Abstract:
A voltage at an output terminal of a photodiode is reset to a fixed voltage and an output signal of the photodiode and a dummy signal matching a dark output signal is output by a dummy photodiode comprising an identical component as that of the photodiode. The voltage difference between an input side and an output side of an amplifier is made to match the difference between a reset voltage of the photodiode and a reset voltage of a common signal line and a reset voltage of an output terminal to optimize the size of a MOS transistor forming the amplifier. The offset voltage is set to a constant which does not depend on the size of the MOS transistor. The amplifier is formed with CMOS devices and is selectively operated only during an output operation to suppress the current consumption.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an one-dimensional image sensor for converting the reflected light from a manuscript which is irradiated with light into an electrical signal and can be applied to an image reading apparatus such as a FAX. 
     FIG. 10 is a circuit diagram showing a configuration of a contact type one-dimensional image sensor IC which is used in a conventional image reading apparatus for use in a FAX. As shown in FIG. 10, the conventional image reading apparatus is such that the output signals outputted from phototransistors P 1  to Pn are read out to a common signal line  1  by turning on successively switches S 1  to Sn. In this connection, reference symbol AMP designates an amplifier for amplifying the output signals which are outputted from the phototransistors P 1  to Pn. 
     The contact type phototransistor one-dimensional image sensor as described above is disclosed in Japanese unexamined patent application publication S61-124171(1986). 
     SUMMARY OF THE INVENTION 
     However, in such a one-dimensional image sensor, there arises a problem in that since the carriers remaining in the base of the phototransistor are removed through the emitter of the phototransistor, the base electric potential can not be sufficiently reset to the initial state and hence the residual image is remarkable. 
     In the light of the foregoing, the present invention was made in order to solve the above-mentioned problem associated with the prior art, and it is therefore an object of the present invention to provide an image sensor which is inexpensive, has smaller residual image and is capable of obtaining the effective bright output with accuracy. 
     In order to solve the above-mentioned problem, the image sensor according to the present invention is designed as follows. 
     According to the present invention, there is provided an image sensor which is constructed by arranging and mounting linearly a plurality of linear image sensor ICs, wherein a unit light receiving circuit includes: a photodiode acting as a light receiving element; an amplifier for amplifying a signal outputted from the photodiode; and a reading switch for reading out an output signal of the amplifier, and the image sensor IC is constructed in such a way that output signals, each varying depending on the amount of received light, from the plurality of unit light receiving circuits are read out successively in the order of the arrangement of the unit light receiving circuits to a signal output terminal therethrough. 
     In addition, in the above-mentioned image sensor IC, a first electrode of a reset switch is connected to a common signal line, an electric potential at a second electrode of the reset switch is fixed to an electric potential giving an initial electric potential by which the state of the photodiode is reset to the initial state, and the reset switch is temporarily closed when an image signal is outputted therethrough and is opened or closed in such a way that the electric potential of the common signal line is rapidly brought close to the initial electric potential. 
     In addition, preferably, a reference voltage circuit may be configured in terms of a configuration of the amplifier circuit in such a way that the difference between the electric potential inputted to the amplifier and the electric potential outputted from the amplifier is equal to the difference between the electric potential by which the state of the photodiode is made the initial state and the initial electric potential by which the common signal line is reset due to the back gate effect of a MOS transistor. 
     In addition, a first electrode of another reset switch is connected to an output terminal, an electric potential at a second electrode of the another reset switch is fixed to the electric potential giving an initial electric potential by which the state of another amplifier is reset to the initial state, and the another reset switch is temporarily closed when outputting therethrough an image signal and is opened or closed in such a way that the electric potential at the signal output terminal is rapidly brought close to the initial electric potential. As a result, against even the increase of the external load capacity which occurs when connecting a plurality of linear image sensor ICs linearly, the electric potential at the signal output terminal can be rapidly brought close to the initial electric potential. 
     In addition, if the amplifiers start, after having been sufficiently stabilized, their amplifying operations a fixed time period before an image signal of the light receiving element which is arranged in the first stage of the image sensor starts to be outputted, and complete their amplifying operations after the operation of outputting an image signal of the light receiving element which is arranged in the final stage thereof has been completed and the state of that light receiving element is returned back to the initial state, since those amplifiers are operated stably and selectively, those amplifiers serve so as to provide the less current consumption. 
