TW201509186A - Pixel sensing unit - Google Patents

Abstract

A pixel sensing unit includes a light sensor and a pixel amplifier. The amplifier includes a float diffusion area, a reset gate, a source follower and a voltage control unit. The float diffusion area is coupled to the light sensor. The reset gate has a control end, a first connection end and a second connection end, wherein the reset gate is arranged for performing a reset operation on the float diffusion area which is on the second connection end of the reset gate according to a reset signal received at the control end of the reset gate. The source follower is coupled to the float diffusion area and arranged for generating the voltage signal according to a voltage level of the float diffusion area. The voltage control unit is coupled to the source follower and provides a first bias voltage during the reset operation while provides a second bias voltage after the reset operation.

Description

Pixel sensing unit

The present invention relates to a pixel sensing unit, and more particularly to a pixel sensing unit that can compensate for a charge injection effect to increase an output swing.

With the development and advancement of technology, the distance between pixels and pixels on a liquid crystal display is getting smaller and smaller. In the design of small pixels, since the pixel amplifier allows more pixels to share a floating node, it can be improved. The light fills the effective fill factor while maintaining a high conversion gain. However, since the parasitic capacitance in the circuit is used to feed back the gain, the output swing of the pixel amplifier is affected by the clock feedthrough and charge injection effects.

For general image pixel sensing, the capacitance coupled between the gate and source of the reset transistor results in a reset charge injection. At the lower edge of the reset clock applied to the gate of the reset transistor, this coupled capacitor pulls down the voltage of the floating node of the pixel. The lower the voltage of this floating node is pulled, the smaller the voltage swing that a pixel can have. Because of this phenomenon, the dynamic range of general image pixel sensing will be reduced.

Therefore, how to avoid the influence of the clock feed effect on the output swing of the pixel sensing unit is still a major issue in this field.

Accordingly, it is an object of the present invention to provide a pixel sensing unit that compensates for the charge injection effect to increase the output swing to solve the above problems.

According to an embodiment of the invention, it is disclosed that a pixel sensing unit is disclosed. The pixel sensing unit includes a light sensor and a pixel amplifier. The pixel amplifier includes a floating diffusion region, a reset gate, a source follower, and a bias control unit. The floating diffusion region is coupled to the photo sensor. The reset gate has a control end point, a first connection end point and a second connection end point, wherein the reset gate is used to reset the signal according to one of the control terminals received by the control gate The floating diffusion region on the second connection terminal of the reset gate performs a reset operation. The source follower is coupled to the floating diffusion region for generating an output voltage signal according to the level of the floating diffusion region. The bias control unit is coupled to the source follower, and provides a first bias voltage during the reset operation, and a second bias voltage is provided after the reset operation ends.

As can be seen from the above, the present invention provides a pixel sensing unit that can compensate for the effects of the charge injection effect to increase the output swing of the pixel amplifier and improve the quality of pixel reading.

110‧‧‧Pixel Amplifier

100‧‧‧pixel sensing unit

102‧‧‧Light sensor

112‧‧‧Remove the brake

114‧‧‧Selecting the gate

116‧‧‧Source follower

118‧‧‧Capacitive components

115‧‧‧Transmission gate

120‧‧‧ bias control unit

122‧‧‧current source

124‧‧‧Operational Amplifier

126‧‧‧reference voltage generator

FIG. 1 is a schematic diagram of an embodiment of a pixel sensing unit of the present invention.

FIG. 2 is a timing diagram of control signals of the pixel sensing unit of the present invention.

Certain terms are used throughout the description and following claims to refer to particular elements. It should be understood by those of ordinary skill in the art that hardware manufacturers may refer to the same elements by different nouns. The scope of this specification and the subsequent patent application do not use the difference of the names as the means for distinguishing the elements, but the difference in function of the elements as the criterion for distinguishing. The term "including" as used throughout the specification and subsequent claims is an open term and should be interpreted as "including but not limited to". In addition, the term "coupling" is used in this context to include any direct and The electrical connection means. Therefore, if a first device is coupled to a second device, it means that the first device can be directly electrically connected to the second device or indirectly electrically connected to the second device through other devices or connection means.

An image sensor includes a matrix in which a plurality of pixel sensing units are arranged. Here, a pixel sensing unit is taken as an example for illustration. Please refer to FIG. 1. FIG. 1 is an embodiment of a pixel sensing unit 100 of the present invention. The pixel sensing unit 100 includes a photo sensor 102, a pixel amplifier 110, and a bias control unit 120.

