JP2003228427A - Constant-current circuit and solid-state image pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide high-precise constant current, without being influenced by variations of threshold in a transistor. <P>SOLUTION: At first, a switch SW1 and a switch SW3 are turned on. In this state, the gate potential of a MOS transistor Tr1 is set so as to make a current determined by a resistance R1 to flow, and then the switch SW1 is turned off. In this state, the gate of the MOS transistor Tr1 is merely separated, and the current will not change so much. Subsequently, the switch SW3 is turned off and the switch SW2 is turned on, to shift voltage of a voltage applying terminal 1 by delta V1 in the high direction. This allows gate voltage of the MOS transistor Tr1 to shift by a quantity proportional to delta V1 in the high direction through capacity coupling of a capacitor C1, and then the current larger by the shifted quantity flows into a circuit 2. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a constant current circuit capable of supplying a highly accurate constant current and a solid-state image pickup device using the constant current circuit.

[0002]

2. Description of the Related Art FIG. 7 is a circuit diagram showing a configuration example of a conventional constant current circuit using a current mirror. Three MOS transistors Tr11 and Tr that form a current mirror
Gates of 12 and Tr13 are connected to each other, and sources thereof are connected to the ground potential GND. The drain of the drive-side MOS transistor Tr11 is connected to the drive potential Vdd via the resistor R11, and the gate and drain are connected. Also, the two Ms on the load side
The drains of the OS transistors Tr12 and Tr13 are
Various circuits 10A and 10B to which a constant current is supplied are connected. The drive power supplies on the circuits 10A and 10B side are omitted. In such a constant current circuit, a current is determined by the resistor R11 and the MOS transistor Tr11, and a voltage corresponding to the current appears at the gate of the MOS transistor Tr11. Therefore, the MOS transistors Tr12 and Tr13 whose gates are commonly connected are also connected. , If the transistor size is the same, almost the same current flows.

[0003]

However, the above-mentioned conventional constant current circuit has a problem that the current value varies due to the variation in the threshold value of the transistor. That is, the threshold value of a transistor varies by several tens of mV even if adjacent transistors having the same shape. Therefore, this variation is caused by the MOS transistor Tr11.
It becomes the electric current variation between ~ Tr13. In particular, when the power supply voltage is 3 V or more, a variation of about several tens of mV has been allowed as an error. However, as the power supply voltage decreases, such a variation in threshold cannot be ignored as a factor that narrows the operation margin. There is. For example, as a CMOS type solid-state imaging device, there is one that has a constant current source for each column of unit pixels arranged in a matrix and drives each unit pixel of the column. However, in such a solid-state imaging device, conventionally, there is a problem that vertical stripe-shaped fixed pattern noise appears in a captured image because the characteristics of the constant current sources of several hundred columns or more are not uniform for the above reasons. It was Further, a design margin corresponding to the variation in the threshold value of the constant current source must be taken, which is an obstacle to the reduction of the power supply voltage.

Therefore, an object of the present invention is to provide a constant current circuit capable of supplying a highly accurate constant current without being affected by variations in the threshold value of a transistor, and a solid-state image pickup device using the constant current circuit. is there.

[0005]

In order to achieve the above object, the present invention provides a field effect transistor for supplying a constant current to a predetermined circuit, and a gate potential of the field effect transistor with a voltage corresponding to a predetermined small current. After the application, the gate potential is changed by capacitive coupling to cause a large current to flow through the field effect transistor, and the control means supplies the predetermined current as a constant current to the predetermined circuit.

