Abstract:
A pixel bias current supply for supplying a stable source of bias current to pixels of an imager includes a current bypass feature for improving stability when one or more pixels of an imager saturates. The pixel bias current supply also features voltage limiters to restrict the output voltage of a pixel in order to prevent reverse video from being output when very strong light is incident upon one or more pixels.

Description:
FIELD OF INVENTION  
       [0001]     The present invention relates generally to semiconductor imagers. More specifically, the present invention relates to methods and apparatus for improving bias current stability in an imager.  
       BACKGROUND OF THE INVENTION  
       [0002]      FIG. 1  illustrates a conventional imager  100  in block diagram form. As illustrated, the imager  100  includes a power source  110 , a pixel array  120  including a plurality of pixels  121 , a plurality of load circuits  130 , a plurality of sample and hold circuits  140 , and a plurality of amplifiers  150 . Typically, there are numerous pixels  121  arranged in a plurality of rows and columns. However, for simplicity, the pixel array  120  illustrated in  FIG. 1  only includes two columns and one row of pixels  121 . The power source  110  produces a bias current IBIAS at an array pixel voltage of VAAPIX. Load circuit  130  produces a bias current IBIAS and this bias current IBIAS is supplied to each pixel  121  of the imager  100 . The imager  100  also includes a control circuit  160  for generating the illustrated control signals (e.g., ROW, TX, RESET, SHR, SHS, and OUT) and additional image processing circuitry  170  for further processing signals output by the pixels  121 .  
         [0003]     Now also referring to  FIGS. 2-5 , the operation of the imager  100  is explained.  FIG. 2  illustrates a conventional four transistor pixel  121 . The pixel  121  includes a photodiode  122 , a transfer transistor  123 , a reset transistor  124 , a source follower transistor  125 , and a row select transistor  126 . The pixel  121  accepts the ROW control signal at node F 1  (coupled to the gate of the row select transistor  126 ), the TX control signal at node D 1  (coupled to the gate of the transfer transistor  123 ) and the RESET control signal at node C 1  (coupled to the gate of the reset transistor  124 ). The pixel  121  accepts at node A 1  power from an output node (e.g., nodes B 01  or B 02 ) of the power source  110  and produces reset “Vrst” and photo “Vsig” signals as outputs at node B 1 . As described in greater detail below, the pixel  121  also includes a charge detection node E 1 .  
         [0004]     Now also referring to the timing diagram of  FIG. 5 , it can be seen that initially (i.e., at time t 0 ), control signals ROW, RESET, TX, SHR, and SHS are not asserted (i.e., at a logical low level).  
         [0005]     At time t 1 , the ROW control signal is asserted and supplied to node F 1  to activate row select transistor  126 .  
         [0006]     At time t 2 , the RESET control signal is asserted, causing voltage VAAPIX supplied from the power supply  110  at node A 1  to be applied to the charge detection node E 1 . Since node E 1  is coupled to the gate of transistor  125  and node F 1  is coupled to the gate of transistor  126 , both transistors  125  and  126  conduct. As a result, the reset signal Vrst is output at node B 1 .  
         [0007]     At time t 3 , the control signal RESET is deasserted and the charge detection node E 1  becomes floating. The photodiode  122  has already accumulated charge at node P 1  while it has been exposed to light since the last initialization. This exposure period is also known as an integration period.  
         [0008]     At time t 5 , the control signal TX is asserted, causing charge transfer from the node P 1  to the charge detection node E 1 . As a result, this charge is dumped to node E 1 . This charge decreases the voltage at node E 1 , and affects the conductivity of the source follower transistor  125 . Since row select transistor  126  is still conducting, a photo signal Vsig based on the transferred charge at node E 1  is output at node B 1 . In  FIG. 5 , the variability of the photo signal Vsig at node B 1  is illustrated by three traces  511 ,  512 ,  513 . The top trace  511  indicates the output at B 1  if the photodiode  122  was exposed to little if any light during the integration period. The middle trace  512  corresponds to moderate light exposure during the integration period. The bottom trade  513  corresponds to strong light exposure during the integration period.  
