Solid state image pickup apparatus

Abstract: A solid state image pickup apparatus having a selector for selecting a plurality of sensor outputs through predetermined circuits includes a difference processing circuit for calculating a difference between a selected sensor signal and a reference signal selected through the same predetermined circuit as that of the selected sensor signal. The difference processing circuit includes a first holding circuit for holding the sensor signal, a second holding circuit for holding the selected reference signal, and a circuit for receiving the output signals from the first and second holding circuits and outputting a signal corresponding to a difference therebetween.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a solid state image pickup apparatus for selectively reading out a plurality of sensor signals and, more particularly, to a solid state image pickup apparatus capable of eliminating unnecessary components such as variations in dark signals and drive noise. Related Background Art Fig. 1A is a schematic circuit diagram of a known solid-state image pickup apparatus. Referring to Fig. 1A, signals from sensors S1 to Sn are respectively amplified by amplifiers A1 and An, and transistors T1 to Tn are sequentially turned on. A dot sequential output appears on an output line 101. The dot sequential signal is amplified by a buffer amplifier 102, and the resultant signal appears as an output signal Vout. In the known image pickup apparatus described above, variations in input/output characteristics of the amplifiers A1 to An are included in the sensor signals forming the dot sequential output appearing on the output line 101. As a result, fixed pattern noise undesirably occurs. Fig. 1B shows a schematic arrangement of another known photoelectric transducer apparatus. Referring to Fig. 1B, signals read out from photosensors S1 to Sn are temporarily stored in storage capacitors C1 to Cn. Transistors T1 to Tn are sequentially turned on at timings of a scanning circuit SH, and the readout signals sequentially appear on an output line 101 and are output to an external device through an amplifier 102. In the above photoelectric transducer apparatus, however, unwanted components such as dark signals and drive noise of the photosensors are undesirably included. Drive noise is defined as noise generated when a photosensor is driven to read out a signal. The drive noise components are noise caused by manufacturing variations such as element shapes, and smear caused by element isolation and depending on radiation amounts. The dark signal is defined as a dark current of a photosensor and greatly depends on accumulation time and on the temperature of the photosensor. This drive noise will be described in detail. Variations in drive capacitance of a drive element for driving a photoelectric transducer element and variations in capacitance of a photoelectric transducer element cause variations in the leakage component of drive pulses. These variation components form an information signal superposed on the desired photoelectric transducer signal, and are read out. The cause of generation of drive noise will be described below. Fig. 1C is a schematic view of a photoelectric transducer element described in JP-A-12764/1985, Fig. 1D is a timing chart of drive pulses for driving the photoelectric transducer element shown in Fig. 1C, and Fig. 1E is a chart showing the base potential of the photoelectric transducer element. Referring to Fig. 1C, the photoelectric transducer element includes a base accumulation type bipolar transistor B, a drive capacitor Cox for reverse- or forward-biasing the transistor B in response to a drive pulse r, and a refresh transistor Qr. The transistor B has junction capacitances Cbc and Cbe. It should be noted that Cox, Cbc, and Cbe are referred to as capacitances or capacitors hereinafter, as needed. The capacitances Cox, Cbc, and Cbe are added to obtain a charge storage capacitance Ctot. The operation of the photoelectric transducer element will be described below. Assume that the initial value of a base potential VB is given as V0. When the drive pulse r is set at a potential Vr at time tl, a voltage Va is applied to the base of the transistor B through the drive capacitor Cox. In this case, the voltage Va can be represented as follows: Va = Cox/(Cox + Cbc + Cbe) x Vr = (Cox/Ctot) x Vr When the refresh pulse rh is set at a high potential at time t2, the transistor Qr is turned on. When the transistor B is forward-biased, the base potential VB abruptly decreases. A time interval TC between time t2 and time t3 is a so-called refresh time interval. The drive pulse r is set to zero at time t3, and a voltage -Va is added to the base voltage VB, so that the base voltage VB is set at V2. This reverse-biased state is the accumulation state. The above description was confined to one photoelectric transducer element. However, a line or area sensor has a large number of photoelectric transducer elements. The capacitances of the capacitors Cox, Cbc, and Cbe between a large number of photoelectric transducer elements vary by a few fractions of 1%. For example, if the following conditions are given: Cox = Cbc = Cbe ≃ 0.014 pF, and Vr = 5 V and the capacitance variation is 0.2 %, then a variation ΔVa in capacitance division voltage Va is about 3 mV. The variation ΔVa can be reduced by refreshing. However, when the refresh mode is changed to an accumulation operation mode (time t3), the variation occurs again to produce ΔVb. The variation ΔVb does not satisfy relation ΔVb = -ΔVa, and no correlation can be established therebetween according to test results. The above fact is assumed to be due to different bias voltage dependencies of Cbc and Cbe. In the next read cycle, when the transistor B is forward-biased, the variation in base potential thereof is approximated as follows: ΔV 2 ≃ ΔVa 2 + ΔVb 2 + 2KΔVaΔb where K is -1 or more. As a result, the variation ΔV becomes steady drive noise of about 4 to 5 mV. The variation in leakage component of such a drive pulse (to be referred to as drive noise hereinafter) is eliminated by a known technique in which the above drive noise is stored in a memory means and is read out and subtracted from the signal read out from the sensor to obtain a true information signal. The known drive noise correction technique described above results in a bulky, expensive photoelectric transducer element. In particular, in case of that the numbers of elements arranged in the horizontal direction and vertical direction are five hundred respectively, an area sensor requires 250,000 photoelectric elements arranged in a matrix form. In addition, when the resolution of the sensor is also taken into consideration, a memory of several megabits is required. The unwanted signals such as drive noise and a dark signal pose serious problems when an image of a dark object is to be picked