Abstract:
A bias sampling device includes a reference current generation unit suitable for generating a reference current; a plurality of multi-staged current mirror circuits suitable for receiving the reference current generated from the reference current generation unit and outputting a bias voltage; and a bias sampling unit suitable for performing sampling on a first bias voltage of a first current mirror circuit of the plurality of multi-staged current mirror circuits, wherein the first bias voltage of the first current mirror circuit, which is located prior to a final-staged current mirror circuit of the plurality of multi-staged current mirror circuits, is preset.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority of Korean Patent Application No. 10-2014-0054890, filed on May 8, 2014, which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    Various exemplary embodiments of the present invention relate to a bias sampling device and a complementary metal-oxide semiconductor (CMOS) image sensor including the same and, more particularly, to a bias sampling device and a CMOS image sensor including the same for sampling and providing a bias voltage. 
         [0004]    2. Description of the Related Art 
         [0005]      FIG. 1A  is a block diagram illustrating a conventional CMOS image sensor having a local bias sampling structure.  FIG. 1B  is a block diagram illustrating a conventional CMOS image sensor having a global bias sampling structure.  FIGS. 2A to 2C  are diagrams illustrating image deterioration caused by global bias sampling. 
         [0006]    Referring to  FIG. 1A , the conventional CMOS image sensor having the local bias sampling structure may include a row decoder and pixel driver  111 , a pixel array  112 , a bias voltage generation unit  113 , a pixel amplifying unit  114  and a read-out processing unit  115 . 
         [0007]    The row decoder and pixel driver  111  drives pixels, which are selected by a row decoder, included in the pixel array  112 . 
         [0008]    The pixel array  112  senses light using an optical element and generates a pixel signal corresponding to sensed light. The pixels, which are selected by a row decoder and driven by the pixel driver, of the pixel array output a pixel signal. The pixel signal is an electrical analog signal and includes a reset voltage and a signal voltage. 
         [0009]    The bias voltage generation unit  113  generates and provides a bias voltage to a load transistor  116  of the pixel amplifying unit  114  through transmission line VBIAS 1 . 
         [0010]    The pixel amplifying unit  114  amplifies the pixel signal of the pixel array  112  and transfers an amplified pixel signal to the read-out processing unit  115 . 
         [0011]    The read-out processing unit  115  reads-out the amplified pixel signal of the pixel amplifying unit  114  and outputs read-out data. 
         [0012]    Since the bias voltage applied to the load transistor  116  of the pixel amplifying unit  114  is directly provided from the bias voltage generation unit  113 , circuit noise generated from the bias voltage generation unit  113  or external noise may be applied to the load transistor  116 , and quality deterioration may occur. 
         [0013]    In order to prevent quality deterioration caused by circuit or external noise, a local bias sampling structure, using one sampling switch  117  and one sampling capacitor  118  included in the pixel amplifying unit  114  of each column, is widely used. 
         [0014]    However, as shown in  FIG. 1A , with the local bias sampling structure, since a plurality of sampling capacitors having a large capacity and thus occupying a large space are needed to prevent linearity deterioration, which is caused by the coupling of an output node of the pixel array  112 , the CMOS image sensor having the local bias sampling capacitor structure has an increased size. 
         [0015]    Referring to  FIG. 1B , the conventional CMOS image sensor having the global bias sampling structure may include a row decoder and pixel driver  121 , a pixel array  122 , a bias voltage generation unit  123 , a pixel amplifying unit  124 , a read-out processing unit  125 , a single sampling switch  127  and a single sampling capacitor  128 . The row decoder and pixel driver  121 , the pixel array  122 , the bias voltage generation unit  123 , and the read-out processing unit  125  shown in  FIG. 1B  are the same as the row decoder and pixel driver  111 , the pixel array  112 , the bias voltage generation unit  113 , and the read-out processing unit  115  described with reference to  FIG. 1A . To prevent quality deterioration caused by circuit noise or external noise, the global bias sampling structure using the single sampling switch  127  and the single sampling capacitor  128  at an entire column is also widely used. 
