A current source
By employing an equivalent resistance unit and a BJT nonlinear current mirror in the current source, the accuracy problem of traditional current sources when PVT changes is solved, and the stability and accuracy of the current output are improved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- SG MICRO CORP
- Filing Date
- 2022-09-20
- Publication Date
- 2026-06-19
AI Technical Summary
The output current of a traditional Widlar current source is affected by the resistance value and the nonlinear current mirror of the MOSFET, making it difficult to maintain high accuracy when the PVT changes.
Equivalent resistance units are used to replace ordinary resistors, and BJT transistors are used as nonlinear current mirrors. Constant voltage difference is generated to control the source and drain voltage of cascaded MOSFETs through mirror connection, ensuring the accuracy of current output.
It reduces the negative impact of PVT changes on the current, improves the output accuracy of the current source, avoids unsolvable circuit states, and ensures the stability of the current over a wide temperature range.
Smart Images
Figure CN115562414B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of integrated circuits, and more specifically, to a current source. Background Technology
[0002] Currently, traditional Widlar current sources often use a nonlinear current mirror composed of a pure resistor and a MOSFET to obtain the output current. The current generated by this type of current source is affected by both the resistance value changing with the process angle and the gate-source voltage difference of the MOSFET's nonlinear current mirror changing with PVT (process angle, voltage, and temperature). When the PVT conditions change significantly during circuit operation, the current output by the current source is difficult to keep in line with the ideal current designed in advance, and this difference can be quite large. This makes it difficult for traditional current sources to achieve high-precision current output.
[0003] To address this problem, the present invention provides a novel current source. Summary of the Invention
[0004] To address the shortcomings of existing technologies, this invention provides a current source that replaces ordinary resistive elements with an equivalent resistance unit and further improves the resistance accuracy of the equivalent resistance unit using a BJT nonlinear current mirror, thereby achieving accurate output.
[0005] The present invention adopts the following technical solution.
[0006] This invention relates to a current source, which includes a first branch, a second branch, an equivalent resistance unit, and an output unit. The equivalent resistance unit uses cascaded MOSFETs operating in the linear region to form an equivalent resistance, thus overcoming the influence of process corners. The first branch is connected to the equivalent resistance unit and mirrored with the second branch to generate a constant voltage difference to control the source-drain voltage of the cascaded MOSFETs. The output unit is mirrored with the first and second branches, and collects and outputs the mirrored currents of the first and second branches.
[0007] Preferably, the equivalent resistance unit includes a fourth mirror transistor Mp4, a first cascade transistor Mn1, and a second cascade transistor Mn2; wherein, the gate of the fourth mirror transistor Mp4 is connected to the first branch, the second branch, and the output unit in a mirror manner, the source is connected to the power supply voltage, and the drain is connected to the drain and gate of the first cascade transistor Mn1 and the gate of the second cascade transistor Mn2; the source of the first cascade transistor Mn1 is connected to the drain of the second cascade transistor Mn2 used to form the equivalent resistance, and the source of the second cascade transistor Mn2 is grounded.
[0008] Preferably, the first branch includes a first mirror transistor Mp1 and a first transistor Qn1; wherein the gate and drain of the first mirror transistor Mp1 are connected to each other and connected to the collector of the first transistor Qn1, and the source of the first mirror transistor Mp1 is connected to the power supply voltage; the emitter of the first transistor Qn1 is connected to the source of the first cascade transistor Mn1 and the drain of the second cascade transistor Mn2, and the base of the first transistor Qn1 is connected to the base and collector of the second transistor Qn2.
[0009] Preferably, the second branch includes a second image transistor Mp2 and a second transistor Qn2; wherein, the source of the second image transistor Mp2 is connected to the power supply voltage, the gate is connected to the gate and drain of the first image transistor, the drain is connected to the gate and collector of the second transistor Qn2; and the emitter of the second transistor Qn2 is grounded.
[0010] Preferably, the output unit is an output transistor Mp3, with the source of the output transistor connected to the power supply voltage, the drain serving as the output terminal of the current source, and the gate connected to the gate and drain of the first mirror transistor Mp1, the gate of the second mirror transistor Mp2, and the gate of the fourth mirror transistor Mp4.