up, i.e., image pickup at a low intensity. In the low-intensity image pickup mode, an information signal level is low and accordingly the S/N ratio is degraded. As a result, image quality is degraded. In order to improve image quality, the unwanted signals must be reduced. As described above, however, the dark signal primarily depends on temperature and charge accumulation time, although the drive noise rarely depends thereon. If these unwanted signals are to be eliminated, the dark signal must be separated from the drive noise and a correction coefficient must be determined, thus requiring a large-capacity memory. As a result, signal processing is complicated and expensive, and an image pickup apparatus is undesirably bulky. EP-A-0108308 discloses an arrangement in which a plurality of photodiodes are connected to a common output line through a respective plurality of switches. In operation, each photodiode is read out twice in succession. The output line is cleared between the two times each photodiode is read out. It is stated that the photodiode is discharged when it is first read out, so that the first signal is a light information signal and the second signal is a noise signal. When each signal is read, it is output to one terminal of a differential amplifier. Additionally, the first signal is supplied to a sample-and-hold circuit, the output of which is provided to the other input of the differential amplifier, so that when the second signal is read out the differential amplifier acts to output the difference between the two signals, thereby removing the noise signal from the light information signal. In "100 x 100 CID Imager with Integrated Fixed Pattern Noise Suppression", Rudolph Koch and Heiner Herbst, Proceedings of 5th International Conference on Charge-Coupled Devices, University of Edinburgh, September 1979, pages 92 to 97, a system is proposed in which the light sensitive part of an imager consists of pairs of MOS-capacitors forming the picture elements. Signal charges are stored in potential wells, and at read-out the charges for the elements of a selected row flow onto column lines. The charge is transferred from the column lines in parallel into elements of a first CCD. Then the charge is moved from the first CCD into a second CCD, the picture elements of the selected row are re-set, and immediately thereafter the same row is selected again and a zero signal, with fixed pattern noise, is read into the first CCD. The two CCD's are clocked out synchronously and a differential amplifier removes the fixed pattern noise from the signal. EP-A-0172474 discloses a CCD output signal generating circuit for use in a solid state image pickup device or the like. Signals are clocked along the CCD and are transferred in turn from the CCD to a floating diffusion region. After a signal has been transferred to the floating diffusion region, a reset pulse is applied to a reset gate through which the signal charges in the floating diffusion region flow into a drain region to which a predetermined reset voltage is applied. Then the reset pulse is removed so that the diffusion region floats again, before the next signal is transferred to it from the CCD. The signal on the floating diffusion region passes through a source follower circuit to a correlated double sampling circuit. The correlated double sampling circuit comprises a clamping circuit which has a DC-cut capacitor connected between the output of the source follower and the input of a buffer amplifier. A switch is connected to the line between the capacitor and the buffer amplifier, and it is turned on (grounding the input terminal of the buffer amplifier) during a reference voltage set period. In this way, it clamps the reference voltage in the output signal from the source follower, and thereby removes the effect of the reset voltage on the signal received from the floating diffusion region. Since the reset voltage may have a slightly different effect on the floating diffusion region during different reset periods, the removal of the effect of the reset voltage results in the removal of a source of noise in the output signal. SUMMARY OF THE INVENTION According to the present invention there is provided image pickup apparatus as set out in claim 1. The remaining claims set out optional features. An embodiment of the present invention helps to eliminate variations in drive noise in units of sensor cells. An embodiment of the present invention helps to compensate for variations in electrical characteristics of a plurality of amplifiers arranged for sensor cells. In an embodiment of the present invention, there is provided a solid state image pickup apparatus having a selector for selecting a plurality of sensor signals through corresponding amplifiers, comprising a processing circuit for calculating a difference between a selected sensor signal and a reference signal selected through the same amplifier as the selected sensor signal. The sensor signal selected by the selector, therefore, includes a noise component caused by variations in amplifier characteristics since the sensor signal is amplified by the corresponding amplifier. For this reason, the reference signal is selected through the same amplifier which has amplified the sensor signal, so that the amplifier noise is superposed on the reference signal. A difference between the selected sensor signal and the selected reference signal is calculated to eliminate the noise component. In another embodiment of the present invention, there is provided a photoelectric transducer apparatus having storage means for storing a signal from a photoelectric transducer element, wherein the storage means comprises first storage means for storing the signal read out from the photoelectric transducer element and second storage means for storing a residual signal after the photoelectric transducer element is refreshed, and further comprising difference processing means for calculating a difference between the readout and residual signals respectively stored in the first and second storage means. Since the residual signal obtained upon completion of refreshing is subtracted from the readout signal, the unwanted components such as a dark signal and drive noise of the photoelectric transducer element can be eliminated. A MOS, electrostatic induction, or base accumulation type photosensor may be used as a photoelectric transducer element. "Refreshing" of the photoelectric transducer element means erasure of optical information of the photoelectric transducer element. In some photosensors, optical information is erased simultaneously when the information is read out. However, in some photosensors, optical information is kept unerased even after the information is read out. In an embodiment of the present invention, there is provided a solid state image pickup apparatus comprising a plurality of photoelectric transducer elements, first storage means, arranged