Further, according to the present invention, a pixel section configured by arranging a plurality of unit pixels two-dimensionally, a pixel driving means for sequentially reading pixel signals from each unit pixel of the pixel section, and a pixel driving section And a constant current unit that supplies a constant current to each unit pixel of the pixel unit for each column or each row, and the unit pixel is configured to detect incident light. In the solid-state imaging device including a photoelectric conversion element that generates a signal charge according to the amount of light and a gate circuit that converts the signal charge generated by the photoelectric conversion element into an electric signal and outputs the electric signal, The current section has a constant current circuit for each column or row of each unit pixel of the pixel section, and the constant current circuit includes a field effect transistor for supplying a constant current to the unit pixel, and a field effect transistor of the field effect transistor. Predetermined to gate potential After applying a voltage corresponding to a small current, the gate potential is changed by capacitive coupling to cause a large current to flow through the field effect transistor, and the control unit supplies the unit pixel of the pixel unit as a constant current. And

In the constant current circuit of the present invention, a voltage corresponding to a predetermined small current is applied to the gate potential of the field effect transistor serving as a constant current source, and then the gate potential is changed by capacitive coupling to increase the potential of the field effect transistor. An electric current was passed to obtain a constant current. Therefore, a highly accurate constant current can be supplied without being affected by variations in the threshold value of the transistor serving as the constant current source.

Further, in the solid-state image pickup device of the present invention, as a constant current circuit of the unit pixel of the pixel portion, after applying a voltage corresponding to a predetermined small current to the gate potential of the field effect transistor serving as a constant current source, A structure in which a constant current is obtained by flowing a large current through the field effect transistor by changing the gate potential by coupling is used. Therefore, it is possible to supply a highly accurate constant current to each unit pixel of the pixel portion without being affected by the variation in the threshold value of the transistor serving as a constant current source, and it is possible to obtain a pixel signal with uniform characteristics. it can.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a constant current circuit and a solid-state image pickup device according to the present invention will be described below. First, the constant current circuit according to the present embodiment is configured such that the gate voltage corresponding to a small current is stored in the gate of the MOS transistor serving as a constant current source, and then the gate voltage is increased by capacitive coupling. The influence of the threshold variation in the MOS transistor on the current value is eliminated.

FIG. 1 is a block diagram showing the configuration of a constant current circuit according to the first embodiment of the present invention. In the figure, the structure surrounded by a broken line a is a feature of the present embodiment. This constant current circuit has a MOS transistor T
r1, switches SW1, SW2, SW3, and resistor R1
And a capacitor C1 and a voltage application terminal 1, and supplies a constant current to the circuit 2. The ground potential GND (first reference potential) is connected to the source (first channel end) of the MOS transistor Tr1. Also,
The switch SW1 (first switch) is connected between the gate (gate end) and the drain (second channel end) of the MOS transistor Tr1.

Further, switches SW2 (second switch) and S are connected in parallel to the drain of the MOS transistor Tr1.
One end of W3 (third switch) is connected. Further, one end of the capacitor C1 is connected to the gate of the MOS transistor Tr1, and the voltage application terminal 1 is connected to the other end of the capacitor C1. Also, switch SW
The circuit 2 is connected to the other end of the circuit 2. Circuit 2
Is connected to the drive potential Vdd. The drive potential Vdd is connected to the other end of the switch SW3 via the resistor R1.

FIG. 2 is a timing chart showing the operation of the constant current circuit configured as described above. The operation of each switch SW1, SW2, SW3 is ON at the upper level and OFF at the lower level. First, in the period (1) shown in FIG. 2, the switch SW1 and the switch S
Turn on W3. As a result, the GND, MOS transistor Tr1, switch SW3, resistance R1, and Vdd lines are connected. Further, the voltage of the voltage application terminal 1 is Lo
It is w. In this state, the MOS transistor Tr
The gate potential of 1 is set so that a current determined by the resistor R1 flows, like the MOS transistor Tr11 of the conventional example described above. Next, in the period (2) shown in FIG. 2, the switch SW1 is turned off. In this state, only the gate of the MOS transistor Tr1 is cut off, and the current hardly changes. Next, in the period (3) shown in FIG.
Switch SW3 is turned off, switch SW2 is turned on,
The voltage of the voltage application terminal 1 is increased by ΔV1 in the positive direction (high
shift to h).