     An output signal is read out to a dummy signal output terminal through a dummy photodiode, an amplifier for amplifying a signal outputted from the dummy photodiode and a reading switch for reading out therethrough the output signal. An electric potential at an electrode of a reset switch is fixed to the electric potential giving the initial electric potential by which the state of the dummy photodiode is reset to the initial state at all times. The reset switch is temporarily closed when an image signal is outputted and is opened or closed in such a way that the electric potential of the common signal line is rapidly brought close to the initial electric potential. In addition, a first electrode of another reset switch is connected to a dummy signal output terminal, and an electric potential at a second electrode of the reset switch is fixed to the electric potential giving the initial electric potential by which the reset is always made to the initial state. These circuit configurations and the circuit constants are respectively made identical to those of the circuit for outputting an image signal from the photodiode acting as the light receiving element, and the same signal as the output signal in the dark state is outputted. 
     An image sensor according to the present invention is designed in such a way that a photodiode is used as a light receiving element of a unit block and an amplifier is connected to the photodiode within the block so that the terminal voltage of the photodiode varying depending on the amount of received light is outputted successively in the form of an image signal through the amplifier, and also a dummy photodiode is provided which has the same configuration as that of the unit block of the light receiving element and the state of which is reset to the initial state at all times and which outputs a dummy signal equal to an output signal in the dark state of the photodiode as the light receiving element, thereby enabling the difference between the image signal output and the dummy output signal to be obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing a configuration of an image sensor according to the present invention. 
     FIG. 2 is a timing chart useful in explaining the operation of the image sensor according to the present invention. 
     FIG. 3 is a circuit diagram showing a configuration of each of a first amplifier  3  and an amplifier  11  of the image sensor according to the present invention. 
     FIG. 4 is a circuit diagram showing a configuration of a first reference voltage generating circuit  1  of the image sensor according to the present invention. 
     FIG. 5 is a circuit diagram showing a configuration of a second reference voltage generating circuit  2  of the image sensor according to the present invention. 
     FIG. 6 is a circuit diagram showing a configuration of each of a second amplifier  5  and an amplifier  12  of the image sensor according to the present invention. 
     FIG.  7 A and FIG. 7B is a graphical representation useful in explaining the input/output characteristics of each of the amplifiers  3  and  11  and each of the amplifiers  5  and  12  of the image sensor according to the present invention. 
     FIG. 8 is a circuit diagram showing a configuration of an example of a circuit using a dummy output signal of the image sensor according to the present invention. 
     FIG. 9 is a timing chart useful in explaining the operation of the circuit using the dummy output signal of the image sensor according to the present invention. 
     FIG. 10 is a circuit diagram showing a configuration of a conventional image sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will hereinafter be described in detail with reference to the accompanying drawings. 
     FIG. 1 is a circuit diagram showing a configuration of an image sensor according to the present invention. 
     An N type region underlying a first electrode of a photodiode  1  acting as a light receiving element is connected to a power source having a power source voltage VDD, and a P type region of a second electrode of the photodiode  1  is connected to a drain of a first electrode of a MOS transistor  2  acting as a first reset switch by which the state of the photodiode  1  is returned back to the initial state and is connected to an input terminal of a first amplifier  3 . A first reference voltage VREF  1  is supplied to a source of a second electrode of the MOS transistor  2 . An output terminal of the amplifier  3  is connected to a common signal line  15  through a MOS transistor  4  acting as a reading switch. In this connection, a plurality of blocks each of which has perfectly the same configuration as that of the block of the above-mentioned unit light receiving circuit are provided by the number which is equal to at least the number of output bits. 
     The signal on the common signal line  15  is, in order to further amplify the signal which has been outputted to the common signal line  15 , inputted to a second amplifier  5 . The common signal line  15  is connected to a drain of a first electrode of a MOS transistor  7  acting as a second reset switch by which the state of the common signal line  15  is returned back to the initial state, and a second reference voltage VREF  2  is supplied to a source of a second electrode of the MOS transistor  7 . The output terminal of the amplifier  5  is connected to an output terminal  16  for outputting an image signal through a MOS transistor  6  acting as a chip selecting switch for selectively functioning the second amplifier. Against the increase of the external load capacity which occurs in the case where a plurality of image sensor ICs are connected, the output terminal  16  is connected to a drain of a first electrode of a MOS transistor  8  acting as a third reset switch by which the state of the output terminal  16  is returned back to the initial state. Also, the reference voltage VREF  2  is supplied to a source of a second electrode of the MOS transistor  8 . In this connection, in the present invention, since change in the electric potential of the photodiode depending on the amount of received light is outputted through the amplifier to the outside, the operating speed can also be remarkably increased due to the performance of the amplifier, and since the common