JP2005354317A - Filter circuit and pll - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a filter circuit the area of which is reduced more than that of prior area and a PLL with a small area size employing the filter circuit. <P>SOLUTION: The filter circuit is provided with: a first capacitor 1 to one electrode of which a first input current Iin1 is given and the other electrode of which is connected to ground; a voltage-current converter 2 connected in parallel with the first capacitor 1 when viewed from a first input section 30 and for converting a voltage produced by the first capacitor 1 into a current inversely proportional to an internal resistance of the converter 2; and a second capacitor 3 and a resistor 4 to which a sum current of an output current of the voltage-current converter 2 and a second input current Iin2 is injected. Adjusting the gain of the voltage-current converter 2 can increase the apparent capacitance of the first capacitor 1. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a filter circuit and a PLL (Phase Locked Loop) including the filter circuit.

  The filter circuit selectively passes signals in a specific frequency range, and is used for a PLL that oscillates a stable sine wave.

  FIG. 9 is a circuit diagram showing a conventional filter circuit, and FIG. 10 is a block circuit diagram showing a conventional PLL including the conventional filter circuit shown in FIG.

  As shown in FIG. 9, a conventional filter circuit (114 in FIG. 10) includes an input unit 120 to which an input current Iin is input, an output unit 124 for outputting an output voltage Vout, A resistor 104 connected to the output unit 124 for flowing the input current Iin, a first capacitor 121 connected in series with the resistor 104 and having one electrode grounded, and one electrode serving as the input unit 120 and the output And a second capacitor 123 connected to the portion 124 and having the other electrode grounded.

  As shown in FIG. 10, a conventional PLL includes a phase comparator 106 to which an input signal is input, a charge pump 107 that receives an output from the phase comparator 106, and a filter circuit that receives an output from the charge pump 107. 114, a voltage-current converter (VIC) 110 that receives an output from the filter circuit 114, a current-controlled oscillator (ICO) 111 that receives an output from the voltage-current converter 110 and outputs a signal to the outside, and a current A frequency divider 112 that receives an output from the control oscillator 111 and outputs a signal to the phase comparator 106 is provided.

  The operation of the conventional filter circuit and PLL configured as described above will be described.

  First, in a conventional PLL, an input signal is input to the phase comparator 106 and phase-compared with a feedback signal input to the phase comparator 106 via the frequency divider 112. The phase comparator 106 outputs a pulse having a time width corresponding to the phase difference between the input signal and the feedback signal. Next, the pulse is input to the charge pump 107 and converted into a current pulse corresponding to the time width of the pulse, and the current pulse is output from the charge pump 107. The current pulse is then input to the filter circuit 114 and filtered into a voltage signal. Subsequently, the output voltage of the filter circuit 114 is input to the voltage-current converter 110 for current conversion. Next, the current output from the voltage-current converter 110 is input to the current control oscillator 111 and converted into a frequency corresponding to the amount of current. An output signal from the current control oscillator 111 becomes an output of the PLL. Further, the output signal from the current control oscillator 111 is frequency-divided by the frequency divider 112 and is output from the frequency divider 112 as a feedback signal to the phase comparator 106.

  As described above, the PLL has a feedback loop configuration, and the input signal and the feedback signal of the PLL are automatically controlled so as to have the same frequency and a constant phase difference.

  As described above, the filter circuit 114 has a role of filtering the pulsed current into a continuous voltage, and needs to be appropriately adjusted so that the PLL having the feedback loop does not become unstable. is there.

  In the conventional filter circuit 114, as shown in FIG. 9, the input current Iin input to the input portion 120 directly charges the second capacitor 123 and the current charging the first capacitor 121 via the resistor 104. The voltage that is divided into the current and generated at the node to which the input current Iin is input becomes the output voltage.

  The transfer function of such a filter is expressed as follows by the ratio of the input current Iin (s) and the output voltage Vout (s).

Vout (s) / Iin (s) = (1 + s × C ₁₂₁ × R ₁₀₄ ) / {s × (C ₁₂₁ + C ₁₂₃ ) × (1 + s × Ca × R ₁₀₄ )} (1)

Here, C ₁₂₁ and C ₁₂₃ are capacitance values of the first capacitor 121 and the second capacitor 123, respectively, and R ₁₀₄ is a resistance value of the resistor 104. Further, Ca is 1 / Ca = 1 / C ₁₂₁ + 1 / C ₁₂₃ .

