Wide dynamic range pixel circuit

1. Field of the Invention

The present invention relates, generally, to an image sensor, and, more particularly, to an image sensor and a pixel circuit thereof that has a wide dynamic range due to logarithmic-linear-logarithmic operation.

2. Description of the Prior Art

Complementary metal-oxide-semiconductor (CMOS) image sensors are widely used in a variety of commercial applications. The major advantages of CMOS image sensors are high level of integration, low voltage operation and low production cost. However, CMOS image sensors usually have poor dynamic range when compared to a charge-coupled device (CCD) image sensors.

There are several ways to improve the dynamic range of a CMOS image sensor, one of which is logarithmic pixel. It is widely used in a three-transistor (3T) Photodiode pixel for some applications. Generally, an active or passive pixel responds linearly to the light intensity that enters the pixel. A logarithmic pixel is based on the sub-threshold operation mode of a MOS transistor to provide a logarithmic number of electrons into the photodiode when the photocurrent is so small that the transistor enters the sub-threshold region. However, such operation leads to following disadvantages:

1. Low photosensitivity especially in the low illumination region; 2. Slow response due to sub-threshold operation; and 3. A relatively large variation of the device characteristics due to sub-threshold operation.

Therefore, there is still a need to provide a pixel circuit and an image sensor that can overcome the above-mentioned disadvantages.

With this in mind, it is one objective of the present invention to provide a pixel circuit and an image sensor having a wide dynamic range. The inventive pixel circuit has a logarithmic-linear-logarithmic operation in different illumination conditions.

According to one exemplary embodiment, a pixel circuit is provided, which comprises: a photodetector, a signal adjustment circuit and a switch circuit. The photodetector is employed for generating a output signal in response to light that is incident thereon. The signal adjustment circuit is coupled to the photodetector, and employed for selectively adjusting the output signal to allow the output signal to have a plurality of different logarithmic functions in respect to an intensity of the light. The switch circuit is coupled to the signal adjustment circuit and the photodetector, and employed for coupling the photodetector to the signal adjustment circuit.

According to one exemplary embodiment, an image sensor is provided, which comprises: an array of pixel circuits. Each of pixel circuit comprises: a photodetector, a signal adjustment circuit and a switch circuit. The photodetector is employed for generating a output signal in response to light that is incident thereon. The signal adjustment circuit is coupled to the photodetector, and employed for selectively adjusting the output signal to allow the output signal to have a plurality of different logarithmic functions in respect to an intensity of the light. The switch circuit is coupled to the signal adjustment circuit and the photodetector, and employed for coupling the photodetector to the signal adjustment circuit.

Above-mentioned embodiments provide a pixel circuit and an image sensor have logarithmic operations in a low illumination condition or in a high illumination condition. In addition, the pixel circuit and the image sensor have a linear operation in a moderate illumination condition such that the pixel circuit and the image sensor can have a wide dynamic range and a fast response compared to a log pixel or a linear pixel.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

FIG. 1 is a simplified diagram of a pixel circuit according to one exemplary embodiment of the present invention.

FIG. 2 is a circuit diagram of a pixel circuit according to one exemplary embodiment of the present invention.

FIG. 3 is a circuit diagram of a pixel circuit according to another exemplary embodiment of the present invention.

FIG. 4 is a circuit diagram of a pixel circuit according to still another exemplary embodiment of the present invention.

FIG. 5 is a circuit diagram of a pixel circuit according to yet another exemplary embodiment of the present invention.

FIG. 6 is a circuit diagram of a readout part according to one exemplary embodiment of the present invention.

With reference to FIG. 1 , a simplified diagram of a pixel circuit according to one exemplary embodiment is depicted. As depicted in FIG. 1 , the pixel circuit comprises a photodetector 110 , a signal adjustment circuit 120 and a switch circuit 130 . The photodetector 110 , preferably a photodiode (but not limited to, which may be a phototransistor), is employed for generating an output signal V PHOTO in response to light that is incident thereon. The signal adjustment circuit 120 is coupled to the photodetector 110 , and utilized for selectively adjusting the output signal V PHOTO to allow the output signal V PHOTO to have a plurality of different logarithmic functions in respect to an intensity of the light. The signal adjustment circuit 120 may be operated in a sub-threshold region, which allows the output signal V PHOTO to be the logarithmic function in respect to the intensity of the light if the switch circuit 130 is conductive. The switch circuit 130 is coupled to the signal adjustment circuit 120 and the photodetector 110 is utilized for coupling the photodetector 110 to the signal adjustment circuit 120 . When the switch circuit 130 operates to couple the signal adjustment circuit 120 to the photodetector 110 , the output signal V PHOTO will have the logarithmic function in respect to the intensity of the light. Otherwise, the output signal V PHOTO will have a linear function in respect to the intensity of the light.