The pixel amplifier 110 includes a floating diffusion FD, a reset gate 112, a selection gate 114, a transmission gate 115, a source follower 116, and a capacitor element 118. In other embodiments, the selection of the gate 114 is not necessary, and the purpose of selecting a pixel sensing unit may be achieved in other manners. The capacitive element 118 may be a parasitic capacitance. The reset gate 112 has a control terminal C1, a first connection terminal N11 and a second connection terminal N12, and the reset gate 112 is used to reset the signal S_RST according to the control terminal C1 to the second connection terminal N12. The floating diffusion region FD on the upper side performs a reset operation. The selection gate 114 is used to select a pixel sensing unit for output, but in other embodiments, the selection gate 114 may be omitted, and other operations achieve the same purpose. The selection gate 114 has a control terminal C2, a first connection terminal N21 and a second connection terminal N22, and the selection gate 114 is configured to output a voltage at the first connection terminal N21 according to the selection signal S_SEL received by the control terminal C2. Signal S_V. The source follower 116 is coupled to the second connection terminal N12 of the reset gate 112 and the second connection terminal N22 of the selection gate 114, and the source follower 116 is configured to generate the voltage signal S_V according to the charge signal S_CHG. The capacitive component 118 is, for example, a parasitic capacitor or an actual capacitor coupled between the second connection terminal N12 of the reset gate 112 and the first connection terminal N21 of the selection gate 114. In addition, the transfer gate 115 is coupled to the first connection terminal N11 and the source follower 116 of the reset gate 112 for transmitting the charge signal S_CHG to the source follower 116 according to a transmission signal S_TX.

The bias control unit 120 is coupled to the pixel sensing unit 110, and the bias bias control unit 120 is configured to control one of the voltage levels V of the first connection terminal N21 of the selection gate 114 according to the reset signal S_RST. The bias control unit 120 may be shared by a plurality of pixel amplifiers 110 or shared by a whole row or column of pixel amplifiers 110. For the bias control unit 120, it includes a current source 122, an operational amplifier 124, and a reference voltage generator 126. The current source 122 is coupled to the source follower 116, and the current source 122 is configured to provide a bias current I to the pixel sensing unit 110 according to a control signal S_CTRL. The operational amplifier 124 is coupled to the current source 122 and the source follower 116, and the operational amplifier 124 is configured to generate a control signal S_CTRL according to one of the voltages V_DVI and a reference voltage V_REF generated by the bias current I. The reference voltage generator 126 is coupled to the operational amplifier 124, and the reference voltage generator 126 is configured to adjust the reference voltage V_REF according to the reset signal S_RST. In other embodiments, operational amplifier 124 can control multiple current sources 122 simultaneously.

In this embodiment, the pixel sensing unit 110 first performs a pixel sensing operation, so that the photo sensor 111 receives the optical signal to generate the charge signal S_CHG. At this time, the transfer gate 115 has not transmitted the charge signal S_CHG to the source. Coupler 116. Then, the pixel sensing unit 110 performs a pixel readout operation: first, the reset gate 112 resets the floating diffusion region FD according to one of the reset signals S_RST received by the control terminal C1, for example, the first level. . After the reset gate 112 resets the floating diffusion FD, the source follower 116 outputs according to the reset floating diffusion FD. It is worth noting that after the reset operation, the reset signal S_RST will change to the second level to end the reset operation. At this time, the charge injection effect is easily generated to affect the level of the floating diffusion FD, thereby affecting the source. The output value of the coupler 116 is exemplified by the first level being greater than the second level in this embodiment.

The reference voltage generator 126 adjusts the level of the reference voltage V_REF to compensate for the charge injection effect generated in the pixel sensing unit 100. In the reset operation, the reference voltage V_REF is a higher first reference level, such that the voltage of the divided voltage V_DVI generated by the bias current I is a higher first bias; after the reset operation For a short period of time, the reference voltage V_REF is reduced to a comparison. The low second reference level causes the bias voltage V_DVI generated by the bias current I to be at a lower second bias. When the level of the reference voltage V_REF is lowered, the level of the divided voltage V_DVI generated by the bias current I also decreases, and the transfer gate 115 is turned on/on to transfer the charge signal S_CHG to the floating diffusion. The zone FD is then output by the source follower 116. Since the level of the divided voltage V_DVI drops to a second bias voltage, the voltage level V of the first connection terminal N21 of the selection gate 114 also decreases, so the voltage level V can pass through the capacitive element 118 to affect the floating diffusion region FD. .