[0011] Preferably, the first cascade transistor Mn1 operates in the saturation region based on the control of its gate voltage, and the second cascade transistor Mn2 operates in the linear region based on the control of its gate voltage.
[0012] Preferably, the output current of the current source is proportional to the 4 / 3 power of the temperature.
[0013] Preferably, when the ambient temperature of the circuit where the current source is located is between -55°C and 135°C, the output current of the current source fluctuates between 65% and 150% of the ideal output current.
[0014] The beneficial effects of this invention are as follows: Compared with the prior art, the current source in this invention uses an equivalent resistance unit to replace the resistor, reducing the negative impact of resistance changes under PVT on the current at different process corners and ensuring the accuracy of the current. Furthermore, this invention uses a nonlinear current mirror composed of a BJT (Bipolar Junction Transistor) instead of a nonlinear current mirror composed of a MOSFET, which further improves the resistance accuracy of the equivalent resistance unit and avoids the unsolvable state of the circuit when using a MOSFET nonlinear current mirror. Attached Figure Description
[0015] Figure 1 This is a schematic diagram of the circuit structure of a current source in the prior art;
[0016] Figure 2 This is a schematic diagram of the circuit structure of a current source according to the present invention;
[0017] Figure 3This is a schematic diagram illustrating the saturation characteristics of mirrored MOSFETs and cascaded transistors in the prior art.
[0018] Figure 4 This is a schematic diagram of the saturation characteristics of a mirror BJT and a cascaded tube in a current source according to the present invention. Detailed Implementation
[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, other embodiments obtained by those skilled in the art without creative effort are all within the protection scope of this invention.
[0020] Figure 1 This is a schematic diagram of the circuit structure of a current source in the prior art. For example... Figure 1 As shown, in existing technologies, a Widlar current source is typically used to achieve current output. The principle of this current source is to connect the first and second branches using a current mirror method, and by adding a resistor to one of the branches, a mirrored current is generated within the current source.
[0021] However, this current source has significant problems. Specifically, the output current in this circuit is inversely proportional to the resistance. In this case, when the PVT conditions of the circuit change, the resistive element will be greatly affected, resulting in large differences in the output current of the circuit under different process corners of the chip.
[0022] Therefore, the present invention provides a novel current source.
[0023] Figure 2 This is a schematic diagram of the circuit structure of a current source according to the present invention. (For example...) Figure 2 As shown, this invention relates to a current source, which includes a first branch, a second branch, an equivalent resistance unit, and an output unit. The equivalent resistance unit uses cascaded MOSFETs operating in the linear region to form an equivalent resistance to overcome process corner effects. The first branch is connected to the equivalent resistance unit and mirrored with the second branch to generate a constant voltage difference to control the drain-source voltage of the cascaded MOSFETs. The output unit is connected to the first differential branch and the second differential branch, and collects and outputs the mirrored currents of the first and second branches.
[0024] It is understood that in the method of this invention, one cascaded MOSFET in the equivalent resistance unit can function as a resistor in the original current source circuit, and is connected in series with the first branch and between the power supply voltage and ground. Furthermore, the first and second branches are connected in a mirror manner, each generating its own first branch current and second branch current. Under the action of the equivalent resistance, the mirrored current caused by the first and second branch currents can be output through the output unit. Specifically, due to the different number of BJTs in the first and second branches, a relatively constant voltage difference is generated, which is the difference ΔVbe between the base-emitter voltages of the two BJTs. Since this voltage difference acts on the equivalent resistance, causing the equivalent resistance, i.e., the cascaded MOSFET, to operate in a linear state, the equivalent resistance is no longer affected by process angle variations, and the current source can output a current with high accuracy.
[0025] Preferably, the equivalent resistance unit includes a fourth mirror transistor Mp4, a first cascade transistor Mn1, and a second cascade transistor Mn2; wherein, the gate of the fourth mirror transistor Mp4 is connected to the first branch, the second branch, and the output unit in a mirror manner, the source is connected to the power supply voltage, and the drain is connected to the drain and gate of the first cascade transistor Mn1 and the gate of the second cascade transistor Mn2; the source of the first cascade transistor Mn1 is connected to the drain of the second cascade transistor Mn2 used to form the equivalent resistance, and the source of the second cascade transistor Mn2 is grounded.
