors T5 and T14. On the other hand, one end of the smoothing capacitor C <b> 1 is connected to the base of the transistor 18. Therefore, the conduction state of the transistor T18 changes according to the charging voltage of the smoothing capacitor C1. Specifically, the lower the charging voltage of the smoothing capacitor C1, the more emitter current flows through the transistor T18, so the voltage output from the output terminal OUT1 decreases. At this time, as the emitter current increases, the base current Ib1 'of the transistor T18 also increases. Accordingly, the smoothing capacitor C1 is charged relatively quickly by the charging current I12 which is the sum of the base current Ib1 'and the collector current I1 / n'm' of the transistor T17.

  As the charging voltage of the smoothing capacitor C1 increases, the emitter current of the transistor T18 decreases, and the voltage output from the output terminal OUT1 increases. As the emitter current decreases, the base current Ib1 'also decreases, so the rate of increase of the charging voltage of the smoothing capacitor C1 also decreases. When the voltage output from the output terminal OUT1 rises and becomes larger than the input voltage, the discharge current I11 starts flowing instead of the charging current I12 at that time, and the charging voltage of the smoothing capacitor C1 is discharged. .

  As described above, in the conventional filter circuit 10, the collector current I1 is attenuated by utilizing the multistage mirror circuit composed of a plurality of bipolar transistors and the current amplification characteristics of the bipolar transistors T15 and T18. A discharge current I11 and a charging current I12 are generated. The low-pass filter constant in the filter circuit 10 can be increased by charging and discharging the smoothing capacitor C1 using the attenuated minute discharge current I11 and charge current I12. As a result, it is possible to effectively remove high-frequency components such as noise and output a stable voltage to the comparator 20.

  Here, the configuration and operation of the comparator 20 shown in FIG. 5 will be briefly described. The reference voltage and the filtering voltage output from the filter circuit 10 are applied to the bases of the PNP transistors T23 and T26, respectively. The transistor T23 is connected to the transistor T24 by Darlington, and the transistor T26 is connected to the transistor T25 by Darlington.

  Since the transistors T23 and T24 and the transistors T25 and T26 are connected symmetrically, when the reference voltage is lower than the output voltage of the filter circuit 10, the transistors T23 and T24 are turned on, and conversely, the reference voltage is higher than the output voltage. Are higher, the transistors T25 and T26 are turned on. When the transistors T23 to T26 are turned on, the PNP transistors T20 to T22 that generate a constant collector current are connected to the emitters of the transistors T23 to T26 so that a constant current is supplied to the transistors T23 to T26. It is connected to the.

  A current mirror circuit composed of NPN transistors T26 and T27 is connected to the collectors of the transistors T24 and T25. In this current mirror circuit, since the bases of the transistors T26 and T27 are connected to the collector of the transistor 24, when the transistor T24 is turned on, both the two transistors T26 and T27 are turned on. In contrast, when the transistor 25 is turned on, the two transistors T26 and T27 remain off.

  Therefore, the NPN output transistor T28 whose base is connected to the collector of the transistor T27 is turned off when the transistor T24 is turned on, and turned on when the transistor T25 is turned on.

  As a result, when the output voltage of the filter circuit 10 is larger than the reference voltage, a voltage corresponding to the high potential side power supply Vcc is output from the output terminal OUT2 of the comparator 20, and when the output voltage is smaller than the reference voltage, the low potential side A voltage corresponding to the power supply GND is output.

  In the conventional filter circuit 10 described above, if only the filter circuit 10 is limited, the discharge current I11 and the charging current I12 are kept in balance, and an output voltage corresponding to the voltage input to the filter circuit 10 is output. It can be configured.

