Reconfigurable filter based on active RC network

Abstract

The invention provides a reconfigurable filter based on an active RC network, and belongs to the field of filters. The reconfigurable filter comprises a first active RC network structure, a second active RC network structure, a first capacitor C1, a second capacitor C2, a first inductor L1, a second inductor L2, a third inductor L3, a fourth inductor L4 and a fifth inductor L5. In the active RC network structure, a transistor loaded through an inductor is biased to a proper working point position through resistor voltage division, the active RC network structure can be equivalent to a capacitor and a negative resistor which are connected in series from a grid electrode of the transistor, and the circuit structure can be equivalent to a five-order elliptical filter. The reconfigurable filter is realized, and different working frequency bands can be switched to meet different working requirements. According to the invention, an active RC network structure is introduced, the design of the reconfigurable filter is simplified, and by introducing negative resistance, the sideband selectivity of the filter is improved, and the insertion loss in a filter passband is improved.

Description

Technical Field

[0001] The invention belongs to the field of filters, and in particular relates to a reconfigurable filter based on an active RC network. Background Art

[0002] Filters are an important part of RF circuit design. Taking wireless communication receivers as an example, filters are mainly used to select or suppress signals of specific frequencies. They can allow signals of certain frequencies to pass through as needed, while blocking signals of other frequencies, effectively suppressing out-of-band interference and noise, and ensuring impedance matching to reduce signal reflection and energy loss, thereby improving signal quality and receiver performance. There are various types of filters, which can be divided into low-pass, high-pass, band-pass and band-stop filters according to frequency range; and can be divided into pre-filters, intermediate frequency filters and post-filters according to position.

[0003] Traditional filters are passive devices. Due to the influence of parasitic resistance in the circuit, the quality factor of the filter circuit will be reduced, the sideband selectivity of the filter will be reduced, and the passband insertion loss will be deteriorated, which will cause the filter to suppress adjacent channels. The traditional filter can only work in a single frequency band. When the receiver selects signals in different frequency bands, it is necessary to use multiple traditional filter groups to implement it. The inability to tune between the various filter groups makes the system architecture complicated. In order to adapt to multiple wireless communication standards in civil, military and space communications, and meet the requirements of high data rates, software radio, etc., the entire RF communication system is developing in the direction of ultra-wideband and reconfigurable. Traditional filters have many restrictions when used, which is not conducive to the miniaturization and high integration of modern RF communication systems. Summary of the invention

[0004] The present invention aims to provide a reconfigurable filter based on an active RC network, and realizes a frequency band reconfigurable filter with low insertion loss and strong sideband selectivity, so as to solve the technical problems of limited use of traditional filters and deterioration of passband insertion loss in the prior art.

[0005] In order to solve the above technical problems, the specific technical solutions of the present invention are as follows:

[0006] A reconfigurable filter based on an active RC network, the reconfigurable filter comprises a first active RC network structure, a second active RC network structure, a first capacitor C1, a second capacitor C2, a first inductor L1, a second inductor L2, a third inductor L3, a fourth inductor L4 and a fifth inductor L5. A signal enters from a first port, one end of the first capacitor C1 is connected to the first port, the other end of the first capacitor C1 is connected to one end of the first inductor L1, the other end of the first inductor L1 is connected to one end of the second inductor L2 and one end of the fourth inductor L4, the other end of the second inductor L2 is connected to one end of the third inductor L3 and one end of the fifth inductor L5, and the other end of the fourth inductor L4 is connected to one end of the first active RC network. The other end of the first active RC network is grounded, and the other end of the fifth inductor L5 is connected to one end of the second active RC network. The other end of the second active RC network is grounded, and the other end of the third inductor L3 is connected to the second port through the second capacitor C2, and the second port is an output port.

[0007] Further, the first active RC network structure includes a first DC voltage source V1, a first resistor R1, a sixth inductor L6, a first transistor Q1, a first switched capacitor and a second switched capacitor. In the first active RC network structure, the first DC voltage source V1 is connected to one end of the sixth inductor L6 and one end of the first resistor R1, the other end of the sixth inductor L6 is connected to the drain of the first transistor Q1, the gate of the first transistor Q1 is connected to the other end of the first resistor R1, one end of the first switched capacitor, one end of the second switched capacitor and the other end of the fourth inductor L4, and the source of the first transistor Q1 and the other end of the first switched capacitor and the other end of the second switched capacitor are grounded.