FIG. 2 depicts a detailed circuit diagram of a pixel circuit 100 according to one exemplary embodiment of the present invention. The signal adjustment circuit 120 comprises a first signal adjustment component 122 , which preferably includes a diode-connected transistor M 1 , and is coupled to the photodetector 110 (which comprises a photodiode PD) via the switch circuit 130 which comprises a first switching element M 2 controlled by a control signal x 1 . When the control signal x 1 allows the first switching element M 2 to be conductive, the first signal adjustment component 122 will adjust the output signal V PHOTO , making the output signal V PHOTO to have a first logarithmic function in respect to the intensity of the light. A predetermined voltage Vlog 1 is coupled to one specific electrode and one control electrode of the diode-connected transistor M 1 . The predetermined voltage Vlog 1 determines the characteristics of the first logarithmic function of the output signal V PHOTO caused by the diode-connected transistor M 1 and when the diode-connected transistor M 1 operates in a sub-threshold region to make the output signal V PHOTO have the first logarithmic function. When the switch circuit 130 couples to the diode-connected transistor M 1 to the photodetector 110 , and the diode-connected transistor M 1 starts to operate in the sub-threshold region, charges will be provided by the diode-connected transistor M 1 to increase the current flowing into the node A. In this regards, the pixel circuit 100 acts as a log pixel. The output signal V PHOTO drops more largely than a linear pixel, providing stronger luminance information. When the voltage of node A drops below a certain level, the diode-connected transistor M 1 will be turned off, and the output signal V PHOTO will therefore have a linear function in respect to the intensity of the light. Meanwhile, the first switching element M 2 is not conductive by pulling down the control signal x 1 and the pixel circuit 100 acts as a linear pixel.

By properly setting the predetermined voltage Vlog 1 , it is also available that the diode-connected transistor M 1 operates to restrain the output signal V PHOTO to prevent from being saturated. In a high illumination condition, the output signal V PHOTO may be saturated. Under such condition, the first switching element M 2 couples to the diode-connected transistor M 1 to the photodetector 110 , the diode-connected transistor M 1 operates in the sub-threshold region, providing a sub-threshold current which is much smaller than a high photocurrent generated by the photodiode PD due to high illumination such that the drop of the output signal V PHOTO can be restrained. As a result, the output signal V PHOTO will not be saturated.

The switch circuit 130 may include an optional component, second switching element M 3 controlled by a control signal x 2 . The second switching element M 3 allows the output signal V PHOTO to be read out by a readout part of the pixel circuit 100 . The switching element M 3 is able to prevent the output signal V PHOTO from being interfered with noises during an integration period of the pixel circuit 100 . When the integration period ends, the second switching element M 3 is conductive and the readout part can derive the output signal V PHOTO .

FIG. 3 depicts a detailed circuit diagram of the pixel circuit according to another exemplary embodiment of the present invention. In this embodiment, the signal adjustment circuit 120 comprises a first signal adjustment component 122 as well as a second signal adjustment component 124 . Preferably, the first signal adjustment component 122 includes a diode-connected transistor M 1 while the second signal adjustment component 124 includes a transistor M 4 . Both of the transistors M 1 and M 4 are coupled to the photodetector 110 (which comprises a photodiode PD) via the switch circuit 130 which comprises a first switching element M 2 controlled by a control signal x 1 . When the control signal x 1 allows the first switching element M 2 to be conductive, the first signal adjustment component 122 and the first signal adjustment component 124 will adjust the output signal V PHOTO , making the output signal V PHOTO to have one or more logarithmic function in respect to the intensity of the light.

Both of the transistors M 1 and M 4 have one electrode being coupled to predetermined voltages Vlog 1 and Vlog 2 , respectively. In case of the voltage Vlog 1 identical to voltage Vlog 2 , both of the transistors M 1 and M 4 can provide charges to increase the current flowing into the node A, thereby to enhance the drop of the output signal V PHOTO . The predetermined voltage Vlog 1 as well as Vlog 2 determines the characteristics of the first logarithmic function of the output signal V PHOTO and when the diode-connected transistor M 1 and the transistor M 4 operate in the sub-threshold region to make the output signal V PHOTO have the first logarithmic function. If the voltage Vlog 1 is different from the voltage Vlog 2 , one of the transistors M 1 and M 4 may be used to enhance the drop of the output signal V PHOTO while other is used to restrain the drop of the of the output signal V PHOTO . Such configuration can therefore improve the dynamic range of the pixel circuit 200 . The transistor M 1 may make the output signal V PHOTO have a first logarithmic function in respect to the intensity of the light while the transistor M 4 may make the output signal V PHOTO have a second logarithmic function in respect to the intensity. The characteristics of a second logarithmic function are determined by the predetermined voltage Vlog 2 . This could be useful in a scene having complicated lightings. The pixel circuit 200 has better responses than a linear pixel in low illumination condition and in a high illumination condition due to the first logarithmic function and the second logarithmic function of the output signal V PHOTO . If the illumination is moderate, the pixel circuit 200 will have a linear function by cutting off the control signal x 1 of the first switching element M 2 . The period during which the transistors M 1 operates in the sub-threshold region and the period during which the transistors M 4 operates in the sub-threshold region may be partially overlapped or not overlapped, which depends on the selection of the predetermined voltages Vlog 1 and Vlog 2 .

