Operational amplifier for an active pixel sensor

• US 20060033007A1
• Filed: 07/29/2005
• Published: 02/16/2006
• Est. Priority Date: 08/11/2004
• Status: Active Grant

• Current Assignee: Avago Technologies General IP PTE Limited (Broadcom, Inc.)
• Original Assignee: Broadcom Corporation (Broadcom, Inc.)
• Inventors: Terzioglu, Esin
• Granted Patent: US 7,605,854 B2
• Time in Patent Office: Days
• US Class Current: 250/208.100
• CPC Class Codes: H03F 1/14 by use of neutralising means
• CPC Class Codes: H03F 3/45183 Long tailed pairs H03F3/4521, H03F3/45237 take precedence
• CPC Class Codes: H04N 23/70 Circuitry for compensating brightness variation in the scene
• CPC Class Codes: H04N 23/76 by influencing the image signals
• CPC Class Codes: H04N 25/63 applied to dark current

Abstract

The present invention includes operational amplifier for an active pixel sensor that detects optical energy and generates an analog output that is proportional to the optical energy. The active pixel sensor operates in a number of different modes including: signal integration mode, the reset integration mode, column reset mode, and column signal readout mode. Each mode causes the operational amplifier to see a different output load. Accordingly, the operational amplifier includes a variable feedback circuit to provide compensation that provides sufficient amplifier stability for each operating mode of the active pixel sensor. For instance, the operational amplifier includes a bank of feedback capacitors, one or more of which are selected based on the operating mode to provide sufficient phase margin for stability, but also considering gain and bandwidth requirements of the operating mode.

Claims

1. An active pixel circuit, comprising:

a reset circuit that charges a node during a reset condition;

a photo-diode that discharges said node when optical energy is received by said photo-diode;

a source follower circuit that reproduces a voltage at said node; and

an operational amplifier coupled to an output of said source follower circuit, said operational amplifier having a feedback compensation circuit adjustable for one or more operating modes of said active pixel circuit.

2. The active pixel circuit of claim 1, wherein said reset circuit, said photo-diode, and said source follower are configured on a common CMOS substrate.

3. The active pixel circuit of claim 2, wherein said operational amplifier is configured on said common CMOS substrate.

4. The active pixel circuit of claim 2, wherein said CMOS substrate is a conventional CMOS substrate.

5. The active pixel circuit of claim 1, further comprising a shallow trench isolation between said photo-diode and one of said reset circuit and said source follower circuit.

6. The active pixel circuit of claim 1, further comprising one or more metal traces in proximity to said shallow trench isolation, said metal traces having minimal bends to reduce dark current of said active pixel circuit.

7. The active pixel circuit of claim 1, wherein said CMOS substrate is fabricated with a conventional CMOS process.

8. The active pixel circuit of claim 1, wherein said feedback compensation circuit in said operational amplifier includes a bank of capacitors, one or more of which are selected based on said one or more operating modes.

9. The active pixel circuit of claim 8, wherein said capacitors are selected to provide sufficient stability for said operational amplifier for a corresponding operating mode.

10. The active pixel circuit of claim 9, wherein said operating modes includes one of following:

signal integration mode, reset integration mode, column reset mode, column signal readout mode.

11. The active pixel circuit of claim 1, further comprising a pre-charging circuit that pre-charges an output stage of said operational amplifier.

12. An operational amplifier, comprising:

an input stage receiving an imaging signal;

an output stage producing an output responsive to said imaging signal;

a feedback circuit between said input stage and said output stage, having a response that is adjustable based on a load variation coupled to said output stage.

13. The operational amplifier of claim 12, wherein said feedback circuit includes a bank of capacitors, a subset of which is selectable based on said load variation coupled to said output stage.

14. The operational amplifier of claim 13, wherein said subset of selectable capacitor are selected to ensure stability for a corresponding load variation coupled to said output stage.

15. The operational amplifier of claim 14, further comprising a pre-charging circuit that pre-charges an output stage of said operational amplifier to improve pull-up slew rate performance.

16. The operational amplifier of claim 15, wherein said pre-charging circuit biases said output stage to approximately a positive supply voltage for said operational amplifier.

17. The operational amplifier of claim 15, further comprising an intermediate amplifier stage coupled between said input stage and said output stage, configured to increase gain and provide increased voltage headroom.