[0008] Further, the second active RC network structure includes a second DC voltage source V2, a second resistor R2, a seventh inductor L7, a second transistor Q2, a third switch capacitor, and a fourth switch capacitor; in the second active RC network structure, the second DC voltage source V2 is connected to one end of the seventh inductor L7 and one end of the second resistor R2, the other end of the seventh inductor L7 is connected to the drain of the second transistor Q2, the gate of the second transistor Q2 is connected to the other end of the second resistor R2, one end of the third switch capacitor, one end of the fourth switch capacitor, and the other end of the fifth inductor L4, and the source of the second transistor Q1 and the other end of the third switch capacitor and the other end of the fourth switch capacitor are grounded.

[0009] Furthermore, the first switching capacitor includes a third capacitor C3, a third transistor Q3, a third resistor R3, and a third DC voltage source V3. One end of the third capacitor C3 in the first switching capacitor is connected to the gate of the first transistor Q1, and the other end of the third capacitor C3 is connected to the drain of the third transistor Q3. The gate of the third transistor Q3 is connected to the third DC voltage source V3 through the third resistor R3, and the source of the third transistor Q3 is grounded.

[0010] Furthermore, the second switching capacitor includes a fourth capacitor C4, a fourth transistor Q4, a fourth resistor R4, and a fourth DC voltage source V4; one end of the fourth capacitor C4 in the second switching capacitor is connected to the gate of the first transistor Q1, the other end of the fourth capacitor C4 is connected to the drain of the fourth transistor Q4, the gate of the fourth transistor Q4 is connected to the fourth DC voltage source V4 through the fourth resistor R4, and the source of the fourth transistor Q4 is grounded.

[0011] Furthermore, the third switching capacitor includes a fifth capacitor C5, a fifth transistor Q5, a fifth resistor R5, and a third DC voltage source V3. One end of the fifth capacitor C5 in the third switching capacitor is connected to the gate of the second transistor Q2, the other end of the fifth capacitor C5 is connected to the drain of the fifth transistor Q5, the gate of the fifth transistor Q5 is connected to the third DC voltage source V3 through the fifth resistor R5, and the source of the fifth transistor Q5 is grounded.

[0012] Furthermore, the fourth switching capacitor includes a sixth capacitor C6, a sixth transistor Q6, a sixth resistor R6, and a fourth DC voltage source V4; one end of the sixth capacitor C6 in the fourth switching capacitor is connected to the gate of the second transistor Q2, the other end of the sixth capacitor C6 is connected to the drain of the sixth transistor Q6, the gate of the sixth transistor Q6 is connected to the fourth DC voltage source V4 through the sixth resistor R6, and the source of the sixth transistor Q6 is grounded.

[0013] Compared with the prior art, the present invention has the following beneficial technical effects: the technical solution of the present invention realizes a reconfigurable filter, which can switch different working frequency bands to meet different working requirements. The technical solution of the present invention introduces an active RC network structure, simplifies the design of the reconfigurable filter, and improves the sideband selectivity of the filter by introducing negative resistance, thereby improving the insertion loss in the filter passband. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments of the present invention will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative labor.

[0015] Figure 1 A schematic diagram of the specific structure of a switched capacitor provided in an embodiment of the present invention.

[0016] Figure 2 A schematic diagram of an equivalent circuit structure of an active RC network provided in an embodiment of the present invention.

[0017] Figure 3A schematic diagram of the specific structure of a reconfigurable filter based on an active RC network provided in an embodiment of the present invention.

[0018] Figure 4 A schematic diagram of the equivalent circuit structure of a reconfigurable filter based on an active RC network provided in an embodiment of the present invention.

[0019] Figure 5 A graph showing the variation of insertion loss of a reconfigurable filter based on an active RC network with frequency provided in an embodiment of the present invention.