  However, as shown in FIG. 5, the current I3 flows from the comparator 20 connected to the subsequent stage of the filter circuit 10 into the output terminal OUT1 of the filter circuit 10. That is, in the comparator 20, when the filtering voltage is lower than the reference voltage, the base current I3 of the transistor T26 is generated and flows into the output terminal OUT1 of the filter circuit 10. Since the current I3 is superimposed on the emitter current of the transistor T18, the base current of the transistor T18 increases by I3 / hfe. As a result, the charging current I12 of the smoothing capacitor C1 increases from the discharging current I11 by I3 / hfe, which is an increase in the base current.

  As described above, since the discharge current I11 and the charge current I12 are attenuated and become minute, they are greatly affected by the increase in base current (I3 / hfe). For example, as the charging current I12 increases, the potential level of the charging voltage of the smoothing capacitor increases as a whole. As a result, the output voltage of the filter circuit 10 includes an offset voltage. Furthermore, when the current amplification factor hfe of the PNP transistor T18 and the current I3 flowing from the comparator 20 increase or decrease due to a temperature change, a temperature characteristic is generated in the above-described offset voltage.

  As described above, when the output voltage of the filter circuit 10 includes an offset voltage or the offset voltage has a temperature characteristic, there arises a problem that the voltage comparison accuracy in the comparator 20 is deteriorated.

  The present invention has been made in view of the above points, and an object thereof is to provide a filter circuit capable of outputting an output voltage with a reduced offset voltage.

In order to achieve the above object, the filter circuit according to claim 1 comprises:
A low-pass filter for the voltage to be input to the comparator is provided in a preceding stage of the comparator that compares the input voltage with a predetermined reference voltage and stabilizes the voltage input to the comparator. A filter circuit that performs processing and outputs the filtered voltage to the comparator,
The comparator has an input stage circuit that flows a current toward the filter circuit or receives a current flowing from the filter circuit, depending on a magnitude relationship between the filtering voltage and the reference voltage,
The filter circuit is
A smoothing capacitor;
The current corresponding to the voltage difference between the filter processing voltage and the voltage input to the filter circuit is attenuated at a predetermined attenuation rate using a plurality of bipolar transistors, and the charging current of the smoothing capacitor And a current generation circuit for generating a discharge current;
A filtering voltage generation circuit for generating a voltage corresponding to a charging voltage of the smoothing capacitor as the filtering voltage;
A compensation circuit corresponding to the input stage circuit of the comparator is added to the current generation circuit in order to compensate for the influence on the charging current of the smoothing capacitor due to the current flowing in from the comparator or the current flowing out to the comparator. In addition, the discharge current is increased or decreased.

  As described above, in the filter circuit according to the first aspect, the current flowing from the comparator into the filter circuit is not limited to the filter circuit, and the discharge current and the charging current for the smoothing capacitor are not balanced. In consideration of the influence of the current flowing from the circuit to the comparator on the charging current, a compensation circuit for increasing or decreasing the discharge current is provided.

  As a result, it is possible to achieve a balance between the discharge current and the charging current with higher accuracy than in the conventional filter circuit. Therefore, the offset voltage can be effectively reduced by outputting a voltage corresponding to the charging voltage of the smoothing capacitor that is charged and discharged by such a discharging current and the charging current as the filtering voltage.

  As a specific example of the compensation circuit, as described in claim 2, when the input stage circuit of the comparator includes at least two bipolar transistors connected between a high potential power source and a low potential power source, The discharge current of the current generation circuit is increased or decreased by flowing out of the input stage circuit or flowing out of the input stage circuit as much as the charging source flow is increased or decreased. It is preferable to add a circuit including at least two bipolar transistors connected between the high potential power source and the low potential power source, which is equivalent to the input stage circuit, to the energization path. Thus, by adding a circuit equivalent to the input stage circuit of the comparator as a compensation circuit, the discharge current can be increased or decreased by the same amount as the increase or decrease of the charging current.

  According to the third aspect of the present invention, when a plurality of comparators are connected in parallel to the common filter circuit, the same number of compensation circuits as the number of connected comparators are generated as the current generation circuit. It is preferable to add in parallel to the circuit. When a plurality of comparators are connected in parallel to a common filter circuit, the current flowing from the comparators or the current flowing out to the comparators increases in proportion to the number of comparators. For this reason, it is possible to achieve a balance between the charging current and the discharging current by adding as many compensation circuits as the number of comparators.