signal line  15  is provided in the inside of the linear image sensor IC, the internal wiring capacity is increased to limit the reading speed as compared with the well known CCD image sensor. Then, in order to further increase the reading speed, there is provided the MOS transistor as the second reset switch by which the state of the common signal line  15  is temporarily brought close to the initial state. When outputting the initial electric potential during the operation of resetting each of the unit light receiving circuits through the common signal line  15 , the MOS transistor  7  as the second reset switch is temporarily caused to conduct to be operated in such a way that the electric potential of the common signal line  15  is rapidly brought close to the initial electric potential. In addition, against the increase as well of the external load capacity which occurs in the case where a plurality of linear image sensor ICs are connected, in order to obtain the same effect, the MOS transistor  8  as the third reset switch is temporarily caused to conduct to be operated in such a way that the electric potential at the output terminal  16  is rapidly brought close to the initial electric potential. By adopting such a circuit configuration, the initial electric potential can be stably outputted even in the high speed operation and also the degradation of the picture quality can be suppressed. 
     Next, the dummy output system will hereinbelow be described. An N type region of a first electrode of a dummy photodiode  9  is connected to the power source having the power source voltage VDD, and a P type region of a second electrode of the photodiode  9  is connected to a drain of a first electrode of a MOS transistor  10  acting as a first dummy reset switch by which the state of the photodiode  9  is returned back to the initial state, and is connected to an input terminal of a first dummy amplifier  11 . Also, the reference voltage VREF  1  is supplied to a source of a second electrode of the MOS transistor  10 . The output signal of the amplifier  11  is directly inputted to a second dummy amplifier  12  and the output terminal of the amplifier  12  is connected to an output terminal  17 , through which the dummy signal is outputted, through a MOS transistor  13  acting as a dummy chip select switch by which them amplifier is selectively functioned. Alike the output terminal  16 , against the increase of the external load capacity which occurs in the case where a plurality of image sensor ICs are connected, the output terminal  17  is connected to a drain of a first electrode of a MOS transistor  14  acting as a second dummy reset switch by which the state of the output terminal  17  is returned back to the initial state. Also, the reference voltage VREF  2  is supplied to a source of a second electrode of the MOS transistor  14 . In this connection, the photodiodes  1  and  9  are designed with perfectly the same size, the MOS transistors  2  and  10  are designed with perfectly the same size, the amplifiers  3  and  11  are also designed with perfectly the same size, and the amplifiers  5  and  12  are also designed with perfectly the same size. 
     FIG. 2 is a timing chart useful in explaining the operation of the image sensor according to the present invention. 
     At the time when a pulse signal φS 1  goes to HIGH synchronously with the clock signal, the pulse signals φSl to φSN are inputted successively to the gates of the MOS transistors  4  of the respective bits after a lapse of one clock, and then the output signals of the photodiode  1  in the respective bits are read out to the common signal line  15 . After the delay of a half clock with respect to pulse signal φS 1 , the gate of the MOS transistor  2  in the first bit is turned on on the basis of a pulse signal φR 1  and then the electric potential at the P type region of the photodiode  1  is reset to the electric potential of VREF  1 . Likewise, the P type regions of the photodiodes  1  on and after the second bit are also successively reset to the electric potential of VREF  1 . At the time when φRl goes to LOW, the MOS transistor  2  is turned off and the P type region of the photodiode  1  becomes the floating state. The electric potential at the P type region of the photodiode  1  right after the MOS transistor  2  has been turned off is slightly lower than that of the reference voltage VREF  1 . This is due to the switching noises which are generated when the gate of the MOS transistor  2  is turned off. But, since the difference in the electric potential therebetween is small, the output value which is obtained by reading out to the output terminal  16  the electric potential, when the P type region of the photodiode  1  is reset to the voltage of the reference voltage VREF  1 , through the amplifiers  3  and  5  can be regarded as the value which is substantially the same as the output signal in the dark state. At the time when light is made incident to the photodiode  1  after the gate of the MOS transistor  2  is turned off, a current is caused to flow from the P type region to the N type region of the photodiode  1  due to the generated carriers, and then the electric potential at the P type region of the photodiode  1  is increased gradually. Next time the MOS transistor  4  is turned on, the electric potential of the common signal line  15  nears gradually the output voltage of the amplifier  3  corresponding to the electric potential at the P type region of the photodiode  1  at this time. Next, at the time when φRE goes to HIGH, the MOS transistor  7  is turned on and then the electric potential of the common signal line  15  is reset to the reference voltage VREF  2 . 
