Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a signal processing apparatus having a clamping capacity. 
     2. Related Background Art 
     For the solid-state image pickup apparatus, CCD has been conventionally employed because of its high S/N ratio. On the other hand, there has been developed so-called amplifying-type solid-state image pickup apparatus which is featured by the simplicity of use and the low electric power consumption. The amplifying solid-state image pickup apparatus is to guide a signal charge accumulated in a light receiving pixel to a control electrode of a transistor provided in the pixel portion and to output an amplified signal from a main electrode, and is known in various types such as an SIT image sensor utilizing an SIT (static induction transistor) as the amplifying transistor, a bipolar image sensor utilizing a bipolar transistor, a CMD utilizing a JFET (junction field effect transistor) in which the control electrode (gate) is depleted, and a CMOS sensor utilizing a MOS transistor. Intensive development is being conducted for the CMOS transistor, since it matches well with the CMOS process and allows to form peripheral CMOS circuits on a single chip. In such amplifying solid-state image pickup apparatus however, the output offset of the amplifying transistor in each pixel is different from pixel to pixel, so that a fixed pattern noise (FPN) is superposed with the output signal of the image sensor. There have been proposed various signal output circuits in order to eliminate such FPN. In the following there will be explained a representative example of such CMOS sensor. 
     FIG. 1 is a circuit diagram of a conventional CMOS image sensor and a readout circuit therefor, showing unit pixels  1  illustrated in a 2×2 matrix arrangement for the purpose of simplicity. In FIG. 1 there are shown a photodiode  2  for accumulating a signal charge by receiving light, a MOS transistor  3  for amplifying the signal charge, a transfer MOS transistor  4  for transferring the signal charge accumulated in the photodiode  2  to the gate electrode of the MOS transistor  3 , a reset MOS transistor  5  for resetting the gate electrode potential of the MOS transistor  3 , and a power supply potential supply line  6  to which commonly connected are the drain electrode of the reset MOS transistor  5  and the drain electrode of the amplifying MOS transistor  3 . There are also shown a selector switch MOS transistor  7  for selecting an output pixel, and a pixel output line  8 . When the selector switch MOS transistor  7  is turned on, the source electrode of the amplifying MOS transistor and the output line  8  are connected whereby the signal output of a selected pixel is guided to the output line  8 . A constant current supply MOS transistor  9  for feeding a constant current to the pixel output line  8 , which supplies the amplifying MOS transistor  3  with a load current through the selector switch MOS transistor  7 , thereby causing the amplifying MOS transistor  3  to function as a source follower and outputting to the output line  8  a potential corresponding to the gate potential of the MOS transistor  3  with a constant voltage difference. There are also shown a transfer control line  10  for controlling the gate potential of the transfer MOS transistor  4 , a reset control line  11  for controlling the gate potential of the reset MOS transistor  5 , a selection control line  12  for controlling the gate potential of the selecting MOS transistor  7 , a constant potential supply line  13  for supplying the gate of the MOS transistor  9  with a constant potential thereby causing the MOS transistor  9  to operate in a saturation region thereby constituting a constant current supply source, a pulse terminal  14  for supplying the transfer control line  10  with a transfer pulse, a pulse terminal  15  for supplying the reset control line  11  with a reset pulse, a pulse terminal  16  for supplying the selection control line  12  with a selection pulse, a vertical scanning circuit  17  for selecting in succession rows of the pixels in a matrix arrangement, an output line  18  of the vertical scanning circuit  17 , including a first row selecting output line  18 - 1  and a second row selecting output line  18 - 2 , a switching MOS transistor  19  for guiding the pulse from the pulse terminal  14  to the transfer control line  10 , a switching MOS transistor  20  for guiding the pulse from the pulse terminal  15  to the reset control line  11 , and a switching MOS transistor  21  for guiding the pulse from the pulse terminal  16  to the selection control line  12 . The gates of the MOS transistors  19 ,  20 ,  21  are connected to the row selecting output line  18 , and the row of the pixels to be driven is determined by the state of the row selecting output line  18 . An output readout circuit  22  of a pixel is provided with a capacitance  23  for holding a reset signal output of the pixel, a capacitance  24  for holding a photo signal output of the pixel, a switching MOS transistor  