Similarly, the switch circuit 130 may include an optional component, second switching element M 3 controlled by a control signal x 2 . The second switching element M 3 allows the output signal V PHOTO to be read out by a readout part of the pixel circuit 100 . The switching element M 3 is able to prevent the output signal V PHOTO from being interfered with noises during an integration period of the pixel circuit 100 . When the integration period ends, the second switching element M 3 is conductive and the readout part can derive the output signal V PHOTO .

In one exemplary embodiment, the switching circuit 130 further comprises a third switching element M 5 . This is illustrated by a pixel circuit 300 as shown in FIG. 4 . The third switching element M 5 is coupled between the photodetector 110 and the signal adjustment circuit 120 , which is controlled by a control signal x 3 . When the control signal x 3 allows the third switching element M 5 to be conductive, the second signal adjustment component 124 will adjust the output signal V PHOTO , making the output signal V PHOTO to have the logarithmic function in respect to the intensity of the light. In this exemplary embodiment, the first signal adjustment component 122 and the second signal adjustment component 124 are coupled to the photodetector 110 via different switching elements.

To further improve the response of the pixel circuit in a complicated lighting environment, the signal adjustment circuit 120 may comprises a plurality of signal adjustment components such that the enhancement or the restraining of the drop of the output signal V PHOTO can be improved. Please refer to FIG. 5 , which depicts a detailed circuit diagram of a pixel circuit 400 according to one exemplary embodiment of the present invention. In this exemplary embodiment, the pixel circuit 400 has a first signal adjustment component M 1 and two second signal adjustment components M 2 and M 3 . Therefore, the output signal V PHOTO can be adjusted to have a plurality of logarithmic functions in respect to the intensity of the light according to the predetermined voltages Vlog 1 , Vlog 2 and Vlog 3 , respectively. The first signal adjustment component M 1 is coupled to the photodetector 110 via the switching element M 4 controlled by the signal x 1 , the second signal adjustment component M 2 is coupled to the photodetector 110 via the switching element M 5 controlled by the signal x 2 , and the third signal adjustment component M 3 is coupled to the photodetector 110 via the switching element M 6 controlled by the signal x 3 . The switching element M 4 controlled by the signal x 4 is responsible for allowing the signal to be read out. Depending on the value of the predetermined voltages Vlog 1 , Vlog 2 , and Vlog 3 , the output signal V PHOTO will have different logarithmic functions under different illumination condition such that the drop of the output signal V PHOTO can be enhanced in a low illumination condition or be restrained in a high illumination condition. Therefore, the pixel circuit 400 has a very wide dynamic range.

In order to read out the output signal V PHOTO , the pixel circuits as mentioned above further comprise a readout part which may include reset transistor and a source follower. Please refer to FIG. 6 , which illustrated a readout part according to one exemplary embodiment of the present invention. As shown in FIG. 6 , a readout part 500 includes a reset transistor M rst and a source follower M sf . The reset transistor M rst has a first electrode coupled to the photodetector 110 , a second electrode coupled to a supply voltage Vdd, and a control electrode coupled to a reset control signal RST. The source follower M sf has a control electrode coupled to the photodetector 110 , and is employed for generating a pixel data output V Data according to the output signal V PHOTO . Additionally, the readout part 500 may further comprise a selecting circuit M sel which is coupled to the source follower M sf , and utilized for transferring the pixel data output V Data during a read-out period of the pixel circuit (e.g. pixel circuits 100 - 400 ). Please note that only one of the selecting circuit M sel and the second switching element (e.g. M 3 of the pixel circuits 100 , 200 , and 300 , or M 7 of the pixel circuit 400 ) is necessary. Depending on different requirement, it is possible that embodiments of the present invention include either both of the selecting circuit M sel and the second switching element or only one of the selecting circuit M sel and the second switching element. Operations and principles thereof are well known to those skilled in the art. Therefore, detailed descriptions regarding the above-mentioned components are omitted here for the sake of brevity.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.