CN110311650B - Low-pass filter circuit, low-pass filter and CMOS chip - Google Patents
Low-pass filter circuit, low-pass filter and CMOS chip Download PDFInfo
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- CN110311650B CN110311650B CN201910560423.1A CN201910560423A CN110311650B CN 110311650 B CN110311650 B CN 110311650B CN 201910560423 A CN201910560423 A CN 201910560423A CN 110311650 B CN110311650 B CN 110311650B
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H11/00—Networks using active elements
- H03H11/02—Multiple-port networks
- H03H11/04—Frequency selective two-port networks
- H03H11/12—Frequency selective two-port networks using amplifiers with feedback
- H03H11/1213—Frequency selective two-port networks using amplifiers with feedback using transistor amplifiers
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Abstract
The application relates to a low-pass filter circuit, a low-pass filter and a CMOS chip. The low-pass filter circuit comprises a first resistor, a first switching tube, a second switching tube, a node voltage regulating module A, a third switching tube and an equivalent capacitance module B, wherein bias current is generated through the first resistor and the first switching tube, a current mirror structure is formed through the first switching tube and the second switching tube, so that current flowing through the second switching tube is equal to the bias current, and then the node voltage between the input end of the second switching tube and the control end of the third switching tube is regulated through the node voltage regulating module A, so that the on voltage of the third switching tube is increased, a large resistor is formed, and the-3 dB frequency of the low-pass filter circuit can be effectively increased. The devices in the low-pass filter circuit can be directly manufactured on an integrated circuit, and low-cost mass production is facilitated.
Description
Technical Field
The present application relates to the field of analog circuits, and in particular to low pass filter circuits, low pass filters, and CMOS (Complementary Metal Oxide Semiconductor ) chips.
Background
In the field of analog circuits, it is generally necessary to perform a low-pass filtering process on an input signal, i.e., a process of filtering out high-frequency components in the signal.
At present, a common low-pass filter is a first-order passive low-pass filter formed by a resistor and a capacitor, wherein the first-order passive low-pass filter is formed by a resistor R and a capacitor C, an input end and an output end of the low-pass filter are connected through the resistor, the capacitor is connected with one end of the resistor, and the other end of the capacitor is grounded. An important performance index describing a filter is the-3 dB frequency, i.e. the frequency (denoted fc) at which the power of the output signal of the filter drops to half the power of the dc signal, which is disadvantageous for implementation in an integrated circuit when a low pass filter needs to achieve a smaller fc, the values of the resistor R and the capacitor C are relatively large.
Disclosure of Invention
In view of this, it is necessary to provide a low-pass filter circuit, a low-pass filter, and a CMOS chip that can be implemented in an integrated circuit, in order to solve the above-described problem that the low-pass filter is not advantageous to be implemented in the integrated circuit.
A low pass filter circuit, said circuit comprising: the device comprises a first resistor, a first switching tube, a second switching tube, a node voltage regulating module, a third switching tube and an equivalent capacitance module;
the first resistor is connected with the first switching tube to form a bias current generating unit, one end of the bias current generating unit is connected with a power supply, the other end of the bias current generating unit is connected with the control end of the second switching tube, the input end of the second switching tube is connected with one end of the node voltage regulating module and the control end of the third switching tube, the other end of the node voltage regulating module is connected with the power supply, the input end of the third switching tube receives signal input, the output end of the third switching tube is connected with the equivalent capacitor module, the output end of the third switching tube outputs signals, and the first switching tube and the second switching tube are the same type switching tubes.
In one embodiment, the equivalent capacitance module includes a capacitor, a fourth switching tube and a fifth switching tube, one end of the capacitor is connected with the output end of the third switching tube, the other end of the capacitor is connected with the input end of the fourth switching tube and the output end of the fifth switching tube, the control end of the fourth switching tube is connected with the control end of the second switching tube, the control end of the fifth switching tube is connected with one end of the capacitor, the input end of the fifth switching tube is connected with a power supply, and the fourth switching tube and the second switching tube are the same type switching tubes.
In one embodiment, the circuit further comprises a plurality of equivalent capacitance modules, wherein a first end of each equivalent capacitance module is connected with the control end of the fifth switching tube, a second end of each equivalent capacitance module is connected with the output end of the fifth switching tube, and a third end of each equivalent capacitance module is connected with the input end of the fifth switching tube.