25  for turning on/off the conduction between the pixel output line  8  and the capacitance  23 , a switching MOS transistor  26  for turning on/off the conduction between the pixel output line  8  and the capacitance  24 , a noise output line  27  for guiding the reset output held in the capacitance  23 , a signal output line  28  for guiding the signal output held in the capacitance  24 , a switching MOS transistor  29  for turning on/off the conduction between the capacitance  23  and the noise output line  27 , a switching MOS transistor  30  for turning on/off the conduction between the capacitance  24  and the signal output line  28 , a noise output line resetting MOS transistor  31  for resetting the potential of the noise output line  27 , a signal output line resetting MOS transistor  32  for resetting the potential of the signal output line  28 , a power supply terminal  33  for supplying the source electrodes of the resetting MOS transistors  31 ,  32  with a reset potential, and a horizontal scanning circuit  34  for selecting in succession the above-mentioned capacitances  23 ,  24  provided in each column of the pixels in a matrix arrangement, including an output line  35 - 1  for selecting a first column and an output line  35 - 2  for selecting a second column. The output line of the horizontal scanning circuit  34  is connected to the gates of the switching MOS transistors  29 ,  30 . There are further shown a pulse supply terminal  36  for applying a pulse to the gates of the resetting MOS transistors  31 ,  32 , pulse supply terminals  37 ,  38  for respectively applying pulses to the gates of the switching MOS transistors  25 ,  26 , a differential amplifier  39  for amplifying and outputting the differential voltage between the potential of the noise output line  27  and that of the signal output line  28 , and an output terminal  40  of the differential amplifier  39 . 
     In the following there will be explained the operation of the sensor shown in FIG. 1, with reference to a timing chart shown in FIG.  2 . It is assumed that each of the MOS transistors shown in FIG. 1 is N type, which is turned on or off respectively when the gate potential is at the high or low level state. In timing pulse Φ 14  to Φ 38  in FIG. 2, the suffixes  14  to  38  respectively coincide with the numbers of the pulse input terminals shown in FIG. 1, and Φ 14  to Φ 38  indicate the pulses entering the respective input terminals. 
     At first the vertical scanning circuit  17  shifts the pulse Φ 18-1  supplied to the terminal  18 - 1  to the high level state to enable the operation of the first row of the pixel matrix. When the pulse Φ 16  applied to the terminal  16  is shifted to the high level state, the source of the amplifying MOS transistor  3  of the pixel is connected with the constant current power supply  9  through the output line  8  whereby the output of the source follower of the pixel is outputted to the output line  8 . Then the pulse Φ 15  applied to the terminal  15  is shifted to the high level state to reset the gate of the amplifying MOS transistor  3  by the resetting MOS transistor  5 , and, when the pulse Φ 37  applied to the terminal  37  is shifted to the high level state, the reset output of the pixel is accumulated in the capacitance  23  through the MOS transistor  25 . Then the pulse Φ 14  applied to the terminal  14  is shifted to the high level state whereby the signal charge accumulated in the photodiode  2  is transferred, through the transfer MOS transistor  4 , to the gate of the MOS transistor  3 . Subsequently, when the pulse Φ 38  applied to the terminal  38  is shifted to the high level state, whereby a signal output superposed with the reset output of the pixel, is accumulated through the MOS transistor  26  in the capacitance  24 . The reset outputs of the pixels show variety because of the fluctuation of the threshold voltage of the MOS transistors  3  of the pixels. Therefore, the difference of the outputs accumulated in the capacitances  23 ,  24  becomes a pure signal without the noise. With the operation of the horizontal scanning circuit  34 , the pulses Φ 35-1 , Φ 35-2  applied to the output lines  35 - 1 ,  35 - 2  are shifted to the high level state in succession, whereby the outputs accumulated in the capacitances  23 ,  24  of each column are guided, respectively through the MOS transistors  29 ,  30 , to the horizontal output lines  27 ,  28 . Prior to the shifting to the high level state of the control pulses Φ 35-1 , Φ 35-2  applied to the output lines  35 - 1 ,  352 , the pulse Φ 36  applied to the terminal  36  is shifted to the high level state to reset the horizontal output lines  27 ,  28  through the MOS transistors  31 ,  32 . The pixel reset output and the signal output superposed with the pixel reset output, guided to the horizontal output lines  27 ,  28  are input to the differential amplifier  39 , thereby outputting a pixel signal without noise, namely after the deduction of the reset level, from the output terminal  40 . 
