阅读说明：本技术 用于缓冲感光信号的缓冲电路及其图像感测器 (Buffer circuit for buffering photosensitive signal and image sensor thereof ) 是由 林郁轩 王仲益 于 2019-09-19 设计创作，主要内容包括：一种用于缓冲感光信号的缓冲电路及其图像感测器,该缓冲电路包含：一放大电路、一开关元件以及一电容。该放大电路的控制端耦接像素电路的输出端,该放大电路的第一端用以输出缓冲后感光信号,以及该放大电路的第二端耦接于一参考电压。该开关元件具有第一端耦接于该放大电路的控制端,以及第二端耦接于该放大电路的第一端。该电容具有第一端耦接于该放大电路的控制端,以及第二端耦接于该放大电路的第一端。其中,该开关元件在第一阶段中导通,以及在第二阶段中不导通,且该放大电路在第二阶段中产生该缓冲后感光信号。(A buffer circuit for buffering photosensitive signals and an image sensor thereof are provided, the buffer circuit comprising: an amplifying circuit, a switch device and a capacitor. The control end of the amplifying circuit is coupled with the output end of the pixel circuit, the first end of the amplifying circuit is used for outputting the buffered photosensitive signal, and the second end of the amplifying circuit is coupled with a reference voltage. The switch element has a first end coupled to the control end of the amplifying circuit, and a second end coupled to the first end of the amplifying circuit. The capacitor has a first end coupled to the control end of the amplifying circuit, and a second end coupled to the first end of the amplifying circuit. The switch element is conducted in the first stage and is not conducted in the second stage, and the amplifying circuit generates the buffered light sensing signal in the second stage.)

1. A buffer circuit for buffering a photosensitive signal of a pixel circuit, comprising:

a control end of the amplifying circuit is coupled to the output end of the pixel circuit, a first end of the amplifying circuit is used for outputting the buffered photosensitive signal, and a second end of the amplifying circuit is coupled to a reference voltage;

a first switch element having a first end coupled to the control end of the amplifying circuit and a second end coupled to the first end of the amplifying circuit; and

a capacitor having a first end coupled to the control end of the amplifying circuit and a second end coupled to the first end of the amplifying circuit;

wherein the first switching element is turned on in a first phase and is turned off in a second phase; the amplifying circuit generates the buffered light sensing signal in the second stage.

2. The buffer circuit of claim 1, wherein the amplifier circuit is in a common source configuration.

3. The buffer circuit of claim 1, wherein the buffer circuit comprises a current source coupled to the second terminal of the amplifying circuit, and the current source provides a current to the pixel circuit during the first phase.

4. The buffer circuit of claim 1, wherein the capacitor is a parasitic capacitor between the control terminal and the first terminal of the amplifying circuit.

5. The buffer circuit of claim 1, wherein the buffer circuit is coupled to a read circuit through a second switch element; when the second switch element is turned on, the reading circuit is coupled to the first end of the amplifying circuit for reading the buffered photosensitive signal.

6. An image sensor, comprising:

a pixel circuit array including a plurality of pixel circuits; and

at least one buffer circuit for buffering the light sensing signal of at least one of the pixel circuits, comprising:

an amplifying circuit, wherein a control terminal of the amplifying circuit is coupled to the output terminal of the pixel circuit, a first terminal of the amplifying circuit is used for outputting the buffered photosensitive signal, and a second terminal of the amplifying circuit is coupled to a reference voltage;

a first switch element having a first end coupled to the control end of the amplifying circuit and a second end coupled to the first end of the amplifying circuit; and

a capacitor having a first end coupled to the control end of the amplifying circuit and a second end coupled to the first end of the amplifying circuit;

wherein the first switching element is turned on in a first phase and is turned off in a second phase; the amplifying circuit generates the buffered light sensing signal in the second stage.

Technical Field

The present invention relates to image sensing, and more particularly to a buffer circuit for buffering a photosensitive signal of a pixel circuit and an image sensor using the same.

