Second-order Tow-Thomas low-pass filter with independently adjustable gain and bandwidth
By designing the gain and bandwidth independent and adjustable filter based on the Tow-THOMAS structure in the ultrasonic front end, the problem of gain loss caused by VGA bandwidth reduction is solved, and the high integrated design suitable for multiple frequency ultrasound applications is realized.
Patent Information
- Application Number
- CN202411260964.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-09-10
- Publication Date
- 2025-01-24
AI Technical Summary
In the existing ultrasonic front-end design, the bandwidth of VGA is reduced at certain process angles, resulting in gain loss. At the same time, the ultrasonic frequencies used in different application scenarios are different, and there is a lack of a solution that can adjust the system bandwidth like the system gain. plan.
Based on the Tow-Thomas low-pass filter structure and combining the functions of VGA and LPF, a filter whose gain and bandwidth can be adjusted independently is designed. Through the variable resistor network and the slope voltage generating circuit, the filter gain and cutoff frequency are independently controlled.
It achieves independent adjustment of gain and bandwidth, improves the integration of the design, is suitable for multiple frequency ultrasound applications, reduces power consumption, and ensures the stability and functional correctness of signal processing.
Smart Images
Figure CN119363069A_ABST
Abstract
Description
Technical Field
[0001] The invention belongs to the technical field of integrated circuits, and in particular relates to a second-order Tow-Thomas low-pass filter with independently adjustable gain and bandwidth. Background Art
[0002] Ultrasound is widely used in nondestructive testing of materials, medical diagnosis, bioengineering, national defense and military fields due to its short wavelength and strong penetration. It has great theoretical research value and application prospects. The ultrasonic front end is a combination of a series of circuit modules for analog signal processing and digitization of ultrasonic signals, mainly including low noise amplifier (LNA), variable gain amplifier (VGA), low pass filter (LPF), analog-to-digital converter (ADC) and other modules. Since the attenuation of sound waves in the transmission medium is directly related to the total transmission distance, in order to make the electrical signal adapt to the input dynamic range of subsequent processing modules such as analog-to-digital converters, it is necessary to introduce an amplifier with variable gain control to compensate for the attenuation of ultrasound in the propagation medium, so that the signal can adapt to the input dynamic range of the subsequent analog-to-digital converter, and ensure the stability of the ultrasonic system and the correctness of its function. The variable gain amplifier (VGA) can ensure that the reflected electrical signals with different amplitudes can be amplified to a stable amplitude before entering the subsequent circuit processing. It is one of the core components of the ultrasonic front end.
[0003] In addition to the gain of the amplifier, the bandwidth of the amplifier should also be considered during design. In a paper published by Ji-Yong Um in 2021, the bandwidth of the VGA designed by him was reduced to 2MHz under a certain process angle, which is close to the frequency of ultrasound in the application scenario, and will inevitably cause gain loss. On the other hand, the ultrasonic frequencies used in different application scenarios are also different. The frequency of medical therapeutic ultrasound is about 1MHz, the frequency of abdominal diagnostic ultrasound is about 2MHz, and the ultrasonic frequency used by ophthalmic ultrasound diagnostic instruments is as high as 20MHz. If the system bandwidth can be adjusted like the system gain, then an ultrasonic front end will have the potential for application in ultrasound of multiple frequencies. Based on this consideration, the present invention is based on the Tow-Thomas low-pass filter structure, combines the functions of VGA and LPF, and designs a filter whose gain and bandwidth can be independently adjusted, which greatly improves the integration of the design. The gain of the Tow-Thomas low-pass filter depends on the ratio of the two resistors. In order to achieve adjustable gain, the present invention also studies and designs an integrated variable resistor structure. This structure is different from the traditional structure that uses an external control voltage as the transistor gate voltage to control the source-drain resistance, and can achieve a larger control voltage regulation range and better linearity. In order to solve the defect of high power consumption of the Tow-Thomas structure, the present invention also independently designs a low-power, high-gain-bandwidth operational amplifier. Summary of the invention
[0004] The object of the present invention is to provide a second-order Tow-Thomas low-pass filter with high integration, large control voltage regulation range and good linearity, and independently adjustable gain and bandwidth.