FIG. 11 is a diagram illustrating frequency characteristics of a conventional filter circuit. That is, as shown in the figure, the conventional filter circuit 114 has a zero fz frequency of 1 / (2π × C ₁₂₁ × R ₁₀₄ ) and a pole fp frequency of 1 / (2π × Ca × R ₁₀₄ ). It is a kind of low-pass filter.

  Further, the stability of the PLL is better as fp / fz is larger. However, if fp is too high, the high frequency noise is difficult to drop, and the high frequency component of the current pulse of the charge pump remains and becomes the pattern jitter of the PLL. It needs to be set appropriately. fp / fz is determined by the capacitance ratio of the first capacitor 121 and the second capacitor 123. Usually, the capacitance of the first capacitor 121 is required to be about 10 times that of the second capacitor 123 or more. .

Some conventional filter circuits increase the apparent capacitance of the capacitor. For example, Patent Document 1 describes a technique for increasing the apparent capacitance of a capacitor by feeding back the output of a VIC that injects current into the capacitor to the AMP.
JP 2-195711 A

  However, in the conventional filter circuit shown in FIG. 9, when used as a PLL loop filter, the first capacitor 121 needs to be set to about 10 times or more larger than the second capacitor 123. When built in an integrated circuit, there is a problem that the area occupied by the first capacitor 121 increases. Moreover, even if the technique described in Patent Document 1 is used, it has been difficult to miniaturize to the extent that the requirements of recent integrated circuits are satisfied.

  An object of the present invention is to provide a filter circuit having a circuit area smaller than that of the prior art and a PLL including the filter circuit.

  The filter circuit of the present invention is connected to the first input section for inputting the first input current, the second input section for inputting the second input current, and the first input section. A voltage-current having a first capacitor and a first resistor, and converting the voltage generated by the first capacitor into a current whose AC component is inversely proportional to the resistance value of the first resistor A second capacitor that is connected to the second input unit together with the converter, and to which a part of the sum current of the second input current and the output current of the voltage-current converter is shunted; With no resistance.

  With this configuration, by adjusting the gain of the voltage-current converter, the capacitance value of the first capacitor can be made smaller than the conventional one while maintaining the conventional filter function, so that the circuit area is reduced. be able to. Therefore, by using the filter circuit of the present invention, it is possible to realize a PLL having a smaller circuit area than the conventional one.

  The second capacitor and the second resistor are both grounded, and by outputting the voltage generated by the second capacitor and the second resistor, for example, the output voltage of the filter circuit is changed to a current signal. In combination with a voltage-current converter that converts to a PLL, it is possible to create a PLL with a smaller circuit area than in the past.

  The voltage-current converter, the second input unit, and the second capacitor are connected to the voltage-current converter, the second input, and the second capacitor through the second resistor, respectively, and output a current equal to the current flowing through the second resistor. Since the bias means is further provided, the voltage-current converter can be omitted from the configuration when used in the PLL, so that the circuit area of the PLL as a whole can be further reduced.

  The PLL of the present invention includes a phase comparator to which an input signal is input, a first charge pump that receives an output from the phase comparator and outputs a current pulse, and the first comparator as viewed from the phase comparator. A second charge pump disposed in parallel with the charge pump and receiving an output from the phase comparator and outputting a current pulse equal to the first charge pump; and a current pulse from the first charge pump A first capacitor having a first capacitor and a voltage generated by the first capacitor, wherein an AC component is converted into a current that is inversely proportional to a resistance value of the first resistor, and then output; A current converter and a second capacitor to which at least a part of a current obtained by adding an output current from the first voltage-current converter and a current pulse from the second charge pump is shunted. And a filter circuit having a second resistor, a current control oscillator that outputs a signal having a frequency according to a current amount of a current from the filter circuit or a current converted from an output from the filter circuit, and And a frequency divider for receiving an output from the current control oscillator and outputting a signal to the phase comparator.