In one embodiment, the equivalent capacitance module includes a current amplifying switch tube, a control end of the current amplifying switch tube is connected with a control end of the fifth switch tube, an output end of the current amplifying switch tube is connected with an output end of the fifth switch tube, and an input end of the current amplifying switch tube is connected with an input end of the fifth switch tube.
In one embodiment, the equivalent capacitance module further includes first control switches equal to the current amplifying switch tubes in number, and an output end of each current amplifying switch tube is connected with an input end of the fourth switch tube through each first control switch.
In one embodiment, one end of the first resistor is connected with a power supply, and the other end of the first resistor is connected with the input end of the first switching tube, the control end of the first switching tube and the control end of the second switching tube.
In one embodiment, the node voltage adjustment module includes a second resistor.
In one embodiment, the node voltage adjusting module further includes a plurality of secondary variable resistors and second control switches equal to the secondary variable resistors in number, each of the secondary variable resistors is connected in parallel with the node voltage adjusting resistor, and a single secondary variable resistor is connected with the input end of the second switching tube through a single second control switch.
A low pass filter comprising a low pass filter circuit as described in any one of the preceding claims and a voltage source connected to one end of an equivalent resistance module in the low pass filter circuit.
A CMOS chip comprises the low-pass filter and a chip body, wherein the low-pass filter is connected with the chip body.
According to the low-pass filter circuit, the low-pass filter and the CMOS chip, the bias current is generated through the first resistor and the first switching tube, the current mirror structure is formed through the first switching tube and the second switching tube, so that the current flowing through the second switching tube is equal to the bias current, and then the node voltage between the input end of the second switching tube and the control end of the third switching tube is regulated through the node voltage regulating module, so that the conduction voltage of the third switching tube is increased, a large resistor is formed, and the-3 dB frequency of the low-pass filter circuit can be effectively increased. Meanwhile, all devices in the low-pass filter circuit can be directly integrated on the integrated circuit, and large-scale production is facilitated.
Drawings
FIG. 1 is a block diagram of a low pass filter circuit in one embodiment;
FIG. 2 is a block diagram of a low pass filter circuit in another embodiment;
fig. 3 is a block diagram of a low-pass filter circuit in yet another embodiment.
Detailed Description
In order to facilitate an understanding of the present application, a more complete description of the present application will now be provided with reference to the relevant figures. Preferred embodiments of the present application are shown in the drawings. This application may, however, be embodied in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.
As shown in fig. 1, the present application provides a low-pass filter circuit, which includes a first resistor R1, a first switching tube M1, a second switching tube M2, a node voltage adjusting module a, a third switching tube M3, and an equivalent capacitor module B;
the first resistor R1 and the first switching tube M1 form a bias current generating unit, one end of the bias current generating unit is connected with a power supply, the other end of the bias current generating unit is connected with a control end of the second switching tube M2, as shown in the figure, in one embodiment, one end of the first resistor R1 in the bias current generating unit is connected with the power supply VDD, the other end of the first resistor R1 is connected with an input end of the first switching tube M1, a control end of the first switching tube M1 and a control end of the second switching tube M2, an input end of the second switching tube M2 is connected with one end of the node voltage regulating module a and a control end of the third switching tube M3, the other end of the node voltage regulating module a is connected with the power supply VDD, an input end of the third switching tube M3 receives a signal input, an output end of the third switching tube M3 is connected with the equivalent capacitor module B, and the output end of the third switching tube M3 outputs a signal to the first switching tube M1 and the second switching tube M2 are of the same type. The output ends of the first switching tube M1 and the second switching tube M2 are grounded.