         [0009]     At time t 8 , the ROW control signal is deasserted, causing the row select transistor  126  to become non-conducting. As a result, the output at node B 1  is shut off.  
         [0010]     The pixel  121  therefore produces two output signals, namely a reset signal Vrst and a photo signal Vsig. The two signals Vrst and Vsig are produced at different times, but both signals are output at node B 1 . The outputs of the pixel  121  are routed to a load circuit  130 , so that the source follower transistor  125  and the load circuit comprise a voltage follower circuit when the ROW control signal is asserted.  
         [0011]      FIG. 3  illustrates the load circuit  130  used in the imager  100 . The circuit  130  is a current sink for the output of the pixel circuit  121 . As illustrated, the load circuit  130  accepts the output signals Vrst, Vsig of the pixel  121  at node A 2  and produces corresponding outputs at node B 2 . The output of the load circuit  130  is generically referred to as VPIXOUT, which corresponds to a modified form of the reset signal between times t 3  and t 5  and corresponds to a modified form of the photo signal between times t 5  and t 8 . The transistor  131  has its gate coupled to node C 2 , which accepts a control signal VLN. The control signal VLN is used to adjust the bias current generated by the transistor  131  and for optimizing the performance of the source follower circuit with regard to power consumption and speed. This transistor  131  is often referred to as a biasing transistor.  
         [0012]      FIG. 4  is a circuit diagram of the sample and hold circuit  140 . The function of the sample and hold circuit  140  is to sample and hold the reset and photo signals Vrst, Vsig of pixel  121  and output a corresponding differential signal. The differential signal output by the sample and hold circuit  140  has as its components the reset signal Vrst (at node B 31 ) and the photo signal Vsig (at node B 32 ). The operation of the sample and hold circuit  140  is described below.  
         [0013]     At time t 3 , the sample and hold circuit  140  accepts at node A 3  the reset signal Vrst and the SHR control signal transitions from low to high. The OUT control signal and the SHS control signal are both low, so transistors  142 ,  145 , and  146  are non-conducting while transistor  141  conducts. Accordingly, the reset signal Vrst charges capacitor  143 , and is thus stored on capacitor  143 .  
         [0014]     At time t 4 , the SHR control signal transitions from high to low, causing transistor  141  to become non-conducting.  
         [0015]     At time t 5 , the SHS control signal transitions from low to high. The OUT control signal and the SHR control signal are both low, so transistors  141 ,  145 , and  146  are non-conducting while transistor  142  conducts. Accordingly, the photo signal Vsig charges capacitor  144 , and is thus stored on capacitor  144 .  
         [0016]     At time t 7  the SHS control signal transitions from high to low, causing transistor  142  to become non-conducting.  
         [0017]     At time t 8 , the OUT control signal transitions from low to high, causing transistors  145  and  146  to conduct. Since the SHR and SHS control signals are both low, transistors  141  and  142  are non-conducting. Accordingly, charge from capacitors  143  and  144  begins to respectively and simultaneously flow to nodes B 31  and B 32  via transistors  145  and  146 . For simplicity, the signal arising from charge flowing from capacitor  143  to node B 31  is labeled as Vrst while the signal arising from charge flowing from capacitor  144  to node B 32  is labeled as Vsig.  
         [0018]     Now referring back to  FIG. 1 , it can be seen that the Vsig and Vrst signals respectively output from nodes B 31  and B 32  are supplied to a differential amplifier  150 , which produces a single ended output VOUT. The VOUT signal is representative of the output of a pixel  121  and can be supplied to the image processing circuit  170  for digitization, digital processing, and storage.  
         [0019]     At time t 9 , the OUT control signal transitions from high to low, causing transistors  145  and  146  to stop conducting.  
