Photoelectric conversion apparatus having reset noise holding and removing units

[0002] 1. Field of the Invention

[0003] The invention relates to a photoelectric converting apparatus and, more particularly, to a one-dimensional or two-dimensional photoelectric converting apparatus for executing a reading operation of, for example, a facsimile, a digital copying apparatus, an X-ray image pickup apparatus, or the like. More specifically, the invention relates to a removal of a random noise which is caused by a resetting operation of a photosensor.

[0004] 2. Related Background Art

[0005] Hitherto, a CCD has mainly been used as an image pickup device of an image reading system such as facsimile, digital copying apparatus, or the like, a video camera, a digital camera, or the like. In recent years, however, a development of what is called an amplifying type photoelectric converting apparatus having an amplifying function of an MOS transistor or a bipolar transistor on a pixel unit basis is also vigorously being executed.

[0006] In an amplifying type photoelectric converting apparatus, in order to realize a high sensitivity, a removal of a noise becomes important. With respect to the noise removal, however, several methods have conventionally been proposed.

[0007] FIGS. 6A and 6B are circuit diagrams of a one-dimensional photoelectric converting apparatus having a transistor as a photosensor for each pixel and a timing chart for such a circuit (refer to the magazine of The Institute of Television Engineers of Japan, Vol. 47, No. 9, pp. 1180, 1993).

[0008] The operation of such a photoelectric converting apparatus will now be described hereinbelow. When a start pulse (SP) is inputted, accumulating capacitors C_TSand C_TNare reset and a photosignal including a sensor noise is subsequently transferred to the capacitor C_TS.

[0009] After that, a resetting operation of a photosensor is executed and an output in a dark state including the sensor noise is transferred to the capacitor C_TN. The sensor is again reset and an accumulating operation is started. At the same time, a shift register of the first chip starts to scan and data of C_TSand C_TNis respectively capacitance divisionally outputted to C_HSand C_HNas common output lines of the first chip. The outputted signals are respectively inputted to a differential amplifier through voltage followers, so that a signal without any sensor noise is derived as an output of an IC.

[0010] “Sensor reset noise” used here denotes an FPN (Fixed Pattern Noise) which is caused due to a variation in h_FEof a transistor serving as a photosensor of each pixel or a variation in capacitance C_bc, between a base and a collector. Namely, an emitter potential after an emitter was reset by φERS varies due to the variations in h_FEand C_bc, every pixel. Therefore, it appears as an offset in the ordinary reading operation. However, since the offset can be removed by such a method, the FPN can be reduced.

[0011] As another prior art, s solid-state image pickup apparatus as shown in FIGS. 7 and 8 has been proposed in Japanese Patent Application Laid-Open No. 1-154678.

[0012] In the diagram, signal charges which were caused by an incident light and accumulated in each photodiode 101 are outputted by the following procedure. At the start of a horizontal blanking period of an output of the apparatus, when the photodiodes 101 of one line in the horizontal direction to be read out next are selected, a reset line 106 corresponding to such a line is turned on or off. After a resetting operation was performed by a reset switch 103, when a drain line 107 is subsequently turned on, each pixel amplifier 104 of such a line operates as a driver transistor of a source follower. An output of each source follower in this instance is an amplifier output when there is no signal charge. By turning on/off a gate line 116, the output voltage is stored into an accumulating capacitor 111 through a gate switch 109.

[0013] Subsequently, when a vertical gate line 105 corresponding to such a line in the horizontal direction is turned on or off and signal charges are supplied to a gate of each pixel amplifier 104, the output of each source follower has a value corresponding to an amount of signal charges. By turning on or off a gate line 117, the output voltage is stored into an accumulating capacitor 112 through a gate switch 110.

[0014] The operation in the horizontal blanking period is executed as mentioned above. In a horizontal scanning output period of time, a horizontal register 122 sequentially on/off scans horizontal gate switches 113 and 114 corresponding to each pixel, so that the source-follower output charges accumulated in the accumulating capacitors 111 and 112 are sequentially outputted from a horizontal signal line 120.

[0015] The output charges accumulated in the capacitors 111 and 112 are obtained by time-sequencing outputs in both of the case of resetting and the case of inputting the signal charges with regard to one pixel amplifier 104. Further, by obtaining a difference between both of those outputs, a noise which is caused by a variation in input offsets of a plurality of source followers and a 1/f noise of the source followers can be easily suppressed.