         [0016]    However, as shown in  FIG. 1B , in the global bias sampling structure, the CMOS image sensor may be implemented in a relatively small size. But, as shown in  FIGS. 2A and 2C , a pixel output corresponding to a region B is saturated and sharply changes with a large amplitude. This change causes a change of voltage level in a transmission line VBIAS 2  through a coupling capacitor to the transmission line VBIAS 2 . Herein, since the transmission line VBIAS 2  is sampled through a global bias sampling structure, a gate voltage VGS of a load transistor of a region A is accordingly changed and thus a data error occurs to decrease the pixel data value of the region A due to the change of the gate voltage VGS. Therefore, as shown in  FIG. 28 , an output image of the region A is darker than the corresponding real subject. 
       SUMMARY 
       [0017]    Various exemplary embodiments of the present invention are directed to a bias sampling device and a CMOS image sensor including the same for sampling a bias voltage at a bias voltage supply target circuit, e.g., a pixel amplifying unit, of a previous stage and providing a sampled bias voltage to a bias voltage supply target circuit of a post stage. 
         [0018]    Various exemplary embodiments of the present invention may prevent quality deterioration caused by circuit noise or external noise, which is respectively provided from a bias voltage generation unit or an external device, and may solve problems of the local bias sampling structure and the global bias sampling structure. That is, various exemplary embodiments the present invention may be implemented in a small area, and prevent the distortion of data at a pixel amplifying unit. 
         [0019]    An exemplary embodiment of the present invention may include: a reference current generation unit suitable for generating a reference current; a plurality of multi-staged current mirror circuits suitable for receiving the reference current generated from the reference current generation unit and outputting a bias voltage; and a bias sampling unit suitable for performing sampling on a first bias voltage of a first current mirror circuit of the plurality of multi-staged current mirror circuits, wherein the first bias voltage of the first current mirror circuit, which is located prior to a final-staged current mirror circuit of the plurality of multi-staged current mirror circuits, is preset. 
         [0020]    The final-staged current mirror circuit may be of a cascade type. 
         [0021]    Each of the first and final-staged current mirror circuits may comprise two or more transistors. The bias sampling device may further comprise a first sampling switch disposed between the transistors of the first current mirror circuit, and a second sampling switch disposed between the transistors of the final-staged current mirror circuit. 
         [0022]    Each of the first current mirror circuit and the final-staged current mirror circuit may be of a cascade type. 
         [0023]    The bias sampling unit may switch a voltage applied to a gate of the first sampling switch before a rising timing of the voltage applied to the gate of a reset transistor after a read-out of a pixel data is completed. 
         [0024]    In accordance with another exemplary embodiment of the present invention, a complementary metal-oxide semiconductor (CMOS) image sensor includes a row decoder and pixel driver suitable for selecting and driving a pixel of a pixel array; a pixel amplifying unit suitable for amplifying a pixel signal of the pixel array; a bias sampling device suitable for performing sampling on a first bias voltage of a first current mirror circuit of a plurality of multi-staged current mirror circuits, and applying the sampled first bias voltage to the pixel amplifying unit; and a read-out processing unit suitable for reading-out the pixel signal amplified by the pixel amplifying unit, wherein the first bias voltage of the first current mirror circuit, which is located prior to a final-staged current mirror circuit of the plurality of multi-staged current mirror circuits, is preset. 
         [0025]    The bias sampling device may include a reference current generation unit suitable for generating a reference current; the plurality of multi-staged current mirror circuits suitable for receiving the reference current generated from the reference current generation unit and outputting a bias voltage; and a bias sampling unit suitable for performing sampling on the first bias voltage of the first current mirror circuit of the plurality of multi-staged current mirror circuits. 
         [0026]    The final-staged current mirror circuit may be of a cascade type. 
         [0027]    Each of the first and final-staged current mirror circuits may comprise two or more transistors. The bias sampling device may further comprise a first sampling switch disposed between the transistors of the first current mirror circuit, and a second sampling switch disposed between the transistors of the final-staged current mirror circuit. 
         [0028]    Each of the first current mirror circuit and the final-staged current mirror circuit may be of a cascade type. 
         [0029]    The bias sampling unit may switch a voltage applied to a gate of the first sampling switch before a rising timing of the voltage applied to the gate of a reset transistor after a read-out of pixel data is completed. 
         [0030]    The CMOS image sensor may further include a capacitor suitable for preventing image deterioration caused by a coupling capacitor between a second bias voltage node and a pixel output line. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1A  is a block diagram illustrating a conventional CMOS image sensor having a local bias sampling structure. 
           [0032]      FIG. 1B  is a block diagram illustrating a conventional CMOS image sensor having a global bias sampling structure. 
