Abstract:
Disclosed is a horizontal transfer line in which first horizontal transfer pulses are applied to transfer electrodes via a commonly connected electrode line, second horizontal transfer pulses are applied to transfer electrodes via a commonly connected electrode line, and final horizontal transfer pulses are applied to a final transfer electrode via an electrode line independent of the above-mentioned electrode lines. Since the independent electrode line is independent of the other electrode lines, amount of capacitance produced in equivalent terms is reduced. The final transfer pulses exhibit a steep characteristic, thereby improving transfer efficiency. The input/output characteristic of the horizontal transfer line is improved as a result.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a signal charge transfer line, which has been formed on a substrate and is formed to have a number of transfer electrodes, for transferring signal charge by application of transfer pulses to the transfer electrodes via electrode lines.  
           [0003]    2. Description of the Related Art  
           [0004]    An output gate is formed at the output end of a transfer line that transfers signal charge in response to application of transfer pulses. Output of the signal charge that has been transferred is controlled by application of an output gate pulse to the output gate.  
           [0005]    If the amount of signal charge transferred is small, as is the case where signal charge obtained when the image of a low-luminance subject is sensed, the next signal charge may be transferred before sweep-out of the previous signal charge from the output gate is completed. If the amount of signal charge transferred is small, the input/output characteristic may not exhibit linearity.  
           [0006]    There is prior art (e.g., see the specification of Japanese Patent Application Laid-Open No. 8-172179) in which the width of the output gate is reduced in order that the linearity of the input/output characteristic can be maintained even if the amount of signal charge transferred is small.  
           [0007]    If the width of the output gate is reduced, however, there are instances where signal-charge transfer capacity declines.  
         SUMMARY OF THE INVENTION  
         [0008]    Accordingly, an object of the present invention is to improve the input/output characteristic without diminishing transfer capacity.  
           [0009]    According to the present invention, the foregoing object is attained by providing a signal charge transfer line, which has been formed on a substrate and is formed to have a number of transfer electrodes, for transferring signal charge by application of transfer pulses to the transfer electrodes via electrode lines, wherein the number of transfer electrodes are divided into transfer electrodes of a plurality of sets to each of which a common transfer pulse is applied; electrode lines for applying the common transfer pulses to the transfer electrodes of the plurality of sets are formed on the substrate in common for each of the transfer electrodes of the plurality of sets; and a final electrode line for applying a transfer pulse to a final transfer electrode at an output end is formed on the substrate independently of the electrode lines of respective ones of the plurality of sets.  
           [0010]    In accordance with the present invention, a number of transfer electrodes are divided into transfer electrodes of a plurality of sets to each of which a common transfer pulse is applied. Electrode lines that apply the common transfer pulses to the plurality of transfer-electrode sets are formed on the substrate in common for each of the transfer electrodes of the plurality of sets. A final electrode line that applies a transfer pulse to a final transfer electrode is formed on the substrate independently of the other transfer electrodes.  
           [0011]    If an electrode line is connected in common with a plurality of transfer electrodes, the amount of capacitance produced between the transfer electrodes and ground increases. Since the final electrode line is formed on the substrate independently of the other electrode lines (i.e., not in common with the other transfer electrodes), the amount of capacitance declines. When the amount of capacitance declines, the transfer pulse applied to the final transfer electrode becomes steeper. Even if the amount of transferred signal charge is small, the signal charge that has been transferred under the final transfer electrode can be swept out efficiently. The input/output characteristic is improved despite the small amount of signal charge.  
           [0012]    It is possible to adopt an arrangement in which the transfer line further includes a plurality of drivers provided in correspondence with respective ones of the plurality of sets of the transfer electrodes for applying transfer pulses to the corresponding sets of transfer electrodes. In this case a transfer pulse applied by at least one driver among the plurality of drivers would be applied to the final transfer electrode via the final electrode line.  
           [0013]    Further, it is possible to adopt an arrangement in which the transfer line further includes a synchronizing circuit for synchronizing transfer pulses applied to the transfer electrodes of the plurality of sets and the transfer pulse applied to the final transfer electrode.  