[0020] Figure 6 A graph showing the variation of insertion loss with frequency for a reconfigurable filter based on an active RC network and a traditional filter provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0021] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] Figure 1 The specific structural diagram of the switch capacitor provided by the embodiment of the present invention includes a third capacitor C3, a third transistor Q3, a third resistor R3 and a third DC voltage source V3, one end of the third capacitor C3 is connected to the third port, the other end of the third capacitor C3 is connected to the drain of the third transistor Q3, the gate of the third transistor Q3 is connected to the third DC voltage source V3 through the third resistor R3, and the source of the third transistor Q3 is grounded. The third capacitor C3, the third transistor Q3, the third resistor R3 and the third DC voltage source V3 form a first switch capacitor, and the remaining switch capacitor structure is similar to the first switch capacitor structure, and the DC voltage source is used to control the opening and closing of the transistor, thereby controlling whether the capacitor is connected to the circuit.

[0023] Figure 2 A schematic diagram of an equivalent circuit structure of an active RC network provided for an embodiment of the present invention includes a first DC voltage source V1, a first resistor R1, a sixth inductor L6, a first transistor Q1, a first switch capacitor and a second switch capacitor. In the first active RC network structure, the first DC voltage source V1 is connected to one end of the sixth inductor L6 and one end of the first resistor R1, the other end of the sixth inductor L6 is connected to the drain of the first transistor Q1, the gate of the first transistor Q1 is connected to the other end of the first resistor, one end of the first switch capacitor, one end of the second switch capacitor and the third port, and the source of the first transistor Q1 and the other end of the first switch capacitor and the other end of the second switch capacitor are grounded.

[0024] The first switch capacitor includes a third capacitor C3, a third transistor Q3, a third resistor R3, and a third DC voltage source V3, and the second switch capacitor includes a fourth capacitor C4, a fourth transistor Q4, a fourth resistor R4, and a fourth DC voltage source V4. One end of the third capacitor C3 in the first switch capacitor is connected to the gate of the first transistor Q1, the other end of the third capacitor C3 is connected to the drain of the third transistor Q3, the gate of the third transistor Q3 is connected to the third DC voltage source V3 through the third resistor R3, and the source of the third transistor Q3 is grounded. One end of the fourth capacitor C4 in the second switch capacitor is connected to the gate of the first transistor Q1, the other end of the fourth capacitor C4 is connected to the drain of the fourth transistor Q4, the gate of the fourth transistor Q4 is connected to the fourth DC voltage source V4 through the fourth resistor R4, and the source of the fourth transistor Q4 is grounded.

[0025] The active RC network can be equivalent to Figure 2 The simple series structure on the right side includes the thirteenth variable capacitor C13 and the eleventh negative resistor R11. By adjusting the switch capacitor to turn on and off, it is equivalent to adjusting the size of the thirteenth variable capacitor C13, thereby realizing the function of switching frequency bands. The size of the negative resistance is little affected by the switch capacitor, so it can be equivalent to a fixed value of the eleventh negative resistor R11, which can reduce the circuit parasitic resistance and improve the circuit quality factor.

[0026] Figure 3 The specific structural diagram of the reconfigurable filter based on the active RC network provided by the embodiment of the present invention includes a first active RC network structure, a second active RC network structure, a first capacitor C1, a second capacitor C2, a first inductor L1, a second inductor L2, a third inductor L3, a fourth inductor L4 and a fifth inductor L5. The signal enters from the first port, one end of the first capacitor C1 is connected to the first port, the other end of the first capacitor C1 is connected to one end of the first inductor L1, the other end of the first inductor L1 is connected to one end of the second inductor L2 and one end of the fourth inductor L4, the other end of the second inductor L2 is connected to one end of the third inductor L3 and one end of the fifth inductor L5, and the other end of the fourth inductor L4 is connected to one end of the first active RC network. The other end of the first active RC network is grounded, and the other end of the fifth inductor L5 is connected to one end of the second active RC network. The other end of the second active RC network is grounded, and the other end of the third inductor L3 is connected to the second port through the second capacitor C2, that is, the other end of the third inductor L3 is connected to one end of the second capacitor C2, and the other end of the second capacitor C2 is connected to the second port, and the second port is an output port.