[0016] However, the prior art mentioned above has a problem to be solved such that the reset noise which is caused upon resetting of a photoelectric conversion unit cannot be removed.

[0017] Each time the photosensor is reset, an electric potential which was reset fluctuates and a reset noise appears as a random noise.

[0018] For example, in a photodiode having a pn junction, when a light production carrier Q_pis accumulated into a capacitor C_PDof a photodiode unit and converted into a voltage, a photosignal voltage V_pby the light production carrier is

[0019] V_P=Q_P/C_PD  (1)

[0020] On the other hand, since a reset noise V_Nis

[0021] V_N=(kT/C_PD)  (2)

[0022] where, k: Boltzmann's constant

[0023] T: Temperature (° K.)

[0024] an S/N ratio is

[0025] V_P/V_N=Q_PD·(1/(kTC_PD))  (3)

[0026] Therefore, from the equation (3), in order to raise the S/N ratio, although it is desirable to reduce the accumulated capacitance C_PDof the photosensor as small as possible, there is practically a limitation. On the other hand, since there is a tendency such that the signal charges Q_pdecrease in accordance with the realization of a high fineness and a high speed of the photoelectric converting apparatus, the removal of the reset noise becomes an important point when realizing a high S/N ratio of the photoelectric converting apparatus.

[0027] However, in the prior art 1, as shown in the timing chart of FIGS. 6A and 6B, the sensor is reset twice and the photosignal and noise signal to be read out are based on the different sensor resetting operations. Namely, after completion of the second sensor resetting operation, the accumulating operation and the photosignal reading operation are executed. A sensor noise N which is generated by the first sensor resetting operation is subtracted from a read-out signal (S+N′) including a sensor noise N′ generated by the second sensor resetting operation, thereby removing the sensor noise. Therefore, a random noise that is 2 times as large as the sensor reset noise is generated.

[0028] In the prior art 2 as well, since the resetting operation by the reset switch 103 is executed in a closed state of the gate switch 109, the resetting operation of the photodiode 101 and the resetting operation by the reset switch 103 are different. Therefore, a random noise which is caused due to the resetting operation cannot be perfectly removed in a manner similar to the prior art 1.

[0029] In the prior art 2, although an example in which the gate switch 109 is not provided is also disclosed, there is a disclosure such that there is a problem such that the random noise due to the sensor resetting operation is also generated even in such a case.

[0030] Namely, in the conventional techniques, although the FPN can be improved, the random noise due to the resetting operation of the photosensor is not improved yet.

[0031] It is an object of the invention to reduce a random noise due to a resetting operation of a sensor unit in a one-dimensional or two-dimensional photoelectric converting apparatus in which a photosignal accumulation and a signal output are simultaneously executed.

[0032] As means for solving the foregoing problems, it is an object of the invention to provide a photoelectric converting apparatus for outputting a signal during an accumulation of a photosignal, comprising:

[0033] photoelectric conversion means;

[0034] reset means for resetting the photoelectric conversion means;

[0035] noise signal holding means for holding a noise signal when a resetting operation of the photoelectric conversion means is executed; and

[0036] noise signal removing means for removing a noise from the signal accumulated in the photoelectric conversion means after completion of the same resetting operation by using the noise signal held.

[0037] According to the invention, there is also provided a photoelectric converting apparatus comprising:

[0038] a plurality of photoelectric conversion means arranged in a one-dimensional or two-dimensional form;

[0039] reset means for resetting the plurality of photoelectric conversion means;

[0040] noise signal read-out means for reading out noise signal charges just after the resetting operation;

[0041] photosignal read-out means for reading out photosignal charges after the photosignal was accumulated; and

[0042] scanning means for sequentially scanning the noise signal of the noise signal read-out means and the photosignal of the photosignal read-out means,

[0043] in which the signals are read out from the noise signal read-out means and the photosignal read-out means by the scanning means and, at the same time, the photosignal is accumulated in the photoelectric conversion means,

[0044] wherein the photoelectric converting apparatus further comprises

[0045] a noise signal holding unit for holding the noise signal just after the resetting operation at least means a timing before the photosignal after completion of the same resetting operation is read out by the photosignal read-out means, and

[0046] for outputting a difference between the noise signal just after the resetting operation and the photosignal after completion of the same resetting operation.

[0047] According to the invention, by providing the noise holding means, after the noise generated upon resetting of the sensor was held, the accumulating operation and reading operation of the photosignal are executed as they are and the difference between the noise and the photosignal in the same sensor resetting operation can be obtained, so that a random noise which is caused upon sensor resetting can be perfectly removed.