     In the following there will be explained, with reference to FIG. 3, a conventional signal readout circuit of another system. 
     In FIG. 3, there is shown a readout circuit  56  corresponding to the readout circuit  22  shown in FIG. 1, and components equivalent to those in FIG. 1 are represented by corresponding numbers. The configurations other than the readout circuit  56  and the output amplifier are the same as those in FIG.  1  and are therefore omitted in FIG.  3 . 
     In FIG. 3, there are shown a pixel output line  8  equivalent to the output line  8  in FIG. 1, a clamp capacitance  41  for clamping the pixel output, a clamping MOS switch  42 , a power supply terminal  43  for supplying a clamping potential, a terminal  44  for applying a pulse to the gate of the MOS transistor  42 , a capacitance  45  for accumulating a signal output, a switching MOS transistor  46  for connecting the clamping capacitance  41  and the accumulating capacitance  45 , a terminal  47  for applying a pulse to the gate of the MOS transistor  46 , a MOS transistor  48  receiving the output  50  of a horizontal shift register  34  for transferring the signal accumulated in the accumulating capacitance  45 , a horizonal output line  49  for transferring the signal accumulated in the accumulating capacitance  45 , an amplifier  51  for amplifying and outputting the signal appearing on the horizontal output line  49 , and an output terminal  52  of the amplifier  51 . 
     The readout circuit shown in FIG. 3 operates in the following manner. The signals from the pixel are outputted, as in the first conventional example explained with reference to FIGS. 1 and 2, in the order of a reset output and a signal output which is superposed with signal charge transferred on the reset level. The MOS transistors shown in FIG. 3 are assumed to be turned on or off respectively the gate potential thereof is at the high or lower level state. Thus, when the reset output of a pixel appears on the output line  8 , high level potentials are applied to the terminals  44 ,  47  to turn on the MOS transistors  42 ,  46  thereby maintaining potential of each of the clamp portion  41  and the accumulating capacitance  45  at the clamping potential supplied to the terminal  43 . Then, after the terminal  44  is shifted to the low level state to turn off the MOS transistor  42 , the signal output is given to the signal line  8 , whereby the signal voltage appears in the accumulating capacitance  45  through the clamping capacitance  41 . The terminal  47  is shifted to the low level in this state to turn off the MOS transistor  46 . The signals accumulated in the capacitances  45  are outputted in succession through the amplifier  51  to the output terminal  52 , according to the output of the horizontal shift register. 
     However, the first conventional example explained in the foregoing with reference to FIGS. 1 and 2 has been accompanied by the following drawbacks because the output potential from the pixel is directly accumulated in the accumulating capacitance and the pixel output is input to the differential amplifier under a capacitative division by the capacitance of the horizontal output line and the aforementioned accumulating capacitance. 
     A first drawback lies in the loss in the signal output potential. In FIG. 1, it is assumed that the pixel resetting accumulating capacitance  23  has a capacitance C TN , the pixel signal accumulating capacitance  24  has a capacitance C TS , the horizontal output line  27  has a capacitance C HN , the horizontal output line  28  has a capacitance C HS , the reset output potential for the pixel is V N , and the signal voltage superposed on the reset level of the pixel is V S . The input ports of the differential amplifier receive potentials |C TN /(C HN +C TN )|·V N  and |C TS /(C HS +C TS )|·(V N +V S ) resulting from the capacitative division. Since the circuit is so designed that C HN =C HS  and C TN =C TS , a signal |C TS /(C HS +C TS )|·gV S  without the noise component V S  is output to the output terminal  40 , wherein g represents the gain of the differential amplifier  39 . Thus the signal output is lower than the pixel output by a factor C TS /(C HS +C TS ), except for the gain g of the differential amplifier. C HS  and C HN  become larger as the number of pixel columns increases, so that the loss of the signal output becomes more conspicuous. 