Background

Generally, the photosensitive signal generated by the pixel circuit is read out by the reading circuit. Before the reading circuit reads the photosensitive signal, the buffer circuit is used to buffer the photosensitive signal. Referring to fig. 1, a schematic diagram of a buffer circuit is shown. In FIG. 1, the photosensitive signal V _ sense at the terminal VX of the pixel circuit 10 is buffered by the buffer circuit 20, and the buffered photosensitive signal V _ buffered is generated at the terminal VXS before being read out by the reading circuit 30. During a reset phase, the terminal VX is connected to a specific voltage level through the switch element SW2, so that the voltage level of the light sensing signal V _ sense is raised to the specific voltage level. In the subsequent sensing phase, the switching element SW2 is turned off, and the capacitor C _ S is discharged through the sensing element 12, thereby gradually deviating the voltage level of the terminal VX from the specific voltage level. Finally, in the reading phase, the switch SW1 is turned on, and the reading circuit 30 reads out the buffered photo sensing signal V _ buffered. However, in some cases where the lighting condition is not ideal (for example, light reception is weak or the exposure time is short), the variation of the light sensing signal V _ sense is not very different, so that the light reception condition of the pixel circuit 10 cannot be reflected exactly.

Disclosure of Invention

In order to solve the above problem, the present invention provides a buffer circuit for buffering a light sensing signal of a pixel circuit. The buffer circuit of the present invention includes an amplifier configured in a common source configuration. Through the common source configuration amplifier, the change of the photosensitive signal can be effectively amplified, so that the photosensitive signal can better reflect the illumination environment. Therefore, the invention substantially improves the photosensitive effect of the pixel circuit.

The present invention provides a buffer circuit for buffering a photosensitive signal of a pixel circuit, comprising: an amplifying circuit, a first switch element and a capacitor. A control terminal of the amplifying circuit is coupled to the output terminal of the pixel circuit, a first terminal of the amplifying circuit is used for outputting the buffered light sensing signal, and a second terminal of the amplifying circuit is coupled to a reference voltage. The first switch element has a first end coupled to the control end of the amplifying circuit, and a second end coupled to the first end of the amplifying circuit. The capacitor has a first end coupled to the control end of the amplifying circuit, and a second end coupled to the first end of the amplifying circuit. Wherein the first switching element is turned on in a first phase and is turned off in a second phase; the amplifying circuit generates the buffered light sensing signal in the second stage.

An embodiment of the present invention provides an image sensor, including: a pixel circuit array and at least one buffer circuit. The pixel circuit array comprises a plurality of pixel circuits. The buffer circuit is used for buffering the photosensitive signal on at least one of the plurality of pixel circuits. The buffer circuit includes: an amplifying circuit, a first switch element and a capacitor. A control terminal of the amplifying circuit is coupled to the output terminal of the pixel circuit, a first terminal of the amplifying circuit is used for outputting the buffered light sensing signal, and a second terminal of the amplifying circuit is coupled to a reference voltage. The first switch element has a first end coupled to the control end of the amplifying circuit, and a second end coupled to the first end of the amplifying circuit. The capacitor has a first end coupled to the control end of the amplifying circuit, and a second end coupled to the first end of the amplifying circuit. Wherein the first switching element is turned on in a first phase and is turned off in a second phase; the amplifying circuit generates the buffered light sensing signal in the second stage.

Drawings

Fig. 1 is a schematic diagram of a conventional buffer circuit.

Fig. 2 is a schematic diagram illustrating an architecture and an application of a buffer circuit according to an embodiment of the invention.

Fig. 3 to 5 are schematic views illustrating operation states of the buffer circuit of the present invention at different stages.

FIG. 6 is a diagram illustrating the structure and application of a buffer circuit according to another embodiment of the present invention.

FIG. 7 is a block diagram of an image sensor according to an embodiment of the present invention.