[0027] Furthermore, the first active RC network structure includes a first DC voltage source V1, a first resistor R1, a sixth inductor L6, a first transistor Q1, a first switch capacitor and a second switch capacitor, and the second active RC network structure includes a second DC voltage source V2, a second resistor R2, a seventh inductor L7, a second transistor Q2, a third switch capacitor and a fourth switch capacitor. In the first active RC network structure, the first DC voltage source V1 is connected to one end of the sixth inductor L6 and one end of the first resistor R1, the other end of the sixth inductor L6 is connected to the drain of the first transistor Q1, the gate of the first transistor Q1 is connected to the other end of the first resistor R1, one end of the first switch capacitor, one end of the second switch capacitor and the other end of the fourth inductor L4, and the source of the first transistor Q1 and the other end of the first switch capacitor and the other end of the second switch capacitor are grounded. In the second active RC network structure, the second DC voltage source V2 is connected to one end of the seventh inductor L7 and one end of the second resistor R2, the other end of the seventh inductor L7 is connected to the drain of the second transistor Q2, the gate of the second transistor Q2 is connected to the other end of the second resistor R2, one end of the third switch capacitor, one end of the fourth switch capacitor and the other end of the fifth inductor L4, and the source of the second transistor Q1 and the other end of the third switch capacitor and the other end of the fourth switch capacitor are grounded.

[0028] Furthermore, the first switch capacitor includes a third capacitor C3, a third transistor Q3, a third resistor R3, and a third DC voltage source V3, the second switch capacitor includes a fourth capacitor C4, a fourth transistor Q4, a fourth resistor R4, and a fourth DC voltage source V4, the third switch capacitor includes a fifth capacitor C5, a fifth transistor Q5, a fifth resistor R5, and a third DC voltage source V3, and the fourth switch capacitor includes a sixth capacitor C6, a sixth transistor Q6, a sixth resistor R6, and a fourth DC voltage source V4. One end of the third capacitor C3 in the first switch capacitor is connected to the gate of the first transistor Q1, the other end of the third capacitor C3 is connected to the drain of the third transistor Q3, the gate of the third transistor Q3 is connected to the third DC voltage source V3 through the third resistor R3, and the source of the third transistor Q3 is grounded. One end of the fourth capacitor C4 in the second switch capacitor is connected to the gate of the first transistor Q1, the other end of the fourth capacitor C4 is connected to the drain of the fourth transistor Q4, the gate of the fourth transistor Q4 is connected to the fourth DC voltage source V4 through the fourth resistor R4, and the source of the fourth transistor Q4 is grounded. One end of the fifth capacitor C5 in the third switch capacitor is connected to the gate of the second transistor Q2, the other end of the fifth capacitor C5 is connected to the drain of the fifth transistor Q5, the gate of the fifth transistor Q5 is connected to the third DC voltage source V3 through the fifth resistor R5, and the source of the fifth transistor Q5 is grounded. One end of the sixth capacitor C6 in the fourth switch capacitor is connected to the gate of the second transistor Q2, the other end of the sixth capacitor C6 is connected to the drain of the sixth transistor Q6, the gate of the sixth transistor Q6 is connected to the fourth DC voltage source V4 through the sixth resistor R6, and the source of the sixth transistor Q6 is grounded.

[0029] The working mechanism of the present invention is as follows: in the active RC network structure of the present invention, the transistor with inductive load is biased to a suitable working point by resistor voltage division, and it can be equivalent to a capacitor connected in series with a negative resistance when viewed from the gate of the transistor, so that the circuit structure can be equivalent to a 5-order elliptical filter. The negative resistance can reduce the parasitic resistance of the circuit, thereby improving the quality factor of the circuit, improving the filter sideband selectivity and reducing the insertion loss in the passband. By adjusting the bias voltage, the drain inductance and the size of the transistor, its equivalent capacitance and negative resistance reach a suitable size. In order to realize the reconfigurable function, a parallel switch capacitor is introduced into the active RC network, and the size of the equivalent capacitance of the active RC network can be changed by controlling the opening and closing of the switch capacitor. Then the active RC network can be equivalent to a variable capacitor connected in series with a negative resistance to the ground. In the active RC network of the present invention, two parallel switch capacitors, i.e., two switch capacitor groups, can realize the switching of three working frequency bands of the circuit by adjusting the capacitance value to realize the reconfigurable function of the frequency division band, and the number of switchable working frequency bands can also be increased by increasing or decreasing the number of switch capacitor groups.