As a result, the MOS transistor Tr1
Of the gate voltage of Δ due to capacitive coupling of the capacitor C1
The current is shifted in the positive direction by an amount proportional to V1, and a current larger than that in the periods (1) and (2) flows in the circuit 2 by that amount. In this method, the gate potential can be determined by a current much smaller than the current passed in the period (3), for example, a current such as 1/10 to 1/1000, so that the resistor R1 can be designed to be that large. Therefore, the current value is hardly influenced by the characteristics of the MOS transistor Tr1 and is almost determined only by the value of the resistor R1. Therefore, the gate voltage of the MOS transistor Tr1 at this time is also determined only by the value of the resistor R1, and is almost accurately determined near the threshold value. Further, when a constant current is passed through the circuit 2, the gate voltage is increased from the voltage application terminal 1 through the capacitor C1, but the variation at this time is
By making the capacitance values of the MOS transistor Tr1 and the capacitor C1 large, it is possible to make them sufficiently small.

With the above configuration, a preset increase in gate voltage can be given with reference to the threshold value of the MOS transistor Tr1. As a result, even if the threshold value of Tr1 varies, the flowing current value does not vary. In addition, even when a plurality of the same constant current sources are arranged, the gate voltage is increased after setting the gate voltage at the threshold value of each MOS transistor. Therefore, unlike the conventional example, the current value and the operating margin are not the same. Is not affected by the variation in the threshold value of. Note that such a configuration has a dynamic element, and the gate voltage dynamically changes and the current also changes due to leakage of the switch SW1 over time. However, by making the capacitance of the capacitor C1 large, it is possible to flow an accurate current for a certain period of time. For example, when the switch SW1 is made to have the minimum size and the capacitance value of the capacitor C1 is set to 50 fF, the gate voltage fluctuation occurring during the period of 50 ms can be reduced to 1 mV or less.

FIG. 3 is a block diagram showing the configuration of a constant current circuit according to the second embodiment of the present invention. In the figure, the structure surrounded by a broken line a is a characteristic part of the present embodiment, and this is the same as the first embodiment shown in FIG. The configuration shown in FIG. 3 has the switch SW shown in FIG.
3 and the resistor R1 are eliminated. Others are the same as those in the example shown in FIG.

FIG. 4 is a timing chart showing the operation of the constant current circuit shown in FIG. The operation of each switch SW1 and SW2 is ON at the upper level,
It is OFF at the lower level. First, in the period (1) shown in FIG. 4, the switches SW1 and SW2 are turned on.
However, the voltage of the voltage application terminal 1 is Low. In this state, the gate potential of the MOS transistor Tr1 is
It is set so that a current determined by the MOS transistor Tr1 and the circuit 2 flows. Next, in the period (2) shown in FIG.
The switch SW2 is turned off. As a result, the point A shown in FIG. 3 and the potential of the gate of the MOS transistor Tr1 are rapidly lowered, and the MOS transistor Tr1 is turned off.
Stop where you do. At this time, the MOS transistor T
The gate voltage of r1 is almost exactly determined near the threshold value.
Finally, even if the switch SW1 is turned off, the situation is not changed because the gate is disconnected.

Next, in the period (3) shown in FIG. 4, the switch S
W2 is turned on and the voltage application terminal 1 is swung by ΔV1 in the positive direction (high). As a result, a voltage proportional to ΔV1 is applied to the gate of the MOS transistor Tr1 from the state in which the variation in the threshold is removed, and the current flows in the circuit 2. In this example, the influence of the resistance is not included and the gate voltage is determined only by the threshold value, so that the effect of removing the variation in the threshold value is greater than that in the configuration shown in FIG. For example, the configuration shown in FIG. 1 can obtain an accuracy of about 1 mV, but the configuration shown in FIG. 3 can obtain an accuracy of about 0.1 mV. Further, when a plurality of constant current sources are used, the current value in the period (1) varies depending on the state of each circuit side, but in the period (2), the gate voltage rapidly converges to the threshold value of each transistor. I'm going, so period (1)
The difference in (2) disappears in the period (2), and neither the current value nor the operation margin is affected by the variation in the threshold value. Also in this example, the circuit operation is dynamic and the method of designing the capacitor C1 is the same. Further, in this example, the switch SW2 may be in the circuit or may be on the Vdd side.