     A second drawback lies in a loss in the noise eliminating ability resulting from the unevenness in the capacitances C HS  and C HN  and in those C TS  and C TN , eventually resulting in an increased noise level. As explained in the foregoing, the input ports of the differential amplifier receive the potentials respectively corresponding to C TS /(C HS +C TS ) and C TN /(C HN +C TN ) times of the pixel output voltage, and, even through C TS  and C TN  are designed with an identical pattern, they inevitably show certain fluctuation in size in the practical fubrication. Also C TS  and C TN  tend to show a difference in the parasite capacitance, resulting for example from a fact that one of the output lines  27 ,  28  is closer to the horizontal shift register  34  while the other is farther therefrom, as will be apparent from the arrangement of such output lines shown in FIG.  1 . Thus, if C TS /(C HS +C TS ) and C TN /(C HN +C TN ) are mutually different because of these factors, residual of the pixel reset level cancellation will be contained in the output of the differential amplifier. Since the pixel reset level is different from pixel to pixel because of the fluctuation in the threshold voltage of the MOS transistor of each pixel, such residual represents a fixed pattern noise (FPN). 
     A third drawback lies in the slow signal transfer to the horizontal output line. The reset output potential of a pixel is determined by the gate reset level of the MOS transistor  3  of the source follower amplifier of the pixel and the gate-source potential difference V gs  in the source follower operation. As the gate reset level is given by (VDD−V th ) in which VDD is the potential of the power supply line  6  in FIG. 1 and V th  is the threshold voltage of the resetting MOS transistor  5  of the pixel, the pixel reset output is given by (VDD−V th −V gs ) which is usually at about the middle of VDD and ground level and which will be represented by V RS . Since the reference output level of the solid-state image pickup device is taken at a dark state when V S =0, namely at V RS , the potential of the resetting power supply terminal  33  for the horizontal output line is also selected at V RS . Consequently, when the high level potential VDD is applied to the gate of the transfer MOS transistor at the signal transfer from the capacitances  23 ,  24  to the horizontal output lines, the gate-source potential becomes (VDD−V RS ), thus showing a higher on-resistance of the channel in comparison with a state where the gate-source potential is VDD and resulting in a slower signal transfer to the horizontal output line. 
     The above-described first drawback is more conspicuous in the second conventional example explained with reference to FIG.  3 . More specifically, this is because the signal voltage accumulated in the capacitance  45  is subjected to a capacitative division C O /(C O +C T ) on the pixel output signal voltage, wherein C O  and C T  are magnitudes of the capacitances  41 ,  45 , and such signal voltage is further subjected to a capacitative division C T /(C H +C T ) at the entry into the amplifier  51  wherein C H  is the capacitance of the horizontal output line  49 . 
     The second drawback, namely the generation of the fixed pattern noise resulting from the fluctuation of C T  in each column, remains same also in the second conventional example. 
     The third drawback can however be avoided in the second conventional example by suitable selection of the clamping potential. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a signal processing apparatus capable of efficient signal transfer. 
     The above-mentioned object can be attained, according to one aspect of the present invention, by a signal processing apparatus comprising clamp capacitance means for receiving, at one electrode thereof, first and second signals outputted from a signal source; a signal transfer transistor of which one main electrode is connected to an other electrode of the clamp capacitance means; signal accumulating capacitance means connected to an other main electrode of the signal transfer transistor; and reset means for fixing the potential of the signal accumulating capacitance means; and 
     control means for fixing the potential of the signal accumulating capacitance means by the reset means when the first signal is outputted from the signal source and maintaining the signal accumulating capacitance means in a floating state when the second signal is outputted from the signal source; and 
     while the signal charges are transferred from the clamp capacitance means through signal transfer transistor during the output of the first and second signals, controlling the signal transfer transistor in such a manner that the potential of the one main electrode thereof is different from that of the other main electrode thereby causing the signal charge to be transferred by a saturation current. 
