Wide dynamic range active pixel with knee response
A pixel circuit, and a method for operating a pixel circuit, to provide a multiple knee response characteristic. In one embodiment of the invention, one or more feed-through pulse (FTP) signals are transmitted to an integration node to end a first linear integration time period. The FTP signal allows electrons to drain from the integration node to a reset node through a transfer gate. After the first integration period, a second linear integration period is conducted on the pixel circuit, where the photo conversion gain of the pixel circuit becomes reduced under higher illumination conditions due to the drained node. Such operation creates a pixel with a photo response having multiple “knee” points, where each “knee” in the photo response curve will create separate regions whose photo sensitivities can be independently controlled with minimal thermal interference. By setting different voltage levels for the FTP signal and by controlling the integration time periods, the photo-response of the pixel circuit can be easily controlled.
1. A pixel circuit, comprising:
a photoconversion device for accumulating charge during a first integration period and second integration period, said device being connected to an integration node;
a first transistor, having one terminal connected to said integration node and another terminal connected to a reset line; and
a feed-through pulse capacitor, having one terminal coupled to a feed-through pulse signal line, and a second terminal coupled to said integration node, said feed-through pulse signal line providing an intermediate pulse between said first and second integration periods to generate an overflow current in said first transistor during medium and high illumination conditions.
2. The pixel circuit of claim 1 , further comprising an output circuit, connected to said integration node, for reading out a signal at said integration node.
3. The pixel circuit of claim 2 , wherein said output circuit further comprises a second transistor operating as a source-follower readout transistor.
4. The pixel circuit of claim 1 , wherein said photoconversion device is a photodiode.
5. The pixel circuit of claim 4 , wherein the first transistor has a gate coupled to a supply voltage.
6. The pixel circuit of claim 5 , wherein said reset signal line and said feed-through pulse line are operable to apply a reset voltage to said integration node.
7. The pixel circuit of claim 6 , further comprising an output circuit to read out said reset voltage to an output line.
8. The pixel circuit of claim 7 , wherein a pixel select line coupled to the integration node, is operable to begin a first charge integration period.
9. The pixel circuit of claim 8 , wherein said pixel select line is operable to begin a second charge integration period following said first integration period.
10. A CMOS imaging device, comprising:
at least one pixel circuit;
a sample and hold circuit for storing a reset signal and an image signal produced by said pixel circuit;
an amplifier for subtracting the reset signal from the image signal;
a digitizer for receiving the output of said amplifier; and
an image processor for receiving the output of said digitizer, wherein said pixel circuit further comprises:
a photoconversion device for accumulating charge during a first integration period and second integration period, said device being connected to an integration node;
a first transistor, having one terminal connected to said integration node and another terminal connected to a reset line; and
a feed-through pulse capacitor, having one terminal coupled to a feed-through pulse signal line, and a second terminal coupled to said integration node, said feed-through pulse signal line providing an intermediate pulse between said first and second integration periods to generate an overflow current in said first transistor during medium and high illumination conditions.
11. The imaging device of claim 10 , further comprising an output circuit, connected to said integration node, for reading out a signal at said integration node.
12. The imaging device of claim 11 , wherein said output circuit further comprises a second transistor operating as a source-follower readout transistor.
13. The imaging device of claim 10 , wherein said photoconversion device is a photodiode.
14. The imaging device of claim 13 , wherein said first transistor has a gate coupled to a supply voltage.
15. The imaging device of claim 14 , wherein said reset signal line and said feed-through pulse line are operable to apply a reset voltage to said integration node.
16. The imaging device of claim 15 , further comprising an output circuit to read out said reset voltage to an output line.
17. The imaging device of claim 16 , wherein a pixel select line coupled to the integration node, is operable to begin a first charge integration period.
18. The imaging device of claim 17 , wherein said pixel select line is operable to begin a second charge integration period following said first integration period.
19. A method of operating a pixel circuit, comprising:
collecting photogenerated charge at an integration node in response to a first pixel signal during a first part of a charge integration period;
draining photogenerated charge from said integration node to a reset node through a first transistor at the conclusion of the first part of the integration period;
collecting photogenerated charge at the integration node in response to a second pixel signal during a second part of the charge integration period; and
injecting an intermediate pulse between said first and second integration periods to generate an overflow current in said first transistor during medium and high illumination conditions.
20. The method of claim 19 , wherein the first part of the integration period produces a first linear photoconversion response in the pixel circuit, and the second part of the integration period produces a second linear photoconversion response in the pixel circuit.
21. The method of claim 19 , wherein the step of draining photogenerated charge from said integration node to the reset node at the conclusion of the first part of the integration period occurs through the activation of a bias transistor.
22. The method of claim 19 , further comprising injecting a feed-through pulse to the integration node at the end of the first part of the integration period to initiate the draining of photogenerated charge.
23. A method of operating a CMOS imaging device, said method comprising the steps of:
(a) sequentially selecting a row or column of a pixel array;
(b) starting a first integration period for collecting photo-generated charge from an integration node for each row or column;
(c) transmitting a feed-through pulse at an intermediate level to the integration node of said imaging device during each row or column horizontal blanking period to end the first integration period and to generate an overflow current in a transistor connected to the integration node during medium and high illumination conditions;
(d) reading out the photo-generated charge;
(e) activating an additional integration period for collecting photo-generated charge from the integration node for each row or column after the photo-generated charge is read out; and
(f) repeating steps (a)–(e) sequentially for each row or column line.
24. The method of claim 23 , wherein the first integration period and the additional integration time period are of the same length.
25. The method of claim 23 , wherein step (f) repeats for every row or column cycle.
26. A processing system, comprising:
a processor; and a CMOS imaging device, coupled to said processor, said imaging device comprising:
a photoconversion device for accumulating charge during a first integration period and second integration period, said device being connected to an integration node;
a first transistor, having one terminal connected to said integration node and another terminal connected to a reset line; and
a feed-through pulse capacitor, having one terminal coupled to a feed-through pulse signal line, and a second terminal coupled to said integration node, said feed-through pulse signal line providing an intermediate pulse between said first and second integration periods to generate an overflow current in said first transistor during medium and high illumination conditions.
27. The processing system of claim 26 , further comprising an output circuit, connected to said integration node, for reading out a signal at said integration node.
28. The processing system of claim 27 , wherein said output circuit further comprises a second transistor operating as a source-follower readout transistor.
29. The processing system of claim 26 , wherein said photoconversion device is a photodiode.
30. The processing system of claim 29 , wherein the first transistor has a gate coupled to a supply voltage.
31. The processing system of claim 30 , wherein said reset signal line and said feed-through pulse line are operable to apply a reset voltage to said integration node.
32. The processing system of claim 31 , further comprising an output circuit to read out said reset voltage to an output line.
33. The processing system of claim 32 , wherein a pixel select line is coupled to the integration node and is operable to begin a first charge integration period.
34. The processing system of claim 33 , wherein said pixel select line is operable to begin a second charge integration period following said first integration period.