Apparatus of high dynamic-range CMOS image sensor and method thereof
The present invention discloses an apparatus of high dynamic-range CMOS image sensor and method thereof. The present invention utilize a pixel circuit outputting an output signal, wherein the pixel circuit has a photodiode and a plurality of transistors; and utilize a current source as a charge supplement unit to supply current into one end of the photodiode, and providing charges to the parasitic capacitor of the photodiode to delay saturation in the pixel circuit. In addition, a feedback circuit can be further designed connecting the pixel circuit. The feedback circuit receives the output signal from the pixel circuit and then outputs a control signal according to the output signal of the pixel circuit to control status of the charge supplement unit, and thereby increasing the dynamic range of the CMOS image sensor.
This application is a Divisional patent application of co-pending application Ser. No. 11/211,670, filed on 26 Aug. 2005. The entire disclosure of the prior application, Ser. No. 11/211,670, from which an oath or declaration is supplied, is considered a part of the disclosure of the accompanying Divisional application and is hereby incorporated by reference.
BACKGROUND OF INVENTION1. Field of the Invention
The invention relates to an apparatus of high dynamic-range CMOS image sensor and method thereof, and more particularly, to an apparatus of high dynamic-range CMOS image sensor and method thereof that providing charge supplement to the photodiode.
2. Description of the Prior Art
Dynamic range is an important norm to evaluate quality of an image sensor. Larger the dynamic range, larger the sensible range from lowest sensible signal to highest unsaturated signal of the image sensor is.
Technologies of designing image sensors with high dynamic ranges can be classified into various methods: photonic signal compression, multiple sampling, exposure-time adjustment and so on. In the conventional technologies, the photonic signal compression method will lower the sensitivity of an image sensor, especially the signal noise ratio will be smaller under low illumination, and the Correlated Double Sampling (CDS) circuit cannot be used to reduce the Fixed Pattern Noise (FPN) due to no reset signal, so the image quality is limited. In addition, this method combining linear and logarithmic integration modules will make extra power consumption and increase design complexity of the signal processing circuit. The methods of multiple sampling and exposure-time adjustment can achieve high sensitivity and increase the dynamic range, but the exposure time must be adequately controlled by the complex backend circuit. This backend circuit will enlarge the chip size, and its processing time will reduce the reading speed that makes the sensor cannot capture images immediately.
Hence, the present invention discloses an apparatus of high dynamic-range CMOS image sensor and method thereof to overcome the above-mentioned drawbacks.
SUMMARY OF INVENTIONIt is therefore a primary objective of the claimed invention to provide an apparatus of high dynamic-range CMOS image sensor and method thereof that have a charge supplement unit in the pixel circuit to provide charges to the parasitic capacitor of the photodiode for the purpose of supplying charges and delaying saturation.
It is therefore another objective of the claimed invention to provide an apparatus of high dynamic-range CMOS image sensor and method thereof that utilize a feedback circuit to duly send a trigger signal to the charge supplement unit when the node voltage of photodiode descends to a predetermined value.
It is therefore a further objective of the claimed invention to provide an apparatus of high dynamic-range CMOS image sensor and method thereof that retain the reset signal for the correlated double sampling circuit to effectively reduce the fixed pattern noise.
It is therefore a further objective of the claimed invention to provide an apparatus of high dynamic-range CMOS image sensor and method thereof that utilize the feedback circuit and charge supplement unit to make the image sensor applicable in the high contrast environment without changing exposure time and make each pixel having an independent high dynamic range.
According to the claimed invention, an apparatus of high dynamic-range CMOS image sensor and method thereof utilize a pixel circuit outputting an output signal, wherein the pixel circuit has a photodiode and a plurality of transistors; and utilize a current source as a charge supplement unit to supply current into one node of the photodiode, and providing charges to the parasitic capacitor of the photodiode to delay saturation in the pixel circuit. In addition, a feedback circuit can be further designed connecting the pixel circuit. The feedback circuit has a PMOS source follower and a PMOS inverter, and after receiving the output signal from the pixel circuit, the feedback circuit will generate a control signal according to this output signal to control status of the charge supplement unit and then enhance the dynamic range of the CMOS image sensor.
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 implement that is illustrated in the various figures and drawings.
The present invention discloses an apparatus of high dynamic-range CMOS image sensor and method thereof. The present invention provides charge supplement with a charge supplement unit, and the charge supplement unit can be controlled by using an external bias voltage or connecting a feedback circuit to enable that the saturation time of the parasitic capacitor of the photodiode can be delayed and the dynamic range of the image sensor can be enlarged. The present invention also has the advantages of low noise, high sensitivity and low complexity.