(First embodiment)
Hereinafter, a filter circuit according to a first embodiment of the present invention will be described with reference to the drawings. Note that, similarly to the conventional filter circuit 10, the filter circuit according to the present embodiment, for example, performs a low-pass filter process on the divided power supply voltage to reduce the high frequency component, thereby reducing the voltage input to the comparator. Used to stabilize. Therefore, the overall configuration including the filter circuit is the same as the conventional configuration shown in FIG. Further, the comparator in the present embodiment has the same circuit configuration as that of the conventional comparator 20, and the filter circuit is mostly shared with the conventional filter circuit 10. Therefore, in the description regarding the filter circuit and the comparator according to the present embodiment, the same reference numerals are given to the same components as those in the related art, and the description will be omitted or simplified.

  FIG. 1 shows a circuit configuration of a filter circuit 10 according to the first embodiment and a comparator 20 connected to the subsequent stage of the filter circuit 10. As shown in FIG. 1, in the present embodiment, a difference from the conventional filter circuit 10 is that a compensation circuit 40 is added to the circuit portion that generates the discharge current I11 in the filter circuit 10.

  The compensation circuit 40 includes PNP transistors T30 and T31 connected in series between the high potential side power supply VCC and the low potential side power supply GND. In the compensation circuit 40, the constant voltage from the constant voltage circuit 30 is applied to the base of the transistor T30. The transistor T30 has the same characteristics as the transistor T22 in the comparator 20. For this reason, the collector current of the transistor T30 is the same as that of the transistor T22.

  The collector current of the transistor T30 constituting the compensation circuit 40 is supplied to the emitter of the transistor T31. The transistor T31 is configured to have the same characteristics as the transistor T26 of the comparator 20. As a result, the base current I3 'of the transistor T31 is substantially the same as the base current I3 of the transistor T26 of the comparator 20.

  That is, in the present embodiment, the comparator 20 is added with a configuration equivalent to the input stage circuit composed of the two transistors T22 and T26 as the compensation circuit 40 in which the current I3 flows into the output terminal OUT1 of the filter circuit 10. is there.

  The compensation circuit 40 is added to the filter circuit 10 so that the current from the compensation circuit 40 flows through a connection line connecting the collector of the transistor T14 and the emitter of the transistor T15. As a result, the emitter current in the transistor T15 becomes I1 + I3 ', and the base current Ib1 becomes (I1 + I3') / hfe. Therefore, since the discharge current I11 is a current obtained by adding the base current Ib1 and the collector current of the transistor T11, it becomes I1 / nm + (I1 + I3 ') / hfe.

  Therefore, it is possible to increase the discharge current I11 by an amount corresponding to an increase I3 / hfe of the charging current I12 due to the current I3 flowing from the comparator 20 to the output terminal OUT1 of the filter circuit 10. As a result, the discharge current I11 and the charging current I12 are kept in balance regardless of the current I3 flowing from the comparator 20, so that the offset voltage included in the filter processing voltage output from the filter circuit 10 can be reduced. .

  In the above-described embodiment, the case where one comparator 20 is connected to the subsequent stage of the filter circuit 10 has been described. However, as shown in FIG. 2, for example, in order to determine the level of the power supply voltage VCC1 in more detail, a common filter is used. A plurality of comparators 20, 20A,... Having different reference voltages may be connected to the circuit 10. In such a case, the current flowing into the output terminal OUT1 of the filter circuit 10 also increases in proportion to the number of comparators 20 and 20A connected to the filter circuit 10.