  With this configuration, the output current from one charge pump is input to the first capacitor of the filter circuit, and the output current from the other charge pump and the output from the first voltage-current converter are input to the second capacitor. Since a part of the output current can be flowed, the capacitance value of the first capacitor can be suppressed to be small, and the area of the filter circuit can be made smaller than before. Since the area that can be reduced by reducing the capacitance value of the first capacitor is larger than the increase in area by adding a charge pump, the area of the PLL as a whole can be reduced.

  The second capacitor and the second resistor are both grounded, and the filter circuit outputs a voltage generated by the second capacitor and the second resistor, outputs a voltage, and The PLL may further include a second voltage-current converter that converts the output voltage of the filter circuit into a current and outputs the current to the current controlled oscillator.

  The filter circuit is connected to the voltage-current converter, the second input unit, and the second capacitor through the second resistor, and has a current equivalent to the current flowing through the second resistor. Further, the current control oscillator can omit the second voltage-current converter from receiving the output current of the filter circuit, so that the circuit area similar to the conventional one can be reduced. It can be greatly reduced.

  According to the filter circuit of the present invention, since the apparent capacitance value of the first capacitor 1 can be increased by adjusting the gain of the VIC, the capacitance value of the first capacitor 1 can be reduced to 1/10, for example. Can be kept small.

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

(First embodiment)
As a first embodiment of the present invention, a filter circuit and a PLL using the filter circuit will be described.

  FIG. 1 is a block circuit diagram showing the configuration of the filter circuit of this embodiment, and FIG. 2 is a block circuit diagram showing the configuration of the PLL of this embodiment using the filter circuit shown in FIG.

  As shown in FIG. 1, the filter circuit 9 includes a first input unit 30 to which a first input current Iin1 is input, a second input unit 31 to which a second input current Iin2 is input, and a voltage signal. , The first capacitor 1 with one electrode connected to the first input unit 30 and the other electrode grounded, the first input unit 30 and the first capacitor 1 A voltage-current converter (VIC) 2 having a resistor and converting a voltage generated by the first capacitor 1 into a current whose AC component is inversely proportional to the resistance value of the resistor; One electrode is connected to the voltage-current converter 2 and the second input part 31, and the other electrode is connected to the grounded second capacitor 3, the second input part 31 and the voltage-current converter 2. A resistor 4 grounded in parallel with the second capacitor 3 Eteiru. The filter circuit 9 of the present embodiment obtains an output voltage by flowing a part of the current obtained by adding the output current of the voltage-current converter 2 and the second input current Iin2 into the second capacitor 3 and the resistor 4. It has a configuration.

  As shown in FIG. 2, the PLL according to this embodiment includes a phase comparator 6 to which an input signal is input, a first charge pump 7 that receives an output from the phase comparator 6 and outputs a current pulse. A second charge pump 8 disposed in parallel with the first charge pump 7 as viewed from the phase comparator 6 and receiving the output from the phase comparator 6 and outputting the same current pulse as the first charge pump 7; From the filter circuit 9 that receives the outputs from the first charge pump 7 and the second charge pump, the voltage-current converter (VIC) 10 that receives the output from the filter circuit 9, and the voltage-current converter 10 The current control oscillator (ICO) 11 that outputs the signal and outputs the signal to the outside, and the frequency divider 12 that receives the output from the current control oscillator 11 and outputs the signal to the phase comparator 6 are provided. The PLL of this embodiment is different from the conventional PLL in that there are a plurality (two) of charge pumps that output a pulse current to the filter circuit 9. Usually, only two charge pumps are provided, and the timing of outputting pulses is the same as each other. In addition, the filter circuit 9 of the present embodiment has two input units (first input unit 30 and second input unit 31) for inputting a pulse current. 1 is an output current from the first charge pump 7, for example, and Iin2 is an output current from the second charge pump 8, for example.

  Next, the operation of the filter circuit and PLL configured as described above will be described.

  First, in the PLL of the present embodiment, an input signal is input to the phase comparator 6 and phase-compared with a feedback signal input to the phase comparator 6 via the frequency divider 12. The phase comparator 6 outputs a pulse having a time width corresponding to the phase difference between the input signal and the feedback signal. Next, the pulse is input to the first charge pump 7 and the second charge pump 8 and converted into a current pulse corresponding to the time width of the pulse, and the current pulse is converted into the first charge pump 7 and the second charge pump. Each is output from the charge pump 8. The two current pulses are then input to the filter circuit 9 and filtered into a voltage signal. Subsequently, the output voltage of the filter circuit 9 is input to the voltage-current converter 10 for current conversion. Next, the current output from the voltage-current converter 10 is input to the current control oscillator 11 and converted into a frequency corresponding to the amount of current. The output signal from the current control oscillator 11 becomes the output of the PLL. Further, the output signal from the current control oscillator 11 is divided by the frequency divider 12 and is output from the frequency divider 12 as a feedback signal to the phase comparator 6.