     For a time period when φS 1  goes to HIGH and φCLK goes to LOW right after φS 1  goes to HIGH, and the output of the final bit is completed, and further for 1.5 clocks thereafter φACS is held LOW, the amplifiers  3  and  5  carry out their amplifying operations at all times. Since this leads to that in the case where a plurality of ICs are connected, the amplifiers are selectively operated, it is also effective to the reduction of the current consumption. Since a time period is required to stabilize the output signals of the amplifiers  3  and  5 , φACS goes to LOW one clock before the photodiode  1  starts to output the signal, and then goes to HIGH after the operation of resetting the final stage photodiode N has been completed in order to prevent the influence by the electric potential fluctuation when the amplifying operation has been completed. In addition, in the case where a lapse of one clock is insufficient for the time period required to stabilize the output signals of the amplifiers  3  and  5  in the high speed operation, that time period may also be set longer than one clock in correspondence with the operating frequency. 
     For a time period ranging from a time point a half clock before φS 1  goes to HIGH up to a time point when the output of the final bit is completed, φCS is held HIGH and the MOS transistor  6  is turned on, and also the output signal of the common signal line  15  is amplified by the amplifier  5  to be read out to the output terminal  16 . 
     In such a way, the photodiode is employed as the light receiving element and the electric potential at the second electrode of the photodiode which varies depending on the amount of received light is read out to the outside through the amplifiers, whereby the photodiode is hardly influenced by the switching noises due to the operation of the reading switch and the chip selecting switch and the noises due to the capacity across the lines. In addition, the photodiode is employed as the light receiving element, and the second electrode the electric potential at which varies depending on the amount of received light is given the initial electric potential through the reset switch, after having read out the signal therefrom, to return the state of the second electrode back to the initial state, whereby the degradation of the picture quality such as the residual image due to the residual electric charges which has been common among conventional image sensors having the phototransistor as the light receiving element is greatly improved, and hence the excellent picture quality is obtained. 
     The gate of the MOS transistor  10  is, on the basis of the pulse signal φRD, held in the on state at all times for a time period when CLK is HIGH, and also even if light is made incident to the photodiode  9 , the P type region of the photodiode  9  is reset to the voltage of the reference voltage VREF  1  to be in the initial state at all times. At the time when φRD goes to LOW, the MOS transistor  10  is turned off and then the P type region of the photodiode  9  becomes the floating state. The electric potential at the P type region of the photodiode  9  right after the MOS transistor  10  is turned off is slightly lower than that of the reference voltage VREF  1 . This results from the switching noises which are generated when the gate of the MOS transistor  10  is turned off. But, since the difference in the electric potential therebetween is small, the value of the output signal which is obtained by reading out the electric potential of the reference voltage VREF  1  to which the P type region of the photodiode  9  is reset to the output terminal  17  through the amplifiers  11  and  12  can be regarded as substantially the same as the voltage of the output signal in the dark state which is read out to the output terminal  16 . 
     The output signal of the photodiode  9  is inputted to the amplifier  12  through the amplifier  11  at all times, and the amplifiers  11  and  12  carry out, similarly to the amplifiers  3  and  5 , their amplifying operations continue at all times for a time period when φACS is held LOW. For a time period ranging from a time point a half clock before φS 1  goes to HIGH up to a time point when the operation of outputting the final bit has been completed, φCS is held HIGH and the MOS transistor  13  is turned on so that the output signal of the photodiode  9  is read out to the output terminal  17 . 
     FIG. 3 is a circuit diagram showing a configuration of each of the amplifiers  3  and  11  of the image sensor according to the present invention. A MOS transistor  301  is an enhancement type P-channel MOS transistor, and a source of a first electrode of the MOS transistor  301  and a substrate thereof are both connected to VDD, and also a gate of a second electrode thereof is connected to φACS. 
     A MOS transistor  302  is a depletion type N-channel MOS transistor and a source of a first electrode of the MOS transistor  302  is connected to a drain of a third electrode of the MOS transistor  301  and a substrate thereof is connected to GND. 
     A MOS transistor  303  is a depletion type N-channel MOS transistor and a drain of a first electrode of the MOS transistor  303  is connected to a drain of a second electrode of the MOS transistor  302  and a gate of a second electrode, a substrate and a source of a third electrode thereof are all connected to GND. 
     In this connection, a gate of a third electrode of the MOS transistor  302  forms an input terminal  304  of the amplifier  3 , and the drains of the MOS transistors  302  and  303  form an output terminal  305  of the amplifier  3 . 
     The setting is carried out in such a way that as shown in FIG. 2, for a time period when φACS is held LOW, the gate voltage of the MOS transistor  301  is GND and during the operation, the MOS transistor  301  is in the on state at all times, and the diffusion current is always predominant in the current which is caused to flow through the MOS transistors  302  and  303 . Since the MOS transistors  302  and  303  are operated in the saturation region, substantially the fixed current I 3  is caused to flow therethrough irrespective of the source to drain voltage thereof. 
     Assuming that the channel width of the MOS transistor is W, the channel length thereof is L, the mobility of carriers is μ and the electrostatic capacity per unit area of the gate oxide film is Cox, then the electric conductivity coefficient K of the MOS transistor is expressed as follows. 
     
