Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-03-16
2003-07-15
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140RC
Reexamination Certificate
active
06593560
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to image sensors for converting an optical image into electrical signals and, in particular, to a pixel element in such a sensor.
BACKGROUND
One common type of image sensor, commonly found in digital and video cameras, includes an array of photoelements, where each photoelement generates a signal approximately proportional to the light impinging upon the photoelement area. As shown in
FIG. 1
, such a photoelement array
10
outputs its signals, typically pursuant to an addressing operation, to an analog-to-digital converter
12
to produce digital signals. Processing circuitry
14
then performs the required processing of the digital data to, for example, display the image on a screen or store the image in a memory.
FIG. 2
illustrates one photoelement
20
(or pixel element) in the photoelement array
10
and serves to illustrate a common drawback of photoelements. The circuit of
FIG. 1
is described in detail in U.S. Pat. No. 6,037,643, assigned to Hewlett-Packard Company and Agilent Technologies. A similar circuit is described in U.S. Pat. No. 5,769,384, also assigned to Hewlett-Packard Company and Agilent Technologies.
Typically photoelements, such as shown in
FIG. 2
, generate a charge on an integrating capacitor
22
proportional to the light impinging upon the photoelement and the time the shutter is open (i.e., the integration time). The charge is converted to a voltage outside of the photoelement during a reading cycle. The voltage output is then applied to an analog-to-digital converter, as shown in FIG.
1
.
In
FIG. 2
, a bias current is set up by a bias signal PBB controlling a transistor
24
. Transistors
26
and
28
form a bias point amplifier for setting the base bias of a phototransistor
30
at a fixed level with respect to its emitter. Transistors
26
and
28
operate as a negative feedback loop, wherein an increased emitter voltage pulls up the gate of transistor
26
, which causes transistor
28
, connected as a source follower, to lower the emitter voltage. Transistor
28
also provides isolation of the phototransistor
30
emitter from fluctuations at node
32
.
In operation, the integrating capacitor
22
is assumed to be initially charged to a reset voltage by coupling the capacitor to the summing node of the transfer amplifier
44
while read transistor
36
is on. A shutter signal is high during the initial charging of capacitor
22
so that the shutter transistor
38
is off and transistor
40
is on. Transistor
40
, when on, provides a path for phototransistor
30
to draw current from the power supply.
When the shutter signal goes low, transistor
40
is turned off and transistor
38
is turned on, discharging capacitor
22
through phototransistor
30
at a rate depending on the light impinging on the base of phototransistor
30
. At the end of the shutter period (e.g., 20 microseconds), the shutter signal goes high, decoupling phototransistor
30
from capacitor
22
. Since the rate of discharge of capacitor
22
during the shutter period is approximately proportional to the light incident upon the phototransistor
30
, the charge on capacitor
22
after the shutter is closed now reflects the integral of the light intensity during the time that the shutter was open.
A read signal NRD then goes low to couple capacitor
22
to an output line
34
and to the input of a transfer amplifier
44
. Transfer amplifier
44
converts the charge on capacitor
22
to a voltage signal. The transfer amplifier
44
pulls the output line
34
up to Vref (basically a reset level of capacitor
22
), resulting in the charge that was removed from capacitor
22
by the light-induced current during the shutter open time being transferred to a transfer capacitor
48
. The read signal is now raised to turn off transistor
36
.
The output of the transfer amplifier
44
now corresponds to the amount of light that impinged on phototransistor
30
while the shutter was open. This voltage is processed as shown in
FIG. 1
for that particular pixel position. The output line
34
may be connected to all pixel elements in a column, where only one row of photoelements is addressed at a time by the NRD line being common to a row of pixels.
One problem with such image sensors that convert a charge on an integrating capacitor internal to the pixel area to a voltage outside the pixel area is that the transfer capacitor
48
and integrating capacitor
22
must be fairly large to prevent the capacitors' signals from being significantly distorted by stray capacitances that are coupled to the transfer capacitor
48
, the integrating capacitor
22
, or any of the interconnects between the two when the read transistor
36
is turned on. Further, the additional charge-to-voltage conversion circuitry takes up chip area.
Accordingly, the design of the pixel element is relatively inflexible, and its sensitivity (ability to produce large signals in low light conditions) is limited due to the required size of the transfer capacitor
48
. The size of the transfer capacitor
48
has an inverse relationship to both the settling time of the transfer amplifier
44
and the substrate noise coupling into the signal. This means that as the transfer capacitor is made smaller to increase the sensitivity of the photodetector, the settling time and noise get worse.
What is needed is a photoelement that does not suffer from the drawbacks of the prior art.
SUMMARY
A photoelement (or pixel element) for an image sensor is described that does not require a charge-to-voltage conversion, but instead outputs a voltage directly related to the intensity of the light impinging on the photoelement. Hence, a relatively large integrating capacitor is not needed. In one embodiment, only parasitic capacitance is used for the integrating function. Additional capacitance may be added to control the gain of the photoelement.
The integrating capacitance is initially charged to a reset voltage. A shutter signal closes a switch that couples the capacitance to a phototransistor or photodiode to discharge the capacitance. The switch is opened after the shutter period so that the remaining charge corresponds to the integral of the light that impinged on the photoelement during the shutter period.
An MOS transistor, connected in a source follower configuration, has its gate connected to the integrating capacitance and its source coupled to an MOS read transistor. The read transistor is also connected to an output pin of the photoelement. When the read transistor is turned on, the voltage at the source of the source follower is applied to the output pin. There is no external charge-to-voltage transfer circuitry used.
The source follower shields the integrating capacitance from any other parasitic capacitances not intended to be part of the integrating capacitance, thus making the output of the photoelement highly accurate with high gain.
REFERENCES:
patent: 5769384 (1998-06-01), Baumgartner et al.
patent: 6037643 (2000-03-01), Knee
patent: 6380530 (2002-04-01), Afghahi
Agilent Technologie,s Inc.
Le Que T.
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