Image sensor

• US 6,583,398 B2
• Filed: 12/12/2000
• Issued: 06/24/2003
• Est. Priority Date: 12/14/1999
• Status: Expired due to Fees

• Current Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Harkin, Gerard F.
• Primary Examiner(s): Kim, Robert H.
• Assistant Examiner(s): Song, Hoon K.
• Application Number:
  US09/736,663
• Publication Number:
  US 20010019080A1
• Time in Patent Office: 924 Days
• Field of Search: 250/208.1, 250/208.4, 250/214.L, 250/208.2, 250/214.R, 257/223, 257/222, 257/229, 257/234, 257/292, 257/291
• US Class Current: 250/208.1
• CPC Class Codes: H04N 25/30 for transforming X-rays into image signals
• CPC Class Codes: H04N 25/51 Control of the gain
• CPC Class Codes: H04N 25/77 Pixel circuitry, e.g. memories, A/D converters, pixel amplifiers, shared circuits or shared components
• CPC Class Codes: H04N 25/771 comprising storage means other than floating diffusion
• CPC Class Codes: H04N 25/78 Readout circuits for addressed sensors, e.g. output amplifiers or A/D converters
• CPC Class Codes: H04N 3/155 Control of the image-sensor operation, e.g. image processing within the image-sensor

Abstract

An image sensor (39) comprises a plurality of pixels (10), each pixel comprising a light sensor element (12), wherein a sensor voltage across the element varying depending on the light incident on the element. First and second transistors (14,16) are connected in series between voltage supply lines (18,20). A gate voltage (Vg1) on the first transistor is dependent upon the sensor voltage so that the current flowing through the first transistor (14) is a function of the sensor voltage. The gate voltage (Vg2) of the second transistor is supplied by a feedback circuit (22,24,26) which provides that the current through the first and second transistors (14,16) is substantially equal. The output of the pixel (10) is the gate voltage (Vg2) of the second transistor (16). The image sensor design of the invention avoids the need for storage capacitors to provide pixel gain.

Claims

1. An image sensor comprising a plurality of pixels, each pixel comprising:

a light sensor element, a sensor voltage across the element varying depending on the light incident on the element; and

first and second transistors, the first and second transistors being connected in series between voltage supply lines, a gate voltage on the first transistor being dependent upon the sensor voltage so that the current flowing through the first transistor is a function of the sensor voltage, and wherein the gate voltage of the second transistor is supplied by a feedback circuit which provides that the current through the first and second transistors is substantially equal, and wherein the output of the pixel is the gate voltage of the second transistor.

2. An image sensor as claimed in claim 1, wherein the first and second transistors have different current versus gate voltage characteristics so that the pixel provides gain.

3. An image sensor as claimed in claim 1, wherein the feedback circuit includes an open loop amplifier.

4. An image sensor as claimed in claim 3, wherein the input to the amplifier is the junction between the transistors, and the output from the amplifier is coupled to the gate of the second transistor.

5. An image sensor as claimed in claim 4, wherein first and second switches are provided between the junction and the amplifier input and between the gate of the second transistor and the amplifier output, respectively.

6. An image sensor as claimed in claim 3, comprising rows and columns of pixels, and wherein one amplifier is provided for each column of pixels.

7. An image sensor as claimed in claim 1 , wherein the light sensor element comprises a photodiode.

8. An image sensor as claimed in claim 7, wherein the photodiode is coupled between one of the voltage supply lines and the gate of the first transistor.

9. An image sensor as claimed in claim 8, wherein the or each pixel is provided with a switch for providing a predetermined voltage to the gate of the first transistor, thereby charging the photodiode to a predetermined level.

10. A method of measuring light intensity of an image to be detected using a plurality of light sensor elements each forming a pixel of an image sensor, a sensor voltage across the elements varying depending on the light incident on the elements, the method comprising:

applying the sensor voltage to a gate of an associated first transistor thereby causing the current flowing through the first transistor to be a function of the light sensor voltage;

supplying a gate voltage to an associated second transistor using a feedback circuit which provides that the current through the first and second transistors is substantially equal; and

measuring the gate voltage of the second transistor as output.

11. A method as claimed in claim 10, wherein an output is obtained for a dark field when the image sensor is shielded from the image to be detected, and an output is obtained for a light field when the image sensor is exposed to the image to be detected, and wherein the outputs are combined to obtain a light level signal.