Next, a supplementary description will be given of a specific example of the configuration of the portion surrounded by the broken line a in FIGS. 1 and 3 described above. FIG. 5 is a block diagram showing a specific configuration example of the portion surrounded by the broken line a. This circuit has a configuration in which the capacitance C2 and the MOS transistors Tr2, T are added to the configurations shown in FIGS.
r3, an inverter 3, and a switch control terminal 4 are added. The capacitor C2 is formed as a capacitor element, but here the parasitic capacitance generated at the gate node of the MOS transistor Tr1 is also considered. The MOS transistors Tr2 and Tr3 have their channels connected in series and are inserted between the gate and drain of the MOS transistor Tr1. The gate of the MOS transistor Tr2 is connected to the switch control terminal 4, and the gate of the MOS transistor Tr3 is connected to the switch control terminal 4 via the inverter 3.

Next, the detailed configuration of such a circuit will be described. First, the voltage change amount ΔV1 of the voltage application terminal 1 in FIGS. 1 and 3 may be a small voltage of about 0.3 V, but the configuration shown in FIG. 5 is used, and the capacitance of the capacitors C1 and C2 is changed. It can be adjusted by setting the value to an appropriate value. For example, when the capacitance ratio of C1: C2 is 1: 9, if terminal 1 is swung from 0V to 3V,
A voltage change of 0.3 V occurs at the gate of the transistor Tr1. Further, the switch SW1 in FIG. 1 may be made of a CMOS switch, but may be made of an NMOS.
In FIG. 5, the transistor Tr2 corresponds to the switch SW1. At that time, a transistor Tr3 having the same gate length as the transistor Tr2 and a gate width of ½ is inserted,
The feedthrough of the transistor Tr2 can be canceled by short-circuiting the source and the drain and applying a gate pulse whose polarity is opposite to that of the transistor Tr2.

FIG. 6 is a block diagram showing an example of a solid-state image pickup device provided with a constant current circuit according to this embodiment. This solid-state imaging device is an example of a so-called CMOS image sensor, and includes a pixel unit 21 and a CDS unit 22.
, Constant current unit 23, V selection means 24, H selection means 2
5, the horizontal signal line 26, the output unit 27, and the communication unit 28.
And a timing generator (TG) 29 are provided on the semiconductor chip. The pixel portion 21 is provided with a large number of unit pixels in a two-dimensional array. Then, in each unit pixel, a photoelectric conversion element such as a photodiode for picking up an image of a subject and a signal charge corresponding to the amount of received light generated by this photoelectric conversion element are converted into an electric signal and output as a pixel signal. Each MOS transistor for transfer, selection, amplification, reset, etc. is provided.

Further, the CDS unit 22 performs C conversion for each pixel column for the pixel signal output from each unit pixel of the pixel unit 21.
A signal processing circuit (signal processing means) such as DS (correlated double sampling) or A / D conversion is provided to perform signal processing for each pixel column. The signals processed by the CDS unit 22 are sequentially selected by the H selection unit 25, sent to the output unit 27 through the horizontal signal line 26, amplified by the output unit 27, and output to the outside from the output terminal.
The V selection unit 24 scans the pixel portion 21 in the vertical direction, selects a pixel row from which a pixel signal is read, and drives each unit pixel. The communication unit 28 is for receiving control signals and the like from the outside, and the timing generator (TG) 29 is for generating various timing signals necessary for the operation of the image sensor and supplying them to each unit. . The V selection means 24, the TG 29, etc. constitute pixel drive means.