[0026] It is understood that the fourth mirror transistor in the equivalent resistance unit of this invention can be mirrored with the first branch, the second branch, and the output branch in the circuit, respectively. Therefore, this connection method allows the mirror transistors to output mirrored current through the width-to-length ratio of the MOS transistors. In one embodiment of this invention, the ratio between Mp4 and Mp1 is M:1. Therefore, in this way, Mp4 can provide a current that is a multiple of Mp1 to power Mn1 and Mn2.
[0027] In this invention, the fourth mirror transistor Mp4 and the first cascade transistor Mn1 control the gate-source voltage of the cascade transistor Mn2, and the constant voltage difference ΔV generated by the nonlinear current mirror formed by the first branch Qn1 and the second branch Qn2 is... BE =V T lnn controls the drain-source voltage of Mn2.
[0028] It should be noted that, due to the reasonable design, the Mn1 transistor in this invention operates in the saturation region, while the Mn2 transistor operates in the linear region. As the source drain currents of Mp1 and Mp2 increase or decrease, the mirror current of Mp4 also increases or decreases accordingly, thereby causing the internal resistance of Mn2 to change with the drain current of Mp4.
[0029] Specifically, the drain-source current of Mn2 is affected not only by the current of Qn1 but also by the mirror current of Mp1, which is the source-drain current of Mp4. Since the circuit only includes MOSFETs and does not involve resistors or other components, it is unaffected by factors other than temperature. In this case, when other circuit parameters are fixed, the magnitudes of the drain current and gate voltage of Mn2 are also essentially determined. At this point, the resistance of Mn2 is inversely proportional only to a power of 1 / 3 of the temperature. The derivation of this part will be described in detail later.
[0030] Preferably, the first branch includes a first mirror transistor Mp1 and a first transistor Qn1; wherein the gate and drain of the first mirror transistor Mp1 are connected to each other and connected to the collector of the first transistor Qn1, and the source of the first mirror transistor Mp1 is connected to the power supply voltage; the emitter of the first transistor Qn1 is connected to the source of the first cascade transistor Mn1 and the drain of the second cascade transistor Mn2, and the base of the first transistor Qn1 is connected to the base and collector of the second transistor Qn2.
[0031] Preferably, the second branch includes a second image transistor Mp2 and a second transistor Qn2; wherein, the source of the second image transistor Mp2 is connected to the power supply voltage, the gate is connected to the gate and drain of the first image transistor, the drain is connected to the gate and collector of the second transistor Qn2; and the emitter of the second transistor Qn2 is grounded.
[0032] It is understood that the present invention incorporates existing technology, namely... Figure 1 The Mn1 and Mn2 tubes in the text have been modified to... Figure 2 Qn1 and Qn2 in the equation. Since the base-emitter voltage difference between the two BJT transistors can be expressed by the formula ΔV... BE =V T The formula lnn = kTlnn / q is used to express this. According to this formula, when the circuit parameters are determined, the voltage difference between transistors Qn1 and Qn2 is only affected by the temperature T.
[0033] Preferably, the output unit is an output transistor Mp3, with the source of the output transistor connected to the power supply voltage, the drain serving as the output terminal of the current source, and the gate connected to the gate and drain of the first mirror transistor Mp1, the gate of the second mirror transistor Mp2, and the gate of the fourth mirror transistor Mp4.
[0034] It is understandable that, since the output unit is connected to the other mirror tubes, it can also obtain the current from the first mirror tube, the second mirror tube, and the third mirror tube and output it accordingly.
[0035] Preferably, the first cascade transistor Mn1 operates in the saturation region based on the control of its gate voltage, and the second cascade transistor Mn2 operates in the linear region based on the control of its gate voltage.
[0036] A detailed analysis of the circuit's operation reveals that the first and second image transistors provide current input to the current source formed by the equivalent resistances of Qn1, Qn2, and Mn2. The ratio of Q1 to Q2 can be pre-designed to be n:1, ensuring that the actual current of each BJT in Q1 is 1 / n times that of Q2. In this case, the voltage difference between the collector and emitter of Q1 is relatively smaller than that of Q2, and this voltage difference biases the drain-source voltage of Mn2.
