Gate Coupled Input Circuit

This invention relates to an infrared imaging system, and more particularly to a solid state imaging system utilizing pyroelectric detectors in an array.

In our copending applications, Ser. Nos. 912,883 and 912,884, filed on Sept. 26, 1986, and both entitlted "Gate Coupled Input Circuit," we disclose and claim alternate embodiments of the invention disclosed herein.

In the prior art it has been recognized that infrared imaging systems having few or mo moving or fragile parts are desirable. Such infrared imaging systems should be very small because detector arrays would be fabricated on a single large scale integrated circuit chip along with associated electronics. Basically, two types of detectors are typically considered for such a solid state infrared imaging system. They are ferroelectric detectors which must be operated above the Curie temperature of the ferroelectric material, as is known in the art, and typically require that the detectors be heated. Above the Curie temperature the ferroelectric detectors have a cubic crystal structure that is very susceptible to polarization, but is not spontaneously polarized as it is below the Curie temperature. Thus, above the Curie temperature ferroelectric detectors require an external electric field in order to generate a signal. The other type of detector is a pyroelectric detector that is operated below its Curie temperature and exhibits a spontaneous dipole moment that produces an internal electric field, so an externally generated electric field is not required for these detectors to work.

In the prior art, infrared imaging systems utilizing ferroelectric detectors have been built. They have separate detector heating means and electric field biasing means for the detectors, and rows of detectors are sequentially enabled and connected to a common preamplifier. These preamplifiers are made up of a combination of integrated circuits and discrete components. The separate field biasing means and separate preamplifiers require increased space and increase the cost of the infrared imaging system.

To minimize circuit cost, a single preamplifier is used to amplify the signal from all detectors in each row of an array of detectors. With this common preamplifier arrangement the funadamental sensitivity limit for each individual detector cannot be achieved.

Other infrared detector array arrangements have been provided in the prior art that do not need to be heated, but they have another problem. They have to be cooled by a cryogenic cooler to temperatures of in the order of 77 Kelvin and below. In addition, such cryogenic cooling arrangements often utilize a bottle of a liquified gas such as nitrogen. This is bulky and the bottle must be changed frequently as the gas is used up.

Thus, there is a need in the art for an infrared imaging system having an infrared detector array that does not have to be heated or cooled (which requires power), does not require separate electric field detector biasing circuits, and does not utilize shared amplifiers.

The previously described needs of the prior art are met by the present invention. A pyroelectric detector array is utilized that operates well below its Curie temperature so does not require heating. In addition, shared preamplifiers are not used to amplify the output from the detectors. Rather, a preamplifier is manufactured along with each detector of the pyroelectric detector array. Problems of not being able to manufacture discrete resistors of values needed for biasing the integrated preamplifiers are circumvented by biasing or providing a bias impedance by other innovative techniques.

The invention will be better understood upon reading the following detailed description in conjunction with the drawing in which:

FIG. 1 is a block diagram of an infrared focal plane system utilizing the present invention;

FIG. 2 is a schematic drawing of a pyroelectric detector connected to the input of a transistor amplifier and the two of them are biased from a voltage supply through a very high resistance that is fabricated into the detector; and

FIG. 3 shows a pyroelectric detector connected to the input of a transistor amplifier and the two of them are biased by using a transistor switch to periodically apply voltage to both the gate of the amplifier transistor and the detector.

Pyroelectric detectors are in essence temperature sensitive capacitors having two conductive plates, and the dielectric between the plates is a material such as lithium tantalate which is used in the preferred embodiment of the invention. Lithium tantalate has a Curie temperature of 620 Centigrade, well above the temperature at which the detector is operated. These pyroelectric materials have a dielectric constant that is temperature sensitive. Thus, the detectors are variable capacitors that change capacitance with temperature. Lithium tantalate has a dielectric constant around 43 at room temperature. When these temperature sensitive capacitors (pyroelectric detectors) are placed in an array made up of rows and columns of a large number of detectors and an infrared (IR) image is focues on them by an appropriate IR lens system, the capacitance of each detector is varied by the intensity of the infrared energy impinging upon the detector. Thus, the voltage across each detector in an array directly represents a part of the infrared image that is focused on the array. When each of the detectors are connected to an amplifier, and the voltage across the detectors is indicated by an electrical signal output from the amplifiers, the output signal from the amplifiers is a representation of the IR image and can be formatted and used to display the infrared image on a raster scan video display.

