Amplifiers – With semiconductor amplifying device – Including atomic particle or radiant energy impinging on a...
Reexamination Certificate
1999-06-30
2001-06-26
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including atomic particle or radiant energy impinging on a...
C250S2140AG
Reexamination Certificate
active
06252462
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to amplifier circuitry for use with detectors of electromagnetic radiation and, in particular, to capacitive transimpedance amplifiers of a type used in readout integrated circuits (ROICs) that are electrically connected to a focal plane array (FPA) containing solid state detectors of infrared (IR) radiation.
BACKGROUND OF THE INVENTION
The CTIA (Capacitor Transimpedance Amplifier) is utilized in infrared and other sensing applications to integrate the current from a detector for a specified period of time, referred to as the integration time. Referring to
FIG. 1
, an exemplary CTIA
3
of a particular unit cell contains a high gain inverting amplifier having a driver
2
with a capacitor in the feedback loop (C
FB
). The inverting amplifier typically contains, as a minimum, two active transistors or MOSFETs. A first transistor or MOSFET is used to provide a constant current source (typically referred to as the load
1
), while the second transistor or MOSFET is used to implement the driver
2
. A reset switch (typically another transistor) is placed across the feedback capacitor and is closed to discharge the capacitor and is then opened to begin the integration time. The output voltage of the CTIA
3
is proportional to the product of the detector current (I
D
) and the integration time, and is inversely proportional to the value of the feedback capacitor C
FB
. The input voltage is maintained near the reset value by the feedback loop, which maintains a nearly constant bias on the radiation detector. At the end of the integration time the output voltage is sampled by closing an output multiplexer (MUX) switch, the reset switch is then closed, and the CTIA
3
is ready for the next integration.
FIG. 1
shows a conventional case where a two dimensional array of detectors and unit cells are arranged in a row and column (x by y) matrix (only a part of one column is depicted). Typically the MUX switches are closed and then opened one after another to readout in sequence the x unit cell outputs from each of the rows connected to a single one of the y column output lines. Also connected to the column line may be an input of a sample and hold (S/H) circuit (not shown), followed by a voltage follower (not shown). The output voltages may eventually be converted to a digital form and then operated on by a data processing system for performing any desired image processing, or to simply store the image(s) for subsequent transmission to another location. This latter type of operation is typical in space-based and other types of astronomy applications.
One drawback to the use of the conventional CTIA is that it is an active amplifier that requires continuous current. This current is a dominant source of power dissipation in the conventional CTIA, and is also a source of light emission. That is, it is known that, when powered on and operating, silicon-based MOSFET circuits will generate a small amount of IR radiation (typically in the one micron range). Both of these effects of normal operation (i.e., power dissipation and IR light generation) are disadvantageous, especially so when the CTIA is used in a low temperature system with limited cooling capacity, and/or in those systems intended to detect low light levels.
For example, in some astronomy applications, such as deep field galaxy surveys, one may be imaging distant objects over a period of hours or even days, literally on a photon-by-photon basis. As may be appreciated, in such low light level applications it is important to reduce or eliminate any extraneous sources of detectable energy which may deteriorate the signal to noise ratio of the imaging system.
A further drawback to the use of the conventional CTIA is the complexity of the amplifier at each detector element, commonly referred to as the unit cell. That is, since each CTIA
3
of each unit cell has its own associated load
1
(which can be a resistance but is more typically implemented as a transistor (e.g., a MOSFET) connected so as to form the constant current source), the circuit area required to lay out the unit cell is increased, and the overall yield in large arrays is thus also reduced. Reference in this regard can be had to, by example,
FIG. 6
of U.S. Pat. No.: 4,978,872, “Integrating Capacitively Coupled Transimpedance Amplifier”, by Morse et al., where a MOSFET load
122
is shown.
Further reference with regard to various aspects of CTIAs may be had to the following U.S. Patents, namely U.S. Pat. No.: 4,956,716, “Imaging System Employing Charge Amplifier”, by Hewitt et al.; U.S. Pat. No.: 5,043,820, “Focal Plane Array Readout Employing One Capacitive Feedback Transimpedance Amplifier For Each Column”, by Wyles et al.; U.S. Pat. No.: 5,602,511, “Capacitive Transimpedance Amplifier Having Dynamic Compression”, by Woolaway; and U.S. Pat. No. 4,786,831, entitled “Integrating Capacitively Coupled Transimpedance Amplifier”, by Morse et al. The disclosures of these U.S. Patents are incorporated by reference herein in their entireties.
OBJECTS AND ADVANTAGES OF THE INVENTION
It is a first object and advantage of this invention to provide an improved unit cell for use in radiation detection applications.
It is a further object and advantage of this invention to provide a unit cell that exhibits reduced complexity, reduced circuit area, reduced power dissipation, and reduced light emission characteristics as compared to conventional unit cells.
It is an other object and advantage of this invention to provide a common load that is switchably connected into and shared between a plurality of unit cells, each comprising a CTIA.
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome and the objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention, wherein there is provided a CTIA with a multiplexed load (CTIA w/ML). The multiplexed load refers to a load that is shared (at different times) by a plurality of CTIAs, e.g., by a hundred or a thousand or more CTIAs. The driver for each radiation detector element is connected to the load only when an output multiplexer select switch is closed for that detector element. The remaining components of the CTIA circuit, i.e., the driver, the feedback capacitor, and the reset switch, are still present for each amplifier and are not shared. When the output multiplexer select switch for a particular amplifier is closed, current flows from the common load (current source) through the particular amplifier, which then operates as a “normal” CTIA. When the output multiplier switch is open, no current flows in the amplifier circuit, and the radiation detector current integrates instead on the input capacitance, in a manner similar to a self-integrating amplifier, also known as a SFD (Source Follower per Detector).
A readout circuit in accordance with this invention has an x-row by y-column array of readout unit cells each having an input for coupling to an output of a radiation detector and an output that is switchably coupled to one of y column output lines. Each unit cell includes a driver transistor having a gate terminal coupled to the detector, a source terminal coupled to a source voltage, and a drain terminal switchably coupled to a drain voltage through one of y loads, such as one of y current sources. The drain terminal is switchably coupled to the one of y current sources through an output multiplexer switch. The current source is a common current source for all of the x unit cells coupled to a same one of the y column output lines. The unit cell further includes a capacitance coupled between the gate and drain terminals, and a reset switch also coupled between the gate and drain terminals.
REFERENCES:
patent: Re. 34908 (1995-04-01), Wyles et al.
patent: 4365209 (1982-12-01), Yamauchi
patent: 4786831 (1988-11-01), Morse et al.
patent: 4857725 (1989-08-01), Goodnough et al.
patent: 4956716 (1990-09-01), Hewitt et al.
patent: 4978872 (1990-12-01), Morse et al.
patent: 5043820 (1991-08-01), Wyles et al.
patent: 5
Lenzen, Jr. Glenn H.
Nguyen Khanh Van
Pascal Robert
Raytheon Company
Schubert William C.
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