in units of photoelectric transducer elements, for storing a video signal, second storage means, arranged in units of photoelectric transducer elements, for storing noise components, first readout means for simultaneously and independently reading out signals of photoelectric transducer elements of a plurality of horizontal lines from the first storage means, and second readout means for combining signals of the photoelectric transducer elements of the plurality of horizontal lines from the second storage means with the signals from the first storage means and reading out combined signals. With the above arrangement, it is assumed that the drive noise is generated as a sum of noise components generated in the refresh, charge accumulation, and readout modes of the photoelectric transducer element and the drive noise level is substantially identical in each mode. A difference between the photoelectric transducer signal read out upon completion of exposure and drive noise read out in the photoelectric transducer signal readout mode is calculated in order to eliminate the drive noise. In an embodiment of the present invention, there is provided a solid state image pickup apparatus comprising photoelectric transducer elements, a plurality of storage capacitors for storing readout signals when the photoelectric transducer elements are read-accessed a plurality of times, dot sequential processing means for converting signals from the storage capacitors into a dot sequential signal, and clamping means for clamping some components of the dot sequential signal from the dot sequential processing means. With the above arrangement, it is assumed that the drive noise is generated as a sum of noise components generated in the refresh, charge accumulation, and readout modes of the photoelectric transducer element and the drive noise level is substantially identical in each mode. The photoelectric transducer signal read out upon completion of exposure and drive noise read out in the photoelectric transducer signal readout mode are converted into a dot sequential signal, and the drive noise component is clamped, thereby eliminating the drive noise included in the photoelectric transducer signal components. Embodiments of the present invention, given by way of non-limiting example, will now be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a schematic circuit diagram of a known solid state image pickup apparatus; Fig. 1B is a schematic view of another known conventional solid state image pickup apparatus; Figs. 1C to 1E are views for explaining the principle of generation of drive noise of a photoelectric transducer element; Fig. 2A is a schematic circuit diagram showing a solid state image pickup apparatus according to an embodiment of the present invention; Fig. 2B is a timing chart for explaining the operation of the difference processing circuit shown in Fig. 2A; Fig. 3 is a circuit diagram showing an arrangement of switches SW1 to SWn in the apparatus of Fig. 2A; Fig. 4 is a circuit diagram showing another arrangement of switches SW1 to SWn in the apparatus of Fig. 2A; Fig. 5A is a schematic circuit diagram showing a solid state image pickup apparatus according to another embodiment of the present invention; Fig. 5B is a timing chart for explaining the operation of the apparatus shown in Fig. 5A; Fig. 6 is a block diagram showing an image pickup system using the apparatus (of any embodiment described above) as an image pickup device; Fig. 7A is a schematic sectional view of a photoelectric transducer cell described in JP-A-12759/1985 to 12765/1985. Fig. 7B is an equivalent circuit diagram thereof; Fig. 8 is a graph showing the relationship between a width t of a refresh pulse applied to the photoelectric transducer cell and a photoelectric transducer cell output after refreshing; Fig. 9 is a circuit diagram of a solid state image pickup apparatus according to another embodiment of the present invention; Fig. 10 is a timing chart for explaining the operation of the apparatus shown in Fig. 9; Fig. 11 is a schematic view showing an arrangement when the apparatus in Fig. 9 is applied to an area sensor; Fig. 12 is a timing chart for explaining the operation of the area sensor shown in Fig. 11; Fig. 13 is a circuit diagram of a solid state image pickup apparatus according to still another embodiment of the present invention; Fig. 14 is a detailed circuit diagram of a readout circuit Ri in the apparatus shown in Fig. 13; Fig. 15 is a timing chart for explaining the operation of the apparatus shown in Fig. 13; and Fig. 16 is a block diagram showing an image pickup system using the apparatus (Fig. 9) as an image pickup device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described with reference to the accompanying drawings hereinafter. Fig. 2A is a schematic circuit diagram of a solid state image pickup apparatus according to an embodiment of the present invention. Referring to Fig. 2A, switches SW1 to SWn are arranged to select corresponding inputs in response to pulses c1 to cn. The switches SW1 to SWn respectively receive sensor signals S1 to Sn from photosensors S1 to Sn arranged in a line or a matrix form. The switches SW1 to SWn also receive signals E from reference signal sources E, respectively. The output terminals of the switches SW1 to SWn are respectively connected to the input terminals of amplifiers A1 to An. The output terminals of the amplifiers A1 to An are connected to an output line 101 through corresponding transistors T1 to Tn. Pulses 1 to n from a scanning circuit SH such as a shift register are respectively supplied to the gate electrodes of the transistors T1 to Tn. The transistors T1 to Tn are turned on in response to the pulses 1 to n, The output line 101 is grounded through a transistor 103. A pulse hrs is applied to the gate electrode of the transistor 103. The output line 101 is also connected to a difference processing circuit 1. An output signal Vout free from noise components is output from the difference processing circuit 1. The operation of this embodiment will be described below. When the reference E is input to the amplifier A1 upon operation of the switch SW1, the reference signal E is amplified by the amplifier A1, and an amplified signal E1' is output to the transistor T1. In this case, only the transistor T1 is kept on in response to the pulse 1, and other transistors T2 to Tn are kept off. The reference signal E1' is selected by the transistor T1 and appears on the output line 101. The reference signal E1' is a signal reflecting variation characteristics of the amplifier A1, i.e., a signal including a noise component N1 which becomes a fixed pattern noise. In other words, E1' = E + N1. Subsequently, the transistor 103 is turned on in response to the pulse hrs to remove the charge left on the output line 101. An output signal from the sensor S1 is input to the amplifier A1 through the switch SW1. In