           [0033]      FIGS. 2A to 2C  are diagrams illustrating image deterioration caused by a global bias sampling. 
           [0034]      FIGS. 3A and 3B  are block diagrams illustrating a bias voltage generation unit. 
           [0035]      FIG. 4  is a circuit diagram illustrating a bias sampling device and a CMOS image sensor including the same in accordance with an exemplary embodiment of the present invention. 
           [0036]      FIG. 5A  is a circuit diagram illustrating a general pixel unit. 
           [0037]      FIG. 58  is a timing diagram illustrating sampling of a bias voltage of a CMOS image sensor in accordance with an exemplary embodiment of the present invention. 
           [0038]      FIG. 6  is a block diagram illustrating a bias sampling device in accordance with another exemplary embodiment of the present invention. 
           [0039]      FIG. 7  is a block diagram illustrating a bias sampling device in accordance with still another exemplary embodiment of the present invention. 
           [0040]      FIG. 8  is a block diagram illustrating a bias sampling device in accordance with still another exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    Exemplary embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. Throughout the disclosure, reference numerals correspond directly to the like parts in the various figures and embodiments of the present invention. 
         [0042]    The drawings are not necessarily to scale and, in some instances, proportions may have been exaggerated in order to dearly illustrate features of the embodiments. In this specification, specific terms have been used. The terms are used to describe the present invention, and are not used to qualify the sense or limit the scope of the present invention. 
         [0043]    It is also noted that in this specification, ‘and/or’ represents that one or of components arranged before and after ‘and/or’ is included. Furthermore, “connected/coupled” refers to one component not only directly coupling another component but also indirectly coupling another component through an intermediate component. In addition, a singular form may include a plural form as long as it is not specifically mentioned in a sentence. Furthermore, ‘include/comprise’ or ‘including/comprising’ used in the specification represents that one or more components, steps, operations, and elements exist or are added. 
         [0044]      FIGS. 3A and 3B  are block diagrams illustrating a bias voltage generation unit. 
         [0045]      FIG. 3A  shows a bias voltage generation unit having a current mirror circuit of a single stage structure, and  FIG. 36  shows a bias voltage generation unit having a current mirror circuit of a multi stage structure. 
         [0046]    As shown in  FIG. 3A , the bias voltage generation unit having the single-stage current mirror circuit may include a reference current generation unit  311  and a first current mirror circuit  312 . 
         [0047]    The reference current generation unit  311  generates a reference current, and may be implemented with a band gap reference circuit, which is widely used. The first current mirror circuit  312  receives the reference current generated from the reference current generation unit  311  and provides a bias voltage to a pixel amplifying unit (not shown). A transistor  313  included in the first current mirror circuit  312  may be the load transistor of the pixel amplifying unit described with reference to  FIGS. 1A and 16 . 
         [0048]    As shown in  FIG. 36 , the bias voltage generation unit having the multi-stage current mirror circuit may include a reference current generation unit  321 , a first current mirror circuit  322  and a second current mirror circuit  323 . 
         [0049]    The reference current generation unit  321  is the same as the reference current generation unit  311  described with reference to  FIG. 3A . The first current mirror circuit  322  and the second current mirror circuit  323 , which are included in the bias voltage generation unit having the multi-stage current mirror circuit, receive the reference current generated from the reference current generation unit  321  and provide a bias voltage to a pixel amplifying unit (not shown). A transistor  324  included in the second current mirror circuit  323  may be the load transistor of the pixel amplifying unit described with reference to  FIGS. 1A and 1B . 
         [0050]      FIG. 4  is a circuit diagram illustrating a bias sampling device  430  and a CMOS image sensor including the same in accordance with an exemplary embodiment of the present invention. 
         [0051]    Referring to  FIG. 4 , the CMOS image sensor may include a row decoder and pixel driver  410 , a pixel array  420 , a bias voltage generation unit  430 , a pixel amplifying unit  440 , a read-out processing unit  450 , and a capacitor  460 . The row decoder and pixel driver  410 , the pixel array  420 , the pixel amplifying unit  440 , and the read-out processing unit  450  shown in  FIG. 4  are the same as the row decoder and pixel drivers  111  and  121 , the pixel arrays  112  and  122 , the pixel amplifying units  114  and  124 , and the read-out processing units  115  and  125  described with reference to  FIGS. 1A and 1B . 