[0048] The above and other objects and features of the present invention will become apparent from the following detailed description and the appended claims with reference to the accompanying drawings.

[0049] FIGS. 1A and 1B are circuit diagrams of the first embodiment of the invention and a timing chart showing the operation of such a circuit respectively;

[0050] FIGS. 2A and 2B are circuit diagrams of the second embodiment of the invention and a timing chart showing the operation of such a circuit respectively;

[0051] FIGS. 3A and 3B are circuit diagrams of the third embodiment of the invention and a timing chart showing the operation of such a circuit respectively;

[0052] FIG. 4 is a circuit diagram of the fourth embodiment of the invention;

[0053] FIG. 5 is a timing chart showing the operation of the circuit of the fourth embodiment of the invention;

[0054] FIGS. 6A and 6B are circuit diagrams of a prior art 1 and a timing chart showing the operation of such a circuit respectively;

[0055] FIG. 7 is a circuit diagram of a prior art 2; and

[0056] FIG. 8 is a circuit diagram of the prior art 2.

[0057] An embodiment of the invention will now be described in detail hereinbelow with reference to the drawings.

[0058] FIGS. 1A and 1B are circuit diagrams of a photoelectric converting apparatus according to the first embodiment of the invention and its timing chart.

[0059] As shown in the diagram, the photoelectric converting apparatus of the embodiment is constructed by:

[0060] a photodiode 1 serving as photoelectric conversion unit; MOS transistors 3 and 3′; an MOS switch 2 serving as a reset unit of the photoelectric conversion unit;

[0061] MOS transistors 4, 6, and 6′ and a capacitor (C_M) 5 serving as a noise signal holding unit for holding a noise signal upon resetting of the photoelectric conversion unit; and

[0062] a noise signal removing unit (7 to 16) for removing a noise from the signal accumulated in the photoelectric conversion unit after completion of the same resetting operation by using the noise signal held.

[0063] There is also provided a photoelectric converting apparatus constructed by:

[0064] an MOS switch 7 and a capacitor (C_TN) 9 serving as a noise signal read-out unit for reading out noise signal charges just after the resetting operation;

[0065] an MOS switch 8 and a capacitor (C_TS) 10 serving as a photosignal read-out unit for reading out photosignal charges after the photosignal was accumulated;

[0066] a shift register 13 serving as a scanning unit for sequentially scanning the noise signal of the noise signal read-out unit and the photosignal of the photosignal read-out unit,

[0067] in which the signals are read out from the noise signal read-out unit (7, 9) and the photosignal read-out unit (8, 10) by the scanning unit and, at the same time, the photosignal is accumulated in the photoelectric conversion unit,

[0068] wherein the photoelectric converting apparatus further has

[0069] a noise signal holding unit (4, 5, 6, 6′) for holding the noise signal just after the resetting operation until a timing before the photosignal accumulated after completion of the same resetting operation is read out to the photosignal read-out unit (8, 10), and

[0070] buffer amplifiers 14 and 14′ and a differential amplifier 15 serving as a unit for outputting a difference between the noise signal held just after the resetting operation and-the photosignal after completion of the same resetting operation.

[0071] The MOS transistors (6, 6′) and (3, 3′) construct MOS source followers, respectively.

[0072] Inputs 16 and 16′ of the buffer amplifiers 14 and 14′ are common output lines and as for the component elements other than the buffer amplifiers 14 and 14′ and differential amplifier 15, the apparatus has those component elements of the number as many as the number of bits.

[0073] In the embodiment, all of the portions shown in the diagram are formed on a same semiconductor substrate.

[0074] The operation and a construction of the embodiment will now be described hereinbelow with reference to the timing chart.

[0075] When the start pulse SP is first inputted, the capacitors C_TS10 and C_TN9 for accumulating the photosignal and the noise signal are first reset.

[0076] Subsequently, a drive pulse φTN is turned on and the noise signal held in the capacitor C_M5 is read out to the capacitor C_TN9. In this instance, the noise signal that is read out from the capacitor C_M5 is a noise signal Just after the sensor was reset in the previous field. After the noise signal was read out to the capacitor C_TN9, a drive pulse φT1 is turned on and the photosignal is read out to the capacitor C_M5. Further, a drive pulse φTS is turned on and the photosignal is read out to the capacitor C_TS10.

[0077] After that, a drive pulse φR is turned on and the sensor is reset. Subsequently, the drive pulse φT1 is turned on and the signal Just after the sensor was reset is read out as a noise signal to the capacitor C_M5. The sensor starts to accumulate.

