High output impedance current driving circuit with adjustable transconductance function

Abstract

The application relates to an analog integrated circuit design, and discloses a high-output-impedance current driving circuit with adjustable transconductance. The circuit comprises a full-differential differential rail-to-rail input transconductance amplifier, a transconductance amplifier, a differential rail-to-rail input transconductance amplifier, a charge pump module and a feedback resistance array, and works in a closed-loop negative feedback architecture. By controlling the switch combination in the feedback resistance array, continuous or segmented dynamic adjustment of the transconductance is realized to adapt to different load conditions and signal strength requirements; the charge pump module raises the power supply voltage, expands the input common-mode range and ensures the linearity of the circuit during transconductance adjustment. The circuit still maintains a high output impedance of more than 10 MΩ at a frequency of 1 MHz, and the total harmonic distortion is less than 1%, thereby solving the trade-off limitation between bandwidth, output impedance and linearity of traditional current driving and being applicable to bioimpedance measurement in a resistive impedance tomography system.

Description

Technical Field

[0001] This application relates to the field of analog integrated circuits, and in particular to bioimpedance measurement current-driven circuit technology. Background Technology

[0002] Electrical impedance tomography (EIT) is a non-invasive medical imaging technology with wide applications in medical settings such as lung function monitoring and brain imaging. In an EIT system, current drive is the core module, primarily used to inject high-frequency alternating current (e.g., in the range of 50 kHz to 1 MHz) into biological tissues. Its performance directly affects the accuracy of impedance measurements and the quality of the imaging.

[0003] However, conventional current-driven methods face several technical challenges in bioimpedance measurements. First, bandwidth limitation is a significant issue. At high frequencies (especially above 100 kHz), the output impedance of conventional current-driven methods drops significantly, typically below 1 MΩ, leading to reduced output current accuracy and consequently affecting measurement accuracy and imaging resolution.

[0004] Secondly, linearity degradation is also a critical issue. When current-driven parameters need to be adjusted (such as increasing transconductance or current amplitude), harmonic distortion (THD) increases, directly affecting the accuracy of bioimpedance demodulation. Since the transconductance of conventional current-driven circuits is usually fixed, dynamic adjustment of the output current can only rely on external gain or dynamic input voltage regulation, such as through an instrumentation amplifier (IA) and a programmable gain amplifier (PGA), which increases system power consumption and complexity.

[0005] Third, high resource consumption is a problem that traditional solutions cannot avoid. Relying on gain adjustment of the acquisition circuit or feedback compensation after demodulation requires additional power consumption and hardware resources, such as high-precision PGAs and dedicated demodulation modules. This not only increases the system cost, but also limits the portability and real-time performance of the EIT system.

[0006] The root of these technical problems lies in the difficulty of achieving an optimal combination of bandwidth, output impedance, and linearity simultaneously using traditional current drives. This contradiction becomes even more pronounced when the output current needs to be dynamically adjusted to adapt to different measurement scenarios. Therefore, there is an urgent need for a current drive solution that can maintain high output impedance at high frequencies while supporting dynamic transconductance adjustment and maintaining low distortion, in order to meet the requirements of modern EIT systems for high precision, multi-scenario adaptability, and efficient resource utilization. Summary of the Invention

[0007] The purpose of this application is to provide a high output impedance current drive circuit with transconductance adjustable function to solve the problems mentioned in the background art.

[0008] This application discloses a high output impedance current drive circuit with adjustable transconductance, comprising:

[0009] As a pre-amplification module, the fully differential rail-to-rail input transconductance amplifier (FDDTA) has a first differential input terminal (V1P / V1N / V2P / V2N) and a first differential output terminal (VO1 / VO2).

[0010] As a transconductance amplifier (OTA) in a voltage-to-current conversion module, the differential input terminal of the transconductance amplifier is connected to the first differential output terminal of the fully differential differential rail-to-rail input transconductance amplifier, and the transconductance amplifier has a single-ended output terminal (VO).

[0011] The differential rail-to-rail input transconductance amplifier (DDTA), which serves as a current sink, has its differential input terminal V1P connected to the second differential input terminal V2P of the fully differential rail-to-rail input transconductance amplifier, and both connected to the output of the feedback resistor array. The differential rail-to-rail input transconductance amplifier has a single-ended output terminal (VO). The differential input terminals V1N and V2P of the differential rail-to-rail input transconductance amplifier are connected to the common-mode input level of the system. The differential input terminal V2N of the differential rail-to-rail input transconductance amplifier is connected to the single-ended output terminal (VO), forming a unity-gain negative feedback configuration.

[0012] A charge pump module, wherein the input terminal of the charge pump module is connected to the input common-mode level, and the output terminal (VDDX) is connected to the voltage of the folded common-source cascode tail current source in the fully differential rail-to-rail input transconductance amplifier and the differential rail-to-rail input transconductance amplifier; and

[0013] A feedback resistor array, comprising multiple parallel switchable resistors (Rf1-Rf6) and multiple control switches (Sf1-Sf6), is connected in parallel to the single-ended output (VO) of the transconductance amplifier. The resistance values ​​of the switchable resistors cover a predetermined range, enabling the current drive circuit to achieve multi-level adjustment of the transconductance. For example, the resistance value range of the switchable resistors is 200Ω to 1200Ω.

[0014] In a preferred embodiment, the first differential output signal output by the fully differential rail-to-rail input transconductance amplifier is converted into a single-ended output source current (Isource) by the transconductance amplifier and the feedback resistor array.

[0015] The differential rail-to-rail input transconductance amplifier is connected in a unity-gain negative feedback configuration. Based on the voltage signal flowing through the feedback resistor array, it is connected to the second differential input signal V2P to generate a sink current (Isink) that matches the source current (Isource).

[0016] The charge pump module boosts the tail current source voltage VDD of the folded cascode transistor to a boosted supply voltage (VDDX), which is then supplied to the fully differential rail-to-rail input transconductance amplifier and the differential rail-to-rail input transconductance amplifier to extend their input common-mode range and ensure the linearity of the circuit during transconductance adjustment; and

[0017] The feedback resistor array selects resistors (Rf1-Rf6) with different resistance values ​​by switching the control switches (Sf1-Sf6), thereby realizing continuous or segmented dynamic adjustment of transconductance to adapt to different load conditions and signal strength requirements. For example, it can realize dynamic adjustment of transconductance in the range of 0.83ms to 4.92ms.

[0018] In a preferred embodiment, the fully differential rail-to-rail input transconductance amplifier serves as a pre-amplification module, providing loop gain and output impedance extension.

[0019] The transconductance amplifier, as a voltage-to-current conversion module, converts a two-terminal voltage into a single-terminal current, while providing loop gain and output impedance extension, and has the ability to drive a load.

[0020] The differential rail-to-rail input transconductance amplifier serves as a current sink module, achieving unity-gain negative feedback and ensuring stable common-mode levels across the load; and

[0021] The charge pump module is used to provide a boost power supply to ensure linearity during transconductance adjustment.

[0022] In a preferred embodiment, the fully differential rail-to-rail input transconductance amplifier includes:

[0023] The first input stage is used to receive the first differential input signal (V1P / V1N);

[0024] The first folded-cascode output stage is connected to the output of the first input stage and is used to output the first differential output signal.

[0025] The first common-mode feedback output stage is connected to the output of the first folded cascode output stage and is used to stabilize the common-mode level of the first differential output signal.

[0026] In a preferred embodiment, the transconductance amplifier includes:

[0027] The second input stage is used to receive the first differential output signal;

[0028] The second active load stage is connected to the second input stage;

[0029] The second common-source, common-gate output stage is connected to the output of the second active load stage and is used to output the source current (Isource) to provide the ability to drive the load.

[0030] In a preferred embodiment, the differential rail-to-rail input transconductance amplifier includes:

[0031] The third input stage is used to receive the voltage signal flowing through the resistor array connected to the second differential input signal (V2P);

[0032] The third folded-cascode output stage is connected to the output of the third input stage and is used to amplify the input signal, thereby increasing the gain and bandwidth.