  Next, the operation of the filter circuit of this embodiment will be described with reference to FIG.

  First, the first input current Iin1 charges the first capacitor 1, and a voltage is generated in the first capacitor 1. Next, the voltage-current converter 2 converts the voltage generated by the first capacitor 1 into a current. At this time, the voltage-current converter 2 converts the input voltage into a current inversely proportional to the resistance value of the resistor provided therein and outputs the current.

  5 and 6 are circuit diagrams showing examples of the configuration of the voltage-current converter 2 in the filter circuit 9 of the present embodiment, respectively.

  In the example shown in FIG. 5, the input voltage is input to the negative input portion of the operational amplifier 40, and the resistor 41 and the first p-channel MOSFET 42 are connected to the positive input portion of the operational amplifier 40. The output of the operational amplifier 40 is input to the gate electrode of the first p-channel MOSFET 42 and the gate electrode of the second p-channel MOSFET 43. In this example, feedback is applied so that the same voltage as the input voltage is applied to the resistor 41, and the output current I (= aV / R), which is the converted current, is output from the second p-channel MOSFET 43. The Here, a is a constant, V is an input voltage of the voltage-current converter 2, and R is a resistance value of the resistor 41.

  In addition, as shown in FIG. 6, there is a method in which an input voltage I is received by a source follower circuit, and a voltage proportional to the input voltage V is applied to a resistor 51 to obtain an output current I. In this case, an offset is applied to the output current as I = aV / R + α, but the object can be achieved if the AC current is inversely proportional to the resistance. Note that V is an input voltage, R is a resistance value of the resistor 51, and a and α are constants. However, the configuration of the voltage-current converter 2 is not limited to these two specific examples.

  Next, the second input current Iin2 is added to the output current of the voltage-current converter 2, and the total current is divided and injected into the second capacitor 3 and the resistor 4 that are grounded. The voltage generated in the second capacitor 3 becomes the output voltage Vout of the filter circuit 9. Such a transfer function of the filter circuit is expressed as a ratio of the input current Iin (s) and the output voltage Vout (s) as follows.

Vout (s) / Iin (s) = {1 + s × (C ₁ / m) × R ₄ ) / {(s × C ₁ ) × (1 + s × C ₃ × R ₄ )} (2)

Here, m is the voltage-current conversion ratio (gain) of the voltage-current converter 2 and can be expressed using R ₄ as I = m × V / R ₄ . The input current Iin denotes the first input current Iin1 and second input current Iin2 (Iin = Iin1 = Iin2) , the capacity of the C ₁ and C ₃ are the first capacitor 1 and the second capacitor 3, respectively R ₄ means the resistance value of the resistor 4.

Expression (2) is the same type as Expression (1) described above. Then, the expression (2) indicates that the frequency of the zero fz is 1 / (2π × C ₁ / m × R ₄ ) and the frequency of the pole fp is 1 / (2π × C ₃ ×). R ₄ ) is a kind of low-pass filter. The difference between Formula (2) and Formula (1) is that
_{C 1 / m = C 121 ···} (3)
C ₃ = Ca = 1 / {1 / C ₁₂₁ + 1 / C ₁₂₃ }} (4)
Is replaced. Here, in the equation (3), by reducing the m without increasing the C _1, show that it is possible the apparent volume equivalent size and C _121. In the formula (4), since C ₁₂₁ >> C ₁₂₃ , C ₃ is slightly smaller than C ₁₂₃ . The value of C ₁ / m is preferably about 10 times or more than C ₃ or C ₁₂₃ as described above.