Further, the constant current section 23 supplies a constant current to each unit pixel for each pixel column, and is provided with a constant current source for each pixel column. Then, the constant current circuit according to the above-described embodiment can be used as the constant current source provided for each pixel column in the constant current unit 23. When such a constant current unit 23 is configured by the conventional constant current circuit shown in FIG. 7, the load side MOS transistor Tr12,
Several hundreds of Trs 13, ... Are arranged for each pixel column of the pixel section 21. In this configuration, there are variations of several tens of mV in the threshold values of several hundreds of load-side MOS transistors Tr12, Tr13, ... And these variations directly result in variations in the current value of the column. This causes a problem that vertical stripe-shaped fixed pattern noise occurs in a captured image as a slight variation in characteristics of each pixel column. Further, there is a problem that an operating margin of about 100 mV must be taken into consideration at the time of design.

On the other hand, in the present embodiment, the threshold current of each MOS transistor is set by arranging hundreds of the constant current circuits described in FIGS. 1 to 5 in the constant current section 23. Since a current having a constant voltage applied to the gate flows from, the influence of the variation in the threshold value of each MOS transistor can be eliminated and the above-mentioned conventional problem can be solved. Note that in the solid-state imaging device as described above, the pixel is driven with a constant current for a period of about 10 μs of the horizontal blanking period, and at the beginning of the horizontal blanking period, the period (1) in FIG. 2 or 4 is used. The operation of (2) is performed and the period (3)
Since the pixel is driven by, even if the constant current circuit of this embodiment has a dynamic circuit configuration as described above, since the potential is stable during this period, the operation can be performed without any problem.

As described above, in the present embodiment, it is possible to obtain a constant current source that eliminates the influence of variations in the threshold value of MOS transistors. Therefore, it is possible to eliminate variations in current value. Further, it is possible to eliminate the need for a design margin corresponding to the variation of the threshold value. When applied to the solid-state imaging device as shown in FIG. 5, it is possible to eliminate fixed pattern noise in the form of vertical stripes due to variations in the constant current source, and reduce the design margin to reduce the low voltage of the solid-state imaging device. Can be realized. In the above-described embodiment, an example in which an N-channel MOS transistor is used as the field effect transistor forming the constant current circuit has been described. However, by changing the polarity of the power supply, the P-channel MOS transistor and other It is also possible to use field effect transistors. Further, the circuit device provided with the constant current circuit is not limited to the solid-state imaging device as described above, but can be used in various circuit devices.

[0025]

As described above, according to the constant current circuit of the present invention, after the voltage corresponding to a predetermined small current is applied to the gate potential of the field effect transistor serving as the constant current source, the gate potential is capacitively coupled. Since a large current is made to flow in the field effect transistor by changing the constant current to obtain a constant current, it is possible to supply a high-precision constant current without being affected by the variation in the threshold value of the transistor serving as the constant current source. it can.

Further, according to the solid-state image pickup device of the present invention, after a voltage corresponding to a predetermined small current is applied to the gate potential of the field effect transistor serving as a constant current source as a constant current circuit of the unit pixel of the pixel section. Since the configuration in which a large current is made to flow in the field effect transistor by changing the gate potential by capacitive coupling to obtain a constant current is used, the variation in the threshold value of the transistor serving as the constant current source in each unit pixel of the pixel portion It is possible to supply a high-precision constant current without being affected, a pixel signal with uniform characteristics can be obtained, and a solid-state imaging device with good image quality can be provided.

[Brief description of drawings]

FIG. 1 is a block diagram showing a constant current circuit according to a first exemplary embodiment of the present invention.

FIG. 2 is a timing chart showing the operation of the constant current circuit shown in FIG.

FIG. 3 is a block diagram showing a constant current circuit according to a second exemplary embodiment of the present invention.

FIG. 4 is a timing chart showing the operation of the constant current circuit shown in FIG.

5 is a block diagram showing a specific configuration example of the constant current circuit shown in FIG. 1 or FIG.

FIG. 6 is a block diagram showing an example of a solid-state imaging device according to an embodiment of the present invention.

FIG. 7 is a circuit diagram showing a configuration example of a constant current circuit using a conventional current mirror.

[Explanation of symbols]

Tr1 ... MOS transistor, SW1, SW2, SW
3 ... switch, C1 ... condenser, R1 ... resistance,
1 ... Terminal for voltage application, 2 ... Circuit.