[0033] The third output stage, connected to the third folded cascode output stage, outputs a sink current Isink to the single-ended output Vout. The output stage also includes a compensation resistor Rm and a compensation capacitor Cm to ensure circuit stability. The differential rail-to-rail input transconductance amplifier generates a sink current that matches the source current, serving as the current sink.

[0034] In a preferred embodiment, the charge pump module includes:

[0035] A multi-stage cascaded charge pump unit, each charge pump unit including PMOS transistors Mp2 and Mp1 and NMOS transistors Mn2 and Mn1, constitutes a cross-coupled charge pump unit;

[0036] A non-overlapping clock circuit provides complementary and non-overlapping clock signals (CLK and CLKn) to the charge pump unit.

[0037] The input terminal is connected to the common-mode input level; and

[0038] At the output terminal, the boosted power supply voltage (VDDX) is output to the fully differential rail-to-rail input transconductance amplifier and the differential rail-to-rail input transconductance amplifier.

[0039] In a preferred embodiment, when the feedback resistor is greater than a preset threshold, the output impedance of the current drive circuit can reach at least 5MΩ at a frequency of 1MHz, preferably 10MΩ or higher. For example, when the feedback resistor is greater than 600Ω, the output impedance of the current drive circuit is greater than 10MΩ at a frequency of 1MHz.

[0040] The total harmonic distortion (THD) of the current drive circuit is less than 1%.

[0041] The feedback resistor array achieves continuous or segmented dynamic adjustment of transconductance through different resistor combinations to adapt to different load conditions and signal strength requirements. For example, it can achieve dynamic adjustment of transconductance in the range of 0.83ms to 4.92ms.

[0042] The current drive circuit is suitable for electrical impedance tomography (EIT) systems for injecting high-frequency differential sinusoidal alternating current into biological tissues. The high-frequency range includes, but is not limited to, tens of kilohertz to several megahertz, such as differential sinusoidal alternating current of 50 kHz to 1 MHz.

[0043] In a preferred embodiment, the six switchable resistors (Rf1-Rf6) in the feedback resistor array are arranged in ascending order of resistance value to form a resistor sequence covering the required transconductance adjustment range, for example, 200Ω, 400Ω, 600Ω, 800Ω, 1000Ω and 1200Ω respectively.

[0044] The control switches (Sf1-Sf6) control the connection and disconnection of the corresponding resistors respectively, and different equivalent feedback resistance values ​​are achieved through different switch combinations.

[0045] The embodiments of this application specifically achieve the following technical effects:

[0046] First, the bandwidth-impedance trade-off was resolved. Through impedance boosting techniques in the amplifier, such as negative resistance in the active load and a cascode structure, a high output impedance exceeding 10MΩ was achieved at 1MHz, while traditional solutions typically fall below 1MΩ at the same frequency. Figure 7 As shown, when the feedback resistor is greater than 600Ω, the output impedance can reach more than 10.5MΩ at 1MHz. Even with a smaller feedback resistor, it can reach 5.59MΩ, which meets the stringent requirements for high-frequency bioimpedance measurement.

[0047] Secondly, it achieves dynamic adjustment of transconductance. For example... Figure 6 As shown, by using different combinations of feedback resistor arrays (Rf1-Rf6) and control switches (Sf1-Sf6), the transconductance can be flexibly adjusted within the range of 0.83 ms to 4.92 ms, effectively improving the dynamic range of the circuit and avoiding the problem of saturation in the acquisition circuit. This adjustment meets the signal-to-noise ratio (SNR) optimization requirements of far-field signals in electrical impedance tomography (EIT) systems.

[0048] Third, it reduces system complexity and resource consumption. This solution integrates a charge pump module to regulate transconductance, and by increasing the power supply voltage VDDX, it expands the input common-mode range (ICMR) of FDDTA and DDTA, eliminating the need for external control to regulate the current and reducing the system's hardware resource requirements, such as high-precision programmable gain amplifiers (PGA) and demodulation modules.

[0049] Fourth, it maintains low distortion characteristics. For example... Figure 8 As shown, even with dynamic transconductance adjustment, the source current Isource and sink current Isink of the circuit output remain highly linear, as... Figure 9 As shown, when the feedback resistor is 600Ω, the third total harmonic distortion (THD) of the 1 mA peak-to-peak output current can be controlled to below 1% under the premise of having an output impedance greater than 10MΩ, which improves the resolution and measurement accuracy of impedance imaging.

[0050] Finally, through a fully differential output architecture and negative feedback loop design, the circuit exhibits excellent common-mode interference immunity and stability, making it suitable for various complex bioimpedance measurement environments. The overall solution overcomes the limitations of traditional current-driven circuits in balancing bandwidth, output impedance, and linearity, providing a high-performance current-driven solution for electrical impedance tomography (EIT) systems.

[0051] The specification of this application contains numerous technical features distributed across various technical solutions. Listing all possible combinations of these technical features (i.e., technical solutions) would make the specification excessively lengthy. To avoid this problem, the various technical features disclosed in the above-described invention, the various technical features disclosed in the following embodiments and examples, and the various technical features disclosed in the accompanying drawings can be freely combined to form various new technical solutions (all of which are considered to have been described in this specification), unless such a combination of technical features is technically infeasible. For example, one example discloses feature A+B+C, and another example discloses feature A+B+D+E. Features C and D are equivalent technical means that serve the same function, and technically only one needs to be used; they cannot be used simultaneously. Feature E can technically be combined with feature C. Therefore, the solution A+B+C+D should not be considered as described because it is technically infeasible, while the solution A+B+C+E should be considered as described. Attached Figure Description

[0052] Figure 1 This is a schematic diagram of a high output impedance current drive circuit with transconductance adjustable function according to an embodiment of this application.

[0053] Figure 2This is a detailed circuit diagram of the fully differential rail-to-rail input transconductance amplifier (FDDTA) in a current-driven configuration according to an embodiment of this application.

[0054] Figure 3 This is a detailed circuit diagram of the transconductance amplifier (OTA) portion in a current-driven circuit according to an embodiment of this application.

[0055] Figure 4 This is a detailed circuit diagram of the differential rail-to-rail input transconductance amplifier (DDTA) portion in a current-driven circuit according to an embodiment of this application.

[0056] Figure 5 This is a detailed circuit diagram of the charge pump section (a) and the non-overlapping clock section (b) in the current drive according to an embodiment of this application.

[0057] Figure 6 This is a simulation result diagram of adjustable transconductance over a wide bandwidth according to an embodiment of this application.

[0058] Figure 7 This is a simulation result graph showing the output impedance changing with frequency under different feedback resistors according to the embodiments of this application.

[0059] Figure 8 The figure shows the simulation results of the current-driven output of a 1 mA peak-to-peak sinusoidal differential current according to the embodiments of this application.

[0060] Figure 9 The total harmonic distortion (THD) Monte Carlo simulation results are based on an embodiment of this application, which shows a current-driven sinusoidal differential current with a peak-to-peak value of 1 mA output when the feedback resistance is 600Ω. Detailed Implementation

[0061] In the following description, many technical details are presented to help the reader better understand this application. However, those skilled in the art will understand that the technical solutions claimed in this application can be implemented even without these technical details and various variations and modifications based on the following embodiments.

[0062] Explanation of some concepts:

[0063] Fully Differential Rail-to-Rail Input Transconductance Amplifier (FDDTA): An integrated circuit amplifier with differential input and differential output, capable of processing input signals close to the power supply voltage rail and amplifying the differential portion of the input signal.

[0064] Transconductance amplifier (OTA): An analog amplifier that converts input voltage into output current. The output current is proportional to the difference between the input voltage and the output current, and the proportionality coefficient is called the transconductance.

[0065] Differential rail-to-rail input transconductance amplifier (DDTA): A buffer amplifier that can handle input signals close to the power supply voltage rail, providing signal isolation and maintaining signal integrity.

[0066] Transconductance: In electronic circuits, it represents the ratio of output current to input voltage, and the unit is Siemens (S).