[0078] Simultaneously with the accumulating operation of the sensor, the photosignal held in the capacitor C_TS10 and the noise signal held in the capacitor C_TN9 are sequentially outputted to the common output lines. Finally, the difference between the photosignal and the noise signal is obtained from a differential circuit or the like (not shown) and is outputted as a true photosignal.

[0079] In the invention, therefore, the noise signal for a sensor reset denoted by (1) in the timing chart is held in the capacitor C_M5 for an accumulation period of time (A) and is inputted to the capacitor C_TN9 before the photosignal is read out (A′). Therefore, since a difference between the noise signal (A′) for the same sensor reset of (1) and a photosignal (B′) can be outputted as a true photosignal, a sensor reset noise can be perfectly removed.

[0080] The noise removing unit is not limited to that in the foregoing embodiment but, for example, a clamping circuit or the like can be also used.

[0081] As disclosed in Japanese Patent Application No. 61-219666 (1986), a circuit for holding a sensor signal, a circuit for holding a reference signal, and a differential amplifier for outputting a difference signal of them can be also used as a differential circuit.

[0082] FIGS. 2A and 2B are circuit diagrams showing the second embodiment of the invention and its timing chart.

[0083] The second embodiment differs from the first embodiment with respect to a point that a photoelectric conversion unit by a bipolar transistor 1′ is used. In the embodiment as well, a portion surrounded by a broken line corresponds to the noise signal holding unit and its construction is the same as that of the first embodiment.

[0084] The operation will now be described hereinbelow with reference to the timing chart.

[0085] When a start pulse (not shown) is inputted, a drive pulse φCR is turned on and the photosignal accumulating capacitor C_TS10 and noise signal accumulating capacitor C_TN9 are reset.

[0086] Subsequently, the drive pulse φTN is turned on and the noise signal is read out to the capacitor C_TN9.

[0087] The drive pulse φT1 is turned on and the photosignal is read out to the capacitor C_M5.

[0088] The drive pulse φTs is turned on and the photosignal is read out-to the capacitor C_TS10.

[0089] A drive pulse φBRS is turned on and a base of the bipolar transistor 1′ is clamp-reset to V_BB. Further, by turning on a drive pulse φERS, the sensor is reset.

[0090] The drive pulse φT1 is turned on and the noise signal generated upon resetting of the sensor is inputted to the capacitor C_M5. After that, the accumulating operation of the sensor is executed.

[0091] Simultaneously with that the sensor executes the accumulation, the photosignal held in the capacitor C_TS10 and the noise signal held in the capacitor C_TN9 are sequentially outputted to the common output lines. Finally, the difference between the photosignal and the noise signal is derived by the differential amplifier 15 and is outputted as a true photosignal.

[0092] According to the invention as well, therefore, the noise signal for the sensor reset is held in the capacitor C_M5 for the accumulation period of time and is inputted to the capacitor C_TN9 before the photosignal is read out. Consequently, since a difference between the noise signal and the photosignal for the same sensor reset can be outputted as a true photosignal, the sensor reset noise can be perfectly removed.

[0093] It is not always necessary to use the noise holding capacitor C_M5. In such a case, a parasitic capacitance of a wiring or an element can be used in place of the capacitor C_M5.

[0094] In the embodiment, the component elements other than the differential amplifier 15 are formed on the same semiconductor substrate.

[0095] FIGS. 3A and 3B are circuit diagrams of a photoelectric converting apparatus and its timing chart according to the third embodiment of the invention.

[0096] In the diagram, reference numeral 1 denotes the photodiode and a portion surrounded by a broken line corresponds to a noise signal holding unit as a feature of the invention and is constructed by: the capacitor C_M5 to hold a noise signal; MOS transistor switches 4, 18, and 17; and source-follower circuits 6 and 6′ of the MOS transistors. The noise signal holding unit also has the capacitor C_TS10 for accumulating the photosignal and the capacitor C_TN9 for accumulating the noise signal.

[0097] The operation and construction of the embodiment will now be described hereinbelow with reference to the timing chart.

[0098] First, when a start pulse SP (not shown) is inputted, the drive pulse φCR is first turned on and the capacitor C_TS10 for accumulating the photosignal and the capacitor C_TN9 for accumulating the noise signal are reset.

[0099] Subsequently, a drive pulse φT2 is turned on, the drive pulse φTN is turned on during such an ON period of time, and the noise signal is inputted to the capacitor C_TN9.