     According to another aspect of the present invention, there is also provided a signal processing apparatus comprising clamp capacitance means for receiving, at one electrode thereof, a signal from a signal source; signal accumulating capacitance means of which one main electrode is connected to an other electrode of the clamp capacitance means, signal accumulating capacitance means connected to an other main electrode of the signal transfer transistor; reset means for fixing the potential of the signal accumulating capacitance means; and control means for controlling the potential of the control electrode of the signal transfer transistor in such a manner that, among the charges on the aforementioned main electrode of the signal transfer transistor at the side of the clamp capacitance means of which potential varies according to the change in the potential of the output signal from the signal source, a charge in a potential level exceeding the channel potential of the signal transfer transistor is transferred to the signal accumulating capacitance means by a saturation current or a sub-threshold current of the signal transfer transistor. 
     The above and other objects, features and technological advantages of the present invention will become apparent from the following detailed description of preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a first conventional example; 
     FIG. 2 is a timing chart showing the function of the first conventional example; 
     FIG. 3 is a circuit diagram of a second conventional example; 
     FIG. 4 is a circuit diagram of a first embodiment of the present invention; 
     FIG. 5 is a timing chart showing the function of the first embodiment of the present invention; 
     FIG. 6 is a chart showing the potentials of the first embodiment of the present invention; 
     FIG. 7 is a timing chart showing the function of a second embodiment of the present invention; 
     FIG. 8 is a circuit diagram of a third embodiment of the present invention; and 
     FIG. 9 is a block diagram of an image processing apparatus employing the solid-state image pickup device of the first to third embodiments. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now the present invention will be clarified in detail by embodiments thereof, with reference to the accompanying drawings. 
     FIG. 4 is a partial circuit diagram of a signal readout circuit in a first embodiment of the solid-state image pickup element of the present invention, wherein a readout circuit  53  of the present embodiment corresponds to the readout circuit  22  in FIG. 1 or that  56  in FIG.  3 . The configuration of the photoelectric converting pixel is same as that of the unit pixel  1  shown in FIG.  1 . 
     Referring to FIG. 4, there are shown an amplifier  54  for amplifying the pixel output, having an output of a positive phase where the amplifier  54  shows a higher output potential for a larger signal, a PMOS transistor  57  constituting a signal transfer transistor for transferring the signal charge to a capacitance  45 , a gate electrode input terminal  58  for the PMOS transistor  57 , a MOS transistor  59  constituting reset means for resetting the accumulating capacitance  45 , a reset potential supply terminal  60  for the capacitance  45 , and a gate input terminal  61  for the reset MOS transistor  59 . In FIG. 4, components the same as those in FIGS. 1 and 3 are represented by same numbers and will not be explained further. The amplifier  54  in FIG. 4 constitutes a buffer for obtaining a driving power required for driving the clamp capacitance  41  constituting the clamp capacitance means in case the output resistance of the pixel is large, and the present invention may dispense with such the buffer if the output resistance of the pixel is small enough. The transfer MOS transistor  57  is composed of a PMOS transistor for the signal output of a positive phase but an NMOS transistor for the signal output of an inverse phase. This MOS transistor operates as a charge transfer gate, of which part is different from the switch MOS transistor in the conventional configuration shown in FIG.  3 . 
