Photosensor and ambient light sensor with constant bias voltage
A method of operating a photosensor comprising: applying a bias voltage to a photosensor ( 12 ) comprising n (n>1) photo-sensitive elements ( 8 ) connected in series, and determining the photocurrent in the photosensor ( 12 ) at a time when the applied bias voltage across the photosensor maintains the photosensor at or close to the point at which it has the greatest signal-to-noise ratio. This may conveniently be done by determining the current in the photosensor at a time when the applied bias voltage across the photosensor is equal or approximately equal to n×Vbi, where Vbi is the bias voltage about which the current in a single one of the photo-sensitive elements ( 8 ), in the dark, changes sign. In an embodiment in which the photo-sensitive elements ( 8 ) are photodiodes, the bias voltage Vbi is the “built-in” voltage of the photodiodes. The photocurrent generated when n series-connected photodiodes are illuminated is approximately equal to the photocurrent generated when one photodiode is illuminated. However, the leakage current (i.e., the dark current) for the n series-connected photodiodes is significantly lower than the leakage current for one photodiode. The signal-to-noise-ratio is therefore significantly increased.
1. A method of operating a photosensor comprising: applying a bias voltage to a photosensor comprising n photo-sensitive elements connected in series, where n is an integer greater than one; determining the current in the photosensor at a time when the applied bias voltage across the photosensor maintains the photosensor at or close to the point at which it has the greatest signal-to-noise ratio; and maintaining a constant bias voltage across the photosensor irrespective of the light level on the photosensor.
2. A method as claimed in claim 1 and comprising determining the current in the photosensor at a time when the applied bias voltage across the photosensor is equal or approximately equal to n×V bi , where V bi is the bias voltage about which the current in a single one of the photo-sensitive elements, in the dark, changes sign.
3. A method as claimed in claim 2 wherein the step of applying the bias voltage comprises applying a constant bias voltage equal or approximately equal to n×V bi .
4. A method as claimed in claim 2 and comprising determining the current in the photosensor at a time when the applied bias voltage across the photosensor is equal or approximately equal to zero.
5. A method as claimed in claim 4 wherein the step of applying the bias voltage comprises applying a constant bias voltage equal or approximately equal to zero.
6. A method as claimed in claim 1 , wherein n≧10.
7. A method as claimed in claim 1 , wherein n≧20.
8. A method as claimed in claim 1 , wherein n≧50.
9. A method as claimed in claim 1 , wherein n≧100.
10. A method as claimed in claim 1 wherein each photo-sensitive element is a photodiode.
11. A method as claimed in claim 10 wherein each photo-sensitive element is a lateral photodiode.
12. A method as claimed in claim 1 wherein each photo-sensitive element is a phototransistor.
13. A method as claimed in claim 1 wherein each photo-sensitive element is a thin film photo-sensitive element.
14. A method as claimed in claim 13 wherein each photo-sensitive element is a silicon thin film photo-sensitive element.
15. A method as claimed in claim 1 wherein the photo-sensitive elements are nominally identical to one another.
16. A method as claimed in claim 1 and comprising connecting the n photo-sensitive elements in series between a first input of an operational amplifier and the second input of the operational amplifier.
17. A method as claimed in claim 1 and comprising connecting the n photo-sensitive elements in series between a first input of an operational amplifier and an output of the operational amplifier.
18. A method of operating a display device comprising the steps of: sensing an ambient light intensity according to a method as defined in claim 1 ; and controlling the intensity of a backlight of the display device in accordance with an output from the photosensor.
19. A light-sensing system comprising: a photosensor comprising n photo-sensitive elements connected in series, where n is an integer greater than one; bias means for applying a bias voltage to the photosensor; and determining means for determining the current in the photosensor at a time when the applied bias voltage across the photosensor maintains the photosensor at or close to the point at which it has the greatest signal-to-noise ratio, wherein the bias means are adapted to maintain a constant bias voltage across the photosensor irrespective of the light level on the photosensor.
20. A system as claimed in claim 19 wherein the determining means are adapted to determine the current in the photosensor at a time when the bias means applies a bias voltage across the photosensor that is equal or approximately equal to n×V bi , where V bi is the bias voltage about which the current in a single one of the photo-sensitive elements, in the dark, changes sign.
21. A system as claimed in claim 19 wherein the bias means applies, in use, a constant bias voltage equal or approximately equal to n×V bi .
22. A system as claimed in claim 19 , wherein the determining means are adapted to determine the current in the photosensor at a time when the bias means applies a bias voltage across the photosensor that is equal or approximately equal to zero.
23. A system as claimed in claim 22 , and further comprising a second photosensor connected in parallel to the first photosensor, the second photosensor comprising n photo-sensitive elements connected in series, wherein the second photosensor is connected in an opposite sense to the first photosensor.
24. A system as claimed in claim 23 wherein at least one cross-connection is provided between an inter-element node of the first photosensor and a corresponding inter-element node of the second photosensor.
25. A system as claimed in claim 19 wherein the bias means are adapted to apply a constant bias voltage equal or approximately equal to zero.
26. A system as claimed in claim 19 wherein n≧10.
27. A system as claimed in claim 19 wherein n≧20.
28. A system as claimed in claim 19 wherein n≧50.
29. A system as claimed in claim 19 wherein n≧100.
30. A system as claimed in claim 19 wherein each photo-sensitive element is a photodiode.
31. A system as claimed in claim 30 wherein each photo-sensitive element is a lateral photodiode.
32. A system as claimed in claim 19 wherein each photo-sensitive element is a phototransistor.
33. A system as claimed in claim 19 wherein each photo-sensitive element is a thin film photo-sensitive element.
34. A system as claimed in claim 33 wherein each photo-sensitive element is a silicon thin film photo-sensitive element.
35. A system as claimed in claim 19 wherein the photo-sensitive elements are nominally identical to one another.
36. A system as claimed in claim 19 wherein the n photo-sensitive elements are connected in series between a first input of an operational amplifier and the second input of the operational amplifier.
37. A system as claimed in claim 19 wherein the n photo-sensitive elements are connected in series between a first input of an operational amplifier and an output of the operational amplifier.
38. A system as claimed in claim 19 wherein the bias means and the determining means are comprised in a single circuit.
39. A display system comprising: a display device having a backlight; a light-sensing system as defined in claim 19 for sensing an ambient light intensity; and a controller for controlling the intensity of the backlight of the display device in accordance with an output from the light-sensing system.