Wherein the reference numerals are as follows:

10. 100 pixel circuit

12. 120 photosensitive element

20. 110, 210 buffer circuit

121. 221 amplifier circuit

122 current source

30. 130 read circuit

SW1, SW2, SW11 and SW12 switching elements

M1, M2 transistor

C _ S, C _ GD capacitor

300 image sensor

310 pixel circuit array

Detailed Description

Certain terms are used throughout the description and following claims to refer to particular components. As one skilled in the art will appreciate, manufacturers may refer to a component by different names. The present specification and claims do not intend to distinguish between components that differ in name but not function. In the following description and in the claims, the terms "include" and "comprise" are used in an open-ended fashion, and thus should be interpreted to mean "include, but not limited to. Furthermore, the term "coupled" is intended to encompass any direct or indirect electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.

The inventive concept will be described with reference to various embodiments and with reference to the accompanying drawings. Wherein elements or devices having the same reference numerals in different figures represent similar principles of operation and technical effects. Therefore, the repetitive description will be omitted hereinafter. Furthermore, reference herein to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation of the invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Thus, although the description above refers to different features and/or methodological acts, it is to be understood that the various features may be implemented in the same or different embodiments, with appropriate modifications.

Referring to fig. 2, a buffer circuit according to an embodiment of the invention is shown. As shown in fig. 2, the pixel circuit 100 includes a light sensing element 120 and a parasitic capacitor C _ S. The light sensing element 120 is sensitive to illumination and its impedance varies with the intensity of the illumination. In various embodiments of the present invention, the light sensing element 120 can be a photo resistor (photo resistor) or a photo diode (photodiode), or any element that is sensitive to light and changes its impedance value based on the change of light, which is not limited in the present invention. The pixel circuit 100 is coupled to the buffer circuit 110. The buffer circuit 110 is used for buffering the photosensitive signal V _ sense at the output terminal VX of the pixel circuit 100, so as to output a buffered photosensitive signal V _ buffered at the terminal VXS. The buffer circuit 110 includes an amplifier circuit 121, a switching element SW12, a capacitor C _ GD, and a current source 122. In the present invention, the amplifying circuit 121 may include one or more transistors. The amplifying circuit 121 includes a control terminal C, a first terminal E1, and a second terminal E2. The control terminal C of the amplifying circuit 121 is coupled to the switch SW12 and the first terminal of the capacitor C _ GD. The first terminal E1 of the amplifying circuit 121 is coupled to the current source 122, the switch SW12 and the second terminal of the capacitor C _ GD. The second end E2 of the amplifying circuit 121 is coupled to a ground (or a reference voltage). Note that in the following description, the amplifying circuit 121 includes only one transistor, but this is not a limitation of the present invention. In the present embodiment, the amplifying circuit 121 includes a transistor M1 provided as a common source (common source) amplifier. The gate of the transistor M1 is coupled to the control terminal C of the amplifier circuit 121, the drain of the transistor M1 is coupled to the first terminal E1 of the amplifier circuit 121, and the source of the transistor M1 is coupled to the second terminal E2 of the amplifier circuit 121.

It is noted that although the amplifying circuit 121 in fig. 2 only includes the transistor M1, in other embodiments of the present invention, the amplifying circuit 121 may additionally include one or more main/passive elements (e.g., a transistor, a capacitor, or a resistor). For example, the amplifying circuit 121 may be a plurality of transistors connected in series (cascode), and is configured in a common source architecture.

In an embodiment, the capacitor C _ GD may be implemented by a parasitic capacitor between the control terminal C and the first terminal E1 of the amplifying circuit 121. For example, in the embodiment of fig. 2, the amplifying circuit 121 includes only the transistor M1, and thus, the capacitor C _ GD may be a parasitic capacitor between the gate and the drain of the transistor M1. In addition, in one embodiment, the buffer circuit 110 is coupled to the reading circuit 130 through the switch element SW 11. When the switch SW11 is turned on, the readout circuit 130 reads the buffered photo-sensing signal V _ buffered at the node VXS.