           [0014]    Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a block diagram illustrating the electrical structure of part of a CCD and a horizontal driver according to an embodiment of the present invention;  
         [0016]    [0016]FIGS. 2A to  2 C illustrate horizontal transfer pulses applied to the CCD of FIG. 1 according to this embodiment;  
         [0017]    [0017]FIG. 3 is a circuit diagram illustrating an equivalent circuit of a horizontal transfer line according to this embodiment;  
         [0018]    [0018]FIGS. 4A and 4B illustrate first horizontal transfer pulses and final horizontal transfer pulses, respectively;  
         [0019]    [0019]FIG. 5 illustrates a CCD output signal;  
         [0020]    [0020]FIG. 6 is a graph illustrating an input/output characteristic of a horizontal transfer line;  
         [0021]    [0021]FIG. 7 illustrates the electrical structure of a synchronizing circuit according to this embodiment; and  
         [0022]    [0022]FIGS. 8A and 8B illustrate clock pulses input to the synchronizing circuit of FIG. 7 according to this embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    A preferred embodiment of the present invention will now be described in detail with reference to the drawings.  
         [0024]    [0024]FIG. 1 illustrates a horizontal driver  20  and part of a CCD  1  according to an embodiment of the present invention, and FIGS. 2A, 2B and  2 C illustrates first horizontal transfer pulses, second horizontal transfer pulses and final horizontal transfer pulses applied to the CCD  1  of FIG. 1.  
         [0025]    The CCD  1  has a horizontal transfer line  2  formed on a substrate. The horizontal transfer line  2  transfers, in the horizontal direction, signal charge that has been accumulated in photodiodes (not shown) of the CCD  1  and applied via a vertical transfer line (not shown).  
         [0026]    The horizontal transfer line  2  is formed to have horizontal transfer electrodes  3 ,  4  and  5 . Neighboring ones of the horizontal transfer electrodes  3 ,  4  and 5 partially overlap each other. A number of the horizontal transfer electrodes  3  and a number of the horizontal transfer electrodes  4  are formed and alternate along the transfer line. The horizontal transfer electrode  5  is a transfer electrode (final horizontal transfer electrode) formed last on the output side. First horizontal transfer pulses ΦH 1  are applied to all of the horizontal transfer electrodes  3  via a first electrode line  11  (see FIG. 2A), and second horizontal transfer pulses ΦH 2 , which are obtained by inverting the first horizontal transfer pulses ΦH 1 , are applied to all of the horizontal transfer electrodes  4  via a second electrode line  12  (see FIG. 2B). Final horizontal transfer pulses ΦH 1 L are applied to the final horizontal transfer electrode  5  via a third electrode line  13  (see FIG. 2C). The final transfer pulses ΦH 1 L correspond to the first horizontal transfer pulses ΦH 1 .  
         [0027]    A first input terminal  14 , second input terminal  15  and third input terminal  16  are formed on the CCD  1 . The first electrode line  11  is connected to the first input terminal  14 . By applying the first horizontal transfer pulses ΦH 1  to the first input terminal  14 , the first horizontal transfer pulses ΦH 1  are applied commonly to all of the horizontal transfer electrodes  3  via the first electrode line  11 . Further, the second electrode line  12  is connected to the second input terminal  15 . By applying the second horizontal transfer pulses ΦH 2  to the second input terminal  15 , the second horizontal transfer pulses ΦH 2  are applied commonly to all of the horizontal transfer electrodes  4  via the second electrode line  12 . Furthermore, the third electrode line  13  is connected to the third input terminal  16 . By applying the final horizontal transfer pulses ΦH 1 L to the third input terminal  16 , the final horizontal transfer pulses ΦH 1 L are applied to the final horizontal transfer electrode  5 . The third electrode line  13  applies transfer pulses only to the one transfer electrode  5  and does not apply the transfer pulses to a plurality of transfer electrodes. The third electrode line  13  is independent of the first electrode line  11  and second electrode line  12 .  