In order to describe the technical principle and effects of the low-pass filter circuit in detail, the first switching tube M1 and the second switching tube M2 are NMOS (Negative channel Metal Oxide Semiconductor, N-type metal oxide semiconductor) tubes, the third switching tube M3 is a PMOS (positive channel Metal Oxide Semiconductor, P-type metal-oxide semiconductor) tube, and in the NMOS tube, the input end is a drain electrode, the control end is a gate electrode, and the output end is a source electrode. In the PMOS transistor, the input terminal is the source, the control terminal is the gate, and the output terminal is the drain. The switching tube works in a cut-off area and a saturation area, which are equivalent to cut-off and turn-on of a circuit respectively. It has the function of completing the switching-off and switching-on. The first resistor R1 and the first switching tube M1 form a bias current generating unit, configured to generate a bias current, and set a current flowing through the first resistor R1 as I1, where the size of I1 satisfies:
in the above formula, vgs1 represents the gate drain voltage of the first switching transistor M1, and VDD is the output voltage of the power supply. The first switching tube M1 and the second switching tube M2 are the same in model and size, a current mirror structure is formed, and the current flowing through the input end of the second switching tube M2 is I1. The node voltage Vx of the input node of the second switching tube M2 is connected to the node voltage regulating module a. The node voltage adjustment module a is configured to adjust a magnitude of the node voltage Vx, in one embodiment, the node voltage adjustment module a includes a node voltage adjustment resistor, where a resistance R2 of the resistor is the magnitude of the node voltage Vx:
Vx＝VDD-I1*R2
at this time, the on-resistance R3 of the third switching transistor M3 satisfies:
in the above formula, β is a constant determined by the process and the size of the third switching tube M3, vth3 is the threshold voltage of the third switching tube M3, and it can be seen from the above formula that the size of R3 is determined by R2, and when R2 is smaller, R3 is larger, but R2 is less than or equal to Vth3/I1, R3 is theoretically even approaching infinity. While the third switching tube M3 is equivalent to the resistance in a usual low-pass filter, an equivalent large resistance can be achieved by providing a small resistance R2.
As shown in fig. 2, in one embodiment, the equivalent capacitor module B includes a capacitor C1, a fourth switching tube M4 and a fifth switching tube M5, one end of the capacitor C1 is connected to the output end of the third switching tube M3, the other end of the capacitor C1 is connected to the input end of the fourth switching tube M4 and the input end of the fifth switching tube M5, the control end of the fourth switching tube M4 is connected to the control end of the second switching tube M2, the control end of the fifth switching tube M5 is connected to one end of the capacitor C1, the input end of the fifth switching tube M5 is connected to the power supply VDD, the fourth switching tube M4 and the second switching tube M2 are the same type switching tubes, and the output end of the fourth switching tube M4 is grounded.
In order to explain the technical principle and the effect of the low-pass filter circuit in detail, a fifth switch tube M5 is taken as a PMOS tube, the model of a fourth switch tube M4 is the same as that of the first switch tube M1 and the second switch tube M2, the sizes of the fourth switch tube M4 and the second switch tube M2 are consistent, and NMOS tubes are taken as examples for illustration. Since the model of the fourth switching tube M4 is identical to the model of the first switching tube M1 and the second switching tube M2, the fourth switching tube M4 can be also regarded as a direct current source with the size I1. Let the node of signal output, i.e. the output node of the third switching tube M3 has a varying voltage Vm, since Vm acts on the control end of the fifth switching tube M5, according to the characteristic that the fifth switching tube M5 is a PMOS tube, the fifth switching tube M5 generates varying output current Im, im satisfying:
Im＝-Vm*Gm5
in the above formula, gm5 is the transconductance of the fifth switching tube M5, and its size is determined by the size and bias state of the fifth switching tube M5, and the negative sign indicates that the current direction is the direction of the input end flowing into the fourth switching tube. The voltage of the node V4 at the other end of the capacitor C1 is satisfied
V4＝Im*R4
Wherein R4 is an equivalent resistance at the node V4, and the size of the equivalent resistance is determined by the sizes of the fourth switching transistor M4 and the fifth switching transistor M5. The magnitude Ic1 of the alternating current flowing through the capacitor C1 satisfies:
Ic1＝(Vm-V4)*C1*s
where s is a laplace variation operator, and C1 is the size of the capacitor C1, so looking into from the Vm node, the equivalent capacitor C1e of the Vm node to ground satisfies:
as can be seen from the above formula, C1e corresponds to a C1 increase by Gm5 x R4.
When the input signal Vin enters the input terminal of the third switching tube M3, the input signal Vin passes through a resistor equivalent to R3 and outputs the signal Vout on the node of the capacitor C1 equivalent to Cle.