       
           K=W/L·μ·Cox   (1) 
       
     
     Assuming that the input voltage to the amplifier is Vin, the output voltage therefrom is Vout, the electric conductivity coefficient of the MOS transistor  302  is K 2 , the threshold voltage thereof is Vtnd 2 , the electric conductivity coefficient of the MOS transistor  303  is K 1 , and the threshold voltage thereof is Vtnd, then the current I 3  which is caused to flow through the MOS transistors  302  and  303  is expressed as follows. 
     
       
           I   3 =½ ·K   2  ( V in− V out+ Vtnd   2 ) 2 =½ ·K   1 · Vtnd   2   (2) 
       
     
     Also, the output voltage Vout of the amplifier  3  is expressed as follows. 
     
       
           V out= V in+ Vtnd   2 −{square root over ( K +L  1 / K +L  2 )}· Vtnd   (3) 
       
     
     In addition, assuming that the threshold voltage which varies due to the back gate effect of the MOS transistor  302  is Δ Vtnd, then the following equation is established. 
     
       
           Vtnd   2 =( Vtnd−ΔVtnd )  (4) 
       
     
     Therefore, Expression can be transformed as follows. 
     
       
           V out= V in+ Vtnd−ΔVtnd−{square root over (K+L  1 / K +L  2 )}·   Vtnd   (5) 
       
     
     FIG.  4 . is a circuit diagram showing a configuration of a first reference voltage generating circuit of the image sensor according to the present invention. 
     A MOS transistor  401  is a depletion type N-channel MOS transistor and a drain of a first electrode of the MOS transistor  401  is connected to VDD. A MOS transistor  402  is an enhancement type N-channel MOS transistor, and a drain of a first electrode and a gate of a second electrode of the MOS transistor  402  are both connected to a substrate as well as a gate of a second electrode and a source of a third electrode of the MOS transistor  401 . Also, a source of a first electrode and a substrate of the MOS transistor  402  are both connected to GND. An output terminal  403  of the reference voltage generating circuit  1  is connected to the gate of the second electrode, the source of the third electrode and the substrate of the MOS transistor  401  as well as to the gate of the second electrode and the drain of the third electrode of the MOS transistor  402 . In this connection, when the MOS transistors  401  and  402  are operated in the saturation region, substantially the fixed current I 4  is caused to flow therethrough irrespective of the source to drain voltage thereof. 
     Assuming that the output voltage of the reference voltage generating circuit is VREF  1 , the electric conductivity coefficient of the MOS transistor  402  is K 4 , the threshold voltage thereof is Vtn, the electric conductivity coefficient of the MOS transistor  403  is K 3 , and the threshold voltage thereof is Vtnd, then the current I 4  which is caused to flow through the MOS transistors  402  and  403  is expressed as follows. 
     