[0100] A drive pulse φR2 is turned on and the source followers 6 and 6′ are reset.

[0101] The drive pulse φT1 is turned on and the photosignal is inputted to the capacitor C_M5.

[0102] The drive pulse φT2 is turned on, the drive pulse φTS is turned on during such an ON period of time, and the photosignal is inputted to the capacitor C_TS10.

[0103] After that, drive pulses φR1 and φR2 are turned on, thereby resetting the sensor and the source followers. The drive pulse φT1 is turned on and the signal just after the sensor was reset is read out as a noise signal to the capacitor C_M5. The sensor starts the accumulating operation.

[0104] Simultaneously with the accumulating operation of the sensor, the photosignal held in the capacitor C_TS10 and the noise signal held in the capacitor C_TN9 are sequentially outputted to the common output lines. Finally, the difference between the photosignal and the noise signal is derived by a differential circuit (not shown) and is outputted as a true photosignal.

[0105] Therefore, even in the invention as well, the noise signal for the sensor reset is held in the capacitor C_M5 during the accumulation period of time and is inputted to the capacitor C_TN9 before the photosignal is read out. Since a difference between the noise signal and the photosignal for the same sensor reset, consequently, can be outputted as a true photosignal, the sensor reset noise can be perfectly removed.

[0106] FIG. 4 is a circuit diagram showing the fourth embodiment of the invention. FIG. 5 is a timing chart thereof. In the embodiment, there is shown an image reading apparatus in which a plurality of photoelectric converting elements of the invention mentioned above are arranged in a two-dimensional form and a horizontal shift register H.S.R. and a vertical shift register V.S.R. are connected.

[0107] As shown in the diagram, each pixel has: a photodiode serving as a photoelectric conversion unit; and an MOS transistor 44 forming a source follower as a noise signal holding unit together with an MOS transistor switch 43. A gate capacitor of the MOS transistor 44 is used as a noise holding capacitor mentioned above.

[0108] One photosignal accumulating capacitor C_TSand one noise signal accumulating capacitor C_TNmentioned above are connected to a plurality of pixels.

[0109] The operation and construction of the embodiment will now be described hereinbelow with reference to the timing chart.

[0110] When the start pulse SP (not shown) is inputted, the photosignal accumulating capacitor C_TSand the noise signal accumulating capacitor C_TNare first reset.

[0111] Subsequently, the drive pulse φTN is turned on and the noise signal is read out from the gate capacitor of the MOS transistor 44 to the capacitor C_TN. Further, the drive pulse φTS is turned on and the photosignal is read out to the capacitor C_TS.

[0112] After that, the drive pulse φR is turned on, thereby resetting the sensor. Subsequently, the drive pulse φT1 is turned on and the signal just after the sensor was reset is read out to the gate capacitor of the MOS transistor 44 through a source follower of an MOS transistor 42, and the sensor starts the accumulating operation.

[0113] Simultaneously with the accumulating operation of the sensor, the photosignal held in the capacitor C_TSand the noise signal held in the capacitor C_TNare sequentially outputted to the common output lines. Finally, the difference between the photosignal and the noise signal is derived by a differential circuit (not shown) and is outputted as a true photosignal.

[0114] In the invention, therefore, the noise signal for the sensor reset is held in the gate capacitor of the MOS transistor 44 during the accumulation period of time and is inputted to the capacitor C_TNbefore the photosignal is read out. Therefore, since the difference between the noise signal and the photosignal for the same sensor reset can be outputted as a true photosignal, the sensor reset noise can be perfectly removed.

[0115] In the area sensor or the like as shown in the embodiment, in order to improve a numerical aperture, as a photodiode 40, for instance, a photoconductive film such as amorphous silicon is laminated onto the semiconductor substrate and a resultant device can be also used.

[0116] According to the invention as described above, by using the noise holding unit, after the noise generated at the time of the first sensor reset was held, the accumulating operation and reading operation of the photosignal are executed as they are and the difference between the noise and the photosignal in the first reset can be obtained, so that the sensor reset noise can be perfectly removed.

[0117] Namely, in the invention, the noise signal for the sensor reset is held in the noise signal holding unit during the accumulation period of time and is inputted to the capacitor C_TNbefore the photosignal is read out. Therefore, since the difference between the noise signal and the photosignal for the sensor reset can be outputted as a true photosignal, the sensor reset noise can be perfectly removed.

[0118] Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.