[0005] The Tow-Thomas second-order low-pass filter with independently adjustable gain and bandwidth provided by the present invention is composed of two low-power fully differential amplifiers and a symmetrical dual second-order resistor-capacitor network around them. The filter topology adopts the Tow-Thomas filter architecture, and can realize low-pass, high-pass, band-pass, full-pass, and notch filtering functions through reasonable configuration. Its specific structure is as follows Figure 1 As shown: The symmetrical resistor-capacitor network consists of symmetrical resistor pairs R 11 With R 12 , the symmetrical resistance pair R 21 With R 22 , the symmetrical resistance pair R 31 With R 32 , the symmetrical resistance pair R 41 With R 42 , and the symmetrical capacitance pair C 11 With C 12 , the symmetrical capacitor pair C 21 With C 22Composition: The two resistors of the resistor pair have the same resistance value, and the two capacitors of the capacitor pair have the same capacitance value; the symmetrical peripheral resistor and capacitor network forms a feedback loop around the two operational amplifiers OA1 and OA2; the resistor R 11 With R 12 Connect in series to the two circuit inputs (V INN ,V INP ) and the first operational amplifier OA1 input; resistor R 31 With R 32 Respectively connected between the negative output and positive input of OA1 to form negative feedback; resistor R 21 With R 22 Connected between the output of OA1 and the input of OA2 respectively; capacitor C 11 With C 12 , capacitor C 21 With C 22 Respectively connected between the negative output and positive input of OA1 and OA2; Finally, the resistor R 41 With R 42 Connected between the output of OA2 and the input of OA1; the low-frequency gain of the filter is proportional to the resistor R 41 and R 11 The resistance ratio (also known as resistance R 42 and R 12 The square of the cut-off frequency is inversely proportional to the resistance R 21 , R 41 Resistance, capacitance C 11 , C 21 The product of the capacitance (also known as the resistance R 22 , R 42 Resistance, capacitance C 12 , C 22 The square of the quality factor is proportional to the capacitance C 11 , C 21 Capacitance ratio (also known as capacitance C 12 and C 22 Therefore, the resistor R 11 (That is, R 12 )Change the filter gain and adjust the capacitor C synchronously 11 , C 12 , C 21 , C 22 The filter cutoff frequency is changed while the quality factor is kept constant. From the principle of adjusting the gain and the cutoff frequency, it can be seen that the gain and the cutoff frequency can be adjusted independently without interfering with each other.
[0006] The low-frequency gain of the filter is determined by a peripheral biquad resistor-capacitor network, and the resistance value is determined by a voltage-controlled variable resistor structure to adjust the low-frequency gain of the filter; the cutoff frequency of the filter is determined by a peripheral biquad resistor-capacitor network, and the on-off of the complementary metal oxide semiconductor field effect transistor transmission gate is controlled by digital logic to determine the capacitance of the capacitor connected to the loop, thereby adjusting the low-pass cutoff frequency of the filter.
[0007] When the filter low frequency gain needs to be adjusted, only the resistor R 41 and R 42 The resistance remains unchanged, and the change of resistance has an effect on R 11 and R 12 The resistance value is . 11 and R 12 A variable resistor network is used to achieve this. The structure of the variable resistor network is as follows Figure 2 As shown, the variable resistor network consists of 8 parallel branches, each of which contains a resistor R Ci , i = 1, 2, ... 8, among which MOS tubes (M C1 ~M C7 ), so by controlling the gate voltage (V C1 ~V C7 ) to control the on / off of the branch. When the gate voltage is lower than the transistor threshold voltage, the MOS tube is turned off and the branch is disconnected; when the gate voltage is higher than the transistor threshold voltage, the MOS tube is turned on and the branch is turned on. The total resistance of the branch is equivalent to the sum of the on-resistance of the transistor and the fixed resistance on the branch. The total resistance of all the conducting branches in parallel is the equivalent resistance of the variable resistance network.
[0008] The multi-gate control voltage required in the variable resistor network is given by the ramp voltage generating circuit. The structure of the ramp voltage generating circuit is as follows: Figure 3 As shown. Two transistors NMOS (M N1 、M N2 ) are connected to the gate of the external control voltage V ctrl and reference voltage V ref , drain terminal connected to load M L1 、M L2 , the source end is connected to a constant current source, and through the resistor R R1 Connected; transistor M P1 ~M P7 With M L1 、M L2 A current mirror is formed to copy the current and make it flow through the resistor R R2 and constant current sources of different sizes to generate a series of voltages V C1 ~V C7, as the gate control voltage in the variable resistor network.