[0030] Figure 4Schematic diagram of the equivalent circuit structure of the reconfigurable filter based on the active RC network provided in an embodiment of the present invention, the eleventh capacitor C11, the twelfth capacitor C12, the eleventh inductor L11, the twelfth inductor L12, the thirteenth inductor L13, the fourteenth inductor L14, the fifteenth inductor L15, the thirteenth variable capacitor C13, the fourteenth variable capacitor C14, the eleventh negative resistor R11, and the twelfth negative resistor R12. The signal enters from the first port, one end of the eleventh capacitor C11 is connected to the first port, the other end of the eleventh capacitor C11 is connected to the eleventh inductor L11, one end of the eleventh inductor L11 is connected to one end of the twelfth inductor L12 and one end of the fourteenth inductor L14, the other end of the twelfth inductor L12 is connected to one end of the thirteenth inductor L13 and one end of the fifteenth inductor L15, the other end of the fourteenth inductor L14 is connected to one end of the thirteenth variable capacitor C13, the other end of the thirteenth variable capacitor C13 is grounded through the eleventh negative resistor R11, and the other end of the fifteenth inductor L15 is connected to one end of the fourteenth variable capacitor C14. The other end of the fourteenth variable capacitor C14 is grounded through the twelfth negative resistor R12, and the other end of the thirteenth inductor L13 is connected to the second port through the twelfth capacitor C12, and the second port is a signal output port.

[0031] The eleventh inductor L11, the twelfth inductor L12, the thirteenth inductor L13, the fourteenth inductor L14, the fifteenth inductor L15, the thirteenth variable capacitor C13 and the fourteenth variable capacitor C14 form a 5th-order elliptical low-pass filter, and the eleventh capacitor C11 and the twelfth capacitor C12 are DC-isolating capacitors. The function of reconfigurable frequency division band is realized by adjusting the thirteenth variable capacitor C13 and the fourteenth variable capacitor C14. On the basis of the basic elliptical filter, the eleventh negative resistor R11 and the twelfth negative resistor R12 can reduce the parasitic resistance of the fourteenth inductor L14 and the fifteenth inductor L15, thereby improving the quality factor of the circuit, reducing the insertion loss in the passband, and improving the filter sideband selectivity.

[0032] Figure 5 A graph showing the variation of insertion loss of a reconfigurable filter based on an active RC network with frequency provided in an embodiment of the present invention. Figure 6 The insertion loss of the reconfigurable filter based on the active RC network and the traditional filter provided by the embodiment of the present invention varies with frequency. Figure 5 It can be seen that the filter provided by the present invention has a frequency division and reconfigurable function, and has three working frequency bands of 0.1-2GHz, 0.1-2.6GHz and 0.1-3.2GHz, and has good performance in each working frequency band. Since the active RC network has similar effects on performance in different working frequency bands, the performance of a certain working frequency band is compared with the traditional filter structure. The comparison results are shown in Figure 1. Figure 6The conventional filter has poor sideband selectivity and high insertion loss in the working frequency band. The reconfigurable filter based on the active RC network provided by the present invention can reduce the insertion loss in the working frequency band and improve the sideband selectivity.

[0033] In summary, the reconfigurable filter based on the active RC network designed in this embodiment is superior to the traditional filter, has a simple structure, strong sideband selectivity, low insertion loss, and has a frequency-band reconfigurable function.

[0034] It is to be understood that the present invention is described by some embodiments, and it is known to those skilled in the art that various changes or equivalent substitutions may be made to these features and embodiments without departing from the spirit and scope of the present invention. In addition, under the teachings of the present invention, these features and embodiments may be modified to adapt to specific circumstances and materials without departing from the spirit and scope of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the scope of protection of the present invention.