the same manner as described above, a sensor signal S1' amplified by the amplifier A1 appears on the output line 101 through the ON transistor T1. The sensor signal S1' also reflects variation characteristics, i.e., a signal including the noise component N1 (S1' = S1 + N1). Difference processing is performed by a clamping circuit in the difference processing circuit 1. When the reference signal E1' amplified by the amplifier A1 appears on the output line 101, it is input to a clamp circuit through an amplifier comprising transistors 15 and 16. The clamp circuit comprises a capacitor 17 and a transistor 18. Since the transistor 18 in the clamp circuit is kept on in response to a clamp pulse s, the level of the reference signal E1' is clamped as the reference level. As a result, the sensor signal S1' subsequently appearing on the output line 101 is amplified by an amplifier of transistors 19 and 20 using the reference signal E1' as a reference level. Thus the resulting output signal Vout is the difference (S1'-E1') obtained by removing the reference signal E1' from the sensor signal S1', thereby obtaining a signal (S1-E) free from the noise component N1. Similarly, the clamp pulse s is generated at a read timing of the reference signal E2', and the sensor signals S2 to Sn free from the noise components are sequentially output. Fig. 2B shows a timing chart. In this case, the reference signal E represents the reference level of the sensor signal S1, so that E = 0 is established. Therefore, the output signal Vout is the sensor signal S1 before being subjected to the influence of the amplifier A1. When the sensor signal S1 is output in this manner, the residual charge on the output line 101 is eliminated by the transistor 103. At the timings in the same manner as described above, the sensor signals S2 to Sn free from the noise components N2 to Nn are sequentially output. In the above description, the reference signal E is read out prior to the corresponding sensor signal. However, each sensor signal may be read out prior to the reference signal E. Fig. 3 is a circuit diagram showing an arrangement of switches SW1 to SWn in the apparatus shown in Fig. 2A. Referring to Fig. 3, a transistor 201 is turned on in response to a pulse t to store the sensor signal S1 in a capacitor C1. Subsequently, a transistor 203 is turned on in response to a pulse cb to output the reference signal E to the amplifier A1. When the reference signal E1' is held in the clamp circuit, the transistor 202 is turned on in response to a pulse ca to output the sensor signal S1 from the capacitor C1 to the amplifier A1. The switches SW2 to SWn have the same arrangement as that of the switch SW1, and operations of the switches SW2 to SWn are also the same as that of the switch SW1. Fig. 4 is a circuit diagram showing another arrangement of the switches SW1 to SWn in the apparatus of Fig. 2A. In this arrangement, the reference signal E is generated by drive noise caused by variations in leakage component of the sensor. Referring to Fig. 4, a transistor 301 is turned on in response to a pulse t1 to store the sensor signal S1 in a capacitor C11. Subsequently, a transistor 303 is turned on in response to a pulse t2. A sensor signal representing absence of optical information or the dark state thereof serves as the reference signal E. This reference signal E is stored in a capacitor C12. In this state, a drive noise component of the corresponding sensor is stored in the capacitor C12. In the same manner as described above, a transistor 304 is turned on to output the reference signal E from the capacitor C12 to an amplifier A1 and then a transistor 302 is turned on to output the sensor signal S1 from the capacitor C11 to the amplifier A1. By using the sensor drive noise component as the reference signal E, the output Vout (= S1' - E1') is free from the sensor drive noise component as well as the noise component N1 of the amplifier A1. Fig 5A is a schematic circuit diagram of a solid state image pickup apparatus according to another embodiment of the present invention, and Fig. 5B is a timing chart for explaining the operation thereof. Referring to Fig. 5A, sensors B1 to Bn (to be referred to as B hereinafter) are base accumulation type phototransistors. The base potential of each transistor is controlled through a capacitor, and the carriers excited upon incidence of light are accumulated in the base region of the transistor. The accumulated voltage is read out as a sensor signal, or the accumulated carriers are removed, when a read or refresh pulse r is applied to the capacitor electrodes of the sensors B. The emitter electrodes of the sensors B which are adapted to read out sensor signals S1 to Sn (to be referred to as S hereinafter) are grounded through transistors Qr1 to Qrn (to be referred to as Qr hereinafter), respectively. The emitter electrodes are connected to temporary storage capacitors C11 to Cn1 through transistors Qa1 to Qan (to be referred to as Qa herein after) and to temporary storage capacitors C12 to Cn2 through transistors Qc1 to Qcn (to be referred to as Qc hereinafter), respectively. The capacitors C11 to Cn1 are connected to the gate electrodes of amplifiers A1 to An through transistors Qb1 to Qbn, respectively. The capacitors C12 to Cn2 are connected to the gate electrodes of the amplifiers A1 to An through transistors Qd1 to Qdn, respectively. A voltage Vcc is applied to the first terminals of the amplifiers A1 to An, and an output line 501 is commonly connected to the second input terminals thereof. A pulse a1 is applied to the gate electrodes of the transistors Qb1 to Qbn through transistors Qe1 to Qen. A pulse b1 is applied to the gate electrodes of the transistors Qd1 to Qdn through transistors Qf1 to Qfn. Pulses 1 to n from a scanning circuit SH are sequentially supplied to the gate electrodes of the transistors Qe1 to Qen, Qf1 to Qfn, and T1 to Tn, respectively. A transistor 502 is connected to the output line 501, and a voltage Vss is applied to the output line 501 through the transistor 502. A signal S' amplified by each amplifier and appearing on the output line 501 is input to the difference processing circuit 1, and difference processing as described above is performed. The operation of the apparatus of this embodiment will be described with reference to Fig. 5B. Assume that carriers corresponding to the intensity levels of the incident light are stored in the base regions of the sensors B, respectively. For a time interval Tml, the transistors Qr are kept on in response to the pulse rh, and the emitter electrodes of the sensors B and the vertical lines are grounded. At the same time, the transistors Qa and Qc are turned on in response to the pulses t1 and t2 to clear the carriers from the capacitors C11 to Cn1 and C12 to Cn2, respectively. For a time interval Tm2, the transistors Qa are kept on in response to the pulse t1 and the read pulse