       
           I   4   =K   3   Vtnd=K   4  ( V REF  1   −Vtn ) 2   (6) 
       
     
     Also, the first reference voltage VREF  1  of the reference voltage generating circuit  1  is expressed as follows. 
     
       
           V REF  1   =Vtn+K   3 / K   4   Vtnd   (7) 
       
     
     Now, since as shown in FIG. 2, the input voltage Vin of the amplifier  3  in the reset operation is reset to the electric potential of the reference voltage VREF  1  of the reference voltage generating circuit  1 , the output voltage Vout of the amplifier  3  which is in the initial state is expressed as follows. 
     
       
           V out= Vtn+{square root over (K+L  3 / K +L  4 )}·   Vtnd+Vtnd−ΔVtnd−{square root over (K+L  1 / K +L  2 )}·   Vtnd   (8) 
       
     
     Now, by setting the ratio to a constant in such a way that for the values of Ks of the MOS transistors, K 1 :K 2 =K 3 :K 4  is established, Expression 8 can be transformed as follows. 
     
       
           V out= Vtn+Vtnd−ΔVtnd   (9) 
       
     
     In such a way, the MOS transistors  302  and  303  of the amplifier  3  can be laid out within the common P type well by making each of the electric potentials at the substrates of the MOS transistors  302  and  303  the common electric potential, i.e., GND. As a result, the area of the block of the unit light receiving circuit can be decreased. In addition, since there is no dependency on the channel width and the channel length of the MOS transistor, the fluctuation can be reduced. 
     FIG. 5 is a circuit diagram showing a configuration of a second reference voltage generating circuit of the image sensor according to the present invention. 
     A MOS transistor  501  is a depletion type N-channel MOS transistor and a drain of a first electrode thereof is connected to VDD and a substrate thereof is connected to GND. A MOS transistor  502  is an enhancement type N-channel MOS transistor, and a drain of a first electrode and a gate of a second electrode are both connected to a gate of a second electrode and a source of a third electrode of the MOS transistor  501 . Also, a source of a third electrode and a substrate of the MOS transistor  502  are both connected to GND. An output terminal  503  of the reference voltage generating circuit is connected to a gate of a second electrode, a source of a third electrode and a substrate of the MOS transistor  501  as well as to the gate of the second electrode and the drain of the third electrode of the MOS transistor  502 . 
     Since the MOS transistors  501  and  502  are operated in the saturation region, substantially the fixed current I 4  is caused to flow therethrough irrespective of the source to drain voltage thereof. 
     Assuming that the output voltage of the reference voltage circuit is VREF  2 , the electric conductivity coefficient of the MOS transistor  502  is K 6 , the threshold voltage thereof is Vtn, the electric conductivity coefficient of the MOS transistor  503  is K 5 , and the threshold voltage thereof is Vtnd 5 , then the current I 5  which is caused to flow through the MOS transistors  502  and  503  is expressed as follows. 
     
       
           I   5 = K   5   Vtnd   5   2   =K   6  ( V REF  2 − Vtn ) 2   (10) 
       
     
     Also, the second reference voltage VREF  2  of the reference voltage generating circuit is expressed as follows. 
     
       
           V REF  2 = Vtn+{square root over (K+L  5 / K +L  6 )}   Vtnd   5   (11) 
       
     
     Now, assuming that the threshold voltage which varies due to the back gate effect of the MOS transistor  501  is Δ Vtnd, then the following equation is established. 
     
