Compact pixel reset circuits using reversed current readout
View Patent ↗Compact CMOS pixel sensors containing three or four total transistors and four or five control lines provide a high percentage of sensor area for the photodiode that measures light intensity. The CMOS pixel sensors thus have good light sensitivity. The CMOS pixel sensors also provide active reset operations yielding low noise when resetting node voltages. The low transistor count is achieved using the same transistors during both reset operations and readout operation. Reversing the current direction through a pixel sensor during readout allows the row selection transistor to act as a buffer for a transistor having a gate coupled to the photodiode node.
1. A pixel sensor comprising:
a photodiode coupled to a first node;
a first transistor having a gate coupled to the first node, a first terminal coupled to a first control line, and a second terminal coupled to a second node;
a second transistor having a gate directly coupled to the second node, a first terminal coupled to the first node, and a second terminal coupled to a second control line; and
a third transistor having a gate coupled to a third control line, a first terminal directly coupled to the second node, and a second terminal coupled to a fourth control line, wherein
each of the first control line, the second control line, the third control line, and the fourth control line is connected for application of signals at different levels for different operations that the pixel sensor performs.
2. The sensor of claim 1 , wherein the first, second, and third transistors are the only transistors in the pixel sensor.
3. The sensor of claim 1 , wherein each of the first, second, and third transistors is an NMOS transistor.
4. A pixel sensor comprising
a photodiode coupled to a first node;
a first transistor having a gate coupled to the first node, a first terminal coupled to a first control line, and a second terminal coupled to a second node;
a second transistor having a gate directly coupled to the second node, a first terminal coupled to the first node, and a second terminal coupled to a second control line;
a third transistor having a gate coupled to a third control line, a first terminal directly coupled to the second node, and a second terminal coupled to a fourth control line; and
a fourth transistor having a gate coupled to a fifth control line, a first terminal coupled to the second node, and a second terminal coupled to the fourth control line.
5. The sensor of claim 4 , wherein the first, second, third, and fourth transistors are the only transistors in the pixel sensor.
6. The sensor of claim 4 , wherein each of the first, second, and third transistors is an NMOS transistor, and the fourth transistor is a PMOS transistor.
7. An image sensor comprising:
a first set of control lines:
a second set of control lines:
a third set of control lines:
a fourth set of control lines:
a fifth set of control lines; and
an array of pixel sensors, wherein each pixel sensor in the array is coupled to a corresponding first control line from the first set, a corresponding second control line from the second set, a corresponding third control line from the third set, and a corresponding fourth control line from the fourth set, and wherein
each pixel sensor comprises:
a photodiode coupled to a first node;
a first transistor having a gate coupled to the first node, a first terminal coupled to the first control line, and a second terminal coupled to a second node;
a second transistor having a gate coupled to the second node, a first terminal coupled to the first node, and a second terminal coupled to the second control line;
a third transistor having a gate coupled to the third control line, a first terminal coupled to the second node, and a second terminal coupled to the fourth control line; and
a fourth transistor having a gate coupled to a corresponding fifth control line from the fifth set of control lines, a second terminal coupled to the second node, and a first terminal coupled to the corresponding fourth control line.
8. The sensor of claim 7 , wherein for each pixel sensor, the first, second, third, and fourth transistors are the only transistors in the pixel sensor.
9. The sensor of claim 7 , wherein in each pixel sensor, each of first, second, and third transistors is an NMOS transistor, and the fourth transistor is a PMOS transistor.
10. An image sensor comprising:
an array of pixel sensors, wherein the pixel sensors in the array are arranged in rows and columns;
a first set of control lines, wherein each control line in the first set couples only to the pixel sensors that are in a corresponding column of the array;
a second set of control lines:
a third set of control lines, wherein each control line in the third set couples only to the pixel sensors that are in a corresponding row of the array; and
a fourth set of control lines, wherein each control line in the fourth set couples only to the pixel sensors that are in a corresponding column of the array,
wherein each pixel sensor in the array is coupled to a corresponding first control line from the first set, a corresponding second control line from the second set, a corresponding third control line from the third set, and a corresponding fourth control line from the fourth set, and wherein
each pixel sensor comprises:
a photodiode coupled to a first node;
a first transistor having a gate coupled to the first node, a first terminal coupled to the first control line, and a second terminal coupled to a second node;
a second transistor having a gate coupled to the second node, a first terminal coupled to the first node, and a second terminal coupled to the second control line; and
a third transistor having a gate coupled to the third control line, a first terminal coupled to the second node, and a second terminal coupled to the fourth control line.
11. The sensor of claim 10 , wherein for each pixel sensor, the first, second, and third transistors are the only transistors in the pixel sensor.
12. The sensor of claim 10 , wherein for each pixel sensor, each of the first, second, and third transistors is an NMOS transistor.
13. The sensor of claim 10 , further comprising a control circuit outside the array of pixel sensors, wherein for each column of the array, the control circuit comprises:
a current source; and
a switching circuit to a corresponding one of the control lines from the first set and a corresponding one of the control lines in the fourth set, wherein the switching circuit connects the current source to create a current in one direction through a selected one of the pixel sensors during a reset operation, and the switching circuit connects the current source to create a current in an opposite direction through the selected one of the pixel sensors during a readout operation.
14. The sensor of claim 10 , further comprising a set of amplifiers that are external to the array, wherein during a reset operation, each of the amplifiers has a first input terminal coupled to a corresponding one of the control lines in the first set, a second input terminal coupled to receive a reference voltage, and an output terminal coupled to drive a corresponding one of the control lines in the fourth set.
15. A method for operating a pixel sensor, comprising:
driving a first current in a first direction through a first transistor and a second transistor in the pixel sensor to control resetting a voltage of a node of a photodiode in the pixel sensor, wherein the first transistor has a gate coupled to the node and the second transistor has a gate to which a selection signal for the pixel sensor is applied;
performing an integration operation that changes the voltage on the node according to an intensity of incident light on the photodiode;
driving a second current in a second direction through the first and second transistors, the second direction being opposite to the first direction; and
determining the voltage on the node from the effect of the first transistor on the second current.
16. The method of claim 15 , wherein the first and second transistors are connected in series.
17. The method of claim 15 , wherein driving the first current comprises:
driving a third current that is split between flowing through the first transistor in the pixel sensor and a reference transistor in a control circuit; and
applying a voltage to a first terminal of a third transistor in the pixel sensor, the third transistor having a second terminal coupled to the node and a gate coupled to a terminal of the first transistor, wherein the second transistor pulls a voltage on the node to a level corresponding to a gate voltage of the reference transistor.
18. The method of claim 17 , wherein a portion of the third current flowing through the first transistor changes until the voltage on the node reaches the level corresponding to the gate voltage of the reference transistor.
19. The method of claim 15 , wherein driving the first current comprises connecting an amplifier that is external to the pixel sensor so that an output terminal of the amplifier is coupled to the second transistors, a first input terminal of the amplifier is coupled to a terminal of the first transistor, and a second input terminal of the amplifier is coupled to receive a reference voltage.