[0067] Output impedance: Represents the impedance characteristic of the circuit output terminal to AC signals. The output impedance of an ideal current source should be infinite.

[0068] Charge pump: A circuit used to convert low voltage to high voltage by charging and discharging a capacitor to boost the voltage.

[0069] Electrical impedance tomography (EIT): a non-invasive imaging technique that reconstructs an image of the impedance distribution inside a biological tissue by injecting a small current into the tissue and measuring the surface voltage distribution.

[0070] Input Common Mode Range (ICMR): The range of input signal common mode voltages within which the amplifier can operate normally.

[0071] Total Harmonic Distortion (THD): The ratio of harmonic components to fundamental components in a signal, used to measure the degree of signal distortion.

[0072] Closed-loop negative feedback: a control technique that feeds a portion of the output signal back to the input, creating a negative difference with the input signal to improve system stability and accuracy.

[0073] It should be noted that, in the context of this application, "biological tissue" specifically refers to human tissue objects measured and imaged by an electrical impedance tomography (EIT) system. Exemplary examples of such biological tissues include:

[0074] Lung tissue: The current-driven circuit of this application is used for lung function monitoring. By injecting high-frequency alternating current into lung tissue and measuring impedance changes, real-time monitoring of lung physiological functions can be achieved.

[0075] Brain tissue: This application is also applicable to medical brain imaging, using electrical impedance tomography (EIA) technology to perform non-invasive measurement and monitoring of brain tissue.

[0076] Human tissues in other bioimpedance measurement scenarios: This application can be applied to a variety of bioimpedance measurement scenarios and can be extended to other human tissues that require impedance measurement.

[0077] The current-driven circuit of this application enables imaging and analysis of the internal structure and functional state of biological tissues by injecting high-frequency alternating current in a specific frequency range (e.g., 50kHz-1MHz) into these biological tissues and measuring the impedance change after the current passes through the tissue.

[0078] The following is a brief summary of some of the innovative aspects of this application:

[0079] In summary, this application addresses the bandwidth and impedance trade-off in current-driven electrical impedance tomography (EIT) systems by employing a unique multi-stage asymmetric amplification architecture and interactive adjustment mechanism, presenting a non-obvious solution. The stacked combination of a fully differential rail-to-rail input transconductance amplifier (FDDTA) and a transconductance amplifier (OTA), along with a differential rail-to-rail input transconductance amplifier (DDTA), forms an unconventional closed-loop negative feedback architecture, breaking the inherent constraints between bandwidth and output impedance in traditional current-driven systems. In particular, by connecting the boost power supply VDDX output from the charge pump module to the power supply terminals of the folded cascode current sources of the FDDTA and DDTA, a stable operating environment is created for the dynamic switching of the feedback resistor array (e.g., Rf1-Rf6, with resistance values ​​from 200Ω to 1200Ω) (via control switches Sf1-Sf6) while expanding the input common-mode range. This allows for continuous or segmented dynamic adjustment of the transconductance without introducing conventional multi-stage amplification distortion, to adapt to different load conditions and signal strength requirements. For example, nonlinear dynamic adjustment of the transconductance in the range of 0.83mS to 4.92mS is achieved. This scheme, through the complex collaborative operation of the first and third folded-cascode amplification stages, the second active load stage, and the corresponding common-source cascode output stage, maintains an output impedance as high as 13.43MΩ at a frequency of 1MHz (compared to <1MΩ in traditional schemes), while controlling the total harmonic distortion to below 1%. It resolves the technical contradiction between current accuracy and bandwidth in high-frequency bioimpedance measurement, and provides a highly integrated, low-resource-consumption, and highly adaptable non-traditional current excitation scheme for EIT systems. This combination of technologies and the specific effects it achieves do not provide explicit or implicit teachings or inspirations in existing technologies.

[0080] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application will be described in further detail below with reference to the accompanying drawings.

[0081] In this application's specification, to make the description clearer and more concise, some technical features are represented using English letter codes. It should be clarified that the technical features represented solely by letter codes in this application have the same meaning as the corresponding technical features represented by their Chinese names plus letter codes. For example, "FDDTA" and "Fully Differential Rail-to-Rail Input Transconductance Amplifier FDDTA" refer to the same technical feature, and "OTA" and "Transconductance Amplifier OTA" refer to the same technical feature. Other similar technical features represented by English letter codes are also equivalent to their corresponding technical features represented by their Chinese names plus letter codes. When reading and understanding this application, please treat the technical features represented solely by letter codes as equivalent to their corresponding technical features represented by their Chinese names plus letter codes. The technical features involving English letter codes include, but are not limited to:

[0082] Fully differential rail-to-rail input transconductance amplifier (FDDTA); transconductance amplifier (OTA);

[0083] Differential rail-to-rail input transconductance amplifier (DDTA);

[0084] Electrical impedance tomography (EIT);

[0085] Input common-mode range (ICMR);

[0086] Signal-to-noise ratio (SNR);

[0087] Total Harmonic Distortion (THD)

[0088] Programmable gain amplifier (PGA);

[0089] Power supply voltage VDD;

[0090] The increased power supply voltage VDDX;

[0091] Differential input terminals V1P / V1N;

[0092] Differential output terminals VO1 / VO2;

[0093] Single-ended output terminal VO;

[0094] Input V2P / V2N;

[0095] Feedback resistors Rf1 to Rf6;

[0096] Control switches Sf1 to Sf6;

[0097] Source current Isource;

[0098] Sink current;

[0099] Compensation resistor Rm;

[0100] Compensation capacitor Cm;

[0101] Common-mode voltage Vcm;

[0102] Flying capacitors Cfly and Cflyn;

[0103] Output capacitor Cout;

[0104] Clock signals CLK and CLKn;

[0105] Clock input CLKin.

[0106] In this specification, to maintain brevity, the same technical feature is presented in the form of "Chinese name + English code" when it first appears, and may be presented in the form of only its English code when it appears thereafter. These different forms of expression refer to the same technical feature.

[0107] The first embodiment of this application relates to a high output impedance current drive circuit with transconductance adjustable function, such as Figures 1 to 5 As shown, it includes:

[0108] As a pre-amplification module, the fully differential rail-to-rail input transconductance amplifier FDDTA has a first differential input terminal V1P / V1N / V2P / V2N and a first differential output terminal VO1 / VO2.

[0109] As a transconductance amplifier OTA in a voltage-to-current conversion module, the differential input terminal of the transconductance amplifier OTA is connected to the first differential output terminal VO1 / VO2 of the fully differential differential rail-to-rail input transconductance amplifier FDDTA, and the transconductance amplifier OTA has a single-ended output terminal VO;

[0110] The differential rail-to-rail input transconductance amplifier (DDTA) serves as a current sink. Its differential input terminal V1P is connected to the second differential input terminal V2P of the fully differential rail-to-rail input transconductance amplifier (FDDTA), and both are connected to the output of the feedback resistor array. The differential input terminals V1N and V2P of the differential rail-to-rail input transconductance amplifier are connected to the common-mode input level Vcm of the system. The differential rail-to-rail input transconductance amplifier (DDTA) has a single-ended output terminal VO.

[0111] A charge pump module, wherein the input terminal of the charge pump module is connected to the input common-mode level, and the output terminal VDDX is connected to the fully differential rail-to-rail input transconductance amplifier FDDTA and the differential rail-to-rail input transconductance amplifier DDTA; and

[0112] A feedback resistor array, comprising multiple parallel switchable resistors Rf1-Rf6 and multiple control switches Sf1-Sf6, is connected in parallel to the single-ended output terminal VO of the transconductance amplifier OTA. The resistance values ​​of the switchable resistors cover a predetermined range, enabling the current drive circuit to achieve multi-level adjustment of the transconductance. For example, the resistance value range of the switchable resistors is 200Ω to 1200Ω.