  Therefore, when a plurality of comparators 20 and 20A are connected to the filter circuit 10, the same number of compensation circuits 40 and 40A as the connected comparators 20 and 20A are added to the filter circuit 10. Specifically, when the compensation circuit 40A is further added to the compensation circuit 40, the compensation circuit 40A is connected in parallel with the transistors T30 and T31 in the compensation circuit 40 as indicated by a dotted line in FIG. Are connected to each other.

  As a result, the current from each compensation circuit 40, 40A is added to the emitter current of the transistor T15 in the same manner as the current from each comparator 20, 20A is added to the emitter current of the transistor T18. Therefore, even when a plurality of comparators 20 and 20A are connected to the filter circuit 10, the balance between the discharge current I11 and the charge current I12 of the smoothing capacitor C1 can be maintained.

(Second Embodiment)
Next, a filter circuit according to a second embodiment of the present invention will be described. In the first embodiment described above, the example in which the current I3 flows from the input stage circuit of the comparator 20 to the output terminal of the filter circuit 10 has been described.

  However, depending on the configuration of the comparator, current may flow from the output terminal OUT1 of the filter circuit 10 to the input stage circuit of the comparator 20. FIG. 3 shows a circuit configuration of such a comparator.

  In the comparator 20 shown in FIG. 3, the filtering voltage and the reference voltage output from the filter circuit 10 are applied to the bases of the NPN transistors TT42 and T43, respectively. Since the emitters of the transistors T42 and T43 are connected to the power supply VCC via a common line, when the filtering voltage is higher than the reference voltage, only the transistor T42 to which the filtering voltage is applied is turned on. To do. On the other hand, when the filtering voltage is lower than the reference voltage, only the transistor T43 to which the reference voltage is applied to the base is turned on.

  The emitters of the PNP transistors T44 and T45 are connected to the emitters of the transistors T42 and T43, respectively. Further, a constant current circuit 51 is connected to these transistors T44 and T45. Thus, when the transistor T44 or T45 is turned on, the base currents of the respective transistors T44 and T45 become constant values defined by the constant current circuit 51. As a result, the collector current that flows when the transistors T44 and T45 are turned on is also constant.

  When the transistor T43 is turned on to turn on the transistor T45, a current is passed from the transistor T43 through the transistor T45. Then, the base voltage of the NPN output transistor T28 increases due to the terminal voltage of the resistor R15, and the output transistor T28 is turned on. On the other hand, when the transistor T42 is on, the output transistor T28 remains off.

  As a result, when the filtering voltage output from the filter circuit 10 is larger than the reference voltage, the voltage corresponding to the high potential side power source Vcc is output from the output terminal OUT2 of the comparator 20 and is smaller than the reference voltage. A voltage corresponding to the low potential side power supply GND is output.

  In the comparator 20 described above, the current I3 flows from the output terminal OUT1 of the filter circuit 10 to the input stage circuit of the comparator 20 including the transistor T42 and the transistor T44.

  In such a case, the charging current I12 of the smoothing capacitor C1 decreases due to the outflow current I3. Since the amount of decrease in charging current I12 is I3 / hfe, charging current I12 is (I1-I3) / hfe.

  Therefore, in the present embodiment, a compensation circuit 40 that can reduce the discharge current I11 by an amount corresponding to the reduction amount of the charging current I12 is added to the filter circuit 10. Specifically, the compensation circuit 40 includes an NPN transistor T40 and a PNP transistor T41 connected in series between the high potential side power supply VCC and the low potential side power supply GND. Furthermore, a constant current circuit 50 connected to the base of the transistor T41 is also provided.

  Since the base of the transistor T40 of the compensation circuit 40 is connected to the collector of the transistor T14, a part (I3 ') of the collector current of the transistor T14 flows as the base current of the transistor T40. Here, the transistor T40 has the same characteristics as the transistor T42 in the comparator 20. The emitter current of the transistor T40 constituting the compensation circuit 40 is supplied to the emitter of the transistor T41. The transistor T41 has characteristics equivalent to those of the transistor T44 of the comparator 20, and the constant current circuit 50 in the compensation circuit 40 and the constant current circuit 51 in the comparator 20 are configured to pass the same constant current. For this reason, the base current I3 'of the transistor T40 is substantially the same as the base current I3 of the transistor T42.