  As described above, the filter circuit according to the present embodiment has a function equivalent to that of the conventional filter circuit, and the circuit area can be significantly reduced by adjusting the gain of the voltage-current converter 2 as compared with the conventional filter circuit. It is possible. In the PLL using the filter circuit of the present embodiment, the area that can be reduced by reducing the capacitance of the first capacitor 1 is larger than the area that is increased by newly providing a charge pump. Is smaller than the conventional PLL. Therefore, if the filter circuit of this embodiment is used, an integrated circuit that is smaller than the conventional one can be realized.

(Second Embodiment)
As a second embodiment of the present invention, a filter circuit capable of outputting current and a PLL using the same will be described. In the following description, components having the same functions as those of the filter circuit of the first embodiment are denoted by the same reference numerals and description thereof is omitted.

  FIG. 3 is a block circuit diagram showing the configuration of the filter circuit according to the second embodiment. As shown in the figure, the filter circuit 13 of the present embodiment includes a first input unit 30 to which a first input current Iin1 is input and a second input unit 31 to which a second input current Iin2 is input. An output unit 32 for outputting a current signal; a first capacitor 1 having one electrode connected to the first input unit 30 and the other electrode grounded; and the first input unit 30 and the first input unit 30 A voltage-current converter 2 that is connected to one capacitor 1 and converts the voltage generated by the first capacitor 1 into a current that is inversely proportional to the internal resistance, and one electrode is the voltage-current converter 2 and The second capacitor 3 is connected to the second input unit 31 and the other electrode is grounded. The second capacitor 31 is connected to the second input unit 31 and the second capacitor 3, and the output of the voltage-current converter 2 is The resistance 4 to be received, and between the resistance 4 and the output part 32 Vignetting, and a biasing means (5) for biasing the one end of the resistor 4. In this case, the bias unit 5 is connected to the voltage-current converter 2, the second input unit, and the second input unit 31 via the resistor 4. The bias means 5 is provided for fixing one end of the resistor 4 to a predetermined voltage and taking out a current flowing through the resistor 4.

  The filter circuit 13 of this embodiment feeds the current obtained by adding the output current of the voltage-current converter 2 and the second input current Iin2 separately to the second capacitor 3 and the resistor 4, and flows through the resistor 4. An equal amount of current is output from the bias means 5.

  FIG. 4 is a block circuit diagram showing a configuration of a PLL using the filter circuit according to the second embodiment. As shown in the figure, in the PLL using the filter circuit 13 of this embodiment, it is not necessary to provide the voltage-current converter 10 included in the PLL of the first embodiment shown in FIG. That is, in the PLL of the present embodiment, the output current from the filter circuit 13 is directly input to the current control oscillator 11.

  Next, the operation of the filter circuit and PLL configured as described above will be described.

  Here, the same reference numerals as those of the filter circuit or PLL according to the first embodiment are given to portions that perform the same operation as the conventional example, and the description thereof is omitted.

  In the filter circuit 13 of the present embodiment shown in FIG. 3, the first capacitor 1, the voltage-current converter 2, and the second capacitor 3 are the same as the filter circuit of the first embodiment. However, one end of the resistor 4 is biased not by the ground but by the bias means, and the bias means 5 outputs the same amount of current as the current flowing through the resistor 4.

  7 and 8 are circuit diagrams showing examples of the configuration of the bias means 5 in the filter circuit 13 of the present embodiment.

  In the example of the bias means 5 shown in FIG. 7, the input stage is virtually grounded to a predetermined voltage using an operational amplifier, the input current is injected, and the output current is taken out by a current mirror. It is also possible to create the bias means 5 using a current mirror as shown in FIG.

  As described above, the filter circuit 13 of the present embodiment can also serve as the voltage-current converter 10 that is necessary in the first embodiment or the conventional filter circuit by outputting the output current Iout. As a result, the voltage-current converter 10 can be omitted.

  Further, the transfer function of the filter circuit of the present embodiment is expressed as follows by the ratio of the input current Iin (s) and the output current Iout (s).

Iout (s) / Iin (s) = {1 + s × (C ₁ / m) × R ₄ ) / {(s × C ₁ × R ₄ ) × (1 + s × C ₃ × R ₄ )} (5) )

The formula (5) is the same type as the formula (2) except that the dimensions are different. That is, from this equation, in the filter circuit of the present embodiment, as in the first filter circuit, the apparent capacitance can be expressed as C ₁₂₁ by reducing m without increasing the capacitance of the first capacitor 1. It can be seen that the size can be equivalent.