[0113] In this embodiment, the current-driven circuit adopts a closed-loop negative feedback architecture, combining the three core modules of FDDTA, OTA, and DDTA with charge pump power boosting technology and an adjustable feedback resistor array to achieve high output impedance and adjustable transconductance characteristics. This structure can overcome the trade-off between bandwidth and output impedance in traditional current drives, and is particularly suitable for high-precision current injection in electrical impedance tomography systems.

[0114] Optionally, the first differential output signal VO1 / VO2 output by the fully differential rail-to-rail input transconductance amplifier FDDTA is converted into a single-ended output source current Isource by the transconductance amplifier OTA and the feedback resistor array;

[0115] The differential rail-to-rail input transconductance amplifier (DDTA) is connected in a unity-gain negative feedback configuration based on the second differential input signal V2P and a sink current Isink generated by the output voltage of the feedback resistor array, which matches the source current Isource.

[0116] The charge pump module boosts the tail current source voltage VDD of the folded cascode transistor to a boosted voltage VDDX, which is then supplied to the fully differential rail-to-rail input transconductance amplifier FDDTA and the differential rail-to-rail input transconductance amplifier DDTA to extend their input common-mode range and ensure the linearity of the circuit during transconductance adjustment; and

[0117] The feedback resistor array achieves dynamic transconductance adjustment within the range of 0.83mS to 4.92mS by switching the control switches Sf1-Sf6 to select resistors Rf1-Rf6 with different resistance values.

[0118] More specifically, the circuit forms a complete signal path during operation: the input differential signal is amplified by the FDDTA, and the OTA converts the voltage signal into a current output. This current output is then fed back to the FDDTA input terminals V2P / V2N via a feedback resistor array, forming a negative feedback loop. This means the current-driven transconductance is primarily determined by the resistance value of the feedback resistor array. Simultaneously, the DDTA receives the same input signal as the FDDTA, generating a matched sink current, forming a current-driven source-sink structure. This design allows the current to flow precisely through the load without significant changes in load impedance and ensures a stable common-mode level across the load, making it particularly suitable for the complex load conditions present in bioimpedance measurements.

[0119] Optionally, the fully differential rail-to-rail input transconductance amplifier (FDDTA) serves as a pre-amplification module, providing loop gain and output impedance extension.

[0120] The transconductance amplifier OTA serves as a voltage-to-current conversion module, converting a two-terminal voltage into a single-terminal current.

[0121] The differential rail-to-rail input transconductance amplifier (DDTA) serves as a current sink module to achieve unity-gain negative feedback; and

[0122] The charge pump module is used to provide a boost power supply to ensure linearity during transconductance adjustment.

[0123] More specifically, these four modules have clearly defined functions, each with its own responsibilities while working in concert. The FDDTA provides initial gain, output impedance, and bandwidth extension, laying the foundation for subsequent current conversion; the OTA performs the core function of voltage-to-current conversion, combining output impedance and bandwidth extension with the ability to drive loads, and, in conjunction with a feedback resistor, forms a loop to control the current-driven transconductance value; the DDTA, as a current sink, provides a matched return path, forming a complete current loop; and the charge pump module provides a stable power supply after boosting the voltage for the front-end modules, ensuring good linearity during transconductance regulation and reducing harmonic distortion. This modular design allows each part of the circuit to be optimized for specific functions, improving overall performance.

[0124] Optionally, the fully differential rail-to-rail input transconductance amplifier (FDDTA) includes:

[0125] The first input stage is used to receive the first differential input signal V1P / V1N;

[0126] The first folded-cascode output stage is connected to the output of the first input stage and is used to output the first differential output signal VO1 / VO2.

[0127] The first common-mode feedback output stage is connected to the output of the first input stage and is used to stabilize the common-mode level of the first differential output signals VO1 / VO2.

[0128] More specifically, such as Figure 2 As shown, the FDDTA employs a single-stage amplification structure. The first input stage receives the differential input signals V1P / V1N using differential pair transistors (such as Mp1 / Mp2) and converts them into current signals. The first folded-cascode output stage (such as Mp5 / Mp6 / Mp11 / Mp12 and Mn1 / Mn2 / Mn3 / Mn4 / Mn5 / Mn6 / Mn7 / Mn8, etc.) achieves high gain and high output impedance. The first common-mode feedback output stage (such as Mp17 / Mp18 / Mp20 / Mp21 / Mp19 / Mp22 and Mn9 / Mn10, etc.) determines the common-mode level at the fully differential output signals VO1 / VO2, ensuring that the output common-mode point does not change with variations in the input common-mode. This common-mode feedback circuit generates a capacitive load rather than a resistive load for the FDDTA amplifier, and with proper design, it will not significantly affect the frequency characteristics. Therefore, this design can avoid gain attenuation within the bandwidth, which in turn affects the system output impedance and bandwidth.

[0129] This cascaded amplifier structure ensures high gain and high impedance characteristics over a wide bandwidth, providing stable pre-amplification for current drive.

[0130] Optionally, the transconductance amplifier OTA includes:

[0131] The second input stage is used to receive the first differential output signals VO1 / VO2;

[0132] The second active load stage, connected to the second input stage, is used to amplify the input signal and improve its gain and bandwidth.

[0133] The second common-source cascode output stage is connected to the output of the second active load stage and is used to output the source current Isource.

[0134] More specifically, such as Figure 3 As shown, the function of the OTA is to convert voltage into current and enable it to drive a load. The second input stage converts the differential signals VO1 / VO2 from the FDDTA into current.

[0135] The second active load stage (such as Mn1 / Mn3 / Mn4 / Mn5, etc.) employs a negative resistance structure, providing high gain and impedance boost, while the second cascode output stage (such as Mn6 / Mn8 / Mn2 / Mn7 and Mp6 / Mp8 / Mp5 / Mp7, etc.) provides the required current output capability. In particular, in the output stage design, the cascode structure can achieve wide bandwidth characteristics while maintaining high output impedance, which is especially important for high-frequency current drives.

[0136] Optionally, the differential rail-to-rail input transconductance amplifier (DDTA) includes:

[0137] The third input stage is used to receive the second differential input signal V2P / V2N;

[0138] The third folded-cascode output stage is connected to the output of the third input stage and is used to output the sink current Isink.

[0139] More specifically, such as Figure 4 As shown, the DDTA is similar in structure to the FDDTA, but it adds a second-stage amplification output stage and a resistor-capacitor Miller compensation circuit, which consists of a resistor Rm and a capacitor Cm. The two-stage amplification design effectively improves the open-loop gain of the DDTA, while the resistor-capacitor Miller compensation circuit effectively improves the phase margin of the DDTA, enhancing its stability as a current sink. The sink current Isink generated by the third-stage cascode output stage matches the source current Isource of the OTA output, forming a complete current path. This method achieves common-mode feedback of the load by connecting the DDTA as a unity-gain inverter, avoiding the problem of current source and sink mismatch.

[0140] Optionally, the charge pump module includes:

[0141] A multi-stage cascaded charge pump unit, each charge pump unit including PMOS transistors Mp2 and Mp1 and NMOS transistors Mn2 and Mn1, forming a cross-coupled charge pump unit;

[0142] A non-overlapping clock circuit provides complementary and non-overlapping clock signals CLK and inverted clock CLKn to the charge pump unit;

[0143] The input terminal is connected to the common-mode input level; and

[0144] At the output terminal, the boosted power supply voltage VDDX is output to the fully differential rail-to-rail input transconductance amplifier FDDTA and the differential rail-to-rail input transconductance amplifier DDTA.

[0145] More specifically, such as Figure 5 As shown, the charge pump module employs a multi-stage charge transfer structure. Each stage includes PMOS transistors Mp2 and Mp1 and NMOS transistors Mn2 and Mn1, controlled by complementary and non-overlapping clock signals CLK and CLKn. Flying capacitors Cfly and Cflyn are used for charge transfer, while the output capacitor Cout stabilizes the output voltage and reduces its ripple. This design can boost the original supply voltage VDD to a higher VDDX, providing a wider input common-mode range for FDDTA and DDTA, thus maintaining good linearity during transconductance regulation and avoiding distortion caused by the input differential pair operating in the nonlinear or deviating-from-linear region.