Continued front page    (72) Inventor Nobuo Nakamura             6-735 Kita-Shinagawa, Shinagawa-ku, Tokyo Soni             -Inside the corporation (72) Inventor Takashi Abe             6-735 Kita-Shinagawa, Shinagawa-ku, Tokyo Soni             -Inside the corporation (72) Inventor Hiroaki Fujita             6-735 Kita-Shinagawa, Shinagawa-ku, Tokyo Soni             -Inside the corporation (72) Inventor Eiichi Funazu             6-735 Kita-Shinagawa, Shinagawa-ku, Tokyo Soni             -Inside the corporation (72) Inventor Hiroki Sato             134, Kobe-cho, Hodogaya-ku, Yokohama-shi, Kanagawa               Sony LSI Design Stock Association             In-house F-term (reference) 4M118 AB01 BA14 FA06                 5C024 CX04 GY31 HX48                 5F038 AV13 AZ07 DF01 EZ20                 5H430 BB01 BB09 BB12 EE06

Claims (11)

[Claims] 1. A field effect transistor for supplying a constant current to a predetermined circuit, a gate potential of the field effect transistor is applied with a voltage corresponding to a predetermined small current, and then the gate potential is changed by capacitive coupling to generate an electric field. A constant current circuit, comprising: a control unit that applies a large current to the effect transistor and supplies the effect transistor as a constant current. 2. The constant current circuit according to claim 1, wherein the predetermined small current has a current value that dynamically approaches 0. 3. A voltage application terminal connected to the gate end of the field effect transistor via the capacitor for capacitive coupling, and a first reference potential connected to a first channel end of the field effect transistor. And a first switch that opens and closes between the second channel end and the gate end of the field effect transistor.
The described constant current circuit. 4. The control means causes a predetermined small current to flow with the first switch closed and determines the gate potential of the field effect transistor, and then opens the first switch to set the voltage application terminal. The constant current circuit according to claim 3, wherein a large voltage is applied to the circuit. 5. A resistor provided between the second channel end of the field effect transistor and a second reference potential, and a third resistor for opening and closing between the second channel end of the field effect transistor and the resistor. 4. The constant current circuit according to claim 3, further comprising: 6. The control means causes a predetermined small current to flow with the first switch and the third switch closed, determines the gate potential of the field effect transistor, and then opens the first switch, 6. The constant current circuit according to claim 5, wherein the third switch is opened, then the second switch is closed, and a large voltage is applied to the voltage application terminal. 7. The constant current circuit according to claim 3, further comprising a second switch that opens and closes between the second channel end of the field effect transistor and the predetermined circuit. 8. The control means causes a predetermined small current to flow with the first switch and the second switch closed, and then opens the second switch, thereby floating the gate end of the field effect transistor. By setting the state, the predetermined small current value is gradually approached to 0, and then the first
The constant current circuit according to claim 7, wherein the switch is opened to apply a large voltage to the voltage application terminal. 9. The constant current circuit according to claim 1, wherein the field effect transistor is an N-channel MOS transistor. 10. The field effect transistor is a P channel MOS transistor.
The described constant current circuit. 11. A pixel unit configured by arranging a plurality of unit pixels two-dimensionally, a pixel driving unit that sequentially reads pixel signals from each unit pixel of the pixel unit, and a pixel signal read from the pixel unit. And a constant current unit that supplies a constant current to each unit pixel of the pixel unit for each column or each row, and the unit pixel corresponds to the amount of incident light. In a solid-state imaging device configured to include a photoelectric conversion element that generates a signal charge and a gate circuit that converts the signal charge generated by the photoelectric conversion element into an electric signal and outputs the electric signal, the constant current unit, A constant current circuit is provided for each column or row of each unit pixel of the pixel unit, and the constant current circuit has a field effect transistor that supplies a constant current to the unit pixel and a predetermined gate potential of the field effect transistor. Against a small current of And a control means for changing the gate potential by capacitive coupling to cause a large current to flow through the field effect transistor and supplying it as a constant current to each unit pixel of the pixel portion. Imaging device.