As shown in
For reducing the amounts of I.sub.photo and discharging current,
The conception in
Based on the structure of the conventional pixel circuit using the linear integration module, a practical example of an image sensor is realized by using the charge supplement mechanism with the feedback circuit, as shown in
As shown in
Please refer to part of the pixel circuit 10 in
The pixel circuit 10 is constructed at a linear integration structure. For proving that the pixel circuit 10 can effectively enhance the dynamic range, the gate of M1 is addressed by the fixed control voltage V.sub.g, and the variation of terminal voltage of the photodiode 22, or the source voltage V.sub.s of M1, is observed under the simulation of different photo currents I.sub.photo induced by the illuminated photodiode 22 under the same integration time. The simulated result is shown in
As shown in
In addition, the present invention is realized by using the 2P4M 0.35 .mu.m process provided by TSMC, and the measured result of the test chip of the proposed 5.times.5-pixel image sensor is shown in
In conclusion, the present invention is an apparatus of high dynamic-range CMOS image sensor and method thereof. The basic conception is using the charge supplement module to supply charges to the parasitic capacitor in the photodiode to delay the saturation. The charge supplement unit can utilize not only the external bias signal to control turning on/off of the charge supplement unit and amount of charges supplied but also the feedback circuit to control whether the charge supplement unit provides charges to delay the saturation time of the photodiode. The turning on/off of the charge supplement unit and the amount of charges supplied depends on the illumination, so that the output signal of the pixel circuit can be sent to the feedback circuit and then the control signal from the feedback circuit can be used for controlling the operation status of the charge supplement unit. The photodiode will not rapidly discharge the photo current under high illumination to become saturation, so that the dynamic range can be effectively enhanced. The present invention need not change the exposure time under high contrast environment, and each pixel has an independent large dynamic range. In the pixel circuit, the reset signal is also retained and suitable for the correlated double sampling circuit. The present invention has advantages of high sensitivity, large dynamic range, low circuit complexity, only once-required reset and low fixed pattern noise.
Those skilled in the art will readily observe that numerous modifications and alterations of the device 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.
Claims
1. A method for increasing dynamic range of CMOS image sensor, comprising: outputting an output signal through a pixel circuit, wherein the pixel circuit comprises a photodiode and a plurality of transistors; and utilizing a current source as a charge supplement unit to supply current into one end of the photodiode, and providing charges to the parasitic capacitor of the photodiode to delay saturation in the pixel circuit.
2. The method for enhancing dynamic range of CMOS image sensor of claim 1, wherein the output signal of the pixel circuit can be further received by a feedback circuit and the feedback circuit outputs a control signal according to the output signal of the pixel circuit to control status of the charge supplement unit.
3. The method for increasing dynamic range of CMOS image sensor of claim 1, wherein the current source is a constant current source or a variable current source.
4. The method for increasing dynamic range of CMOS image sensor of claim 3, wherein the constant current source provides charge supplement to delay saturation of the pixel circuit.
5. The method for increasing dynamic range of CMOS image sensor of claim 3, wherein the variable current source can be further provided by a bias signal to control the amount of currents and supply applicable charges to delay saturation of the pixel circuit.
6. The method for increasing dynamic range of CMOS image sensor of claim 1, wherein the pixel circuit is linear integration mode.
7. The method for increasing dynamic range of CMOS image sensor of claim 1, wherein the charge supplement unit can supply charges to the parasitic capacitor of the photodiode during discharging to enhance dynamic range of the CMOS image sensor.
8. The method for increasing dynamic range of CMOS image sensor of claim 1, wherein the charge supplement unit further comprises a control node, when the current source is a variable current source, the control node can determine whether turning on the charge supplement unit and amount of the charges supplied according to the control signal of the feedback circuit.
9. The method for increasing dynamic range of CMOS image sensor of claim 8, wherein the control node can be provided by a bias signal and the feedback circuit receives the output signal of the pixel circuit and processes it to yield the output signal to address the control node of the charge supplement unit.
10. The method for increasing dynamic range of CMOS image sensor of claim 2, wherein the feedback circuit can comprise an amplifier with variable active loading, controlled by the output signal of the pixel circuit.
11. The method for increasing dynamic range of CMOS image sensor of claim 2, wherein the feedback circuit controls the charges of the charge supplement unit supplying into the photodiode by a voltage-level way.
Type: Application
Filed: May 12, 2008
Publication Date: Jan 1, 2009
Inventors: Oscal T.-C Chen (Taipei), Wei-Jen Liu (Yuanlin Town), Hsiu-Fen Yeh (Gangshan Town)
Application Number: 12/149,973
International Classification: H04N 5/335 (20060101);