[0146] Optionally, when the feedback resistor is greater than a preset threshold, the output impedance of the current drive circuit can reach at least 5MΩ at a frequency of 1MHz, preferably 10MΩ or higher. For example, when the feedback resistor is greater than 600Ω, the output impedance of the current drive circuit is greater than 10MΩ at a frequency of 1MHz.

[0147] The total harmonic distortion (THD) of the current drive circuit is less than 1%.

[0148] The feedback resistor array achieves dynamic transconductance adjustment within the range of 0.83mS to 4.92mS through different resistor combinations;

[0149] The current drive circuit is suitable for electrical impedance tomography (EIT) systems and is used to inject high-frequency differential sinusoidal alternating current into biological tissues. The high-frequency range includes, but is not limited to, tens of kilohertz to several megahertz, for example, high-frequency alternating current of 50 kHz to 1 MHz.

[0150] More specifically, such as Figures 6 to 9 As shown, this current drive circuit has excellent performance indicators. Figure 7 The results show that when the feedback resistor is greater than 600Ω, the output impedance can reach more than 10.5MΩ at a frequency of 1MHz, which is far superior to the performance of traditional solutions that are usually less than 1MΩ. Figure 9 The harmonic distortion analysis of the output peak-to-peak current of 1 mA with a feedback resistor of 600Ω is presented. The average total harmonic distortion (THD) is only 0.348%, with a standard deviation of 0.139%, far below the design target of 1%. These parameters make this circuit particularly suitable for electrical impedance tomography (EIT) systems, maintaining high-precision current injection over a wide bandwidth from 50kHz to 1MHz, and especially effective for bioimpedance measurement applications such as lung function monitoring and brain imaging. High output impedance ensures insensitivity of the output current to changes in load impedance, while low harmonic distortion guarantees high signal fidelity; both together improve image quality and measurement accuracy.

[0151] Optionally, the six switchable resistors Rf1-Rf6 in the feedback resistor array are arranged in ascending order of resistance value to form a resistor sequence that covers the required transconductance adjustment range, for example, 200Ω, 400Ω, 600Ω, 800Ω, 1000Ω and 1200Ω respectively.

[0152] The control switches Sf1-Sf6 control the connection and disconnection of the corresponding resistors respectively, and different equivalent feedback resistance values ​​are achieved by different switch combinations.

[0153] More specifically, such as Figure 1 As shown, the feedback resistor array includes six resistors Rf1-Rf6 with precise values ​​and corresponding control switches Sf1-Sf6. By controlling different combinations of switch states, various equivalent feedback resistance values ​​can be achieved, thus flexibly adjusting the transconductance of the circuit. For example, when only Sf1 is turned on, the feedback resistance is 200Ω, corresponding to a maximum transconductance of approximately 4.92mS; when only Sf6 is turned on, the feedback resistance is 1200Ω, corresponding to a minimum transconductance of approximately 0.83mS; when multiple switches are turned on, the equivalent value of the parallel resistance is further reduced, allowing for more transconductance levels. This design provides bioimpedance measurement with the ability to adapt to different measurement objects and environments. Especially when processing far-field signals, the signal-to-noise ratio can be improved by increasing the transconductance, while when processing near-field signals, signal saturation can be avoided by reducing the transconductance, greatly enhancing the adaptability and practicality of the circuit.

[0154] To make the technical solution of the present invention clearer and more understandable, it is now combined with Figures 1 to 9 Preferred embodiments of the present invention will be described in detail, but it should be understood that the described embodiments are merely exemplary and not restrictive.

[0155] Figure 1 This is a schematic diagram of a high output impedance current drive circuit with transconductance adjustable function according to an embodiment of this application. The circuit system includes a fully differential rail-to-rail input transconductance amplifier (FDDTA), a transconductance amplifier (OTA), a differential rail-to-rail input transconductance amplifier (DDTA), a charge pump module, and a feedback resistor array.

[0156] As shown in the figure, the FDDTA is located within the "Current Source" module, with differential input terminals V1P / V1N / V2P / V2N and differential output terminals VO1 / VO2. The OTA is cascaded with the FDDTA, its differential input terminal connected to the differential output terminals VO1 / VO2 of the FDDTA, and the OTA has a single-ended output terminal VO. The DDTA is located within the "Current Sink" module, acting as a unity-gain inverting buffer. Its differential input terminal V1P is connected to the V2P terminal of the FDDTA, and both are connected to the voltage signal output through the resistor array. The DDTA outputs a single-ended signal VO. The charge pump module is located on the left, with its input terminal connected to the common-mode level, and its output terminal VDDX connected to the tail current source power supply voltage terminal of the folded cascode output stage of the FDDTA and DDTA, providing the boosted power supply voltage. The feedback resistor array is located in the upper right "Resistor Array" section and includes six switchable resistors Rf1 (200Ω), Rf2 (400Ω), Rf3 (600Ω), Rf4 (800Ω), Rf5 (1000Ω) and Rf6 (1200Ω), as well as corresponding control switches Sf1-Sf6. This resistor array is connected in parallel to the single-ended output VO of the OTA.

[0157] The load on the right side of the circuit receives the source current Isource and the sink current Isink, which are connected to Va and Vb terminals, respectively. The entire circuit adopts a closed-loop negative feedback architecture, and by controlling different combinations of switches Sf1-Sf6, it achieves dynamic transconductance adjustment within the range of 0.83ms to 4.92ms.

[0158] Figure 2 This is a detailed circuit diagram of the fully differential rail-to-rail input transconductance amplifier (FDDTA) in a current-driven circuit according to an embodiment of this application. The FDDTA acts as a pre-amplification module, expanding the loop gain and output impedance within the current source loop.

[0159] The circuit includes: a first input stage, consisting of differential pair transistors Mp1 / Mp2 receiving differential input signals V1P / V1N, differential pair transistors Mp7 / Mp8 receiving voltage signals fed back from both ends of the feedback resistor array, and Mp3 / Mp4 / Mp9 / Mp10; a first folded-cascode output stage, consisting of transistors Mp5 / Mp6 / Mp11 / Mp12 and Mn1 / Mn2 / Mn3 / Mn4 / Mn5 / Mn6 / Mn7 / Mn8, achieving high gain and high output impedance; and a first common-mode feedback output stage, consisting of transistors Mp17 / Mp18 / Mp20 / Mp21 / Mp19 / Mp22 and Mn9 / Mn10, which determines the common-mode level at the fully differential output signals VO1 / VO2, ensuring that the output common-mode point does not change with the input common-mode. The entire circuit operates under the power supply voltages VDD, VDDX, and VSS, and is controlled by the bias current Ibias and the bias voltages VB1, VB2, and VB3.

[0160] Figure 3 This is a detailed circuit diagram of the transconductance amplifier (OTA) portion in the current drive according to an embodiment of this application. The OTA, as a voltage-to-current conversion module, converts the pre-amplified two-terminal voltage into a single-terminal current output.

[0161] The circuit includes: a second input stage, consisting of differential pair transistors Mp1 / Mp2 receiving differential input signals VINP / VINN (corresponding to VO1 / VO2 in FDDTA); a second active load stage, employing a negative resistance structure, composed of transistors such as Mn1 / Mn3 / Mn4 / Mn5, connected to the second input stage to amplify the input signal, achieving high gain and improved impedance and bandwidth; and a second cascode output stage, composed of transistors such as Mn6 / Mn8 / Mn2 / Mn7 and Mp6 / Mp8 / Mp5 / Mp7, outputting the source current Isource to the single-ended output Vout. The circuit also includes a bias circuit controlled by the bias current Ibias and bias voltages VB1 and VB2. The OTA not only implements voltage-to-current conversion but also further expands the loop gain and output impedance, and provides the ability to drive loads.

[0162] Figure 4 This is a detailed circuit diagram of the differential rail-to-rail input transconductance amplifier (DDTA) portion in the current drive according to an embodiment of this application. The DDTA acts as a buffer for the inverting gain in the current drive, achieving unity-gain negative feedback.