r is supplied to the sensors B. Therefore, the sensor signals S from the sensors B are stored in the capacitors C11 to Cn1, respectively. These sensor signals include the drive noise components of the corresponding sensors. For a time interval Tm3, the transistors Qr are kept on in response to the pulse rh to ground the emitters of the sensors B. The sensors B are refreshed in response to the refresh pulse r. Upon completion of refreshing, the transistors Qr are turned off, and the transistors Qc are turned on in response to the pulse t2. During this period, the read pulse r is applied to read out the signals from the sensors B. Their drive noise components, i.e., the above-mentioned reference signals E1 to En are respectively stored in the capacitors C12 to Cn2. Thereafter, the pulse ør falls to cause the sensors B to start charge accumulation. The above operations are performed during a blanking period BLK, and the signals temporarily stored in the corresponding capacitors are sequentially read out onto the output line 501. The transistors T1, Qe1, and Qf1 are turned on in response to the pulse 1. The voltage Vcc is applied to the amplifier A1, and the amplifier A1 is rendered operative (other amplifiers A2 to An are rendered inoperative). The pulse a1 rises in synchronism with the pulse 1 and the transistor Qbl is turned on through the ON transistor Qe1. The sensor signal S1 stored in the capacitor C11 is amplified by the amplifier A1, and the amplified signal appears on the output line 501 and is then input to in a difference processing circuit 1. Subsequently, the pulse b1 rises and then the transistor Qdl is turned on through the transistor Qf1. The reference signal E1 stored in the capacitor C12 is amplified by the amplifier A1, and the amplified signal appears on the output line 501 and then input to the difference processing circuit 1. It can be assumed that the potential of an input to the amplifier A1 can be reset to a reference potential for a period from the time when the sensor signal S1 is output from the capacitor C11 to the amplifier A1 and to the time when the reference signal E1 is output from the capacitor C12. However, most of the input capacitance of the amplifier A1 is an overlap capacitance of the transistors. The input capacitance is sufficiently smaller than the capacitances of the capacitors C11 and C12, and the residual charge can be neglected. Fixed pattern noise caused by variations in amplifier characteristics is typical when the image signal is small. In this case, the residual charge is further decreased. From the above reasons, a means for resetting the input terminals of the amplifiers A1 to An is omitted. However, in an application wherein the residual charge cannot be neglected, a reset means must be connected to the inputs of the amplifiers A. When the sensor and reference signals S1' and E1' have been provided to the difference processing circuit 1, the sensor signal S1 is provided as the output signal Vout free from the drive noise component and the noise component N1. Similarly, the sensor outputs S2 to Sn are sequentially output. The capacitor C12 for storing the reference signal E1 and then the capacitor C11 for storing the sensor signal S1 must be discharged. This also applies to the signal readout operations of the sensor signals S2 to Sn. When all sensor signals are output, the next sensor signals corresponding to the incident light are stored in the sensors B. In the same manner as described above, sensor read access and refreshing are performed for the blanking period BLK. Charge accumulation of the sensors B and dot sequential operation of the sensor signals temporarily stored in the capacitors are simultaneously performed. Fig. 6 shows a schematic arrangement of an image pickup system using any one of the image pickup apparatuses of the embodiments as an image pickup device. Referring to Fig. 6, an image pickup device 601 comprises an image pickup apparatus of any one of the above embodiments. The gain or the like of the output signal Vout is controlled by a signal processing circuit 602, and the resultant signal is output as a video signal. Various pulses  for driving the image pickup device 601 are supplied from a driver 603. The driver 603 is operated under the control of a control unit 604. The control unit 604 controls the gain or the like of the signal processing circuit 602 on the basis of the output from the image pickup device 601 and also controls an exposure control unit 605 to adjust an amount of light incident on the image pickup device 301. As described above, in the solid state image pickup apparatus according to the above embodiments, the difference between the selected sensor signal and the selected-reference signal is calculated to obtain an output signal free from the noise components. Therefore, the variations in readout signal depending on the potential variations of the input/output characteristics of the selector can be corrected. The fixed pattern noise caused by the variations in amplifier characteristics can be eliminated. A photoelectric transducer element used in Figs. 2A to 6 will be described as a supplementary explanation of Figs. 1C to 1E. Fig. 7A is a schematic sectional view of a photoelectric transducer cell described in JP-A-12759/1985 to 12765/1985, and Fig. 7B is an equivalent circuit diagram of the cell. Referring to Figs. 7A and 7B, photo-electric transducer cells are formed on an n + -type silicon substrate 701, and each photoelectric transducer cell is electrically insulated from adjacent photoelectric transducer cells by an element isolation region 702 made of SiO 2 , SiH 3 N 4 , or polysilicon. Each photoelectric transducer cell has the following structure. A p-type region 704 doped with a p-type impurity is formed on an n - -type region 703 formed by an epitaxial technique and having a low impurity concentration. An n + -type region 705 is formed in the p-type region 704 by impurity diffusion or ion implantation. The p-type region 704 and the n + -type region 705 respectively serve as the base and emitter of a bipolar transistor. An oxide film 706 is formed on the n - -type region 703, and a capacitor electrode 707 having a predetermined area is formed on the oxide film 706. The capacitor electrode 707 opposes the p-type region 704 through the oxide film 706 and controls a potential of the p-type region 704 floating upon application of a pulse voltage to the capacitor electrode 707. In addition, an emitter electrode 708 is connected to the n + -type region 705, an n + -type region 711 having a high impurity concentration is formed on the lower surface of the substrate 701, and a collector electrode 712 is formed to apply a potential to the collector of the bipolar transistor. The basic operation of the above arrangement will be described. Assume that the p-type region 704 serving as the base of the bipolar transistor is set at a negative