  As described above, the filter circuit of the present embodiment has not only a smaller area than the conventional filter circuit, but also eliminates the need for the voltage-current converter 10 when used in a PLL. Can do. Therefore, if the filter circuit of this embodiment is used, a PLL having a smaller circuit area than that of the conventional or first embodiment can be realized.

  The filter circuit of the present invention is useful as a PLL loop filter used in various devices.

1 is a block circuit diagram showing a configuration of a filter circuit according to a first embodiment of the present invention. It is a block circuit diagram which shows the structure of PLL which concerns on 1st Embodiment using the filter circuit shown in FIG. It is a block circuit diagram which shows the structure of the filter circuit which concerns on 2nd Embodiment. It is a block circuit diagram which shows the structure of PLL which concerns on 2nd Embodiment using the filter circuit shown in FIG. It is a circuit diagram which shows the structural example of a voltage-current converter among the filter circuits of 1st Embodiment. It is a circuit diagram which shows the structural example of a voltage-current converter among the filter circuits of 1st Embodiment. It is a circuit diagram which shows the structural example of the bias means in the filter circuit of 2nd Embodiment. It is a circuit diagram which shows the structural example of the bias means in the filter circuit of 2nd Embodiment. It is a circuit diagram which shows the conventional filter circuit. FIG. 10 is a block circuit diagram showing a conventional PLL including the conventional filter circuit shown in FIG. 9. It is a figure which shows the frequency characteristic of the conventional filter circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st capacitor 2 Voltage-current converter 3 2nd capacitor 4, 41, 51 Resistance 5 Biasing means 6 Phase comparator 7 1st charge pump 8 2nd charge pump 9, 13 Filter circuit 10 Voltage-current Converter 11 Current control oscillator 12 Frequency divider 30 First input unit 31 Second input unit 32 Output unit 40 Operational amplifier 42 First p-channel MOSFET
43 Second p-channel MOSFET
Iin1 first input current Iin2 second input current

Claims (6)

A first input for inputting a first input current;
A second input section for inputting a second input current;
A first capacitor connected to the first input;
A voltage-current converter having a first resistor, and outputting a voltage generated by the first capacitor by converting an AC component into a current inversely proportional to a resistance value of the first resistor;
A second capacitor and a second resistor, both of which are connected to the second input unit and to which a part of the added current of the second input current and the output current of the voltage-current converter is respectively shunted. Filter circuit provided. The second capacitor and the second resistor are both grounded,
The filter circuit according to claim 1, wherein a voltage generated by the second capacitor and the second resistor is output.   Bias means connected to the voltage-current converter, the second input section, and the second capacitor through the second resistor, respectively, and outputting a current equivalent to the current flowing through the second resistor. The filter circuit according to claim 1, further comprising: A phase comparator to which an input signal is input;
A first charge pump that receives an output from the phase comparator and outputs a current pulse;
A second charge pump disposed in parallel with the first charge pump as seen from the phase comparator, and receiving an output from the phase comparator and outputting a current pulse equal to the first charge pump;
A first capacitor that receives a current pulse from the first charge pump; and a first resistor, wherein the voltage generated by the first capacitor is inversely proportional to the resistance value of the first resistor. A first voltage-current converter that converts the current into a current to be output and at least one of a current obtained by adding an output current from the first voltage-current converter and a current pulse from the second charge pump. A filter circuit having a second capacitor and a second resistor each of which is shunted,
A current-controlled oscillator that outputs a signal having a frequency according to a current amount of a current from the filter circuit or a current converted from an output from the filter circuit;
A frequency divider that receives an output from the current controlled oscillator and outputs a signal to the phase comparator. The second capacitor and the second resistor are both grounded,
The filter circuit outputs a voltage generated by the second capacitor and the second resistor;
The PLL according to claim 4, further comprising a second voltage-current converter that converts an output voltage of the filter circuit into a current and outputs the current to the current controlled oscillator. The filter circuit is connected to the voltage-current converter, the second input unit, and the second capacitor through the second resistor, and has a current equivalent to the current flowing through the second resistor. Is further provided with bias means for outputting
The PLL according to claim 4, wherein the current control oscillator receives an output current of the filter circuit.

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