Let the frequency of the input signal be f, the amplitude of Vout satisfies:
its corresponding-3 dB frequency fc satisfies:
for the above equation, even if C1 is small, R2 need only be small enough, or Gm5 is large enough to achieve smaller fc, even fc approaching 0. While obtaining R2 small enough and Gm5 large enough does not require the use of large devices, in one embodiment, fc as low as 10KHz or less can be achieved when the resistance of the first resistor R1 and the resistance of the resistor R2 in the node voltage adjustment module a are on the order of hundred ohms and the capacitance of the capacitor C1 is on the order of hundred femtofarads. The-3 dB frequency of the low pass filter circuit can be effectively increased.
In one embodiment, the circuit further comprises a plurality of equivalent capacitance modules, wherein a first end of each equivalent capacitance module is connected with the control end of the fifth switching tube M5, a second end of each equivalent capacitance module is connected with the output end of the fifth switching tube M5, and a third end of each equivalent capacitance module is connected with the input end of the fifth switching tube M5. The equivalent capacitance module is used for amplifying the output end current of the fifth switching tube. In one embodiment, the equivalent capacitance module includes a current amplifying switch tube, the current amplifying switch tube is consistent with the fifth switch tube M5 in type, when the fifth switch tube M5 is a PMOS tube, each current amplifying switch tube is also a PMOS tube, a control end of the current amplifying switch tube is connected with a control end of the fifth switch tube M5, an output end of the current amplifying switch tube is connected with an output end of the fifth switch tube M5, and an input end of the current amplifying switch tube is connected with an input end of the fifth switch tube M5. Referring to fig. 3 (fig. 3 only represents an embodiment of the present application), in fig. 3, M5a and M5b are current amplifying switching tubes, which are used to increase the transconductance of the fifth switching tube M5, so as to increase the output current of the fifth switching tube M5, and form a large capacitance.
In one embodiment, the equivalent capacitance module further includes first control switches equal in number to the current amplifying switch tubes, and an output end of the single current amplifying switch tube is connected with an input end of the fourth switch tube through the single first control switch. The first control switch may refer to SM5a and SM5b in fig. 3 in particular. The first control switch is used for controlling whether each current amplifying switch tube is connected with low-pass filtering or not so as to control the output end current of the fifth switch tube M5. The effect of each current amplifying switch tube is to increase output end current Im, and when the switch is closed, the magnitude of output end current Im becomes:
Im＝-Vm*Gme
in the above formula, gme is an equivalent transconductance, and the size of the equivalent transconductance is the sum of the transconductance of the fifth switching tube M5 and the transconductance of the PMOS tube of the switching tube connected to the node Vm after all the switches are closed. The transconductance can be effectively increased by connecting more current amplifying switching tubes in parallel, large-size devices can not be introduced, and meanwhile, the filtering effect of the low-pass filter can be effectively improved.
In one embodiment, the node voltage adjustment module a further includes a plurality of secondary variable resistors, each of which is connected in parallel with the node voltage adjustment resistor. The secondary variable resistor is used for reducing the resistance value of the whole node voltage regulating module A by being connected in parallel with the node voltage regulating resistor. Referring specifically to fig. 3 (fig. 3 represents only one embodiment of the present application), R2a and R2b in fig. 3 are secondary varistors. In one embodiment, the node voltage adjusting module a further includes second control switches equal to the number of the secondary variable resistors, and the single secondary variable resistor is connected to the input terminal of the second switching tube M2 through the single second control switch. The second control switch may specifically refer to SR2a and SR2b in fig. 3. The switching-in of the secondary variable resistor can be controlled by a second control switch. After each second control switch is sequentially closed, due to the parallel effect of the resistors, each secondary node adjusting resistor is sequentially connected into the circuit, so that the equivalent resistance from the node voltage Vx to the VDD is lower and lower, and if the equivalent resistance is Re, the following steps are adopted:
Vx＝VDD-I1*Re
by accessing the secondary variable resistor, the equivalent resistance R3 in the low-pass filter can be effectively reduced, and devices in the low-pass filter circuit can be directly manufactured on an integrated circuit, so that low-cost mass production is facilitated.
