Abstract:
There is provided a signal processing apparatus including a photoelectric conversion element, a compression device which logarithmically compresses an output from the photoelectric conversion element, an expansion device which exponentially expands an output from the compression device, and an integral device which integrates an output from the expansion device, wherein a transistor which performs logarithmic compression in the compression device and a transistor which performs exponential expansion in the expansion device are MOS transistors respectively.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a photoelectric conversion apparatus having a logarithmic compression function.  
           [0003]    2. Related Background Art  
           [0004]    A conventional photoelectric conversion apparatus for modulating electronic flash light from a camera has used a circuit as shown in FIG. 1. In FIG. 1, a photodiode  1  for receiving electronic flash light is connected to an operational amplifier  2 . An npn bipolar transistor  10  logarithmically compresses a photocurrent I p . An npn bipolar transistor  11  applies a gain to a logarithmically compressed signal to expand it. An integral capacitor  7  integrates an expansion current I. This photoelectric conversion apparatus further comprises a comparator  8  and monitoring voltage follower  9 .  
           [0005]    Letting I p  be the photocurrent of the photodiode  1 , an output V 1  from the logarithmic compression circuit is given by  
               V   1     =       V   c     -       kT   q        ln          I   p       I   s                   (   1   )                               
 
           [0006]    where k is the Blotzmann constant, T is the temperature, q is the elementary charge, and I s  is the reverse saturation current of a bipolar transistor Q 1 .  
           [0007]    The output of the logarithmic compression circuit is connected to the emitter of an expansion transistor Q 2 . Letting V DAC  be the base potential of the expansion transistor, the current I flowing through the expansion transistor Q 2  is given by  
             I   =         I   s        exp          q        (       V   DAC     -     V   1       )       kT       =       I   p        exp        q   kT          (       V   DAC     -     V   C       )                 (   2   )                               
 
           [0008]    The expansion current I gains by the potential difference between V DAC  and V c . For example, for V DAC −V c =18 mV, the expansion current is double the photocurrent I p .  
           [0009]    However, the prior art adopts bipolar transistors as elements for logarithmically compressing, expanding, and integrating a current, so the following problems occur.  
           [0010]    a. The bipolar technique is necessary, and is less compatible with a CMOS sensor (sensor manufactured by a CMOS process).  
           [0011]    b. The cost is high in terms of the number of masks and the process.  
         SUMMARY OF THE INVENTION  
         [0012]    It is the first object of the present invention to provide an apparatus which can be manufactured by a CMOS process.  
           [0013]    It is the second object of the present invention to provide a multifunctional CMOS sensor capable of reducing the cost by integrating a modulated light circuit in another CMOS sensor, e.g., an autofocus sensor on a single chip.  
           [0014]    To achieve the above objects, according to an aspect of the present invention, there is provided a signal processing apparatus comprising:  
           [0015]    a photoelectric conversion element;  
           [0016]    a compression device which logarithmically compresses an output from the photoelectric conversion element;  
           [0017]    an expansion device which exponentially expands an output from the compression device; and  
           [0018]    an integral device which integrates an output from the expansion device,  
           [0019]    wherein a transistor which integrates logarithmic compression in the compression device and a transistor which performs exponential expansion in the expansion device are MOS transistors respectively.  
           [0020]    According to another aspect of the present invention, there is provided a signal processing apparatus comprising:  
           [0021]    a photoelectric conversion element;  
           [0022]    a compression device which logarithmically compresses an output from the photoelectric conversion element; and  
           [0023]    an integral device which integrates an output from the expansion device,  
           [0024]    wherein a transistor which performs logarithmic compression in the compression device and a transistor which performs exponential expansion in the expansion device are MOS transistors respectively.  
           [0025]    According to still another aspect of the present invention, there is provided a signal processing apparatus formed by a CMOS process on a single semiconductor substrate, comprising:  
           [0026]    a modulated light circuit including:  
           [0027]    a compression device which logarithmically compresses an output from a photoelectric conversion element;  
           [0028]    an expansion device which exponentially expands an output from the compression device; and  
           [0029]    an integral device which integrates an output from the expansion device,  
           [0030]    wherein a transistor which performs logarithmic compression in the compression device and a transistor which performs exponential expansion in the expansion device are MOS transistors respectively; and  
           [0031]    a focus adjustment circuit.  
           [0032]    The above and other objects and features of the present invention will be apparent from the following description in conjunction with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    [0033]FIG. 1 is a circuit diagram showing the prior art;  
         [0034]    [0034]FIG. 2 is a circuit diagram showing the first embodiment;  
         [0035]    [0035]FIG. 3 is a circuit diagram showing the second diagram;  
         [0036]    [0036]FIG. 4 is a block diagram showing the arrangement of the third embodiment;  
         [0037]    [0037]FIG. 5 is a circuit diagram showing an autofocus circuit;  
         [0038]    [0038]FIG. 6 is a block diagram showing the arrangement of the fourth embodiment; and  
         [0039]    [0039]FIG. 7 is a circuit diagram showing a thermometer circuit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]    [0040]FIG. 2 is a circuit diagram showing a photoelectric conversion apparatus as the first embodiment of the present invention. In FIG. 2, a photodiode  1  photoelectrically converts electronic flash light. An operational amplifier  2  is formed with a CMOS structure. An nMOS transistor  3  performs logarithmic compression, and operates in a subthreshold region. An nMOS transistor  4  performs expansion operation, and similarly operates in the subthreshold region. An operational amplifier  5  is formed with a CMOS structure, and a pMOS transistor  6  negatively feeds back a current to the operational amplifier  5 . An integral capacitor  7  integrates an expanded current, a comparator  8  compares the potential of an integrated charge with a reference potential V REF , and a voltage follower circuit  9  monitors the potential of the integral capacitor  7 .  
         [0041]    When the gate voltage of the MOS transistor is a threshold voltage or less, a subthreshold current flows therethrough. This current value I D  is given by  
               I   D     =       I   DD          exp        [       q   nkT          (       V   G     -     V   S     -     V   T       )       ]                 (   3   )                 I   DD     =         W                   μ   n          C   o       nL            (     nkT   q     )     2          exp        (     -   1     )                 (   4   )                               
 