         [0028]    An output gate  6  is formed on the output side (the right side) of the final horizontal transfer electrode  5 . By applying an output gate pulse to the output gate  6 , signal charge that has been transferred through the interior of the horizontal transfer line  2  is output from the horizontal transfer line  2 .  
         [0029]    A floating diffusion amplifier  7  is formed on the output side of the horizontal transfer line  2 . Signal charge that has been output from the horizontal transfer line  2  is output from the CCD  1  as a video signal via the floating diffusion amplifier  7 .  
         [0030]    The horizontal driver  20  includes a non-inverting buffer circuit  21 , a first inverting buffer circuit  22  and a second inverting buffer circuit  23 .  
         [0031]    Clock pulses H are applied to the horizontal driver  20  from a timing generator (not shown) and are applied to the non-inverting buffer circuit  21  and first inverting buffer circuit  22 . The output of the non-inverting buffer circuit  21  is the signal of the first horizontal transfer pulses ΦH 1  and is applied to the first input terminal  14  of the CCD  1 . Pulses obtained by inverting the entered clock pulses H are output from the first inverting buffer circuit  22 . The pulses output from the first inverting buffer circuit  22  are the second horizontal transfer pulses ΦH 2  and are applied to the second input terminal  15  of the CCD  1 . The pulses output from the first inverting buffer circuit  22  are also applied to the second inverting buffer circuit  23 . The pulses output from the second inverting buffer circuit  23  become pulses corresponding to the pulses that are output from the non-inverting buffer circuit  21 . The pulse outputs from the second inverting buffer circuit  23  are applied to the third input terminal  16  of the CCD  1 .  
         [0032]    [0032]FIG. 3 is a circuit diagram illustrating an equivalent circuit of the horizontal transfer circuit  2 .  
         [0033]    It may be considered that a capacitance C 1  is produced between the horizontal transfer electrodes  3  and ground. Further, since the horizontal transfer electrodes  3  and horizontal transfer electrodes  4  partially overlap, it may be considered that a capacitance C 12  is produced between the horizontal transfer electrodes  3  and the horizontal transfer electrodes  4 . Similarly, it may be considered that a capacitance C 2  is produced between the horizontal transfer electrodes  4  and ground. Further, since the horizontal transfer electrode  4  and the final horizontal transfer electrode  5  partially overlap, it may be considered that a capacitance C 1 L 2  is produced between horizontal transfer electrode  4  and the final horizontal transfer electrode  5 . Furthermore, it may be considered that a capacitance C 1 L is produced between the final horizontal transfer electrode  5  and ground.  
         [0034]    The final horizontal transfer electrode  5  is provided with a horizontal transfer pulse independently, unlike the case with the horizontal transfer electrodes  3  or horizontal transfer electrodes  4 , as mentioned above. The amount of capacitance C 1 L produced between the final horizontal transfer electrode  5  and ground is small in comparison with the amounts of the other capacitances.  
         [0035]    [0035]FIGS. 4A and 4B illustrate the first horizontal transfer pulses ΦH 1  and the final horizontal transfer pulses ΦH 1 L, respectively.  
         [0036]    As mentioned above, the amount of capacitance C 1 L produced between the final horizontal transfer electrode  5  and ground is small in comparison with the amount of capacitance produced between the first horizontal transfer electrodes  3  and ground. This means that the final transfer pulses ΦH 1 L have steeper rising and falling edges than those of the first horizontal transfer pulses ΦH 1 .  
         [0037]    [0037]FIG. 5 illustrates the output signal of the CCD  1 .  
         [0038]    As shown in FIG. 5, the level of the signal from time t2 to time t3 (which shall be referred to as the “video signal interval”) during which the first horizontal transfer pulse ΦH 1  and final horizontal transfer pulse ΦH 1 L are at the L level corresponds to the level of the signal charge that has accumulated in the CCD  1 . The signal in the video signal interval is sampled by a correlated double sampling circuit connected downstage of the CCD  1 .  