[0009] The ramp control voltage generating circuit has an input of an external control voltage V ctrl The output is a series of ramp voltages with the same slope and different intercepts. The control voltage V ctrl With reference voltage V ref Connected to the gates of the two transistors respectively, the resistor R R1 The current flowing through the diodes will be proportional to the difference between the two, and the current flowing through the load connected to the two diodes will also be proportional to the difference between the two, that is, the control voltage V crtl This current is copied to other branches through the PMOS current mirror and flows through the fixed resistance resistor R R2 , generating a ramp voltage with a certain slope. By connecting current sources of different magnitudes in parallel at both ends of the resistor, multiple ramp voltages with the same slope and different intercepts can be generated.
[0010] In the present invention, the voltage-controlled variable resistor structure, under ideal conditions, the MOS tube exhibits infinite impedance when turned off, and works in the linear region when turned on. The resistance between the source and the drain can be obtained by the current-voltage characteristic of the transistor in the linear region. Therefore, the equivalent impedance of the variable resistor network will be affected by the on-state of the MOS tube. Using a series of voltages as gate voltages applied to the gates of each branch MOS tube can control the on-state of the MOS tube, and then control the equivalent impedance of the entire parallel network.
[0011] For this structure, if you want to achieve discrete gain adjustment without intermediate transition states, you need to strictly distinguish the on and off of each MOS tube. If the MOS tube is turned on, the resistance on each branch will appear as a fixed resistance. If the MOS tube is turned off, the resistance on the branch will appear as infinite. To achieve such regulation, you need a MOS gate control voltage that can change suddenly. At this time, the variable resistor will have a discrete gear that matches the number of branches, which corresponds to the gear of the adjustable gain.
[0012] Such a sudden change in gate control voltage is almost impossible to achieve at the circuit level, and it is more desirable to achieve continuous gain adjustment rather than discrete gain adjustment. Therefore, a ramp voltage with a certain slope is used as the gate control voltage of the MOS tube, that is, when the gate voltage is between the threshold voltage and the power supply voltage, the transistor is neither fully turned on nor fully turned off, but is in a half-open and half-closed state. In this state, the resistance on the corresponding branch is between infinity and a fixed resistance. As the gate control voltage increases, the resistance between the source and drain of the MOS tube decreases, resulting in a gradual decrease in the equivalent resistance of the entire network. The specific mathematical relationship between the change in equivalent resistance and gate voltage depends on the various process parameters of the transistor, and it is not easy to give a quantitative relationship, but it is certain that such a change is continuous. In this control method, the equivalent resistance may show "platforms" in sections as the voltage changes. The reason is that the intervals between the ramp signals controlling different branches are large. After the previous branch has been fully turned on, the voltage controlling the next branch has not increased above the threshold voltage, showing infinite impedance under ideal conditions. Before the gate voltage of the MOS tube on this branch increases to the source voltage plus the threshold voltage, this branch is considered to be disconnected, and the equivalent resistance of the network will not change significantly.
[0013] Therefore, the present invention additionally designs a ramp voltage generating circuit, which converts the external input control voltage into the gate voltage for controlling the on state of the MOS tube, and shortens the interval between different ramp signals so that before the current branch is fully turned on, the gate voltage of the next branch is already greater than the source voltage plus the threshold voltage, and current flows through both branches. In this case, the equivalent resistance will always respond to the change of the control voltage, the "platform" of resistance change, that is, the gain change, will no longer exist, and the gain control curve will become smoother.
[0014] The amplifier stage of the low-power operational amplifier uses a sleeve-type common source and common gate structure, and the output stage uses a common source amplifier with a diode load. The load tube of the common source and common gate structure in one branch of the amplifier stage significantly improves the output impedance and achieves high gain. The output stage of the diode load has a smaller voltage gain and low output impedance, providing stronger load capacity and output swing. The use of the output buffer stage also helps us achieve a larger gain-bandwidth product with lower power consumption. BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Figure 1 The structure of the second-order Tow-Thomas low-pass filter with independently adjustable gain and bandwidth is shown in FIG.
[0016] Figure 2 It is a diagram showing the structure of the variable resistor and capacitor network in the low-pass filter of the present invention.
[0017] Figure 3 FIG. 4 is a schematic diagram of the structure of a ramp voltage generating circuit in a low-pass filter of the present invention. DETAILED DESCRIPTION
[0018] The following further describes a second-order Tow-Thomas low-pass filter with independently adjustable gain and bandwidth according to the present invention in conjunction with the accompanying drawings.