         [0020]      FIG. 6  is a block diagram of a conventional power source  110 . As illustrated, the power source  110  includes a power source  111  which is coupled to a resistor  112 . The resistor  112  represents the output resistance of the power source  111  and any parasitic components between the power source  112  and nodes B 01  and B 02 . The circuit  110  outputs at nodes B 01  and B 02  a bias current IBIAS at a predetermined voltage VAAPIX.  
         [0021]     One problem associated with the above described imager  100  is that when a pixel  121  is exposed to very bright light, the charge transferred from node P 1  to node E 1  can decrease the voltage at node E 1  from the reset voltage (i.e., VAAXPIX) to a ground voltage, causing the source follower transistor  125  to become non-conducting. This phenomenon is known as saturation. In an imager  100 , pixel saturation causes a cut-off of the bias current. This results in a fluctuation in the voltage of the output signal at nodes B 01 , B 02  of the power source  110 . Since the power source  110  is coupled to multiple pixels  121 , saturation of one or more pixels  121  can affect the output of other pixels  121 . In particular, saturation of one or more pixels may manifest in the image produced by the imager  100  as unstable horizontal band-wise noise.  
         [0022]     Another problem associated with the above described imager  100  occurs when extremely bright light, such as sunlight, is incident upon the pixel  121 . This saturates the photodiode  122  by causing the photodiode to produce a very large current flow. This current flow can overflow through the transfer transistor  123  to the charge detection node E 1 . Additionally, the source junction of the reset transistor  124  also generates photo current between node E 1  and the substrate, which causes the voltage at node E 1  to drop off when the reset transistor is turned off. In fact, under such strong illumination conditions, the voltage at node E 1  decreases after the reset transistor turns off at time t 3 , causing a drop in the level of the reset signal Vrst between time t 3  and t 4 , which is stored in capacitor  143  when control signal SHR is deasserted at time t 4 . The photo signal Vsig is always saturated under such strong light conditions. That is, the photo signal Vsig is set to a minimum level near ground potential. The decrease in the reset signal Vrst level causes a reduction of the Vout signal since Vout is equal to Vsig-Vrst. Thus, a very bright light incident upon a pixel may result in a decrease in the output signal, which ultimately manifests as a negative photoconversion response. This phenomenon is also known as reversal video noise.  
         [0023]     Accordingly, there is a need for a bias current supply circuit for use in an imager which is capable of producing a more stable pixel bias current independent of the amount of light incident upon the pixels of an imager. There is also a need for a bias current supply circuit which is resistant to reversal video noise when imaging extremely bright objects.  
       SUMMARY OF THE INVENTION  
       [0024]     Exemplary embodiments of the method and apparatus of the present invention provide a power source that provides a stabilized pixel bias current supply in an imager. The power source features a current bypass circuit to prevent bias current cut-off from adversely affecting the stability of the output of the power source. The power source also features a voltage limiter to prevent a pixel from outputting an out of range output voltage when very strong light is incident upon the pixel. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The foregoing and other advantages and features of the invention will become more apparent from the detailed description of exemplary embodiments of the invention given below with reference to the accompanying drawings, in which:  
         [0026]      FIG. 1  is a block diagram of a conventional imager;  
         [0027]      FIG. 2  is a circuit diagram of a conventional pixel;  
         [0028]      FIG. 3  is a circuit diagram of a conventional load circuit;  
         [0029]      FIG. 4  is a circuit diagram of a conventional sample and hold circuit;  
         [0030]      FIG. 5  is a timing diagram regarding the operation of the imager of  FIG. 1 ;  
         [0031]      FIG. 6  is a circuit diagram of a conventional bias current supply circuit;  
         [0032]      FIG. 7A  is a circuit diagram of a bias current supply circuit in accordance with a first exemplary embodiment of the invention;  
         [0033]      FIG. 7B  is a circuit diagram of a bias current supply circuit in accordance with a second and a third exemplary embodiments of the invention;  
         [0034]      FIG. 8  is a supplemental timing diagram for operating the bias current supply circuit of  FIG. 7B  in accordance with a second exemplary embodiment of the invention;  
         [0035]      FIG. 9  is a supplemental timing diagram for operating the bias current supply circuit of  FIG. 7B  in accordance with a third exemplary embodiment of the invention;  
         [0036]      FIG. 10  is a circuit diagram of a bias current supply circuit in accordance with a fourth exemplary embodiment of the invention;  
         [0037]      FIG. 11  is a circuit diagram of a bias current supply circuit in accordance with a fifth exemplary embodiment of the invention; and  
         [0038]      FIG. 12  is a block diagram of a processor based system incorporating the principles of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]     Now referring to the drawings, where like reference numerals designate like elements, there is shown in  FIG. 7A  a circuit diagram of a bias current supply circuit  700  in accordance with a first exemplary embodiment of the invention.  