  That is, in the present embodiment, the compensation circuit 40 has a configuration equivalent to the input stage circuit including the transistor T42, the transistor T26, and the constant current circuit 51 that receives the current I3 flowing out from the output terminal OUT1 of the filter circuit 10 in the comparator 20. It is added.

  By adding such a compensation circuit 40, the current flowing from the transistor T14 to the transistor T15 is reduced by the base current I3 'of the transistor T40 of the compensation circuit 40. As a result, the base current Ib1 in the transistor T15 becomes (I1-I3 ') / hfe. Therefore, the discharge current I11 can be reduced by an amount corresponding to the decrease I3 / hfe of the charging current I12 due to the current I3 flowing out from the output terminal OUT1 of the filter circuit 10 to the comparator 20. Thereby, since the balance between the discharge current I11 and the charging current I12 can be maintained regardless of the current I3 flowing out to the comparator 20, the offset voltage included in the filter processing voltage output from the filter circuit 10 can be reduced. Can do.

  The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

  For example, a plurality of comparators 20 may be connected to the filter circuit 10 described as the second embodiment. In this case, the compensation circuits 40 may be connected in parallel by the same number as the number of comparators 20 to be connected, as in the example described above.

1 is a circuit diagram showing a circuit configuration of a filter circuit 10 according to a first embodiment and a comparator 20 connected to a subsequent stage of the filter circuit 10. FIG. It is a block diagram which shows the whole structure containing a filter circuit when the comparator 20 is provided with two or more. It is a circuit diagram which shows the circuit structure of the filter circuit 10 by 3rd Embodiment, and the comparator 20 connected to the back | latter stage of the filter circuit 10. It is a block diagram which shows the whole structure containing a filter circuit by a prior art. It is a circuit diagram which shows the detailed circuit structure of the conventional filter circuit and comparator shown in FIG.

Explanation of symbols

10 Filter circuit 20 Comparator 30 Constant voltage circuit

Claims (3)

A low-pass filter for the voltage to be input to the comparator is provided in a preceding stage of the comparator that compares the input voltage with a predetermined reference voltage and stabilizes the voltage input to the comparator. A filter circuit that performs processing and outputs the filtered voltage to the comparator,
The comparator has an input stage circuit that flows a current toward the filter circuit or receives a current flowing from the filter circuit, depending on a magnitude relationship between the filtering voltage and the reference voltage,
The filter circuit is
A smoothing capacitor;
The current corresponding to the voltage difference between the filter processing voltage and the voltage input to the filter circuit is attenuated at a predetermined attenuation rate using a plurality of bipolar transistors, and the charging current of the smoothing capacitor is reduced. And a current generation circuit for generating a discharge current;
A filtering voltage generation circuit that generates a voltage corresponding to a charging voltage of the smoothing capacitor as the filtering voltage;
A compensation circuit corresponding to the input stage circuit of the comparator is added to the current generation circuit in order to compensate for the influence on the charging current of the smoothing capacitor caused by the current flowing from the comparator or the current flowing out to the comparator. In addition, a filter circuit characterized in that the discharge current is increased or decreased. The comparator input stage circuit comprises at least two bipolar transistors connected between a high potential power source and a low potential power source,
The discharge current in the current generation circuit is increased or decreased by flowing out of the input stage circuit or flowing into the input stage circuit as much as the charging source flow is increased or decreased. 2. The filter according to claim 1, wherein a circuit including at least two bipolar transistors connected between the high-potential power supply and the low-potential power supply, which is equivalent to the input stage circuit, is added to the energization path of the filter. circuit.   When a plurality of comparators are connected in parallel to the common filter circuit, the same number of compensation circuits as the number of connected comparators are added in parallel to the current generation circuit. The filter circuit according to claim 1 or 2.

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