Since the charge injection effect is generated when the transfer gate 115 transfers the charge signal S_CHG to the source follower 116 to cause the level of the charge signal S_CHG to decrease slightly, the source follower 116 generates a voltage signal according to the charge signal S_CHG. S_V also drops slightly, causing the output swing of the pixel sensing unit 100 to decrease. Therefore, when the voltage level V is transmitted through the capacitive element 118 to charge the charge signal S_CHG, the level of the charge signal S_CHG can be again Pulling high to compensate for the effect of the charge injection effect, thereby restoring the output swing that the pixel sensing unit 100 should have. It should be noted that the capacitor element 118 can be a capacitor or a parasitic capacitor. As long as the voltage level V can be charged through the capacitor element 118 to charge the charge signal S_CHG, the effect of compensating for the charge injection effect can be achieved. For example, when the first connection end point N11 of the reset gate 112 is directly connected to the first connection end point N21 of the selection gate 114, the second connection end N12 of the reset gate 112 and the first connection end of the selection gate 114 are reset. The parasitic capacitance generated between N21 can be used as the capacitive element 118, however, this is for illustrative purposes only and is not a limitation of the present invention.

Please refer to FIG. 2 , which is a timing diagram of the control signals of the pixel sensing unit 100 of the present invention. In FIG. 2, initially, the pixel sensing unit 100 enters an exposure stage. At this stage, the reset signal S_RST first resets the floating diffusion FD, and transmits the signal during the period of performing the reset operation. S_TX will be turned on, causing transfer gate 115 to pass charge signal S_CHG to source follower 116, thus exposing. When the pixel sensing unit 110 performs the pixel readout operation, the reset signal S_RST and the selection signal S_SEL are simultaneously turned on in the period of time T1~T2, so that The reset gate 112 is reset by the charge signal S_CHG. When the reset gate 112 completes the reset operation of the charge signal S_CHG, the reset signal S_RST is turned off at the time point T2, and the selection signal S_SEL continues to remain on. When the reset signal S_RST is turned off, the reference voltage generator 126 starts to lower the level of the reference voltage V_REF at the time point T3 to raise the level of the charge signal S_CHG again. Then, the transmission signal S_TX is turned on at the time point T4, so that the transfer gate 115 transfers the charge signal S_CHG to the source follower 116. When the transfer gate 115 transfers the charge signal S_CHG to the source follower 116, the transmission signal S_TX is turned off at the time point T5, at which time the selection signal S_SEL remains on, and the reference voltage generator 126 controls the reference voltage V_REF to remain in The low level allows the pixel sensing unit 110 to continue the pixel read operation (ie, the charge signal S_CHG is converted to the voltage signal S_V through the source follower 116 and output through the selection gate 114). After the voltage signal S_V is successfully read out, the selection signal S_SEL is turned off at the time point T6, and the reference voltage generator 126 adjusts the reference voltage V_REF to the high level to complete the pixel sensing operation of the pixel sensing unit 110.

Those skilled in the art can easily understand the operation of each signal shown in FIG. 2 after reading the above paragraphs. For detailed description and changes, reference may be made to the foregoing, and for brevity, it will not be repeated here.

110‧‧‧Pixel Amplifier

100‧‧‧pixel sensing unit

102‧‧‧Light sensor

112‧‧‧Remove the brake

114‧‧‧Selecting the gate

116‧‧‧Source follower

118‧‧‧Capacitive components

115‧‧‧Transmission gate

120‧‧‧ bias control unit

122‧‧‧current source

124‧‧‧Operational Amplifier

126‧‧‧reference voltage generator

Claims (5)

A pixel sensing unit includes: a photo sensor; and a pixel amplifier comprising: a floating diffusion region coupled to the photo sensor; a reset gate having a control terminal, a first a connection end point and a second connection end point, wherein the reset gate is used to reset the signal according to the reset end of the reset gate to the second connection end point of the reset gate The floating diffusion region performs a reset operation; a source follower is coupled to the floating diffusion region for generating an output voltage signal according to the level of the floating diffusion region; and a bias control unit, The source follower is coupled to the source follower to provide a first bias voltage during the reset operation, and a second bias voltage is provided after the reset operation ends. The pixel sensing unit of claim 1, wherein the bias control unit comprises: a current source coupled to the source follower for providing a bias current according to a control signal The operation unit is coupled to the current source and the source follower for generating the control signal according to a voltage division generated by the bias current and a reference voltage; And a reference voltage generator coupled to the operational amplifier for adjusting the reference voltage according to the reset signal. The pixel sensing unit of claim 1, wherein in the pixel reading operation of the pixel sensing unit, the control unit performs the reset operation on the charge signal after the reset gate Controlling the voltage level of the first connection end of the selection gate. The pixel sensing unit of claim 4, wherein the bias control unit lowers the voltage level of the first connection end of the reset gate. The pixel sensing unit of claim 1, wherein the pixel sensing unit further comprises: a transmission gate coupled to the reset gate and the source follower, wherein the control unit controls The transfer gate transmits the charge signal to the source follower during the voltage level of the first connection terminal of the selection gate.