         [0040]     The bias current supply circuit  700  includes a plurality of taps  740 . More specifically, there are n taps  740 , where n corresponds to the number of columns that receive power from the bias current supply circuit  700 . The bias current supply circuit  700  provides each tap  740  with the IBIAS current at node  741 , which is coupled to node A 1  ( FIG. 2 ) of each pixel  121 . The voltage VAAPIX is supplied from node  711  of the power source  710 . Node  711  is coupled to line  721 , which is coupled to each node  741 . The power source  710  must be capable of providing the output current of NIBIAS at VAAPIX voltage. The current level of the NIBIAS current must be at least n times the IBIAS current. The power source  710  also produces another output signal VSLICE at node  712 . Node  712  is coupled to line  722 , which is coupled to a gate of each transistor  731  in circuit  700 . Each transistor  731  has one source/drain coupled to line  721  and another source/drain coupled to a respective node  742 . Each node  742  is a output node coupled to node B 1  of an associated pixel  121  ( FIG. 2 ).  
         [0041]     The signal VSLICE is at a voltage level of VS, where VS is greater than VT, the threshold voltage of transistor  731 . When the imager  100  is operated using the power source  700 , when dark or medium light is incident upon the pixel  121 , the photo signal Vsig voltage will be greater than the VS-VT voltage. As a result, transistor  731  will be non-conducting. Thus, when dark or medium light is incident upon the pixel  121 , the imager  100  operates as previously described with respect to  FIG. 5 .  
         [0042]     When bright light is incident upon the pixel  121 , as previously discussed, the source follower transistor  125  in the pixel  121  becomes non-conducting and thus the IBIAS current no longer flows through node  741  to node A 1  of the pixel  121 . However, at the same time the photo signal Vsig voltage will also drop below the VS-VT voltage, thereby causing transistor  731  to conduct. Since one source/drain of transistor  731  is coupled to line  721  and another source/drain is coupled to node B 1  of pixel  121 , the circuit formed by transistor  731  acts as a current bypass from node  741  to node  742  that permits the IBIAS current to continue to flow to the pixel  121  even when source follower transistor  125  is non-conducting. As a result, the output current from the power source  710  remains constant, even when bright light is incident upon the pixel  121 .  
         [0043]     While the  FIG. 7A  embodiment of the invention addresses and reduces the unstable horizontal band-wise noise, it does not address the reverse video noise.  FIG. 7B  is an illustration of a second exemplary embodiment of the present invention, which provides a way to reduce the reverse video noise. The second exemplary embodiment of the present invention utilizes a power source  700 ′, which is similar to the bias current supply circuit  700  of the first exemplary embodiment. However, circuit  700 ′ includes a new power source  710 ′ and is intended to be used with a new control circuit  160 ′. The power source  710 ′ is similar to the power source  710  of the first embodiment, but further includes a new control signal input node  713 , which accepts a new control signal SLICE_R. The power source  710 ′ is modified to control the output of the VSLICE control signal based upon the state of the SLICE_R control signal. The SLICE_R is control signal produced by a modified control circuit  160 ′, which is similar to control circuit  160  ( FIG. 1 ), but which also produces the SLICE_R signal. The characteristics of the SLICE_R signal are described below.  