Claims

1. A reconfigurable filter based on an active RC network, characterized in that: The reconfigurable filter includes a first active RC network structure, a second active RC network structure, a first capacitor C1, a second capacitor C2, a first inductor L1, a second inductor L2, a third inductor L3, a fourth inductor L4 and a fifth inductor L5; a signal enters from a first port, one end of the first capacitor C1 is connected to the first port, the other end of the first capacitor C1 is connected to one end of the first inductor L1, the other end of the first inductor L1 is connected to one end of the second inductor L2 and one end of the fourth inductor L4, the other end of the second inductor L2 is connected to one end of the third inductor L3 and one end of the fifth inductor L5, and the other end of the fourth inductor L4 is connected to one end of the first active RC network; the other end of the first active RC network is grounded, and the other end of the fifth inductor L5 is connected to one end of the second active RC network; the other end of the second active RC network is grounded, and the other end of the third inductor L3 is connected to the second port through the second capacitor C2, and the second port is an output port.

2. The reconfigurable filter based on active RC network according to claim 1, characterized in that: The first active RC network structure includes a first DC voltage source V1, a first resistor R1, a sixth inductor L6, a first transistor Q1, a first switched capacitor and a second switched capacitor. In the first active RC network structure, the first DC voltage source V1 is connected to one end of the sixth inductor L6 and one end of the first resistor R1, the other end of the sixth inductor L6 is connected to the drain of the first transistor Q1, the gate of the first transistor Q1 is connected to the other end of the first resistor R1, one end of the first switched capacitor, one end of the second switched capacitor and the other end of the fourth inductor L4, and the source of the first transistor Q1 and the other end of the first switched capacitor and the other end of the second switched capacitor are grounded.

3. The reconfigurable filter based on active RC network according to claim 1, characterized in that: The second active RC network structure includes a second DC voltage source V2, a second resistor R2, a seventh inductor L7, a second transistor Q2, a third switch capacitor, and a fourth switch capacitor; in the second active RC network structure, the second DC voltage source V2 is connected to one end of the seventh inductor L7 and one end of the second resistor R2, the other end of the seventh inductor L7 is connected to the drain of the second transistor Q2, the gate of the second transistor Q2 is connected to the other end of the second resistor R2, one end of the third switch capacitor, one end of the fourth switch capacitor, and the other end of the fifth inductor L4, and the source of the second transistor Q1 and the other end of the third switch capacitor and the other end of the fourth switch capacitor are grounded.

4. The reconfigurable filter based on active RC network according to claim 2, characterized in that: The first switch capacitor includes a third capacitor C3, a third transistor Q3, a third resistor R3, and a third DC voltage source V3. One end of the third capacitor C3 in the first switch capacitor is connected to the gate of the first transistor Q1, and the other end of the third capacitor C3 is connected to the drain of the third transistor Q3. The gate of the third transistor Q3 is connected to the third DC voltage source V3 through the third resistor R3, and the source of the third transistor Q3 is grounded.

5. The reconfigurable filter based on active RC network according to claim 2, characterized in that: The second switch capacitor includes a fourth capacitor C4, a fourth transistor Q4, a fourth resistor R4, and a fourth DC voltage source V4; one end of the fourth capacitor C4 in the second switch capacitor is connected to the gate of the first transistor Q1, the other end of the fourth capacitor C4 is connected to the drain of the fourth transistor Q4, the gate of the fourth transistor Q4 is connected to the fourth DC voltage source V4 through the fourth resistor R4, and the source of the fourth transistor Q4 is grounded.

6. The reconfigurable filter based on active RC network according to claim 3, characterized in that: The third switch capacitor includes a fifth capacitor C5, a fifth transistor Q5, a fifth resistor R5, and a third DC voltage source V3. One end of the fifth capacitor C5 in the third switch capacitor is connected to the gate of the second transistor Q2, the other end of the fifth capacitor C5 is connected to the drain of the fifth transistor Q5, the gate of the fifth transistor Q5 is connected to the third DC voltage source V3 through the fifth resistor R5, and the source of the fifth transistor Q5 is grounded.

7. The reconfigurable filter based on active RC network according to claim 3, characterized in that: The fourth switch capacitor includes a sixth capacitor C6, a sixth transistor Q6, a sixth resistor R6, and a fourth DC voltage source V4; one end of the sixth capacitor C6 in the fourth switch capacitor is connected to the gate of the second transistor Q2, the other end of the sixth capacitor C6 is connected to the drain of the sixth transistor Q6, the gate of the sixth transistor Q6 is connected to the fourth DC voltage source V4 through the sixth resistor R6, and the source of the sixth transistor Q6 is grounded.

Images