As shown in fig. 3, in one embodiment, the low-pass filter circuit of the present application includes a first resistor R1, a first switching tube M1, a second switching tube M2, a node voltage adjusting module a, a third switching tube M3, and an equivalent capacitance module B, the equivalent capacitance module B includes a capacitor C1, a fourth switching tube M4, a fifth switching tube M5, and current amplifying switching tubes M5a and M5B and first control switches SM5a and SM5B, the node voltage adjusting module a includes a node voltage adjusting resistor R2, secondary variable resistors R2a and R2B, and second control switches SR2a and SR2B, the node voltage adjusting resistor R2 is connected in parallel with the secondary variable resistors R2a and R2B, and a single secondary variable resistor is connected with an input end of the second switching tube M2 through a single second control switch. The control ends of the current amplifying switch tubes M5a and M5b are connected with the control end of the fifth switch tube M5, the output ends of the current amplifying switch tubes M5a and M5b are connected with the output end of the fifth switch tube M5, and the input ends of the current amplifying switch tubes M5a and M5b are connected with the input end of the fifth switch tube M5. The output end of the single current amplifying switch tube is connected with one end of a first resistor R1 through a single first control switch and one end of a fourth switch tube M4, the other end of the first resistor R1 is connected with the input end of the first switch tube M1, the control end of the first switch tube M1 and the control end of a second switch tube M2, the input end of the second switch tube M2 is connected with one end of a node voltage regulating resistor R2 and the control end of a third switch tube M3, the other end of the node voltage regulating resistor R2 is connected with a voltage source power supply VDD, the input end of the third switch tube M3 receives signal input, the output end of the third switch tube M3 is connected with an equivalent capacitor module B, and the output end of the third switch tube M3 outputs signals to the same type of switch tube M1 and the second switch tube M2. One end of a capacitor C1 is connected with the output end of a third switching tube M3, the other end of the capacitor C1 is connected with the input end of a fourth switching tube M4 and the output end of a fifth switching tube M5, the control end of the fourth switching tube M4 is connected with the control end of a second switching tube M2, the control end of the fifth switching tube M5 is connected with one end of the capacitor C1, the input end of the fifth switching tube M5 is connected with a power supply VDD, and the fourth switching tube M4 and the second switching tube M2 are the same type switching tubes. The first switching tube M1, the second switching tube M2, and the fourth switching tube M4 are NMOS tubes, and the third switching tube M3, the fifth switching tube M5, and the current amplifying switching tubes M5a and M5b are PMOS tubes.
In one embodiment, a low-pass filter is further provided, including the low-pass filter circuit in any one of the above embodiments, and a voltage source connected to one end of the equivalent resistor module in the low-pass filter circuit.
A low pass filter is an electronic filtering device that allows signals below the cut-off frequency to pass, but signals above the cut-off frequency cannot. I.e. means for low-pass filtering the input signal. The low-pass filter of the application comprises the low-pass filter circuit in any one of the embodiments, the input signal can be filtered through the low-pass filter circuit, and the voltage source in the low-pass filter can be connected with the first resistor and the equivalent capacitor in the circuit to provide voltage.
According to the low-pass filter, the bias current is generated through the first resistor R1 and the first switching tube M1 in the low-pass filter circuit, the current mirror structure is formed through the first switching tube M1 and the second switching tube M2, so that the current flowing through the second switching tube M2 is equal to the bias current I1, and then the node voltage between the input end of the second switching tube M2 and the control end of the third switching tube M3 is regulated through the node voltage regulating module A, so that the conduction voltage of the third switching tube M3 is increased, a large resistor is formed, and the-3 dB frequency of the low-pass filter circuit can be effectively increased. Meanwhile, all devices in the low-pass filter circuit of the low-pass filter can be directly integrated on the integrated circuit, and large-scale production is facilitated.
In one embodiment, a CMOS chip is provided, including the low-pass filter and a chip body, where the low-pass filter is connected to the chip body.
CMOS is an important chip in computer systems, which stores the most basic data for system booting. CMOS is mainly made of two elements, silicon and germanium, so that N-level and P-level semiconductors coexist in CMOS, and the currents generated by the complementary effects can be recorded and interpreted as images by a processing chip. It was later found that CMOS can be processed as an image sensor in digital photography, and CMOS sensors can be subdivided into passive pixel sensors (Passive Pixel Sensor CMOS) and active pixel sensors (Active Pixel Sensor CMOS). When the low-pass filter needs to be integrated in the CMOS chip, the low-pass filter can be implemented, and the input signal of the chip can be subjected to low-pass filtering by the low-pass filter circuit in the low-pass filter. The output end of the third switching tube M3 in the low-pass filter circuit of the low-pass filter is connected with the signal receiving end of the CMOS chip.