         [0052]    Referring to  FIG. 4 , the bias voltage generation unit  430  may have a reference current generation unit  431 , a first current mirror circuit  432 , a second current mirror circuit  434 , and a bias sampling unit  433 . The bias sampling device  430  may be implemented with the multi stage current mirror circuit described with reference to  FIG. 3B . 
         [0053]    The bias sampling device  430  performs and provides sampling on the first bias voltage VPBIAS of the first current mirror circuit  432  to the pixel amplifying unit  440 . The bias sampling device  430  may perform sampling on a first bias voltage VPBIAS of the first current mirror circuit  432 , which is a previous-staged current mirror circuit to a final-staged current mirror circuit, or the second current mirror circuit  434 , rather than sampling on a second bias voltage VNBIAS provided to a load transistor  441  of the pixel amplifying unit  440 , which is one of transistors included in the second current mirror  434 . The first mirror circuit  432  may be a PMOS current mirror circuit and the second mirror circuit  434  may be an NMOS current mirror circuit. 
         [0054]    The reference current generation unit  431  generates a reference current. The first current mirror circuit  432  and the second current mirror circuit  434  receive the reference current generated from the reference current generation unit  431  and provide a bias voltage to the pixel amplifying unit  440 . 
         [0055]    The bias sampling unit  433  performs sampling on the first bias voltage VPBIAS of the first current mirror circuit  432 , which is coupled to the second current mirror circuit  434 . 
         [0056]    The CMOS image sensor in accordance with an embodiment of the present invention may further include the capacitor  450  for preventing image deterioration caused by coupling capacitance between a pixel output line and a voltage node of a second bias VNBIAS. 
         [0057]      FIG. 5A  is a circuit diagram illustrating a general pixel unit, and  FIG. 55  is a timing diagram illustrating sampling of a bias voltage of a CMOS image sensor in accordance with an exemplary embodiment of the present invention. 
         [0058]    Referring to  FIGS. 5A and 5B , “RX” denotes a voltage applied to a gate of a reset transistor of the pixel unit, “TX” denotes a voltage applied to a gate of a transfer transistor of the pixel unit, “1H” denotes a horizontal data readout period and “SAMPLE” denotes a voltage applied to a gate of a sampling switch of the bias sampling unit  433  of the bias sampling device  430  described with reference to  FIG. 4 . During a correlated double sampling (CDS) operation period, a bias sampling operation is performed to prevent noise caused by the power of the pixel amplifying unit  440 . Thus, a voltage applied to a gate of a sampling switch of the bias sampling unit  433  needs to be dropped and then raised during a time period from the completion of a pixel data readout, including a CDS in a previous 1H period, to the rising timing of the “RX”. 
         [0059]    As described above, the bias sampling device in accordance with an exemplary embodiment of the present invention performs sampling on a first bias voltage of a first current mirror circuit, which is a previous-staged current mirror circuit to a final-staged current mirror circuit, rather than sampling on a second bias voltage provided to a load transistor of the pixel amplifying unit, which is a transistor included in the final-staged current mirror circuit. Thus, pixels and current mirror circuits having various structures may be implemented. 
         [0060]      FIG. 6  is a block diagram illustrating a bias sampling device in accordance with another exemplary embodiment of the present invention. 
         [0061]    Referring to  FIG. 6 , the bias voltage generation unit may have a reference current generation unit  631 , a first current mirror circuit  632 , a second current mirror circuit  634 , and a bias sampling unit  633 . The bias sampling device shown in  FIG. 6  may be implemented with the multi stage current mirror circuit described with reference to  FIG. 3B . The reference current generation unit  631 , the first current mirror circuit  632 , the bias sampling unit  633  and the second current mirror circuit  634 , shown in  FIG. 6 , are same as the reference current generation unit  431 , the first current mirror circuit  432 , the bias sampling unit  433  and the second current mirror circuit  434 , described with reference to  FIG. 4 . A pixel amplifying unit (not shown) may be implemented using a source follower amplification circuit or other amplification circuits. 
         [0062]    However, when a pixel has a PMOS type structure, as shown in  FIG. 6 , instead of the NMOS type structure described with reference to  FIGS. 4 to 5B  the second current mirror circuit  634  of the PMOS type may be disposed at the final stage, the first current mirror circuit  632  may be implemented to be a NMOS type, and the bias sampling unit  633  may be disposed between transistors of the first current mirror circuit  632 . Herein, sampling timing of a bias voltage of a CMOS image sensor including the bias sampling device shown in  FIG. 6  may be an inversion of the timing described with reference to  FIG. 5B . 