         [0044]     The SLICE_R control signal is supplied from the control circuit  160 ′ to the power source  710 ′. When the SLICE_R control signal is asserted, the power source  710  outputs VR to the VSLICE signal. When the SLICE_R control signal is deasserted, the power source  710  does not output the VSLICE signal, that is, VSLICE is kept at ground potential.  
         [0045]     As shown in the supplemental timing diagram of  FIG. 8 , the SLICE_R control signal is controlled so that it is asserted simultaneously with the assertion of the SHR control signal at time t 3 . The SLICE_R control signal is then deasserted at any time between the deassertion of the SHR control signal at t 4  and the assertion of the SHS control signal at time t 5 . Thus, the SLICE_R control signal is asserted only during the reset phase of pixel operation. That is, the power source  710  is controlled to only output the VSLICE signal during the sampling and holding of the reset signal Vrst (the reset phase). The presence of the VSLICE signal does not change the operation of the imager  100  when dark or normal level light is incident upon a pixel. However, when extremely bright light is incident upon a pixel, the presence of the VSLICE signal prevents the reset signal Vrst voltage level from dropping below VSLICE-VT, thereby preventing the reset signal Vrst level from being significantly affected by the extremely bright light. The reverse video noise is therefore addressed by preventing the reset signal Vrst from being significantly affected when extremely bright light is incident upon the pixel  121 .  
         [0046]     Both the unstable load noise and the reverse video noise are addressed by a third exemplary embodiment of the present invention. The third exemplary embodiment utilizes the same bias current supply circuit  700 ′ as the second exemplary embodiments ( FIG. 7B ). Referring now to the supplemental timing diagram of  FIG. 9 , it can be seen that the VSLICE signal is generally at the VS voltage level. However, when the SLICE_R control signal is asserted, the VSLICE signal is at the higher VR voltage level. By setting the VSLICE signal at the VR voltage level during the reset phase, the reverse video noise is reduced as explained above in connection with the second exemplary embodiment. Similarly, by setting the VSLICE signal at the VS voltage level during the photo signal phase, the unstable load noise is reduced as explained above with respect to the first exemplary embodiment. The third exemplary embodiment therefore combines elements of the first and second exemplary embodiments to address both the unstable load noise and reverse video noise.  
         [0047]      FIG. 10  illustrates a bias power supply circuit  1000  in accordance with a fourth exemplary embodiment of the present invention. The power supply circuit  1000  is capable of addressing the reversal video noise. The fourth exemplary embodiment operates similar to the second exemplary embodiment. However, the fourth exemplary embodiment utilizes a different mechanism of generating an output signal at node  742 . While the second exemplary embodiment required power source  710  to be capable of sequencing the VSLICE signal between the VR and ground voltages, and use the VSLICE signal to control transistor  731  to govern the output at node  742 , the power source  1010  of the bias power supply circuit  1000  is configured to output the VSLICE signal at a single voltage level at the VR voltage. An additional transistor  1032  is coupled in series, via its source and drain terminals, between a source/drain of transistor  731  and node  742 . The gate of transistor  1032  is coupled to signal line  1023 , which is also coupled to node  1050 . The control circuit  160  ( FIG. 1 ) is modified to also output the SLICE_EN control signal. The SLICE_EN control signal is used to control the conductivity of transistor  1032 . Since the source/drain terminals of transistor  1032  are coupled in series between the source/drain terminals of transistor  731  and node  742 , transistor  1032  can be used as a control device for the current flowing from the power source  1010  (at node  711 ) to node  742 . More specifically, and referring also back to  FIG. 8 , the control circuit  160  is modified to output the SLICE_EN control signal in a manner so that the output at node  742  in this fourth exemplary embodiment is identical to the output at node  742  of the third exemplary embodiment.  
