Compact active pixel with low-noise image formation
View Patent ↗A low-noise active pixel circuit is disclosed that efficiently suppresses reset (kTC) noise by using a compact preamplifier consisting of a photodetector and only three transistors of identical polarity, in conjunction with ancillary circuits located on an imager's periphery. The use of only three transistors with a tapered reset signal allows the optical area to be increased, while still providing a low-noise imager.
1. An active pixel sensor circuit comprising:
a photodetector;
an access transistor connected to the photodetector;
an electronically reconfigurable transistor, successively operated as a source follower driver and a feedback amplifier, connected to an output of the access transistor and to a signal output bus;
a reset transistor connected between the access transistor and the electronically reconfigurable transistor, wherein the reset transistor is reset with a tapered reset signal; and
a first column buffer connected to the electronically reconfigurable transistor and to the reset transistor, the first column buffer comprising:
a first switch transistor connected to the reset transistor; and a second switch transistor connected to the electronically reconfigurable transistor;
wherein during a reset operation, the first and second switch transistors connect transistor connects the reset transistor with the electronically reconfigurable transistor to form a feedback path.
2. The circuit of claim 1 , wherein the transistors are MOSFETs of identical polarity.
3. The circuit of claim 2 , further comprising a second column buffer connected to the signal output bus.
4. The circuit of claim 3 , further comprising a row disable transistor connected to the reset transistor.
5. The circuit of claim 4 , wherein the first column buffer, second column buffer and row disable transistor are connected to a plurality of active pixel sensor circuits.
6. The circuit of claim 5 , wherein the electronically reconfigurable transistor operates as a driver of a source follower amplifier when a signal from the photodetector is being read out on a row-by-row basis, and operates as a driver of a reset amplifier when the photodetector is being reset.
7. A CMOS imager array comprising a plurality of pixels, each pixel comprising:
a photodetector;
an access MOSFET having a source connected to the photodetector;
an amplifier MOSFET having a gate connected to a drain of the access MOSFET, a source connected to a signal bus, and a drain connected to a column buffer;
a reset MOSFET having a source connected to the drain of the access MOSFET, a drain connected to the column buffer, and a gate connected to a tapered reset signal generator; and
a distributed feedback amplifier comprising the amplifier MOSFET, the reset MOSFET and the column buffer to taper reset the photodetector, wherein the column buffer comprises:
a first switch transistor connected to drain of the reset MOSFET; and
a second switch transistor connected to the drain of the amplifier MOSFET; and
a reset current source connected to the second switch transistor;
wherein during a reset operation, the first and second switch transistors connect transistor connects the drain of the reset MOSFET with the drain of the amplifier MOSFET to form a feedback path, and the second switch transistor connects the reset current source to the amplifier MOSFET.
8. The imager array of claim 7 , further comprising a row disable MOSFET having a source connected to the drain of the reset MOSFET and a drain connected to a row disable signal generator.
9. The imager array of claim 8 , further comprising an access signal generator connected to the gate of the access MOSFET.
10. The imager array of claim 9 , further comprising a second column buffer connected to the signal bus.
11. The imager array of claim 10 , wherein the MOSFETs within each pixel are of identical polarity.
12. The imager array of claim 11 , wherein the photodetector comprises a substrate diode with the silicide cleared.
13. An active pixel sensor circuit comprising:
a photodetector;
an access transistor connected to the photodetector;
an amplifier transistor, connected to an output of the access transistor and to a signal output bus;
a reset transistor connected between the access transistor and the amplifier transistor, wherein the reset transistor is reset with a tapered reset signal; and
a first column buffer connected to the amplifier transistor and to the reset transistor, the first column buffer comprising:
a first switch transistor connected to the reset transistor; and
a second switch transistor connected to the amplifier transistor; and
a reset current source connected to the second switch transistor;
wherein during a reset operation, the first and second switch transistors connect transistor connects the reset transistor with the amplifier transistor to form a feedback path, and the second switch transistor connects the reset current source to the amplifier transistor.
14. An imager array circuit comprising:
a first switch transistor connected to a first column bus;
a second switch transistor connected to a second column bus and the first switch transistor;
a reset current source connected to the second switch transistor;
a signal column bus; and
a plurality of pixel circuits connected to the first column bus, second column bus, and signal column bus, each pixel circuit comprising:
a photodetector;
an access transistor connected to the photodetector;
an amplifier transistor connected to the access transistor, the signal column bus and the second switch transistor; and
a reset transistor connected to the first column bus, the access transistor, and the amplifier transistor, wherein during a reset operation, the first switch transistor connects the reset transistor to the amplifier transistor to form a feedback path, and the second switch transistor connects the reset current source to the amplifier transistor.
15. The circuit of claim 14, wherein the amplifier transistor operates as a driver of a source follower amplifier when a signal from the photodetector is being read out, and operates as a driver of a reset amplifier when the photodetector is being reset.
16. The circuit of claim 1, wherein the first column buffer further comprises:
a second switch transistor connected to the electronically reconfigurable transistor; and
a reset current source connected to the second switch transistor;
wherein during a reset operation, the second switch transistor connects the reset current source to the electronically reconfigurable transistor.