     FIG. 5 is a timing chart showing the operation of the present embodiment 1. In FIG. 5, signals Φ 41 , Φ 58  and Φ 61  have suffixes  44 ,  58 ,  61  respectively coinciding with the number of the pulse input terminals shown in FIG.  4  and respectively indicate potentials (or pulses) at such input terminals. The MOS transistors  42 ,  59  are assumed to be turned on or off respectively when the gate potential is high or low. When the pixel reset output starts to emerge from the amplifier  54 , an electrode of the clamp capacitance  41  is fixed to the potential of the terminal  43  through the MOS transistor  42 . The gate potential of the gate electrode input terminal  58  is selected somewhat lower than a potential, defined by subtracting the threshold voltage of the PMOS transistor  57  from the potential of the terminal  43 . The potential of the capacitance  45  is fixed at the potential of the terminal  60 , but is selected at such a sufficiently low level that the transfer MOS transistor  57  executes a saturation operation or a sub-threshold value operation. When the MOS transistor  42  is turned off, a saturation current or a sub-threshold current flows in the transfer MOS transistor  57 , whereby the potential of an electrode of the clamp capacitance  41 , namely the source potential of the PMOS transistor  57  approaches a potential V CL  determined by the potential of the terminal  58  and the threshold voltage of the PMOS transistor  57 . When the pixel reset output is terminated, the MOS transistor  59  is turned off whereby the accumulating capacitance  45  is maintained in a floating state, and, when the pixel signal output is started, the potential of the clamping portion (namely potential of an electrode of the clamp capacitance  41  at the side of the transfer MOS transistor) tends to be pushed up corresponding to the signal voltage through the clamp capacitance. However, as shown in the potential chart in FIG. 6, the potential of the clamping portion returns to V CL  within the (reset+signal) output period, by the current flowing through the MOS transistor  57 . Consequently the signal charge corresponding to the product C O ·V S  of the clamp capacitance C O  and the signal voltage V S  alone, not including the reset level, is accumulated in the capacitance  45  in the floating state. By shifting the terminal  58  to the high level state to completely turn off the transfer MOS transistor  57  at a certain time during the (reset+signal) output state, the signal charge C O ·V S  is retained in the accumulated state in the capacitance  45 . 
     In the present embodiment, the signal voltage read out to the output line  49  is |C T /(C H +C T )|·(C O ·V S /C T ) or |C O /(C H +C T )|·V S , and the loss of the signal voltage resulting from capacitative division can be maintained smaller than in the conventional examples by selecting C O  to be larger and C T  to be smaller. Also there cannot be generated the fixed pattern noise resulting from the unbalance of the two output paths C HN , C HS  as in the first conventional example shown in FIGS. 1 and 2. Although the fixed pattern noise is surely generated by the fluctuation of C T , its contribution is much smaller than in the conventional examples, so that the fixed pattern noise can be made smaller than in the conventional examples if the fluctuation in C O  can be suppressed. Also by setting the reset potential for the capacitance  45  supplied from the terminal  60  sufficiently low, the on-resistance of the MOS transistor  48  can be made smaller so that the horizontal signal transfer can be achieved at a high speed. 
     The circuit configuration in a second embodiment of the present invention is the same as the first embodiment shown in FIG.  4 . The timing of operation, shown in FIG. 7, is different the first embodiment in that the gate potential of the transfer MOS transistor  57  is made different between the pixel reset output and the (reset+signal) output and is made lower in the latter. 
     In the first embodiment, the potential of the clamping portion in the (reset+signal) output becomes V CL  if the signal voltage is sufficiently large but becomes lower than V CL  if the signal voltage is 0 or very small. This is because even if the signal voltage is 0, the subthreshold current flowing in the MOS transistor  57  at the pixel reset output period, of which magnitude decreases, continues to flow in the (reset+signal) output period. Consequently, in the (reset+signal) output, the final potential of the clamping portion depends on the magnitude of the signal voltage, so that the linearity of the signal in the capacitance  45  is not retained. On the other hand, if the charge flowing in the transfer MOS transistor  57  is more than a certain amount, the potential of the clamping portion eventually reaches a certain value regardless of the initial value thereof. In order to secure a certain amount of such flowing charge, the potential of the terminal  58  is made lower in the (reset+signal) output period than in the reset output period as shown in FIG. 7, whereby the potential of the clamping portion at the end of the (reset+signal) output period assumes a constant value not depending on the magnitude of the signal voltage. In this manner there can be secured the linearity of the transferred signal in the capacitance  45 . 
     FIG. 8 shows a third embodiment of the solid-state image pickup element of the present invention, wherein shown are an operational amplifier  62 , a signal charge integrating capacitance  63 , an amplifier resetting MOS transistor  64 , a gate input terminal  65  of the MOS transistor  64 , and a supply terminal  66  of a reference potential entered into a non-inverted (+) input port of the operational amplifier. An electrode of the signal integrating capacitance  63  is connected to an output line  49  and an inverted (−) input terminal of the operational amplifier  62 , and the other electrode is connected to the output of the operational amplifier  62 . 
