Global shutter CMOS pixel circuit and image capturing method thereof

This application claims priority of International Patent Application Serial No. PCT/CN2017/112353, filed Nov. 22, 2017, which is related to and claims priority of Chinese patent application Serial No. 201611090316.X, filed Nov. 30, 2016. The entirety of each of the above-mentioned patent applications is hereby incorporated herein by reference and made a part of this specification.

The present disclosure generally relates to the field of image sensors, more particularly, to a global shutter CMOS pixel circuit and image capturing method.

Global shutter CMOS image sensors are widely applied in the fields of monitoring, scientific applications, industrial vision, etc. due to its advantage of high data transmission rate. Currently, there exists increasingly demanding for the image sensors of high speed and high dynamic imaging in many applications.

The conventional global shutter CMOS image sensor cannot provide images of high dynamic range by one-time exposure. Although integrating images by multiple exposures may achieve high dynamic image, it requires complicated mathematical algorithms which greatly decreases the frame rate and weakens the characteristic of high speed imaging of the global shutter pixels.

Accordingly, an objective of the present disclosure is to provide a global shutter CMOS pixel circuit with high dynamic range to increase the data transmission rate of the global shutter pixel.

To achieve the above purposes, the present disclosure provides a global shutter CMOS pixel circuit comprising a power supply unit, a pixel signal generating unit, a signal sampling and holding unit, and a signal outputting unit. The power supply unit is connected to the pixel signal generating unit, the signal sampling and holding unit, and the signal outputting unit. An output of the pixel signal generating unit is connected to an input of the signal sampling and holding unit. An output of the signal sampling and holding unit is connected to an input of the signal outputting unit.

The pixel signal generating unit comprises: a first current source, a second current source, a first control terminal, a second control terminal, a bias voltage terminal, a column selection terminal, a photodiode, a first transistor, a second transistor, a third transistor, a fourth transistor, a fifth transistor. The drain of the first transistor is connected to the positive electrode of the power supply unit; the gate of the first transistor is connected to the bias voltage terminal; the source of the first transistor is connected to the drain of the second transistor, the drain of the third transistor, the source of the fourth transistor at a node. The source of the second transistor is connected to cathode of the photodiode, the gate of the second transistor is connected to the column selection terminal. The anode of the photodiode is connected to the negative electrode of the power supply unit. The source of the third transistor is connected to the positive electrode of the first current source, the gate of the third transistor is connected to the first control terminal. The other electrode of the first current source is connected to the negative electrode of the power supply unit. The drain of the fourth transistor is connected to the gate of the fifth transistor, the gate of the fourth transistor is connected to the second control terminal. The drain of the fifth transistor is connected to the positive electrode of the power supply unit, the source of the fifth transistor is connected to the positive electrode of the second current source and is the output of the pixel signal generating unit. The other electrode of the second current source is connected to the negative electrode of the power supply unit.

Preferably, the signal sampling and holding unit comprises a sixth transistor, a seventh transistor, a first capacitor, a second capacitor, a third control terminal and a fourth control terminal. The source of the sixth transistor as the input of the signal sampling and holding unit is connected to the source of the fifth transistor, the drain of the sixth transistor is connected to the source of the seventh transistor and one end of the first capacitor, the gate of the sixth transistor is connected to the third control terminal. The drain of the seventh transistor is connected to one end of the second capacitor and is the output of the signal sampling and holding unit, the gate of the seventh transistor is connected to the fourth control terminal. The other end of the first capacitor and the other end of the second capacitor are connected to the negative electrode of the power supply unit.

Preferably, the first transistor, the second transistor, the third transistor, the fourth transistor, the fifth transistor, the sixth transistor, and the seventh transistor are NMOS transistors.

Preferably, the signal outputting unit comprises an eighth transistor, a ninth transistor, a third current source and a row selection terminal. The gate of the eighth transistor as the input of the signal outputting unit is connected to the drain of the seventh transistor, the drain of the eighth transistor is connected to the positive electrode of the power supply unit, the source of the eighth transistor is connected to the source of the ninth transistor. The drain of the ninth transistor is connected to the positive electrode of the third current source and is the output of the signal outputting unit, the gate of the ninth transistor is connected to the row selection terminal. The other electrode of the third current source is connected to the negative electrode of the power supply unit.

Preferably, the eighth transistor is an NMOS transistor.