In FIG. 1 is shown a schematic block diagram of an infrared focal plane array system that can provide a raster scan signal representing the infrared image focused on the detector array. An infrared lens system (not shown) made of germanium or other suitable material is used in a manner well known in the art to focus an infrared image on pyroelectric detector array 15 made up of rows and columns of individual detector array circuits such as represented as circuits 11 through 14. There can actually be thousands of these detector array circuits. Each representative detector array circuit 11 through 14 is made up of three or four elements each, e.g. bias switch 46, amplifier 50, detector 51 and transfer switch 60, which are those elements seen in FIGS. 2 and 3 and described in detail further in this specification. With reference to FIG. 3, the bias switch is transistor 46, the transfer switch is transistor 60, the detector is capacitor 41, and the amplifier is transistor 40. However, the alternative embodiment of the detector array circuit shown in FIG. 2 may also be used in place of that shown in FIG. 3.

In representative circuit 12 in FIG. 1, the detector amplifier is a metal-oxide-semiconductor field effect transistor (MOSFET) to the gate input of which is connected the pyroelectric detector. The transfer switch is also a MOSFET transistor. When the circuit of FIG. 2 is utilized, in circuits 11 through 14 there is no bias switch; but when the circuit shown in FIG. 3 is used the bias switch transistor is a bipolar transistor. It is easy to fabricate these transistors and the pyroelectric detector as array 15 is fabricated as a whole.

In operation means that are not shown, but are well known in the art, are used for modulating or chopping the infrared image focused on the detector array 15. That is, the image is interrupted at a periodic rate. The purpose that is served is to be able to discriminate between the infrared image signal and various background signals including noise and random temperature change with each of representative circuits 11 through 14 in detector array 15. A reference signal including only the background signals is obtained from the detectors of array 15 while it has no infrared image focused thereon due to the modulating action, and a sample signal is obtained from the detector when it has an image focused thereon. When the reference signal is subtracted from the sample signal the difference signal is only the image signal which is used to display the infrared image. This modulating technique is used with the two embodiments of the invention described hereinbelow.

In operation of the present invention using the detector array circuit shown in FIG. 3 only, the sample signals are actually taken twice before bias switch transistor 46 of FIG. 3 is used to reset the bias on detector amplifier 40 and on detectors 41. This is done to remove uncertainty in the voltage that remains on the detectors and is minimized by operating bias switch transistor 46 a fewer number of times. This operation is described in greater detail further in this specification. Using the detector array circuit shown in FIG. 2, bias is not periodically applied to the detector amplifier and detector. The reason for this is described in detail further in this specification.

In FIGS. 2 and 3 are shown two alternate embodiments of the invention. Preamplifiers are provided with each pyroelectric detector, and special means for biasing the preamplifiers and the detectors are provided in accordance with the teaching of the present invention. The detectors are periodically exposed to the infrared image focused on the detector array as previously described to get the reference and sample signals which are subtracted from each other as previously described to get the difference or image signal which is used to display the infrared image. In response to be being exposed to the infrared image the capacitance of the detectors changes and thereby changes the voltage at the gate input of the associated preamplifier to thereby change its conduction level. The conduction level of all preamplifiers, which reflects the portion of the infrared image focused on the associated detector in detector array 15, is read out in parallel and then converted to a scanning format to create a raster scan video signal of the infrared image focused on the array of detectors. In between being exposed to the infrared image the voltage across the detectors of all detector array circuits 11 through 14 is returned to a quiescent level by use of biasing means or the bias switch and the same read out is again used to obtain the reference signal. As previously described the reference signal is subtracted from the sample signal to obtain the infrared image signal. The circuits used to read out detector array circuits 11 through 14 are not shown herein, but are well known in the art.

In FIG. 2 transistor 20 is a metal-oxide-semiconductor field effect transistor (MOSFET), and pyroelectric capacitor detector 21 is connected to its gate terminal 22. To bias transistor 20 without destroying the small signal created by detector 21A, a large impedance must be connected between ground potential and gate 22. In accordance with the teaching of this first embodiment of the invention, the large impedance is created by doping the detector material. This creates an effective impedance 21B in the order of 10.sup.12 to 10.sup.15 ohms in parallel with the detector material 21A. The required bias is provided and the small signal generated by detector 21 is preserved and is applied to gate terminal 22 of preamplifier transistor 20. As may be seen in FIG. 2, source terminal 23 of transistor 20 is connected to a voltage supply (V.sub.s) 24. On drain terminal 25 of transistor amplifier 20 is output an amplified signal representing the portion of the infrared image impinging upon infrared detector 21 in each of representative detector array circuits 11 through 14. Transfer switch 60 is periodically operated to read out the video signal from amplifier 20 that is used to display the infrared image on a video display. Transfer switch 60 is a transistor gate that is well known in the art so is not described in detail herein.