[0037] In this case, the current in the first branch and the second branch is determined to be the same magnitude, which is represented by current I0 in this invention. Furthermore, since the aspect ratio of the fourth image transistor Mp4 is M times that of the other image transistors, its current is also relatively large, which can be M·I0.
[0038] In this case, since the drain of the cascaded transistor Mn2 simultaneously receives the emitter current I0 of Q1 and the current M·I0 obtained from the fourth mirror transistor through the first cascaded transistor Mn1, its drain current can be (M+1)·I0.
[0039] Since the second-stage transistor Mn2 operates in the linear region, its linear region current calculation formula is as follows: Based on the above formula, we can obtain... In this formula, μ n For electron mobility, C ox Capacitance per unit area This represents the width-to-length ratio of the MOSFET.
[0040] Furthermore, since the first-stage transistor Mn1 operates in the saturation region and its width-to-length ratio is N times that of Mn2, the saturation region current calculation formula is used. It can be solved Since Mn1 and Mn2 are cascaded, and their gates are both connected to the drain of Mn1, it is possible to obtain
[0041] Solving this formula, we get... Furthermore, since the drain-source voltage of Mn2 is exactly equal to the voltage difference between the two BJT transistors in the first and second branches, therefore... Where n is the ratio of the number of Q1 to Q2, V T This is the thermal voltage of the BJT tube. Solving for the current I0 using this formula yields... In the above formula, since all other parameters remain unchanged after the circuit design, I0 only changes with μ. n V T 2It changes with temperature T; therefore, the actual relationship between I0 and temperature T is I0 and T. 4 / 3 Proportional.
[0042] Furthermore, since the equivalent resistance of Mn2 is also related to the linear current calculation formula, further derivation based on the formula in the prior art reveals that the relationship between the linear equivalent resistance of Mn2 during stable circuit operation and temperature T is as follows: -1 / 3 Proportional.
[0043] In one embodiment of the present invention, assuming that the current value of the chip at 27°C is the reference current under ideal process specifications, the current source of the present invention is simulated. When the chip operates between -55°C and 135°C, the output current fluctuates between 65% and 150% of the reference current.
[0044] Figure 3 This is a schematic diagram of the saturation characteristics of mirrored MOSFETs and cascaded transistors in the prior art. Figure 4 This is a schematic diagram illustrating the saturation characteristics of a mirrored BJT and cascaded transistors in a current source according to the present invention. Specifically, the saturation characteristics here mainly refer to the difference ΔV between the mirrored current I0 generated by the current mirror and the gate-source voltage difference of the two MOS transistors constituting the nonlinear current mirror. gs The relationship curve, or the difference ΔV between the mirror current I0 and the base-emitter voltage difference of the nonlinear current mirror formed by the two BJT transistors. be The relationship curves are as follows. Furthermore, for cascaded transistors, the drain-source voltage difference also exhibits a non-linear relationship with the mirror current I0. In this invention, we mainly consider the states of the MOS transistor operating in the saturation region and the BJT transistor operating in the amplification region; therefore, we have extracted the curve in the saturation region from the relationship curves for explanation.
[0045] like Figure 3 and Figure 4 As shown, it should be noted that in the existing technology, that is... Figure 1 The two mirrored MOS transistors, Mn1 and Mn2, exhibit non-ideal saturation characteristics. Due to the influence of substrate material crystal orientation, impurity concentration, gate voltage, and various process conditions during chip manufacturing, the saturation characteristic curves of the MOS transistors will change with ΔV. gs The coiling occurs due to the increase in the value of the MOSFET. When the coiling degree of the mirror MOSFET is higher than that of the cascaded MOSFET, the circuit will have no ideal solution.