     In FIG. 8, components same as those in FIG. 4 are represented by same numbers and will not be explained further. In the present third embodiment, the operation of the readout circuit  53  is same as that in the first or second embodiment. In FIG. 8, the operational amplifier  62 , the signal charge integrating capacitance  63 , the amplifier resetting MOS transistor  64 , the gate input terminal  65  of the MOS transistor  64  and the reference potential supply terminal  66  constitute a charge integrating amplifier, and the signal charge C O ·V S  accumulated in the capacitance  45  is integrated by the signal charge integrating capacitance  63  of a magnitude C S  so that the terminal  52  provides a signal output voltage (C O ·V S )/C S  which is independent from C T . Consequently the fixed pattern noise is caused only by the fluctuation in C O , and can be reduced by suppressing the fluctuation in C O . 
     In the foregoing embodiments, the transfer transistor is composed of a MOS transistor, but it may also be composed of a JFET (junction field effect transistor) or a bipolar transistor as long as a saturation area function is possible. 
     As explained in the foregoing, the first to third embodiments of the present invention firstly allow to output a high signal voltage and secondly allow to increase the readout speed. In addition it is possible to reduce the fixed pattern noise in the solid state image pickup element. 
     In the foregoing first to third embodiments, the control of pulse application to the MOS transistor is executed by a timing generation unit  108  (FIG. 9) to be explained later. 
     In the following there will be explained, with reference to FIG. 9, a fourth embodiment in which the solid-state image pickup element of the first to third embodiments is applied to a signal processing apparatus such as a still camera. 
     In FIG. 9 there are shown a barrier  101  serving as a lens protector and a main switch, a lens  102  for focusing the optical image of an object on a solid-state image pickup element  104 , an iris  103  for varying the amount of light transmitted by the lens  102 , a solid-state image pickup element  104  for fetching the object image, focused by the lens  102 , as an image signal, an A/D converter  106  for executing analog-digital conversion on the image signal outputted from the solid-state image pickup element  104 , a signal processing unit  107  for executing various corrections and data compression on the image data outputted from the A/D converter  106 , a timing generating unit  108  for outputting various timing signals to the solid-state image pickup element  104 , an image signal processing circuit  105 , the A/D converter  106  and the signal processing unit  107 , a system control and operation unit  109  for executing various calculations and controlling the entire still video camera, a memory unit  110 , an interface unit  111  for executing recording on and readout from a recording medium, a detachable recording medium  112  for executing recording or readout of the image data, such as a semiconductor memory, and an interface unit  113  for communication with an external computer or the like. 
     In the following there will be explained the operation of the signal processing apparatus of the above-described configuration in the image taking operation. 
     When the barrier  101  is opened, the main power supply is turned on. Then the power supply for control system is turned on, and the power supply for the image pickup circuits such as the A/D converter  106  etc. is also turned on. 
     Then, in order to control the exposure amount, the system control and operation unit  109  fully opens the iris  103 , and the signal outputted from the solid-state image pickup element  104  is converted by the A/D converter  106  and is input into the signal processing unit  107 . Based on such data, the system control and operation unit  109  calculates the exposure. 
     The luminance is judged from the result of the above-described photometry, and the system control and operation unit  109  controls the iris  103  based on such result. 
     Then a high frequency component is extracted from the signal outputted from the solid-state image pickup element  104 , and the system control and operation unit  109  calculates the distance to the object. Thereafter the lens is driven and there is judged whether the lens is in-focus position, and, if not, the lens is driven again and the distance measurement is repeated. 
     The main exposure is started after the in-focus state is confirmed. 
     When the exposure is terminated, the image signal outputted from the solid-state image pickup element  104  is subjected to A/D conversion by the A/D converter  107 , then passed by the signal processing unit  107  and is written into the memory unit by the system control and operation unit  109 . 
     The data accumulated in the memory unit  110  is thereafter passed by the recording medium control I/F unit and recorded in the detachable recording medium  112  such as a semiconductor memory, under the control of the system control and operation unit  109 . 
     Otherwise the data may be introduced, through the external I/F unit  113 , directly into a computer or the like for image processing. 
     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.

Technology Category: h