potential. Light 713 is incident from the side of the transistor bearing the p-type region 704. Holes in the electron-hole pairs generated upon radiation are accumulated in the p-type region 704 and the potential at the p-type region 704 is increased by the accumulated holes in the positive direction (charge accumulation). Subsequently, a positive read voltage is applied to the capacitor electrode 707, and a read signal corresponding to a change in base potential during charge accumulation is output from the floating emitter electrode 708 (read operation) It should be noted that the amount of accumulated charge is barely reduced in the p-type region 704 serving as the base of the bipolar transistor, so that read access can be repeated. In order to remove the holes from the p-type region 704, the emitter electrode 708 is grounded, and a refresh pulse of a positive voltage is applied to the capacitor electrode 708. Upon application of the refresh pulse, the p-type region 704 is forward-biased with respect to the n + -type region 705, thereby removing the holes. When the refresh pulse falls, the p-type region 704 restores the initial state of the negative potential (refresh operation). Charge accumulation, read access, and refreshing are repeated as described above. In order to restore the initial potential state of the p-type region 704 by refreshing, a refresh pulse having a sufficient pulse width is required. However, the refresh pulse width must be shortened to achieve high-speed operation. In this case, when the refresh pulse width is short, satisfactory refreshing cannot be performed. Unwanted components such as a dark signal and drive noise are added to the after image. Fig. 8 is a graph showing the relationship between a refresh pulse width t applied to the photoelectric transducer cell and the photoelectric transducer cell output. Referring to Fig. 8, an output at t = 0 is a read signal after charge accumulation and represents a read signal having a level corresponding to the intensity of the incident light. The output level of such a photoelectric transducer is reduced by refreshing. However, the rate of change in output level and the level of the residual image upon refreshing vary depending on the intensity of the incident light. When identical refreshing is performed, the levels of the residual signals are not constant. When the intensity of the incident light is high, the level of the residual signal is high. In other words, the after image is typically formed. The residual signal level of high-intensity incident light is higher than that of low-intensity incident light but is greatly lowered as compared with the initial read signal level. The ratio of the unwanted components contained in the read signal is substantially low. However, the residual signal level of the low-intensity incident light is low. The decrease in the residual signal level is small as compared with the initial read signal level. Therefore, the ratio of the unwanted components included in the read signal is high. Even in the photoelectric transducer cell having the above characteristics, by subtracting the residual signal obtain upon refreshing from the initial read signal, the above-mentioned specific after image components as well as the unwanted components such as a dark signal and drive noise can be simultaneously removed. A further embodiment of the present invention will be described below. Fig. 9 is a circuit diagram for explaining the basic arrangement of an image pickup element according to the further embodiment of the present invention. Referring to Fig. 9, the emitter electrode 708 of a photoelectric transducer cell S is connected to a vertical line VL and is grounded through a transistor Qr. The vertical line VL is connected to storage capacitors Ct1 and Ct2 through corresponding transistors Qt1 and Qt2. The capacitors Ct1 and Ct2 are connected to an output line 724 through transistors Qs1 and Qs2, respectively. The output line 724 is connected to the input terminal of a clamp circuit. Pulses s1 and s2 from a scanning circuit SH are applied to the gate electrodes of the transistors Qs1 and Qs2 respectively. Pulses t1 and t2 are applied to the gate electrodes of the transistors Qt1 and Qt2, respectively. A pulse rh is applied to the gate electrode of the transistor Qr. A read or refresh pulse r is applied to a capacitor electrode 707 of the photo-electric transducer cell S. The operation of the above arrangement will be described below. Fig. 10 is a timing chart for explaining the operation of the circuit shown in Fig. 9. Cells S are reverse-biased to perform charge accumulation during a time interval Ts. Upon completion of charge accumulation, the unwanted charges on the vertical transfer lines VL and in the storage capacitors Ct1 are removed prior to transfer of photoelectric transducer signals within a time interval Tvc. The photoelectric transducer signal is transferred to a corresponding storage capacitor Ct1 during the time interval Tt2. Refreshing is performed during a time interval Tc1, and drive noise is transferred to the storage capacitor Ct2 during a time interval Tt2. Thereafter, the cells S are refreshed during a time interval of Tc2, and the next charge accumulation cycle is initiated. The photoelectric transducer signals and the drive noise are independently obtained. The signals stored in the storage capacitors Ct1 and Ct2 are dot-sequentially transferred on a single signal line 724 in response to drive pulses s1 and s2. This operation occurs during time intervals TR1 and TR2. A drive pulse hrs is used to reset the signal line to the reference potential. The signal obtained by the above-mentioned read operation represents a waveform of an output Vout. The drive noise and the photoelectric transducer signal are represented by N and S', respectively. The dot sequential signal S is input to a clamp circuit 1, and only the drive noise N is clamped in accordance with a drive pulse s2. As a result, the drive noise N is eliminated, and a true information signal indicated by a hatched portion in Fig. 10 can be obtained. Fig. 11 is a schematic circuit diagram of an area sensor constituted by the photoelectric transducer elements shown in Fig. 9. Referring to Fig. 11, the area sensor includes a vertical shift register V·SR, a horizontal shift register H·SR, and Smn, base accumulation type transistors arranged in an m x n matrix. The operation of the area sensor is basically the same as that of the photoelectric transducer element shown in Fig. 9, except that the area sensor performs horizontal scanning and vertical scanning, and a detailed description thereof is omitted. Clamping of the read signal will now be described. A schematic waveform of the read signal is shown in Fig. 12. A signal S' appears on a read signal line 724, and a pulse s2 is a drive pulse. Referring to Fig. 12, drive noise and the photoelectric signal of a cell S11 