Image Sensor
AssigneeKoninklijke Philips Electronic, N.V.
InventorMark J. Childs
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Abstract
An image sensor has a plurality of pixels, each pixel having a photodiode (12), a voltage amplifier (16) having gain magnitude greater than 1 and a sampling capacitor (18) charged by the voltage amplifier. In this arrangement, each pixel provides gain through voltage amplification. This enables the sampling capacitor to be kept to a low size, so that the pixel circuitry occupies the smallest possible space, thereby enabling large aperture pixels to be formed. A source-follower buffer transistor (49) is provided at the input to the voltage amplifier. This overcomes the effect of charge sharing resulting from the parasitic capacitances of the output transistor of the voltage amplifier.
Claims19
An image sensor comprising a plurality of pixels, each pixel comprising:
- a light sensor element (12), a sensor voltage across the element varying depending on the light incident on the element (12);
- a voltage amplifier (16) having gain magnitude greater than 1;
- and
- a sampling capacitor (18) charged by the voltage amplifier,
- wherein the voltage amplifier comprises first (40) and second (38) transistors in series, the input to the voltage amplifier being provided to the gate of the first transistor (40), and the output being defined by the junction between the first and second transistors (40,38),
- and wherein each pixel further comprises a third transistor (49), the gate of the third transistor being connected to one terminal of the light sensor element (12), and the source of the third transistor (49) being connected to the gate of the first transistor (40).
An image sensor as claimed in any preceding claim, wherein the gain magnitude of the voltage amplifier (16) is in the range 2 to 5.
An image sensor as claimed in any preceding claim, wherein a bias voltage (44) is connected to the gate of the second transistor (38).
An image sensor as claimed in any preceding claim wherein each pixel further comprises an input switch (20;30) for applying a fixed potential (Vreset) across the light sensor element.
A method of measuring light intensity of an image to be detected using a plurality of light sensor elements (12) each forming a pixel of an image sensor, a sensor voltage (Vin) across the elements varying depending on the light incident on the elements, the method comprising:
- providing the sensor voltage (Vin) to an in-pixel voltage amplifier through a source-follower buffer transistor;
- amplifying the voltage provided by the source-follower buffer transistor using the in-pixel voltage amplifier (16) having a gain magnitude greater than 1;
- charging a sampling capacitor (18) with the amplified voltage (Vout) and measuring the flow of charge required to charge the sampling capacitor (18).
Dependencies are inferred from the claim text.