       
           Vtnd   5   =Vtnd−ΔVtnd   (12) 
       
     
     Therefore, Expression 11 can be transformed as follows. 
     
       
           V REF  2   =Vtn+{square root over (K+L  5 / K +L  6 )}  ( Vtnd−ΔVtnd )  (13) 
       
     
     In addition, by setting the values of Ks of the MOS transistors in such a way that the relation of K 5 =K 6  is established, the reference voltage VREF  2  of the reference voltage generating circuit as the electric potential with which the common signal line  15  is reset is expressed as follows. 
     
       
           V REF  2 = Vtn+Vtnd−ΔVtnd   (14) 
       
     
     As shown in FIGS. 1 and 3, for the output voltage of the photodiode the state of which has become the initial state with the reference voltage VREF  1 , the output voltage Vout of each of the amplifiers  3  and  11  of which output voltage varies due to the back gate effect of the MOS transistor  302  of the internal circuit of each of the amplifiers  3  and  11  can be set equal to the reference voltage VREF  2  of the reference voltage generating circuit as follows. 
     
       
           V out= V REF  2   (15) 
       
     
     FIG. 6 is a circuit diagram showing a configuration of each of the amplifiers  5  and  12  of the image sensor according to the present invention. A MOS transistor  601  is an enhancement type P-channel MOS transistor, and a source of a first electrode and a substrate of the MOS transistor  601  are both connected to VDD and a gate of a second electrode thereof is connected to φACS. 
     In addition, a MOS transistor  602  is a depletion type N-channel MOS transistor, and a source of a first electrode of the MOS transistor  602  is connected to a drain of a third electrode of the MOS transistor  601 . 
     Also, a MOS transistor  603  is a depletion type N-channel MOS transistor, and a drain of a first electrode of the MOS transistor  603  is connected to a drain of a second electrode and a substrate of the MOS transistor  602  and a gate of a second electrode, a substrate and a source of a third electrode thereof are all connected to GND. 
     A gate of a third electrode of the MOS transistor  602  forms an input terminal  604  of the amplifier  3 , and drains of the MOS transistors  602  and  603  form an output terminal  605  of the amplifier  3 . 
     The setting is carried out in such a way that as shown in FIG. 2, for a time period when φACS is held LOW, the gate voltage of the MOS transistor  601  is made GND and during the operation, the MOS transistor  601  is in the on state at all times, and the diffusion current is always predominant in the current which is caused to flow through the MOS transistors  602  and  603 . Since the MOS transistors  602  and  603  are operated in the saturation region, substantially the fixed current I 6  is caused to flow therethrough irrespective of the source to drain voltage thereof. 
     Assuming that the input voltage to each of the amplifiers  5  and  12  is Vin  2 , the output voltage therefrom is Vout 2 , and the electric conductivity coefficient of the MOS transistor  602  is K 8 , then the threshold voltage thereof is Vtnd due to absence of the back gate effect. Also, assuming that the electric conductivity coefficient of the MOS transistor  603  is K 7 , then the threshold voltage thereof is Vtnd. Therefore, the current I 6  which is caused to flow through the MOS transistors  602  and  603  is expressed as follows. 
     
       
           I   6 =½ ·K   8  ( V in 2 − V out 2 + Vtnd ) 2 =½ ·K   7 · Vtnd   2   (16) 
       
     
     Also, an output voltage Vout 2  of each of the amplifiers  5  and  11  is expressed as follows. 
     
       
           V out 2 = Vin   2 + Vtnd−{square root over (K+L  7 / K +L  8 )}·   Vtnd   (17) 
       
     
     In addition, by setting the values of Ks of the MOS transistors in such a way that the relation of K 7 =K 8  is established, the following equation is established. 
     
       
           V out 2 = V in 2   (18) 
       
     
     Therefore, the input voltage to each of the amplifiers  5  and  12  becomes equal to the output voltage therefrom. 
     In addition, each of the amplifiers  3  and  11  may have the same circuit configuration as that of each of the amplifiers  5  and  12 . In this case, the following relation is established. 
     
       
           V out= V in  (19) 
       
     
     Also, when the P type region of the photodiode  1  is reset to the electric potential of the first reference voltage VREF  1  as shown in FIG. 2, the following relation is established. 
     