FIG. 3 shows an alternative embodiment of the invention using a technique for biasing MOSFET preamplifier transistor 40 that doesn't use a high impedance like impedance 21B previously described with reference to FIG. 2. Instea, a MOSFET transistor 46 is provided with every detector 41. Transistor 46 is connected between a reset voltage supply V.sub.r and gate terminal 42 of preamplifier transistor 40, and transistor 46 is switched on and off periodically by a control signal applied to its gate terminal 49 to apply reset voltage V.sub.r to gate terminal 42 and to detector 41. As mentioned previously, this is done after every two read outs. A charge is thereby placed on gate 42 of transistor 40 and on detector 41 and biases both of them to their quiescent operating point. The charge placed on gate 42 leaks off very slowly due to the high internal impedances of detector 41 and gate 42 of transistor 40. While bias switch transistor 46 is turned off, each time after gate 42 and detector 41 have been recharged to their quiescent point, detector 41 is briefly exposed to a portion of the infrared signal focused on detector array 15. In response thereto the capacitance of detector 41 changes and thereby alters the bias voltage at gate terminal 42. This causes the conduction level of transistor 40 to change. Transistor 40 is read out by momentarily placing transfer gate transistor 60 in its conductive state by a signal on its gate 61 in response to a signal from external circuitry that is not shown, but is well known in the art. This is done before transistor 46 is again turned on to reset the bias levels of transistor 40 and detector 41. In operation the bias charge on the gate of transistor 40 changes slowly enough that transistor 40 is read out twice before transistor 46 is turned on to reset the bias level of transistor 40. An advantage to this slower resetting of the bias level is that when transistor 46 is turned off there is an uncertainty in the voltage that remains on the detector 41, which is noise, and which is minimized by turning transistor 46 on and off a fewer number of times.

To remove the switch noise caused by the operation of bias switch transistor 46, in FIG. 3 only, the sample signals read out of transistor 40 for two consecutive read outs when the infrared image is focused on detector 41 are subtracted from each other. As described in the last paragraph bias switch 46 is only operated before one of these two sample signal read outs, and the subtraction of the sample signal before which there was no bias switch operation from the sample signal before which there was a switch operation yield the switch noise which can then be removed from the sample signal right after the switch operation. This is done because for each of these two consecutive read outs the reset transistor 46 switching noise is constant, and the infrared signal sample is the difference of the two read outs. This technique can be utilized because the rate of change of the gate bias voltage is very slow compared to the read out rate of transistor 20. Thus, the first signal sample read out right after the switch operation is a first read out and the next signal sample read out is a reference signal read out. By subtracting these two read outs the signal-to-noise ratio of the infrared signal of interest is improved by removing the switch noise from the first signal sample.

In operation of a complete IR imaging system, the modulating means, not shown, periodically interrupts the infrared image and bias switch transistor 46 resets the bias voltage to both preamplifier 40 and detector 41 while there is no image focused on the detector. In addition, while there is no infrared image focused on detector 41 the system next reads out the conduction level of preamplifier 40 to determine a reference level which includes miscellaneous circuit noise. Then the modulating means causes the infrared image to impinge on detector 41 and two read out samples are taken that are processed as described in the previous paragraph. The reference level read out is subtracted from the two following sample signal read outs to remove the noise.

The transfer switch 60 in each of FIGS. 2 and 3, and shown in circuits 11 through 14 in FIG. 1, are operated under control of external timing circuits that are well known in the art so are not described herein, to read out the conduction level of the associated amplifiers 40 as just described for each of the reference signals and the two sequential signal samples. Transfer switch 60 is a transistor gate that is well known in the art so is not described in detail herein.

In a complete infrared imaging system the drains of the transfer gate transistors 60 are each connected to a corresponding tap of a CCD shift register or another type of analog multiplexer. In operation the pyroelectric detectors accumulate signal and thereby set the operation level of their corresponding preamplifier transistor 40. Thus, preamplifier transistors 40 are enabled to be quickly read out in parallel to CCD shift registers (not shown) that are then shifted out at a slower rate to create a raster scan signal representing the infrared image.

While what has been described herein are the preferred embodiments of the invention, it will be understood that those skilled in the art may make many changes without departing from the spirit and scope of the invention. For example, while N-channel MOSFETs are preferably used in the circuits described herein, the circuits could be simply modified to operate with P-channel MOSFETs. In addition, other pyroelectric materials could be used to fabricate the detectors.