         [0048]      FIG. 11  illustrates a power source  1100  in accordance with a fifth exemplary embodiment of the present invention. The power source  1100  is capable of addressing both the unstable load noise and the reverse video noise (as in the third and fourth exemplary embodiments). The fifth exemplary embodiment operates similar to the third exemplary embodiment. However, the fifth exemplary embodiment utilizes a different mechanism of generating the output signal at node  742 . The third exemplary embodiment controlled the output signal at node  742  by controlling the conductivity of transistor  731  by applying the VSLICE control signal to the gate of transistor  731 . This required the power source  710  to be capable of sequencing the VSLICE control signal between the VR and VS voltages. In the fourth exemplary embodiment, the power source  1010  supplies the VSLICE control signal at a fixed voltage level VR. In the illustrated embodiment, the power source  1110  of the power source  1100  is configured to output the a VSLICE 1  signal at the VR voltage level on an additional signal line  1121  and a VSLICE 2  signal at the VS voltage level on an additional signal line  1123 .  
         [0049]     The power source  1100  utilizes four transistors  1031   a ,  1031   b ,  1032 , and  1033  to control the output at node  742 . More specifically, transistors  1031   a  and  1031   b  each operate similarly to transistor  731  ( FIG. 7A ). Transistor  1032  is coupled in series via its source and drain between a source/drain of transistor  1031   b  and node  742 . Similarly, transistor  1033  is coupled in series via its source and drain between a source/drain of transistor  1031   a  and node  742 . The control circuit  160  ( FIG. 1 ) is modified to provide new SLICE_EN 1  and SLICE_EN 2  control signals respectively to the gates of transistor  1032  and  1033 . The states of the SLICE_EN 1  and SLICE_EN 2  control signals are complementary and set by the control circuit  160  so that node  742  is supplied either the signal flowing from line  711  via transistors  1031   a  and  1033  or the signal flowing from line  711  via transistors  1031   b  and  1032 . In this manner, the bias power supply circuit of  1000  produces an output signal at node  742  identical to that produced third exemplary embodiment.  
         [0050]      FIG. 12  illustrates a processor system  1200 . The processor system  1200  includes a processor device  1210 . The processor device  1210  may be, for example, digital camera, a personal computer, or other image processing apparatus, and includes, for example, a central processing unit  1220 , a memory  1230 , and an I/O controller  1240 . The memory  1230  may be a conventional memory. Alternatively, the memory  1230  may be, or may include, a removable memory, such as a removable flash memory device. The I/O controller is coupled to interconnect  1220 , which couples the processor based device  1210  to an imager  100 ′. The imager  100 ′ is similar to imager  100  ( FIG. 1 ), but incorporates a bias current supply circuit in accordance with the principles of the present invention. As illustrated, the bias current supply circuit is circuit  700  of the first exemplary embodiment, but the bias current supply circuits of the other exemplary embodiments (e.g., circuits  700 ′,  1000 , or  1100 ) may be substituted for circuit  700 .  
         [0051]     The present invention therefore presents a number of embodiments for a pixel power supply circuit with addresses the unstable load and/or reverse video noise which may be encountered in any imaging system. The present invention addresses the unstable load noise by improving bias current stability via current bypass circuits which activate when the source follower is saturated. The present invention addresses the reverse video noise by using a voltage limiter at the pixel output node to limit the reset voltage output from a pixel.  
         [0052]     While the invention has been described in detail in connection with the exemplary embodiments, it should be understood that the invention is not limited to the above disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alternations, substitutions, or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not limited by the foregoing description or drawings, but is only limited by the scope of the appended claims.