[0019] The second-order Tow-Thomas low-pass filter of the present invention is composed of two low-power fully differential amplifiers and a peripheral resistor and capacitor network, and the filter topology adopts the Tow-Thomas filter architecture. Its specific structure is as follows Figure 1 As shown in FIG. 1 , a symmetrical peripheral resistor and capacitor network forms a feedback loop around two operational amplifiers OA1 (110) and OA2 (111). 11 With R 12 Connected in series between the input and the input of the first operational amplifier OA1 (110); resistor R 31 With R 32 Connected between the negative output and positive input of OA1 to form negative feedback; resistor R 21 With R 22 Connected between the output of OA1 and the input of OA2; capacitor C 11 With C 12 , capacitor C 21 With C 12 Respectively connected between the negative output and positive input of OA1 and OA2; Finally, the resistor R 41 With R 42 Connected between the output of OA2 and the input of OA1. The low frequency gain of this filter is proportional to R 41 and R 11 (That is, R 42 and R 12 ), the square of the cut-off frequency is inversely proportional to R 21 , R 41 , C 11 , C 21 (That is, R 22 , R 42 , C 12 , C 22 ), the square of the quality factor is proportional to C 11 , C 21 (That is, C 12 and C 22 ) ratio. Therefore, the resistor R 11 (R 12 ) value changes the filter gain and adjusts the capacitor C synchronously 11 , C 12 , C 21 , C 22The value of changes the filter cutoff frequency and keeps the quality factor constant. From the principle of adjusting the gain and the cutoff frequency, it can be seen that the regulation of the gain and the cutoff frequency does not interfere with each other and can be performed independently.
[0020] When the filter low-frequency gain needs to be adjusted, only R 41 and R 42 The two resistors remain unchanged, and the resistance R is changed 11 and R 12 The resistance value is . 11 and R 12 A variable resistor network is used to achieve this. The structure of the variable resistor network is as follows: Figure 2 As shown,
[0021] The multi-gate control voltage required in the variable resistor network is given by the ramp voltage generating circuit. The structure of the ramp voltage generating circuit is as follows: Figure 3 shown.
[0022] The ramp voltage generating circuit generates a series of ramp voltages with the same slope and different intercepts according to the input external control voltage to control the variable resistor network ( Figure 2 ) is turned on. Using a ramp voltage as the gate control voltage of the MOS tube can achieve continuous resistance control, that is, gain control: when the interval between different ramp signals is shortened to a certain extent, before the current branch is fully turned on, the gate voltage of the next branch is already greater than the source voltage plus the threshold voltage, and current flows through both branches. The equivalent resistance will always respond to the change of the control voltage, so the gain control curve will become smoother.
[0023] The circuit for generating the ramp control voltage is as follows: Figure 3 As shown. The input of the circuit is the external control voltage V ctrl The output is a series of ramp voltages with the same slope and different intercepts. The control voltage V ctrl With reference voltage V ref Connected to two transistors M N1 、M N2 The gate resistor R R1 The current flowing through the two diodes is proportional to the difference between them. L1 、M L2 The current flowing through the control voltage V crtl This current passes through the PMOS current mirror (M P1 ~M P7 ) is mirrored to other branches and flows through a fixed resistance resistor R R2, generating a ramp voltage with a certain slope. By connecting current sources of different magnitudes in parallel at both ends of the resistor, multiple ramp voltages with the same slope and different intercepts can be generated. The specific dependence is as follows: R R1 , R R2 Together they determine the slope of the ramp voltage, V ref Determines the value of the control voltage when the ramp voltage is saturated. Therefore, a larger R R1 , smaller R R2 , Larger V ref Can increase the external input control voltage V ctrl Linear control range of the ramp voltage.
[0024] When the resistance value remains unchanged, only the switch By changing the capacitance value, the cutoff frequency can be adjusted. The quality factor of the filter determines the change in frequency at the main pole of its frequency response. The quality factor is determined by the ratio of the resistance value and the capacitance. Therefore, it is necessary to adjust the two capacitors at the same time through the switch and keep the ratio unchanged so that the quality factor remains unchanged.
[0025] Although the contents and advantages of the present invention have been disclosed in detail as above, it must be noted that the scope of the present invention is not limited to the specific embodiments such as the methods and steps described in the specification. Without departing from the spirit and scope of the present invention, any ordinary technician in the field can make many variations and modifications based on the contents disclosed by the present invention, which should also be regarded as the protection scope of the present invention.