         [0063]    As shown in  FIGS. 4 and 6 , a bias sampling device including two-stage current mirror circuits receiving a reference current generated from the reference current generation unit and providing the bias voltage to the pixel amplifying unit is exemplarily explained. However, according to another embodiment, a bias sampling device having a multi-stage current mirror circuit including three or more current mirror circuits may perform the bias sampling at the earliest stage, thereby obtaining the same effect as the bias sampling device described with reference to  FIGS. 4 to 6  without image deterioration caused by a global bias sampling structure. However, as the number of stages in the multi-stage current mirror circuit is increased, the number of transistors between the sampling switch and the final-staged current mirror circuit may be increased and thus thermal noise may increase. Therefore, it is not preferable for the multi-stage current mirror circuit to have an excessive number of stages of current mirror circuits. 
         [0064]      FIG. 7  is a block diagram illustrating a bias sampling device in accordance with still another exemplary embodiment of the present invention. 
         [0065]    As shown in  FIG. 7 , the bias sampling device in accordance with still another exemplary embodiment of the present invention includes a reference current generation unit  731 , a first current mirror circuit  732 , a bias sampling unit  733  and a second current mirror circuit  734 . 
         [0066]    As shown in  FIG. 7 , the reference current generation unit  731  and the bias sampling unit  733  shown in  FIG. 7  are same as the reference current generation unit  431 , and the bias sampling unit  433  described with reference to  FIG. 4 . A pixel amplifying unit (not shown) may be implemented using a source follower amplification circuit or other amplification circuits. 
         [0067]    The second current mirror circuit  734  may be implemented as in cascade form. When the second current mirror circuit  734  using the cascade form is implemented, since sampling operations of two gate biases may be performed using one sampling switch, it will be more efficiently implemented. That is, the second current mirror circuit  734  may include a second bias voltage line VNBIAS, a third bias voltage line VNBIAS 2  and a second sampling switch SP 2 . The bias sampling is performed on the first bias voltage of the first current mirror  732  using a sampling switch SP 1  of the bias sampling unit  733 . Thus, the bias sampling is performed on a node of the second bias voltage line VNBIAS and a node of the third bias voltage line VNBIAS 2 . This may increase the noise improvement effects, which are obtained through the bias sampling. 
         [0068]    The second sampling switch SP 2  is disposed between transistors of the second current mirror circuit  734 . A first sampling switch SP 1  may be disposed between transistors of the first current mirror circuit  732 . 
         [0069]      FIG. 8  is a block diagram illustrating a bias sampling device in accordance with still another exemplary embodiment of the present invention. 
         [0070]    As shown in  FIG. 8 , a bias sampling device in accordance with still another exemplary embodiment of the present invention includes a reference current generation unit  831 , a first current mirror circuit  832 , a bias sampling unit  833  and a second current mirror circuit  834 . 
         [0071]    The reference current generation unit  831  and the second current mirror circuit  834 , shown in  FIG. 8 , are same as the reference current generation unit  731  and the second current mirror circuit  734  shown in  FIG. 7 . A pixel amplifying unit (not shown) may be implemented using a source follower amplification circuit or other amplification circuits. 
         [0072]    The first current mirror circuit  832  may be implemented in a cascade form. The bias sampling unit  833  may be implemented using two sampling switches SP 1 A and SP 1 B. Ire another embodiment of the present invention, one of the two sampling switches SP 1 A and SP 1 B may be omitted. 
         [0073]    As described above, a bias sampling device in accordance with exemplary embodiments of the present invention may have a load transistor layout in the pixel amplifying unit occupying a small area compared with the local bias sampling structure, which needs a sampling capacitor having a large capacitance on a pitched layout region. 
         [0074]    As shown in  FIG. 2 , in case of the global bias sampling structure, when pixel output lines of column are sharply changed with a large amplitude, data distortion of a neighboring column may be caused by a coupling on a second bias voltage line VNBIAS. But, according to an embodiment of the present invention, data distortion does not occur since sampling of the second bias voltage line VNBIAS is not performed. 
         [0075]    Noise generated from the reference current generation unit, noise induced from power used in the pixel amplifying unit, or noise induced from external devices may be blocked by performing sampling on the first bias voltage line VPBIAS. 
         [0076]    While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.