To achieve the above purposes, the present disclosure also provides an image capturing method by using the above-mentioned global shutter CMOS pixel circuit. The method comprises the following steps:

S01: exposing the photodiode to photons, then turning on the fourth transistor after the exposure;

S02: turning on the second transistor, the sixth transistor, the seventh transistor, at this time the first transistor works in preset sub-threshold region; then, turning off the seventh transistor, the second capacitor keeps a first voltage;

S03: turning on the third transistor, at this time, the first transistor works in saturation region; then, turning off the sixth transistor, the first capacitor keeps a second voltage;

S04: turning off the second transistor and the third transistor, then turning off the fourth transistor;

S05: turning on the ninth transistor, the signal outputting unit outputs a first output voltage; then, turning on the seventh transistor, the signal outputting unit outputs a second output voltage;

S06: turning off the seventh transistor, then turning off the ninth transistor, such that a readout of image signal is accomplished.

According to the present disclosure, the output signal of the pixel and the photo-generated current are set to have a logarithmic relationship, which effectively increases the signal dynamic range. Therefore, image signal transmission with high speed and high dynamic range can be achieved simultaneously. Furthermore, the pixels in the present disclosure can eliminate the process variations, which increases the consistency of the pixels.

FIG. 1 is a circuit diagram of a global shutter CMOS pixel circuit with high dynamic range according to a preferred embodiment of the present disclosure;

FIG. 2 is a sequence diagram of an image capturing method according to a preferred embodiment of the present disclosure;

The present disclosure will now be descried more fully hereinafter with reference to the accompanying drawings. It will be understood that various changes in form and details may be made herein without departing from the spirit and scope of the invention; and the embodiments and drawings are substantially used for illustrating the invention and should not be used as limitation to the present disclosure.

The present disclosure will now be descried more fully hereinafter with reference to FIG. 1 - FIG. 2 . It should be noted that the drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the present disclosure.

Referring to FIG. 1 , the global shutter CMOS pixel circuit according to the embodiment comprises a power supply unit, a pixel signal generating unit 1 , a signal sampling and holding unit 2 and a signal outputting unit 3 . The power supply unit is connected to the pixel signal generating unit 1 , the signal sampling and holding unit 2 and the signal outputting unit 3 . An output of the pixel signal generating unit 1 is connected to an input of the signal sampling and holding unit 2 . An output of the signal sampling and holding unit 2 is connected to an input of the signal outputting unit 3 .

The pixel signal generating unit 1 comprises: a first current source I 1 , a second current source I 2 , a first control terminal S 1 , a second control terminal S 2 , a bias voltage terminal BIAS, a column selection terminal TX, a photodiode PD, a first transistor M 1 , a second transistor M 2 , a third transistor M 3 , a fourth transistor M 4 , a fifth transistor M 5 . The drain of the first transistor M 1 is connected to the positive electrode VDD of the power supply unit; the gate of the first transistor M 1 is connected to the bias voltage terminal BIAS; the source of the first transistor M 1 is connected to the drain of the second transistor M 2 , the drain of the third transistor M 3 , the source of the fourth transistor M 4 at a node. The source of the second transistor M 2 is connected to cathode of the photodiode PD, the gate of the second transistor M 2 is connected to the column selection terminal TX. The anode of the photodiode PD is connected to the negative electrode VSS of the power supply unit. The source of the third transistor M 3 is connected to the positive electrode of the first current source I 1 , the gate of the third transistor M 3 is connected to the first control terminal S 1 . The other electrode of the first current source I 1 is connected to the negative electrode VSS of the power supply unit. The drain of the fourth transistor M 4 is connected to the gate of the fifth transistor M 5 , the gate of the fourth transistor M 4 is connected to the second control terminal S 2 . The drain of the fifth transistor M 5 is connected to the positive electrode VDD of the power supply unit, the source of the fifth transistor M 5 is connected to the positive electrode of the second current source I 2 and is the output of the pixel signal generating unit. The other electrode of the second current source I 2 is connected to the negative electrode VSS of the power supply unit.

The signal sampling and holding unit 2 comprises a sixth transistor M 6 , a seventh transistor M 7 , a first capacitor C 1 , a second capacitor C 2 , a third control terminal S 3 and a fourth control terminal S 4 . The source of the sixth transistor M 6 as the input of the signal sampling and holding unit 2 is connected to the source of the fifth transistor M 5 , the drain of the sixth transistor M 6 is connected to the source of the seventh transistor M 7 and one end of the first capacitor C 1 , the gate of the sixth transistor M 6 is connected to the third control terminal S 3 . The drain of the seventh transistor M 7 is connected to one end of the second capacitor C 2 and is the output of the signal sampling and holding unit 2 , the gate of the seventh transistor M 7 is connected to the fourth control terminal S 4 . The other end of the first capacitor C 1 and the other end of the second capacitor C 2 are connected to the negative electrode VSS of the power supply unit.