Claims
1. A Tow-Thomas second-order low-pass filter with independently adjustable gain and bandwidth, characterized in that: The invention comprises two low-power fully differential amplifiers and a symmetrical biquad resistor-capacitor network around the amplifiers; the filter topology adopts a Tow-Thomas filter architecture, and realizes low-pass, high-pass, band-pass, full-pass, and notch filtering functions through reasonable configuration; wherein the symmetrical resistor-capacitor network is composed of a symmetrical resistor pair R 11 With R 12 , R 21 With R 22 , R 31 With R 32 , R 41 With R 42 , and the symmetrical capacitance pair C 11 With C 12 , C 21 With C 22 Composition: The two resistors of the resistor pair have the same resistance value, and the two capacitors of the capacitor pair have the same capacitance value; the symmetrical peripheral resistor and capacitor network forms a feedback loop around the two operational amplifiers OA1 and OA2; the resistor R 11 With R 12 Connect in series to the two circuit inputs (V INN ,V INP ) and the first operational amplifier OA1 input; resistor R 31 With R 32 Respectively connected between the negative output and positive input of OA1 to form negative feedback; resistor R 21 With R 22 Connected between the output of OA1 and the input of OA2 respectively; capacitor C 11 With C 12 , capacitor C 21 With C 22 Respectively connected between the negative output and positive input of OA1 and OA2; Finally, the resistor R 41 With R 42 Connected between the output of OA2 and the input of OA1; the low-frequency gain of the filter is proportional to the resistor R 41 and R 11 The square of the cut-off frequency is inversely proportional to the resistance R 21 , R 41 Resistance, capacitance C 11 , C 21 The product of the capacitance and the square of the quality factor are proportional to the capacitance C 11 , C 21 capacitance ratio; therefore, by adjusting the resistor R 11 That is, R 12 Change the filter gain and adjust the capacitor C synchronously 11 , C 12 , C 21 , C 22 Vary the filter cutoff frequency while keeping the quality factor constant.
2. The Tow-Thomas second-order low-pass filter according to claim 1, characterized in that: The low-frequency gain is determined by a symmetrical biquad resistor-capacitor network, and the resistance value is determined by a voltage-controlled variable resistor structure to adjust the low-frequency gain of the filter; the cutoff frequency of the filter is determined by a symmetrical biquad resistor-capacitor network, and the on-off of the complementary metal oxide semiconductor field effect transistor transmission gate is controlled by digital logic to determine the capacitance of the capacitor connected to the loop, thereby adjusting the low-pass cutoff frequency of the filter.
3. The Tow-Thomas second-order low-pass filter according to claim 2, characterized in that: When the filter low-frequency gain needs to be adjusted, only the resistor R 41 and R 42 The resistance of R 11 and R 12 The resistance value is , where the resistor R 11 and R 12 The variable resistor network is implemented by using a variable resistor network; the variable resistor network is composed of 8 parallel branches, each branch contains a resistor R Ci , i = 1, 2, ... 8, where MOS tubes M are connected in series on the 7 branches Ci , i = 1, 2, ... 7, by controlling the gate voltage V on the MOS tube C1 ~V C7 To control the on and off of the branch; when the gate voltage is lower than the transistor threshold voltage, the MOS tube is turned off and the branch is disconnected; when the gate voltage is higher than the transistor threshold voltage, the MOS tube is turned on, the branch is turned on, and the total resistance of the branch is equivalent to the sum of the on-resistance of the transistor and the fixed resistance on the branch; the total resistance of all the conducting branches in parallel is the equivalent resistance of the variable resistance network.
4. The Tow-Thomas second-order low-pass filter according to claim 3, characterized in that: The multi-gate control voltage required in the variable resistor network is provided by a ramp voltage generating circuit; in the ramp voltage generating circuit, two transistors NMOS (M N1 、M N2 ) are connected to the gate of the external control voltage V ctrl and reference voltage V ref , drain terminal connected to load M L1 、M L2 , the source end is connected to a constant current source, and through the resistor R R1 Connected; transistor M P1 ~M P7 With M L1 、M L2 A current mirror is formed to copy the current and make it flow through the resistor R R2 and constant current sources of different sizes to generate a series of voltages V C1 ~V C7 , as the gate control voltage in the variable resistor network; the ramp control voltage generating circuit, whose input is the external control voltage V ctrl , the output is a series of ramp voltages with the same slope and different intercepts; the control voltage V ctrl With reference voltage V ref Connected to the gates of the two transistors respectively, the resistor R R1 The current flowing through the diode is proportional to the difference between the two, and the current flowing through the load connected to the two diodes is also proportional to the difference between the two, that is, the control voltage V crtl This current is copied to other branches through the PMOS current mirror and flows through the fixed resistance resistor R R2 , generating a ramp voltage with a certain slope.