Image sensor with reduced column fixed pattern noise
An image sensor may include an array of image pixels arranged in rows and columns. Each column of pixels may be coupled to current source transistors and a threshold voltage mitigation circuit. The threshold voltage mitigation circuit may include a long p-channel device for producing a reference current for the current source transistors. The mitigation circuit also includes an autozero transistor and a sampling transistor for passing a global control voltage to the current source transistors. The global control voltage may be generated using a control voltage generator that includes current mirroring circuits and a replica of the current source transistors and the threshold voltage mitigation circuit.
1. An image sensor, comprising:
an image sensor pixel;
a pixel output line coupled to the image sensor pixel;
a current source transistor coupled to the pixel output line; and
a threshold voltage mismatch mitigation circuit coupled to the current source transistor, wherein the threshold voltage mismatch mitigation circuit comprises:
a reference current generator transistor coupled to a source-drain terminal of the current source transistor and configured to output a reference current for the current source transistor; and
an autozero transistor coupled between the source-drain terminal and a gate terminal of the current source transistor.
2. The image sensor of claim 1 , further comprising:
a cascode transistor coupled between the pixel output line and the current source transistor.
3. The image sensor of claim 1 , wherein the current source transistor has a first gate length and wherein the reference current generator transistor comprises:
a p-type transistor having a second gate length that is greater than the first gate length.
4. The image sensor of claim 3 , wherein the second gate length is at least five times longer than the first gate length.
5. The image sensor of claim 3 , wherein the p-type transistor has a source terminal configured to receive a first power supply voltage and has a gate terminal configured to receive a second power supply voltage less than the first power supply voltage.
6. The image sensor of claim 3 , wherein the threshold voltage mismatch mitigation circuit comprises:
an enable transistor coupled between the p-type transistor and the current source transistor, the enable transistor having a gate terminal configured to receive an enable signal.
7. The image sensor of claim 6 , wherein the threshold voltage mismatch mitigation circuit comprises:
a bypass transistor configured to connect the p-type transistor to a ground line.
8. The image sensor of claim 1 , wherein the threshold voltage mismatch mitigation circuit comprises:
a capacitor having a first terminal coupled to the gate terminal of the current source transistor and having a second terminal coupled to a ground line.
9. The image sensor of claim 8 , wherein the threshold voltage mismatch mitigation circuit comprises:
an additional capacitor having a first terminal coupled to the gate terminal of the current source transistor and having a second terminal.
10. The image sensor of claim 9 , wherein the threshold voltage mismatch mitigation circuit comprises:
a sampling transistor having a first source-drain terminal coupled to the second terminal of the additional capacitor, having a second source-drain terminal configured to receive a control voltage, and having a gate terminal configured to receive a sampling control signal.
11. The image sensor of claim 10 , further comprising:
a control voltage generator configured to generate the control voltage.
12. An image sensor, comprising:
an image sensor pixel;
a pixel output line coupled to the image sensor pixel;
a current source transistor coupled to the pixel output line, the current source transistor having a first gate length and having a threshold voltage; and
a threshold voltage mismatch mitigation circuit coupled to the current source transistor and configured to produce an output current on the pixel output line that does not vary as a function of the threshold voltage of the current source transistor, wherein the threshold voltage mismatch mitigation circuit comprises a reference current generator configured to output a reference current to the current source transistor and wherein the reference current generator comprises a p-type transistor having a second gate length that is greater than the first gate length.
13. The image sensor of claim 12 , wherein the second gate length is at least five times longer than the first gate length.
14. The image sensor of claim 12 , wherein the p-type transistor has a source terminal configured to receive a first power supply voltage and has a gate terminal configured to receive a second power supply voltage less than the first power supply voltage.
15. The image sensor of claim 12 , wherein the threshold voltage mismatch mitigation circuit comprises:
an enable transistor coupled between the p-type transistor and the current source transistor, the enable transistor having a gate terminal configured to receive an enable signal.
16. The image sensor of claim 15 , wherein the threshold voltage mismatch mitigation circuit comprises:
a bypass transistor configured to connect the p-type transistor to a ground line.
17. An image sensor, comprising:
an image sensor pixel;
a pixel output line coupled to the image sensor pixel;
a current source transistor coupled to the pixel output line, the current source transistor having a threshold voltage and having a drain terminal and a gate terminal; and
a threshold voltage mismatch mitigation circuit coupled to the current source transistor and configured to produce an output current on the pixel output line that does not vary as a function of the threshold voltage of the current source transistor, wherein the threshold voltage mismatch mitigation circuit comprises an autozero transistor coupled across the drain and gate terminals of the current source transistor.
18. The image sensor of claim 17 , wherein the threshold voltage mismatch mitigation circuit comprises:
a capacitor having a first terminal coupled to the gate terminal of the current source transistor and having a second terminal coupled to a ground line.
19. The image sensor of claim 18 , wherein the threshold voltage mismatch mitigation circuit comprises:
an additional capacitor having a first terminal coupled to the gate terminal of the current source transistor and having a second terminal.
20. The image sensor of claim 19 , wherein the threshold voltage mismatch mitigation circuit comprises:
a sampling transistor having a first source-drain terminal coupled to the second terminal of the additional capacitor, having a second source-drain terminal configured to receive a control voltage, and having a gate terminal configured to receive a sampling control signal.
21. The image sensor of claim 20 , further comprising:
a control voltage generator configured to generate the control voltage.