[0163] The circuit includes: a third input stage, in which Mp1 of the differential pair transistor receives V1P (corresponding to V2P of FDDTA) from the differential input signal; Mp2 and Mp8 of the differential pair transistor are connected together and connected to the common-mode input level of the system; and Mp7 of the differential pair transistor is connected to the third output stage and receives the output signal VO.

[0164] The third folded-cascode output stage is composed of transistors such as Mn1 / Mn2 / Mn3 / Mn4 and Mp5 / Mp6, and Mn5 / Mn6 / Mn7 / Mn8 and Mp11 / Mp12.

[0165] The third output stage, composed of transistors such as Mp17 and Mn9, outputs a sink current Isink to the single-ended output Vout. The output stage also includes a compensation resistor Rm and a compensation capacitor Cm to ensure circuit stability. The DDTA generates a sink current that matches the source current, serving as the current sink. This method achieves common-mode feedback of the load by connecting the DDTA as a unity-gain inverter, avoiding the problem of current source and sink mismatch.

[0166] Figure 5 This is a detailed circuit diagram of the charge pump section (a) and the non-overlapping clock section (b) in the current drive according to an embodiment of this application. The main function of the charge pump module is to boost the power supply voltage, providing a higher voltage for the FDDTA and DDTA circuits to achieve rail-to-rail input range.

[0167] The charge pump circuit (a) comprises a multi-stage cascaded charge pump unit, each consisting of PMOS transistors (Mp1 / Mp3 / Mp5 / Mp2 / Mp4 / Mp6) and NMOS transistors (Mn1 / Mn3 / Mn5 / Mn2 / Mn4 / Mn6), controlled by complementary clock signals CLK and CLKn. Each unit includes flying capacitors Cfly and Cflyn for charge transfer. An output capacitor Cout is provided at the output terminal to stabilize the output voltage and reduce output voltage ripple. The charge pump boosts the input voltage Vin (Vcm) to the output voltage Vout (VDDX).

[0168] The non-overlapping clock generation circuit (b) provides complementary clock signals CLK and CLKn to control the charge pump, ensuring no overlap between them to avoid short-circuit current. This circuit receives the clock input CLKin and generates the required clock signal through logic gates and buffers.

[0169] Figure 6The figure shows the simulation results of adjustable transconductance over a wide bandwidth according to an embodiment of this application. The figure shows the curves of transconductance versus frequency for different feedback resistance values ​​(R = 200Ω, 400Ω, 600Ω, 800Ω, 1000Ω, 1200Ω).

[0170] As shown in the figure, in the low-frequency region (up to approximately 10^5 Hz), the transconductance remains stable for each resistance value, forming a series of horizontal lines. As the frequency continues to increase, the transconductance begins to decrease. Different resistance values ​​correspond to different transconductance values, ranging from approximately 4.92 ms for R = 200 Ω to approximately 0.83 ms for R = 1200 Ω, achieving a wide range of transconductance adjustment. This result demonstrates that the circuit can provide stable and adjustable transconductance characteristics within the required 1 MHz bandwidth, meeting the needs of various application scenarios.

[0171] Figure 7 The figure shows the simulation results of the output impedance changing with frequency under different feedback resistors according to the embodiments of this application. The figure shows the curves of output impedance changing with frequency under different feedback resistor values ​​(R = 200Ω, 400Ω, 600Ω, 800Ω, 1000Ω, 1200Ω).

[0172] As shown in the figure, the output impedance is high in all configurations in the low-frequency region, gradually decreasing with increasing frequency. Even with a feedback resistor greater than 600Ω, the output impedance remains above 10MΩ at 1MHz. Two typical values ​​are specifically highlighted in the figure: 13.43MΩ with a higher feedback resistor and 5.59MΩ with a lower feedback resistor. These results demonstrate that this circuit successfully solves the problem of decreased output impedance in traditional current drives at high frequencies, ensuring the accuracy of the current drive.

[0173] Figure 8 This is a simulation result graph of a current-driven output of a 1 mA peak-to-peak sinusoidal differential current according to an embodiment of this application. The upper part of the graph shows the input voltage changing with time, and the lower part shows the output current changing with time.

[0174] The figure shows the time-varying curves of the source current Isource (green line) and the sink current Isink (blue line). Both form symmetrical sinusoidal waves with a peak-to-peak value of 1 mA, indicating that the circuit can stably output a high-precision differential current signal. There is a brief startup process at approximately 0.1 × 10⁻⁵ seconds, after which the circuit enters a stable operating state. Simulation results verify the dynamic performance and linearity of this current-driven circuit, with total harmonic distortion controlled within 1%, making it suitable for applications requiring high signal quality, such as bioimpedance measurement.

[0175] Figure 9 The Monte Carlo simulation results of the total harmonic distortion (THD) of the current-driven circuit according to an embodiment of this application, outputting a sinusoidal differential current with a peak-to-peak value of 1 mA at a feedback resistance of 600Ω, are shown in the figure. The figure illustrates the THD distribution obtained from multiple simulations, including histograms and kernel density estimation curves. The results show that when the feedback resistance is 600Ω, the average third-order total harmonic distortion (THD) of the output current is only 0.348%, with a standard deviation of 0.139% and a sample size of 50. This further verifies that the current-driven circuit exhibits extremely low harmonic distortion characteristics under actual operating conditions, far below the design specification of 1%, fully meeting the stringent signal quality requirements of electrical impedance tomography (EIT) systems.

[0176] Working principle:

[0177] The above embodiments operate based on a closed-loop negative feedback architecture. For example... Figure 1 As shown, the circuit includes core components such as a fully differential rail-to-rail input transconductance amplifier (FDDTA), a transconductance amplifier (OTA), a differential rail-to-rail input transconductance amplifier (DDTA), a charge pump module, and a feedback resistor array.

[0178] During operation, the FDDTA receives differential input voltage signals V1P / V1N and generates differential output signals VO1 / VO2 through its internal first input stage and first folded-cascod output stage, which are then transmitted to the OTA. As a pre-amplifier module, the FDDTA provides loop gain and output impedance extension. It employs a differential adder circuit as the common-mode feedback circuit to avoid the additional area consumption caused by resistors and capacitors. Compared to common-mode feedback circuits, such as those using linear transistors, this method generates a capacitive load instead of a resistive load, thus not affecting the DC gain. With proper design, it will not significantly impact frequency characteristics. Therefore, this design avoids gain attenuation within the bandwidth, thereby not affecting the system's output impedance and bandwidth. The detailed circuit is shown below. Figure 2 As shown.

[0179] The OTA receives the differential output signals VO1 / VO2 from the FDDTA (corresponding to VINP / VINN of the OTA) and converts the two-terminal voltage into a single-terminal current output Isource through its internal second input stage, second active load stage, and second cascode output stage. The OTA's output terminal VO implements a negative feedback loop by connecting a feedback resistor array; its detailed circuit is shown below. Figure 3 As shown.

[0180] In the circuit, the DDTA acts as a current sink. Based on the voltage signal fed back to the input V2P of the FDDTA by the feedback resistor array, a sink current Isink is generated that matches Isource, achieving unity-gain negative feedback and thus ensuring the stability of the common-mode level on the load. The detailed circuit of the DDTA is shown below. Figure 4 As shown, a compensation network (Rm, Cm) is included to ensure stability.

[0181] Charge pump module ( Figure 5 The system's common-mode voltage Vcm is boosted to VDDX, providing a higher supply voltage for FDDTA and DDTA. This expands the folded input-to-tail current supply voltage, thereby extending the input common-mode range (ICMR) and ensuring the circuit's linearity during transconductance regulation. The charge pump consists of multiple cascaded charge pump units, controlled by complementary clock signals CLK and CLKn.

[0182] The feedback resistor array includes six switchable resistors Rf1-Rf6 (e.g., resistance values ​​from 200Ω to 1200Ω) and corresponding control switches Sf1-Sf6. By using different combinations of control switches, resistors of varying resistance values ​​can be connected in parallel to the OTA output terminal VO, achieving dynamic transconductance adjustment within the range of 0.83mS to 4.92mS. Figure 6 As shown.