         [0042]    where V G  is the gate voltage, V D  is the gate voltage, V s  is the source voltage, V T  is the threshold voltage, W is the gate width, L is the gate length, μn is the electron mobility, C o  is the gate capacitance, and C D  is the capacitance of the depletion layer, of which C o  and C D  are given by  
             n   =         C   o     +     C   D         C   o               (   5   )                               
 
         [0043]    In FIG. 2, when light is incident on the photodiode  1 , a photocurrent I p  proportional to the light intensity is generated. An output from the logarithmic compression circuit is given by  
               V   1     =       V   C     +     V   T     +       nkT   q          ln        (       I   p       I   DO       )                   (   6   )                               
 
         [0044]    The photocurrent I p  is logarithmically converted and output.  
         [0045]    A source potential V 2  of an expansion nMOS transistor Q 4  is negatively fed back by the operational amplifier  5  and a pMOS transistor Q 5 , which operate to cause the potential V 2  to be equal to V DAC . Hence, a current I flowing through the expansion nMOS transistor Q 4  is given by  
             I   =       I   DO          exp        [       q   nkT          (       V   1     -     V   DAC     -     V   T       )       ]                 (   7   )               I   =       I   p          exp        (         V   c     -     V   DAC         nV   T       )                 (   8   )                               
 
         [0046]    Accordingly, the expansion current I gains by the potential difference between V DAC  and V C .  
         [0047]    The first embodiment can implement a modulated light circuit by only MOS transistors. The number of masks and the process cost can be reduced to attain a low-cost modulated light sensor.  
         [0048]    The circuit of the first embodiment is constituted by nMOS transistors, but the same effects of the present invention can also be obtained with using pMOS transistors depending on the polarity of the photodiode. The photodiode may be formed on the same substrate or may be discretely externally connected.  
         [0049]    [0049]FIG. 3 shows the circuit of a photoelectric conversion apparatus as the second embodiment of the present invention. In the second embodiment, only charge integration is performed without expansion of a logarithmically compressed signal.  
         [0050]    The potential in an integral capacitor can be approximated by  
               V   2     =       V   C     +       nkT   q          ln        (       q   nkTC          ∫       I   p             t           )                   (   9   )                               
 
         [0051]    The potential V 2  is a logarithmically compressed value of the time integral value of the photocurrent I p .  
         [0052]    The second embodiment enables simple integration which does not include expansion. Particularly when the precision can be poor, the number of circuits can be decreased to further reduce the cost.  
         [0053]    [0053]FIG. 4 shows the arrangement of a multifunctional CMOS sensor as the third embodiment of the present invention. In the third embodiment, a modulated light circuit is integrated on the same substrate as an autofocus sensor.  
         [0054]    [0054]FIG. 4 shows an example of an autofocus circuit block  31 . A modulated light circuit  32  is identical to each of those described in the first and second embodiments. A communication circuit  33  communicates with an external CPU. A control circuit  34  controls each internal circuit of the IC. An analog circuit  35  is formed with an auto gain control circuit, amplifier circuit, intermediate power supply, band gap circuit, or the like. A multiplexer circuit  36  selects and externally outputs each output. An externally-connected photodiode  37  is used as a photodiode for the modulated light circuit.  
         [0055]    In the third embodiment, all the circuits are manufactured by a CMOS process, which can implement a low-cost multifunctional CMOS sensor.  
         [0056]    The autofocus sensor and modulated light sensor, which are provided separately in the prior art, can be integrated into one, thereby reducing the cost and size of the camera.  
         [0057]    [0057]FIG. 5 shows the arrangement of a multifunctional CMOS sensor as the fourth embodiment of the present invention. In the fourth embodiment, a thermometer circuit is further integrated in the arrangement of the third embodiment.  
         [0058]    As can be understood from equations (8) and (9), an output from the modulated light sensor changes depending on the temperature owing to the temperature dependency of the threshold value V T . To modulate light at high precision requires correction by the temperature, so that the temperature must be accurately measured. FIG. 6 shows an example of the thermometer circuit using the temperature characteristics of the diode. In the fourth embodiment, since the thermometer is formed on the same substrate as the modulated light circuit, the temperature can be accurately measured. Since no other chip is required, an increase in cost can be suppressed.  
         [0059]    The fourth embodiment can implement at a low cost a multifunctional CMOS sensor capable of high-precision light modulation and autofocusing.  
         [0060]    As has been described above, a logarithmic compression circuit and expansion/integration circuit can be achieved by a CMOS process, so that a lower-cost photoelectric conversion apparatus and modulated light circuit than the prior art can be attained.  
         [0061]    The modulated light circuit can be integrated in a CMOS sensor such as a CMOS autofocus sensor, and thus a small-size, low-cost camera having a small number of components can be obtained.  
         [0062]    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.