correspond to N1 and S1, respectively. A cell S12 outputs signals N2 and S2, a cell S13 outputs signals N3 and S3, a cell S14 outputs signals N4 and S4, .... The drive noise component of the dot sequential signal is clamped in response to the drive pulse s2. As a result, the drive noise is removed, and the true information signal can be obtained. The base accumulation type transistor is exemplified as the photoelectric transducer element. However, a MOS or SIT image pickup device may be used as the photoelectric transducer element. The present invention is not limited to any specific design of the photosensor. In the above embodiment, the drive noise and the photoelectric transducer signal are converted into a dot sequential signal, and clamping can be easily performed, thereby easily removing the drive noise. The above embodiment exemplifies a scheme for reading out a signal of one horizontal line. However, this embodiment may be applied to a scheme for reading out a signal of one horizontal line in a time-divisional manner or a scheme for simultaneously reading signals of a plurality of horizontal lines, as shown in Fig. 13. In the following description, the noise is removed using a differential amplifier, but the skilled reader will understand that the clamping arrangement of Fig. 11 may be used. Fig. 13 is a circuit diagram of a colour image pickup apparatus, and Fig. 14 is a detailed circuit diagram of a readout circuit Ri in this apparatus. This apparatus exemplifies a scheme for processing a signal of two horizontal lines. A similar arrangement can apply to a scheme for processing a signal of three or more horizontal lines. Referring to Fig. 13, photosensors S are arranged in an m x n area. Mosaic R, G, and B filters are arranged on the sensor surface. Column photosensor outputs are respectively output to the readout circuits R1 to Rn through vertical lines VL1 to VLn. Referring to Fig. 14, in any readout circuit Ri (i = 1, 2, ... n), the vertical lines VLi are connected to storage capacitors Ct1 to Ct4 through transistors Qt1 to Qt4, and the capacitors Ct1 to Ct4 are connected to output lines 801 to 804 through transistors Qs1 to Qs4, respectively. Since the scheme for processing a signal of two horizontal lines is used, two capacitors are used for storing the read signals and two capacitors are used for storing residual signals. The gate electrodes of the transistors Qt1 to Qt4 are connected in common through corresponding readout circuits R1 to Rn. Pulses t1 to t4 supplied to the gate circuits of the transistors Qt1 to Qt4. A pulse i from a horizontal scanning circuit SH is supplied to the transistors Qs1 to Qs4 of the readout circuit Ri. The transistors Qs1 to Qs4 are simultaneously turned on/off. The output lines 801 and 802 are connected to the input terminals of a differential amplifier 805, and the output lines 803 and 804 are connected to the input terminals of a differential amplifier 806. Signals OUT1 and OUT2 are output from the differential amplifiers 805 and 806, respectively. Two lines per field are selected by a vertical scanning circuit 807 and an interlace circuit 808. Pairs of two horizontal scanning lines in adjacent fields are selected in response to pulses Vr1 and Vr2. The operation of the above circuit will be described with reference to Fig. 15. Fig. 15 is a timing chart for explaining the operation of the above circuit. Each photosensor read signal and its residual signal for two horizontal lines are read out during a horizontal blanking (HBLK) period and are stored in the storage capacitors in the readout circuits R1 to Rn. Transfer of one of the two horizontal lines is performed during a time interval Ta in response to the pulse Vr1. Transfer of the remaining horizontal line is performed during a time interval Tb in response to the pulse Vr2. The transfer operations are substantially the same as those in Fig. 11. However, since transfer is performed during the HBLK period, the transfer time can be shortened as compared with a scheme for processing a signal of one horizontal line. Clearing of the residual signal storage capacitor and its charge storage are performed during substantially equal time intervals T3' and T3". Smear generated during signal transfer is proportional to the transfer time. In this sense, T2 (T2') and T3' (T3") are shortened to suppress the smearing phenomenon. The capacitors Ct1 and Ct2 are cleared within a time interval T1 in the time interval Ta. During a time interval T2, a pulse r1 is supplied to the first horizontal line in response to the pulse Vr1, and read signals of the photosensors on the first horizontal line are stored in the capacitors Ct1 in the readout circuits R1 to Rn. Subsequently, during a time interval T3', the photosensors of the first horizontal line are refreshed, and the residual signals upon completion of refreshing are stored in the capacitors Ct2. During the next time interval Tb, the same transfer as in the first horizontal line is performed for the second horizontal line in response to the pulse r2 generated in response to the pulse Vr2. The read and residual signal of each photosensor for the second horizontal line are respectively stored in the capacitors Ct3 and Ct4. When the read and residual signals of the first and second horizontal lines are stored in the capacitors Ct1 to Ct4 of the readout circuits R1 to Rn, the pulses 1 to n from the horizontal scanning circuit SH are sequentially output to the readout circuits R1 to Rn, so that an R- and G-dot sequential signal OUT1 and a G- and B-dot sequential signal OUT2 which are free from the unwanted components are output from the differential amplifiers 805 and 806, respectively. It should be noted that the signal OUT1 is a G- and B-dot sequential signal and the signal OUT2 is an R- and G-dot sequential signal in the next field. Fig. 16 is a schematic block diagram of an image pickup system using the solid state image pickup apparatus as an image pickup device. An image pickup device 901 comprises an image pickup apparatus shown in Figs. 13 and 14. The output signals OUT1 and OUT2 from the image pickup device 901 are processed by an image processing circuit 903 through a sample/hold (S/H) circuit 902 to produce a standard television signal such as an NTSC signal. Pulses for driving the image pickup device 901 are supplied from a driver 904. The driver 904 is controlled by a control unit 905. The control unit 905 also controls an exposure control unit 906 to determine an intensity of light incident on the image pickup device 501. In this arrangement the residual signal after refreshing is subtracted from the read signal of the photoelectric transducer cell to remove the unwanted components (e.g., a dark signal and drive noise) of the photoelectric transducer signal, thereby obtaining a video signal having a high S/N ratio. As a result, a low-cost, compact image pickup apparatus can be manufactured.