[0183] This circuit can still maintain a high output impedance of over 10MΩ at a frequency of 1MHz. Figure 7 While maintaining a total harmonic distortion of less than 1%, it can stably output a sinusoidal differential current with a peak-to-peak value of 1 mA. Figure 8 This is suitable for injecting high-frequency differential sinusoidal alternating current into biological tissues in electrical impedance tomography (EIT) systems, wherein the high-frequency range includes, but is not limited to, tens of kilohertz to several megahertz, for example, high-frequency alternating current of 50 kHz to 1 MHz.

[0184] To better understand the technical solution of this application, a specific example is provided below. The details listed in this example are mainly for ease of understanding and are not intended to limit the scope of protection of this application.

[0185] In this example, a transconductance-adjustable current drive based on a closed-loop negative feedback architecture is proposed. By using negative resistance load, impedance boosting output stage, common-mode feedback circuit and charge pump power boosting technology, extremely high output impedance, low harmonic distortion and dynamic transconductance adjustment are achieved over a wide bandwidth, breaking through the traditional trade-off between bandwidth and impedance in current drive.

[0186] Furthermore, this example addresses the trade-off between bandwidth, output impedance, and linearity inherent in traditional current-driven methods, making it applicable to various bioimpedance measurement scenarios and reducing system resource consumption. Specifically, it solves the following technical problems: limited dynamic range due to non-adjustable transconductance; simultaneous achievement of high output impedance and bandwidth: maintaining high output impedance (>10MΩ) and low distortion (THD<1%) over a wide bandwidth (1MHz); achieving dynamic transconductance adjustment (0.83-4.92ms) to meet the SNR optimization requirements of far-field signals in EIT systems, avoiding reliance on gain adjustment in the acquisition circuit; and extending the input common-mode range (ICMR) by boosting the power supply voltage with a charge pump, thus resolving the linearity degradation problem under wide transconductance adjustment.

[0187] More specifically, this example solves the problem of decreased output impedance and linearity of traditional current excitation sources under high bandwidth through innovative circuit design, while realizing dynamic adjustment of transconductance, thereby improving the imaging quality and adaptability of the EIT system.

[0188] See Figure 1 This current-driven system includes a fully differential rail-to-rail input transconductance amplifier (FDDTA), a transconductance amplifier (OTA), a differential rail-to-rail input transconductance amplifier (DDTA), a charge pump, and related circuitry. The current excitation source circuit employs a closed-loop negative feedback architecture to achieve high linearity and accuracy in transconductance. Compared to traditional current excitation sources, this circuit incorporates a charge pump boost circuit combined with a resistor array, enabling flexible and adjustable transconductance and output current amplitude. It can also be applied to a wide range of impedance measurement scenarios.

[0189] Specifically, the internal circuit structure and function of each module are as follows:

[0190] Fully differential rail-to-rail input transconductance amplifier (FDDTA): Its main function is as a pre-amplification module in the current source loop to extend the loop gain and output impedance. See details. Figure 2 .

[0191] Transconductance amplifier (OTA): Its main function is to convert the pre-amplified voltage to current in a current source loop, transforming a two-ended input into a single-ended output. It also expands the loop gain and output impedance, and enables it to drive loads. See details. Figure 3 .

[0192] Differential rail-to-rail input transconductance amplifier (DDTA): In current-driven applications, it acts as a buffer for inverting gain, achieving unity-gain negative feedback and generating a sink current that matches the source current; that is, it functions as a current sink. See details. Figure 4 .

[0193] Charge pump module: Its main function is to provide voltage boosting in current-driven circuits, supplying the tail current source voltage of the P-type MOSFET to achieve rail-to-rail input for FDDTA and DDTA circuits, ensuring the linearity of current drive during transconductance regulation. See details. Figure 5 .

[0194] Feedback resistor array: Adapts to transconductance adjustment requirements for different impedance models and application scenarios. The resistance values ​​of the switchable resistors cover a predetermined range, enabling the current drive circuit to achieve multi-level transconductance adjustment, for example, the resistance value can be varied from 200Ω to 1200Ω. The feedback resistor array includes multiple switchable resistors Rf1-Rf6 connected in parallel and multiple control switches Sf1-Sf6, connected in parallel to the single-ended output terminal VO of the OTA.

[0195] It should be noted that the key output of this example is:

[0196] Adjustable and high-precision transconductance, adjustable over a wide bandwidth: see [link to relevant documentation] Figure 6 By selecting feedback resistors with different resistance values, the transconductance can be continuously or dynamically adjusted in segments to adapt to different load conditions and signal strength requirements. For example, the transconductance can be dynamically adjusted from 0.83mS to 4.92mS.

[0197] Output impedance: See Figure 7 This result satisfies the requirement of an output impedance greater than 1MΩ at the maximum bandwidth frequency. When the feedback resistor is greater than 600Ω, an output impedance of over 10MΩ can be achieved, with typical values ​​of 13.43MΩ and 5.59MΩ shown in the figure.

[0198] Output current: See Figure 8 The waveform of the current-driven output 1 mA peak-to-peak sinusoidal differential current is shown, verifying the dynamic performance and linearity of the circuit.

[0199] It should be noted that this example has the following advantages: it achieves high output impedance and low distortion over a wide bandwidth, supports dynamic adjustment of transconductance, and improves the imaging quality of the EIT system; and it reduces system complexity and supports multiple applications of bioimpedance measurement.

[0200] In summary, the technical effects of this example include at least the following:

[0201] Solving the bandwidth-impedance contradiction: By using negative resistance and common source cascode structure, the impedance can reach 10MΩ at 1MHz (traditional schemes <1MΩ), meeting the requirements of high-frequency bioimpedance measurement.

[0202] Improve dynamic range: Transconductance adjustment avoids saturation of the acquisition circuit.

[0203] Reduce system complexity or resource consumption: Integrated charge pump regulates transconductance, eliminating the need for external control to regulate current.

[0204] Current drive achieves: high output impedance: up to 10MΩ or more at 1MHz, ensuring the accuracy of the output current; low distortion: total harmonic distortion is less than 1%, improving the clarity of the image; dynamic adjustment: the transconductance can be flexibly adjusted within 0.83mS-4.92mS to adapt to different measurement needs; and low power consumption: through dynamic current adjustment, the consumption of system resources is reduced.

[0205] Through the synergistic effect of these technical features, this example provides a high-performance, highly adaptable current-driven solution for EIT systems, which can significantly improve the quality and efficiency of electrical impedance tomography.

[0206] The above embodiments have the following technical effects:

[0207] First, the bandwidth-impedance trade-off was resolved. Through impedance boosting techniques in the amplifier, such as negative resistance and cascode structures, a high output impedance exceeding 10MΩ was achieved at 1MHz, while traditional solutions typically fall below 1MΩ at the same frequency. Figure 7 As shown, when the feedback resistor is greater than 600Ω, the output impedance can reach more than 10.5MΩ at 1MHz. Even with a smaller feedback resistor, it can reach 5.59MΩ, which meets the stringent requirements for high-frequency bioimpedance measurement.

[0208] Secondly, it achieves dynamic adjustment of transconductance. For example... Figure 6 As shown, by using different combinations of feedback resistor arrays (Rf1-Rf6) and control switches (Sf1-Sf6), the transconductance can be flexibly adjusted within a range of, for example, 0.83 ms to 4.92 ms, effectively improving the dynamic range of the circuit and avoiding the problem of saturation in the acquisition circuit. This adjustment meets the signal-to-noise ratio (SNR) optimization requirements of far-field signals in electrical impedance tomography (EIT) systems.

[0209] Third, it reduces system complexity and resource consumption. This solution integrates a charge pump module to regulate transconductance, and by increasing the power supply voltage VDDX, it expands the input common-mode range (ICMR) of FDDTA and DDTA, eliminating the need for external control to regulate the current and reducing the system's hardware resource requirements, such as high-precision programmable gain amplifiers (PGA) and demodulation modules.