       
           V out= V REF  1   (20) 
       
     
     Therefore, the second reference voltage VREF  1  is applied to the source of the second electrode of the MOS transistor  7  acting as the second reset switch by which the state of the common signal line  15  is returned back to the initial state. In addition, the first reference voltage VREF  1  may also be applied to the source of the second electrode of the MOS transistor  8  acting as the third reset switch of the output terminal. 
     FIG.  7 A and FIG. 7B is a graphical representation useful in explaining the input/output characteristics of each of the amplifiers  3  and  11  and each of the amplifiers  5  and  12  of the image sensor according to the present invention. 
     As shown in FIGS. 3 and 6, the depletion type N-channel MOS transistor is employed on the input side of the amplifier, and hence the linear input/output characteristics can not be obtained in the non-saturation operation region. For this reason, as shown in FIG. 1, the reference voltage in the initial state of the photodiode is set to VREF  1  and as a result the operation in the linear region becomes possible. In addition, since for the depletion type N-channel MOS transistor, the diffusion current is predominant in the saturation region of the MOS, the initial electric potential can be stably outputted against the fluctuation of the power supply voltage VDD, and also the degradation of the picture quality can be suppressed by making GND the reference voltage for the output signal. 
     FIG. 8 is a circuit diagram showing a configuration of an example of a circuit in which the dummy output of the image sensor according to the present invention is employed, and FIG. 9 is a timing chart useful in explaining the operation of the circuit shown in FIG.  8 . Also, in FIG. 8, the image sensor IC of the present invention is shown as the unit block. In this connection, the illustration of the input and output terminals including the power source other than the output terminal  16  and the output terminal  17  is omitted here for the sake of simplicity. In FIG. 8, a plurality of image sensor ICs each having the unit block is connected. The output terminals  17  through which the dummy signals are respectively outputted are connected to a common line  18  which is in turn connected to an inverted input terminal of an operational amplifier  20  through a first resistor  19 . In addition, a second resistor  21  is connected between the inverted input terminal of the operational amplifier  20  and an output terminal OUT 3 . Next, the output terminals  16  through which the image signals are respectively outputted are connected to a common line  22  which is in turn connected to a non-inverted input terminal of the operational amplifier  20  through a third resistor  23 . In addition, a fourth resistor  24  is connected between the inverted input terminal of the operational amplifier  20  and GND. In such away, the subtraction circuit of the operational amplifier is applied to the circuit of interest shown in FIG.  8 . Then, assuming that the resistance value of the resistor  19  is R 1 , the resistance value of the resistor  21  is R 2 , and the resistance value of the resistor  23  is R 3 , and the resistance value of the resistor  24  is R 4 , and also the relations of R 1 =R 3  and R 2 =R 4  are established, the following eqution is established. 
     
       
           V OUT 3 = R   3 / R   1 ·( VSIG−VDS )  (21) 
       
     
     Thus, by obtaining the difference output between the image signal output and the dummy signal output, the voltage in the initial state of the photodiode  1  shown in FIG. 1 can be set to substantially the electrical potential of GND. In addition, by removing the switching noises which the MOS transistor  2  generates, the fluctuation due to the process fluctuation in the threshold voltage Vtnd of the depletion type N-channel MOS transistors constituting the amplifier and the reference voltage generating circuit, and the fluctuation due to the process fluctuation in the threshold voltage Vtn of the enhancement type N-channel MOS transistor constituting the reference voltage generating circuit, it is possible to reduce the fluctuation of the dark output. In addition, if the gain of the operational amplifier is increased, then the sensitivity can be enhanced. As a result, the image sensor which is excellent in the S/N ratio can be obtained with the lower cost. 
     As set forth hereinabove, according to the present invention, it is possible to provide an image sensor which is inexpensive and has the small residual image and which is capable of suppressing the dark level deviation by outputting a dummy voltage as a voltage which is obtained when initializing a photodiode and which is substantially the same in level as a dark output voltage to obtain a difference between an image signal output and the dummy signal output to remove the fluctuation of threshold voltages of MOS transistors due to the process fluctuation. In addition, the less power consumption and the high speed operation can be made possible.