Referring to fig. 3 to fig. 5, the operation state of the buffer circuit 110 is further explained. First, in a reset phase shown in fig. 3, the switch SW12 in the buffer circuit 110 is turned on, which makes the terminal VX and the terminal VXS short-circuited, and the current source 122 will supply current to the pixel circuit 100 and charge the parasitic capacitor C _ S in the pixel circuit 100, so that the voltage levels of the terminal VX and the terminal VXS are the same. Then, in a light sensing phase as shown in fig. 4, the switching element SW12 in the buffer circuit 110 is turned off, and the light sensing element 120 in the pixel circuit 100 discharges current along with the light. In this case, the parasitic capacitor C _ S starts to discharge through the photosensitive element 120, which decreases the voltage level of the terminal VX. The voltage level at the node VXS is increased due to the decrease in the voltage level at the node VX, which causes the transistor M1 to turn on less. In one embodiment, assuming that the voltage level change at the terminal VX is Δ V, the voltage level change at the terminal VXS isThen is (C)_S/(C_GD) Δ V (wherein, C)_SAnd C_GDRespectively, the capacitance of the capacitor C _ S and the capacitance of the capacitor C _ GD). In the case of properly controlling the capacitance value of the capacitor C _ GD, (C) can be controlled_S/(C_GD) ) is greater than 1. Thus, the voltage level change at the terminal VXS is greater than the voltage level change at the terminal VX. Finally, in the reading phase shown in fig. 5, the switch SW11 is turned on, and the reading circuit 130 reads the buffered photo-sensing signal V _ buffered from the terminal VXS. Since the buffer circuit 110 amplifies the sensed light signal V _ sensed, the voltage level variation of the node VXS is more obvious relative to the voltage level variation of the node VX, so that the reading circuit 130 can more easily grasp the illumination environment.

Fig. 6 shows an amplifying circuit implemented by transistors of different conduction types according to another embodiment of the present invention. In the buffer circuit 210 shown in fig. 6, the amplifying circuit 221 includes a P-type transistor M2 configured as a common-source configuration amplifier. Since the operation principle of the buffer circuit 210 is not different from that of the buffer circuit 110, it is not described herein.

Fig. 7 illustrates an image sensor 300 according to an embodiment of the invention. As shown, the image sensor 300 includes a pixel circuit array 310, and the pixel circuit array 310 is composed of the pixel circuits 100 shown in FIG. 2. The image sensor 300 further includes one or more buffer circuits 110 for amplifying the light-sensing signals of the pixel circuits 100. The light reception signal buffered by the buffer circuit 110 is read out by one or more read circuits 130. The present invention amplifies the photosensitive signal through the buffer circuit 110, so that the image sensor 300 has a better photosensitive effect.

The invention has the advantages that the common-source configuration amplifier replaces the common-drain configuration amplifier adopted by the prior architecture in the amplifying circuit of the buffer circuit. In an embodiment, since the common-source configuration amplifier has a better voltage gain than the common-drain configuration amplifier, the buffer circuit of the present invention can amplify the photosensitive signal output by the pixel circuit more effectively. On the other hand, in the conventional structure, the change of the photosensitive signal after buffering is inversely proportional to the size of the parasitic capacitor C _ S, so that the capacitance value of the parasitic capacitor C _ S can only be reduced if the photosensitive signal is amplified. However, if the capacitance of the parasitic capacitor C _ S is to be reduced, the size of the photosensitive element is reduced, which will certainly affect the photosensitive effect of the pixel circuit. However, in the present invention, the capacitor C _ GD is added, so that the influence of the magnitude of the parasitic capacitor C _ S is removed, and the change of the buffered photo-sensing signal is inversely proportional to the magnitude of the capacitor C _ GD. Since the capacitance of the capacitor C _ GD is independent of the size of the photosensitive element, reducing the capacitance of the capacitor C _ GD does not affect the photosensitive effect of the pixel circuit. In other words, the invention can effectively amplify the change of the photosensitive signal while ensuring the photosensitive effect of the pixel circuit, so that the subsequent reading circuit can more easily master the illumination environment. Thus, the buffer circuit of the present invention substantially improves the performance of the image sensor.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the concept and principle of the present invention should be included in the protection scope of the present invention.