[0210] Fourth, it maintains low distortion characteristics. For example... Figure 8 As shown, even with dynamic transconductance adjustment, the source current Isource and sink current Isink of the circuit output remain highly linear, as... Figure 9As shown, when the feedback resistor is 600Ω, the third total harmonic distortion (THD) of the 1 mA peak-to-peak output current can be controlled to below 1% under the premise of having an output impedance greater than 10MΩ, which improves the clarity and measurement accuracy of impedance imaging.

[0211] Finally, through a fully differential architecture and negative feedback design, the circuit exhibits excellent common-mode interference immunity and stability, making it suitable for various complex bioimpedance measurement environments. The overall solution overcomes the limitations of traditional current-driven circuits in balancing bandwidth, output impedance, and linearity, providing a high-performance current-driven solution for electrical impedance tomography (EIT) systems.

[0212] It should be noted that in this patent application, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. In this patent application, if it refers to performing an action according to an element, it means performing the action at least according to that element, including two cases: performing the action only according to that element, and performing the action according to that element and other elements. Expressions such as "multiple," "repeatedly," and "various" include two, two times, two kinds, and more than two, more than two times, and more than two kinds.

[0213] All documents mentioned in this application are considered to be incorporated in their entirety into the disclosure of this application so that they can serve as a basis for modifications if necessary. Furthermore, it should be understood that after reading the foregoing disclosure of this application, those skilled in the art can make various alterations or modifications to this application, and these equivalent forms also fall within the scope of protection claimed in this application.

Claims

1. A high output impedance current drive circuit with adjustable transconductance, characterized in that, include: As a pre-amplification module, the fully differential rail-to-rail input transconductance amplifier (FDDTA) has a first differential input terminal (V1P / V1N / V2P / V2N) and a first differential output terminal (VO1 / VO2). As a transconductance amplifier (OTA) in a voltage-to-current conversion module, the differential input terminal of the transconductance amplifier is connected to the first differential output terminal of the fully differential differential rail-to-rail input transconductance amplifier, and the transconductance amplifier has a single-ended output terminal (VO). As a differential rail-to-rail input transconductance amplifier (DDTA) serving as a current sink, the differential input terminal V1P of the differential rail-to-rail input transconductance amplifier is connected to the second differential input terminal V2P of the fully differential rail-to-rail input transconductance amplifier, and both are connected to the output of the feedback resistor array. The differential rail-to-rail input transconductance amplifier has a single-ended output terminal (VO). The differential input terminals V1N and V2P of the differential rail-to-rail input transconductance amplifier are connected to the common-mode input level of the system. The differential input terminal V2N of the differential rail-to-rail input transconductance amplifier is connected to the single-ended output terminal (VO) to form a unity-gain negative feedback. A charge pump module, wherein the input terminal of the charge pump module is connected to the input common-mode level, and the output terminal (VDDX) is connected to the voltage of the folded common-source cascode tail current source in the fully differential rail-to-rail input transconductance amplifier and the differential rail-to-rail input transconductance amplifier; and The feedback resistor array includes multiple switchable resistors (Rf1-Rf6) and multiple control switches (Sf1-Sf6) connected in parallel. The feedback resistor array is connected in parallel to the single-ended output (VO) of the transconductance amplifier. The resistance values ​​of the switchable resistors cover a predetermined range, enabling the current drive circuit to achieve multi-level adjustment of the transconductance.

2. The current driving circuit according to claim 1, characterized in that: The first differential output signal from the fully differential rail-to-rail input transconductance amplifier is converted into a single-ended output source current (Isource) by the transconductance amplifier and the feedback resistor array. The differential rail-to-rail input transconductance amplifier is connected in a unity-gain negative feedback configuration. Based on the voltage signal flowing through the feedback resistor array, it is connected to the second differential input signal V2P to generate a sink current (Isink) that matches the source current (Isource). The charge pump module boosts the tail current source power supply voltage VDD of the folded cascode transistor to a boosted power supply voltage (VDDX), which is then supplied to the fully differential rail-to-rail input transconductance amplifier and the differential rail-to-rail input transconductance amplifier to extend their input common-mode range and ensure the linearity of the circuit during transconductance adjustment. as well as The feedback resistor array selects resistors (Rf1-Rf6) with different resistance values ​​by switching the control switches (Sf1-Sf6), thereby achieving continuous or segmented dynamic adjustment of the transconductance to adapt to different load conditions and signal strength requirements.

3. The current driving circuit according to claim 1, characterized in that: The fully differential rail-to-rail input transconductance amplifier serves as a pre-amplification module, providing loop gain and output impedance extension. The transconductance amplifier, as a voltage-to-current conversion module, converts a two-terminal voltage into a single-terminal current, while providing loop gain and output impedance extension, and has the ability to drive a load. The differential rail-to-rail input transconductance amplifier serves as a current sink module, achieving unity-gain negative feedback and ensuring stable common-mode levels across the load; and The charge pump module is used to provide a boost power supply to ensure linearity during transconductance adjustment.

4. The current driving circuit according to claim 1, characterized in that, The fully differential rail-to-rail input transconductance amplifier includes: The first input stage is used to receive the first differential input signal (V1P / V1N); The first folded-cascode output stage is connected to the output of the first input stage and is used to output the first differential output signal. The first common-mode feedback output stage is connected to the output of the first folded cascode output stage and is used to stabilize the common-mode level of the first differential output signal.

5. The current driving circuit according to claim 1, characterized in that, The transconductance amplifier includes: The second input stage is used to receive the first differential output signal; The second active load stage is connected to the second input stage; The second common-source, common-gate output stage is connected to the output of the second active load stage and is used to output the source current (Isource) to provide the ability to drive the load.

6. The current driving circuit according to claim 1, characterized in that, The differential rail-to-rail input transconductance amplifier includes: The third input stage is used to receive the voltage signal flowing through the resistor array connected to the second differential input signal (V2P); The third folded-cascode output stage is connected to the output of the third input stage and is used to amplify the input signal, thereby increasing the gain and bandwidth. The third output stage, connected to the third folded cascode output stage, outputs a sink current Isink to the single-ended output Vout. The output stage also includes a compensation resistor Rm and a compensation capacitor Cm to ensure circuit stability. The differential rail-to-rail input transconductance amplifier generates a sink current that matches the source current, serving as the current sink.

7. The current driving circuit according to claim 1, characterized in that, The charge pump module includes: A multi-stage cascaded charge pump unit, each charge pump unit including PMOS transistors Mp2 and Mp1 and NMOS transistors Mn2 and Mn1, constitutes a cross-coupled charge pump unit; A non-overlapping clock circuit provides complementary and non-overlapping clock signals (CLK and CLKn) to the charge pump unit. The input terminal is connected to the common-mode input level; and At the output terminal, the boosted power supply voltage (VDDX) is output to the fully differential rail-to-rail input transconductance amplifier and the differential rail-to-rail input transconductance amplifier.

8. The current driving circuit according to claim 1, characterized in that: When the feedback resistor is greater than a preset threshold, the output impedance of the current drive circuit can reach at least 5MΩ, preferably 10MΩ or higher at a frequency of 1MHz. The total harmonic distortion (THD) of the current drive circuit is less than 1%. The feedback resistor array achieves continuous or segmented dynamic adjustment of transconductance through different resistor combinations to adapt to different load conditions and signal strength requirements; The current-driven circuit is suitable for electrical impedance tomography (EIT) systems for injecting high-frequency differential sinusoidal alternating current into biological tissues, wherein the high-frequency range includes, but is not limited to, tens of kilohertz to several megahertz.

9. The current driving circuit according to claim 1, characterized in that: The six switchable resistors (Rf1-Rf6) in the feedback resistor array are arranged in ascending order of resistance value to form a resistor sequence that covers the required transconductance adjustment range. The control switches (Sf1-Sf6) control the connection and disconnection of the corresponding resistors respectively, and different equivalent feedback resistance values ​​are achieved through different switch combinations.

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