Herein, the first transistor M 1 , the second transistor M 2 , the third transistor M 3 , the fourth transistor M 4 , the fifth transistor M 5 , the sixth transistor M 6 , and the seventh transistor M 7 are all NMOS transistors.

The signal outputting unit 3 comprises an eighth transistor M 8 , a ninth transistor M 9 , a third current source I 3 and a row selection terminal RS. The gate of the eighth transistor M 8 as the input of the signal outputting unit 3 is connected to the drain of the seventh transistor M 7 , the drain of the eighth transistor M 8 is connected to the positive electrode VDD of the power supply unit, the source of the eighth transistor M 8 is connected to the source of the ninth transistor M 9 . The drain of the ninth transistor M 9 is connected to the positive electrode of the third current source I 3 and is the output of the signal outputting unit 3 , the gate of the ninth transistor M 9 is connected to the row selection terminal RS. The other electrode of the third current source I 3 is connected to the negative electrode VSS of the power supply unit.

Pleas referring to FIG. 2 and FIG. 1 , the image capturing method by using the above-mentioned global shutter CMOS pixel circuit comprises the following steps:

S01: exposing the photodiode to photons, then turning on the fourth transistor after the exposure;

S02: turning on the second transistor, the sixth transistor, the seventh transistor, at this time the first transistor works in preset sub-threshold region; then, turning off the seventh transistor, at this time, the second capacitor keeps a first voltage;

Specifically, in the step S02, when the first transistor works in sub-threshold region, the source voltage of the fifth transistor equals to the first voltage.

The first voltage is calculated as follows:

Wherein, V a is the value of the first voltage, V bias is the bias voltage, I bias is the bias current, V th,M1 is the threshold voltage of the first transistor, V th,M5 is the threshold voltage of the fifth transistor, n is the ideal sub-threshold factor, I 0 is the drain current when the gate-source voltage of a MOSFET is equal to its threshold voltage; V t is the thermal potential, I p is the photo-generated current of the photodiode, I L is the reverse photo-generated current of the photodiode, β 5 =uC ox , u is the carrier mobility, C ox is the gate oxide capacitance per unit area.

S03: turning on the third transistor, at this time, the first transistor works in saturation region; then, turning off the sixth transistor, at this time the first capacitor keeps a second voltage;

Specifically, in the step S03, when the first transistor works in saturation region, the source voltage of the fifth transistor equals to the second voltage.

The second voltage is calculated as follows:

Wherein, V b is the value of the second voltage, V bias is the bias voltage, I 1 is the current of the first current source, I 2 is the current of the second current source, V th,M1 is the threshold voltage of the first transistor, V th,M5 is the threshold voltage of the fifth transistor, β 1 =u 1 C ox , β 5 =u 5 C ox , u 1 is the carrier mobility of the first transistor, u 5 is the carrier mobility of the fifth transistor, C ox is the gate oxide capacitance per unit area.

S04: turning off the second transistor and the third transistor, then turning off the fourth transistor;

S05: turning on the ninth transistor, the signal outputting unit outputs a first output voltage; then, turning on the seventh transistor, the signal outputting unit outputs a second output voltage;

Specifically, the first output voltage equals to the first voltage. The second output voltage is calculated as: V 2 =(V a *C 2 +V b *C 1 )/(C 1 +C 2 ). Wherein, V a is the value of the first voltage, C 2 is the value of the second capacitor, V b is the value of the second voltage, C 1 is the value of the first capacitor.

S06: turning off the seventh transistor, then turning off the ninth transistor, such that a readout of image signal is accomplished.

After reading out and outputting the image signal to a peripheral circuit, the image signal is further processed. From the above equations (1), (2), it is known that the threshold voltage of the fifth transistor affects both the first voltage and the second voltage. In addition, since the first output voltage equals to the first voltage, the second output voltage has relationship with the first voltage, the threshold voltage of the fifth transistor also affects the first output voltage and the second output voltage, and accordingly affects the image signal. Therefore, the second output voltage can be subtracted from the first output voltage to avoid the influence of the threshold voltage of the fifth transistor. The specific principle will be described as follows:

The difference value of the first voltage V a and the second voltage V b is:

Since the first output voltage equals to the first voltage as: Vout 1 =V a , the second output voltage is calculated as: Vout 2 =(V a *C 2 +V b *C 1 )/(C 1 +C 2 ), then when C 1 =C 2 , Vout 2 =(V a +V b )/2, and Vout 1 −Vout 2 =(V a −V b )/2.

Therefore, the subsequent image processing circuit can avoid the influence of the threshold voltage of the fifth transistor to the image signal by subtracting the second output voltage Vout 2 from the first output voltage Vout 1 , thereby effectively increasing the consistency of the image signal outputted by the pixel array.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.