Claims: Image pickup apparatus comprising:. a plurality of photoelectric transducer elements (S) for converting incident light into electrical signals; reading means for reading a noise signal (E'1) associated with a photoelectric transducer element and for reading a sensor signal (S'1), representing incident light, from the same photoelectric transducer element; and processing means (1) for receiving the noise signal and the sensor signal and performing a difference operation on them to remove noise, represented by the noise signal (E'1), from the sensor signal (S'1), characterised in that : reset means is provided for resetting the said photoelectric transducer element after the sensor signal has been read from it; the processing means comprises a clamp circuit (17, 18) for performing the said difference operation; and the reading means is arranged to provide the noise and sensor signals for the same photoelectric transducer element to the clamp circuit in succession. Apparatus according to claim 1 in which the noise signal associated with a photoelectric transducer element is amplified by an amplifier means (A, 11) before it is received by the processing means, and the sensor signal from the same photoelectric transducer element is amplified by the same amplifier means before it is received by the processing means, whereby the difference operation removes from the sensor signal fixed pattern noise introduced by the amplifier means. Apparatus according to claim 2 which comprises first amplifier means (A1) for amplifying the noise signal associated with a first photoelectric transducer element and for amplifying the sensor signal from the first photoelectric transducer element, and second amplifier means (A2) for amplifying the noise signal associated with a second photoelectric transducer element and for amplifying the sensor signal from the second photoelectric transducer element. Apparatus according to claim 3 in which the photoelectric transducer elements are arranged in a plurality of columns, photoelectric transducer elements of the same column sharing a common said amplifier means and respective columns having respective amplifier means. Apparatus according to claim 3 or claim 4 in which the said noise signals are reference signals applied to the input of the respective said amplifier means. Apparatus according to any one of claims 1 to 4 in which the said noise signals are signals read from the respective photoelectric transducer elements. Apparatus according to any one of the preceding claims in which the clamp circuit comprises a capacitor (17) for receiving the noise signal and the sensor signal in succession at a first side thereof, and switch means (18) for clamping the second side of the capacitor (17) when the earlier of the said signals is received and allowing the capacitor (17) to float when the later of the said signals is received, whereby the capacitor (17) provides at its second side, when the later of the signals is received, the difference between the noise signal and the sensor signal received in succession at its first side. Apparatus according to any one of the preceding claims in which the photoelectric transducer elements comprise transistors. Apparatus according to any one of the preceding claims in which the reading means comprises storage means (C11 to C22, Ct1,Ct2) for storing the noise signal and the sensor signal after they have been read and before they are provided to the clamp circuit. Apparatus according to claim 9 in which the reading means is arranged to read simultaneously the noise signals associated with a plurality of said photoelectric transducer elements and to read simultaneously the sensor signals from a plurality of said photoelectric transducer elements. Apparatus according to claim 9 or claim 10 in which the storage means comprise capacitors. Apparatus according to any one of claims 9 to 11 in which the photoelectric transducer elements are arranged in a plurality of rows, and the reading means reads and stores the sensor signals and the noise signals for the photoelectric transducer elements of a row during a first part (BLK) of the period (1H) taken to read and process the signals for a row, and the processing means receives the stored sensor signals and the stored noise signals for the photoelectric transducer elements of a row during a second part of the said period (1H), the second part of the period following the first part. Apparatus according to any one of claims 1 to 12 which comprises a signal processing circuit (602) for processing a signal from the said processing means, and a control unit (604) for controlling the said signal processing circuit, said reading means and the said reset means.

Inventors: HARADA TADANORI, HASHIMOTO SEIJI, OHZU HAYAO, SUZUKI TSUENO, HARADA, TADANORI, HASHIMOTO, SEIJI, OHZU, HAYAO, SUZUKI, TSUNEO, SUZUKI, TSUENO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1987
Main classification: H04N5
IPC codes: H04N5, H04N3