         [0039]    When the final horizontal transfer pulse ΦH 1 L does not exhibit a characteristic having a steep rising or falling edge, signal charge that has accumulated under the final horizontal transfer electrode  5  cannot be swept out to the output gate  6  rapidly. As a consequence, the signal in the video signal interval also falls gradually, as indicated at reference numeral  31  in FIG. 5, and a signal having a level higher than the rightful signal level happens to be sampled.  
         [0040]    By contrast, when the final horizontal transfer pulse ΦH 1 L does exhibit a characteristic having steep rising and falling edges, signal charge that has accumulated under the final horizontal transfer electrode  5  can be swept out to the output gate  6  rapidly. As a result, the signal in the video signal interval also falls comparatively quickly, as indicated at reference numeral  32  in FIG. 5, and a signal having the rightful signal level can be sampled.  
         [0041]    [0041]FIG. 6 is a graph illustrating the CCD input/output characteristic.  
         [0042]    The amount of signal charge that accumulates in the CCD photodiodes is small in certain cases, e.g., when an image of a subject has a low luminance. The amount of signal charge transferred through the horizontal transfer line also is small. When the amount of signal charge is small, in general the next signal charge will enter the output gate before all of the preceding signal charge is output from the output gate. A signal having a level higher than the rightful signal level is sampled, as pointed out above, and the input/output characteristic in the low-luminance portion often will not have a linear characteristic. Since the final horizontal transfer pulses ΦH 1 L have steep rising and falling edges in this embodiment, as mentioned above, signal charge can be swept out from the output gate quickly even if the amount of the signal charge is small. Accordingly, the input/output characteristic has linearity, even in the low-luminance portion, as indicated at reference character a in FIG. 6.  
         [0043]    [0043]FIG. 7 illustrates the electrical structure of a circuit for synchronizing the first horizontal transfer pulses ΦH 1 , second horizontal transfer pulses ΦH 2  and final horizontal transfer pulses ΦH 1 L. FIGS. 8A and 8B are time charts of first clock pulses ΦH 11  corresponding to the first horizontal transfer pulses ΦH 1  and second clock pulses ΦH 12  corresponding to the second horizontal transfer pulses ΦH 2 , respectively.  
         [0044]    As shown in FIGS. 8A and 8B, the first clock pulses ΦH 11  and the second clock pulses ΦH 12  are clock pulses that are opposite in phase to each other and the periods of the pulses ΦH 11  and ΦH 12  are the same as those of the first and second horizontal transfer pulses ΦH 1 , ΦH 2 .  
         [0045]    The first clock pulses ΦH 11  are inverted in a first inverting buffer circuit  41  and then input to a second buffer circuit  42 . The pulses are inverted again in the second buffer circuit  42 , whereby the first horizontal transfer pulses ΦH 1  are generated, as indicated in FIG. 2A.  
         [0046]    The second clock pulses ΦH 12  are inverted in a third inverting buffer circuit  43 . The second clock pulses ΦH 12  that have been inverted in the third inverting buffer circuit  43  are input to a fourth inverting buffer circuit  44  and to a non-inverting buffer circuit  45 . The pulses are inverted again in the fourth inverting buffer circuit  44 , whereby the second horizontal transfer pulses ΦH 2  are generated, as illustrated in FIG. 2B. Further, the pulses that have been input to the non-inverting buffer circuit  45  are output via the non-inverting buffer circuit  45 , whereby the final horizontal transfer pulses ΦH 1 L are generated, as illustrated in FIG. 2C.  
         [0047]    By utilizing the synchronizing circuit of FIG. 7, the first horizontal transfer pulses ΦH 1 , second horizontal transfer pulses ΦH 2  and final horizontal transfer pulses ΦH 1 L pass through two buffer circuits and therefore all of the transfer pulses ΦH 1 , ΦH 2 , ΦH 1 L are synchronized.  
         [0048]    Though a horizontal transfer line has been described in the above embodiment, it goes without saying that the present invention is similarly applicable also to a vertical transfer line.  
         [0049]    As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.