[0046] The circuit in this invention fully considers the implicit problems existing in existing circuit technologies, using BJT transistors instead of MOSFETs, so that the saturation characteristic curve of the BJT transistor can intersect with the saturation characteristic curve of the cascaded transistor Mn1. The ΔV of the BJT transistor is then mirrored at this intersection. beThe values of Vds and Vds of the cascaded transistors are selected to ensure the reasonable selection of parameters of each transistor in the circuit, thereby making the output current of the circuit more reasonable. The circuit actually manufactured and used fully reflects the circuit design indicators under ideal conditions, and the output accuracy of the circuit is greatly improved.
[0047] Compared to existing technologies where the output current of Widlar current sources fluctuates between 60% and 200% of the reference current, this invention significantly improves the stability of the current output, giving the current certain characteristics.
[0048] The beneficial effects of this invention are that, compared with the prior art, the current source of this invention uses an equivalent resistance unit to replace the resistor, overcoming the problem of resistance value changing with process corners and ensuring the output accuracy of the current source across the entire process corner. Furthermore, this invention uses a nonlinear current mirror composed of a BJT (Bipolar Junction Transistor) instead of a nonlinear current mirror composed of a MOSFET, avoiding unsolvable circuit states and further improving current accuracy.
[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.
Claims
1. A current source, characterized in that: The current source includes a first branch, a second branch, an equivalent resistance unit, and an output unit; wherein... The equivalent resistance unit uses cascaded MOS transistors operating in the linear region to form the equivalent resistance in order to overcome the influence of process corners; The equivalent resistance unit includes a fourth image transistor Mp4, a first cascade transistor Mn1, and a second cascade transistor Mn2; wherein... The gate of the fourth mirror transistor Mp4 is connected to the first branch, the second branch, and the output unit in a mirror manner, respectively. The source is connected to the power supply voltage, and the drain is connected to the drain and gate of the first cascade transistor Mn1 and the gate of the second cascade transistor Mn2. The first cascade transistor Mn1 operates in the saturation region based on the control of its gate voltage, and the second cascade transistor Mn2 operates in the linear region based on the control of its gate voltage. The source of the first cascade transistor Mn1 is connected to the drain of the second cascade transistor Mn2, which forms an equivalent resistance. The source of the second cascade transistor Mn2 is grounded. The fourth mirror transistor Mp4 and the first cascade transistor Mn1 control the gate-source voltage of the second cascade transistor Mn2. The constant voltage difference generated by the nonlinear current mirror formed by the first and second branches is... The drain-source voltage of the second-stage transistor Mn2 is controlled. The first branch is connected to the equivalent resistance unit and is connected to the second branch in a mirror manner to generate a constant voltage difference to control the drain-source voltage of the cascaded MOS transistor; The output unit is mirror-connected to the first branch and the second branch, and collects and outputs the mirror current of the first branch and the second branch.
2. A current source according to claim 1, characterized in that: The first branch includes a first mirror transistor Mp1 and a first transistor Qn1; wherein, The gate and drain of the first mirror transistor Mp1 are connected to each other and connected to the collector of the first transistor Qn1. The source of the first mirror transistor Mp1 is connected to the power supply voltage. The emitter of the first transistor Qn1 is connected to the source of the first cascade transistor Mn1 and the drain of the second cascade transistor Mn2, and the base of the first transistor Qn1 is connected to the base and collector of the second transistor Qn2.
3. A current source according to claim 2, characterized in that: The second branch includes a second mirror transistor Mp2 and a second transistor Qn2; wherein, The source of the second mirror transistor Mp2 is connected to the power supply voltage, the gate is connected to the gate and drain of the first mirror transistor, and the drain is connected to the gate and collector of the second transistor Qn2. The emitter of the second transistor Qn2 is grounded.
4. A current source according to claim 3, characterized in that: The output unit is an output transistor Mp3. The source of the output transistor is connected to the power supply voltage, the drain is used as the output terminal of the current source, and the gate is connected to the gate and drain of the first mirror transistor Mp1, the gate of the second mirror transistor Mp2, and the gate of the fourth mirror transistor Mp4.
5. A current source according to claim 4, characterized in that: The output current of the current source is proportional to the 4 / 3 power of the temperature.
6. A current source according to claim 5, characterized in that: When the ambient temperature of the circuit where the current source is located is between -55°C and 135°C, the output current of the current source fluctuates between 65% and 150% of the ideal output current.