According to the CMOS chip, the bias current is generated through the first resistor R1 and the first switching tube M1 in the low-pass filter circuit, the current mirror structure is formed through the first switching tube M1 and the second switching tube M2, so that the current flowing through the second switching tube M2 is equal to the bias current I1, and then the node voltage between the input end of the second switching tube M2 and the control end of the third switching tube M3 is regulated through the node voltage regulating module A, so that the on voltage of the third switching tube M3 is increased, a large resistor is formed, and devices in the low-pass filter circuit in the CMOS chip can be directly manufactured on an integrated circuit, and low-cost mass production is facilitated.
The technical features of the above embodiments may be arbitrarily combined, and all possible combinations of the technical features in the above embodiments are not described for brevity of description, however, as long as there is no contradiction between the combinations of the technical features, they should be considered as the scope of the description.
The foregoing examples represent only a few embodiments of the present application, which are described in more detail and are not thereby to be construed as limiting the scope of the claims. It should be noted that numerous variations and modifications could be made to those skilled in the art without departing from the spirit of the present application, which would fall within the scope of the present application. Accordingly, the scope of protection of the present application is to be determined by the claims appended hereto.
Claims (9)
1. A low pass filter circuit, said circuit comprising: the device comprises a first resistor, a first switching tube, a second switching tube, a node voltage regulating module, a third switching tube and an equivalent capacitance module;
the first resistor is connected with the first switching tube to form a bias current generating unit, one end of the bias current generating unit is connected with a power supply, the other end of the bias current generating unit is connected with the control end of the second switching tube, the input end of the second switching tube is connected with one end of the node voltage regulating module and the control end of the third switching tube, the other end of the node voltage regulating module is connected with the power supply, the input end of the third switching tube receives signal input, the output end of the third switching tube is connected with the equivalent capacitor module, the output end of the third switching tube outputs signals, and the first switching tube and the second switching tube are switching tubes with the same model;
the equivalent capacitance module comprises a capacitor, a fourth switching tube and a fifth switching tube, one end of the capacitor is connected with the output end of the third switching tube, the other end of the capacitor is connected with the input end of the fourth switching tube and the output end of the fifth switching tube, the control end of the fourth switching tube is connected with the control end of the second switching tube, the control end of the fifth switching tube is connected with one end of the capacitor, the input end of the fifth switching tube is connected with a power supply, and the fourth switching tube and the second switching tube are the same type switching tubes.
2. The low-pass filter circuit of claim 1, further comprising a plurality of equivalent capacitance modules, wherein a first end of the equivalent capacitance modules is connected to the control end of the fifth switching tube, a second end of the equivalent capacitance modules is connected to the output end of the fifth switching tube, and a third end of the equivalent capacitance modules is connected to the input end of the fifth switching tube.
3. The low-pass filter circuit according to claim 2, wherein the equivalent capacitance module comprises a current amplifying switch tube, a control end of the current amplifying switch tube is connected with a control end of the fifth switch tube, an output end of the current amplifying switch tube is connected with an output end of the fifth switch tube, and an input end of the current amplifying switch tube is connected with an input end of the fifth switch tube.
4. A low-pass filter circuit according to claim 3, wherein said equivalent capacitance module further comprises first control switches equal in number to said current amplifying switch tubes, an output terminal of a single one of said current amplifying switch tubes being connected to an input terminal of said fourth switch tube through a single one of said first control switches.
5. The low-pass filter circuit of claim 1, wherein one end of the first resistor is connected to a power supply, and the other end of the first resistor is connected to an input end of the first switching tube, a control end of the first switching tube, and a control end of the second switching tube.
6. The low pass filter circuit of claim 1, wherein the node voltage adjustment module comprises a second resistor.
7. The low pass filter circuit of claim 6, wherein said node voltage adjustment module further comprises a plurality of secondary variable resistors and a second control switch equal in number to said secondary variable resistors, each of said secondary variable resistors being connected in parallel with said node voltage adjustment resistor, a single of said secondary variable resistors being connected to said second switch tube input through a single of said second control switches.
8. A low pass filter comprising the low pass filter circuit of any one of claims 1-7 and a voltage source connected to one end of an equivalent resistor module in the low pass filter circuit.
9. A CMOS chip comprising the low pass filter of claim 8 and a chip body, the low pass filter being coupled to the chip body.
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