Abstract:
An active pixel sensor cell array in which a two-stage amplifier amplifies the output of each cell. The two-stage amplifier design reduces fixed pattern noise in the image data generated by reading the array, by providing increased gain for the output of each cell without impractically increasing the size and complexity of each cell. For each column of cells of the array, one part of the two-stage amplifier for each cell is shared by all cells of the column, and another part of the two-stage amplifier for each cell is included within the cell itself. Preferably, each cell includes only NMOS transistors (no cell includes a PMOS transistor). In preferred embodiments, a differential amplifier within each cell is the primary stage of the cell&#39;s output amplifier, PMOS load circuitry including a secondary output amplifier stage is shared by all cells of the column, and the two amplifier stages for each cell together comprise an op amp. In some such preferred embodiments, the op amp is provided with capacitor feedback for increased gain. Another aspect of the invention is an active pixel sensor cell including a differential amplifier, and PMOS transistor load circuitry coupled to the cell. Preferably, the differential amplifier is the first stage of an op amp, and the remainder of the op amp, including optional capacitor feedback circuitry, is coupled to the cell but is not included within the cell.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to an output amplifier for an active pixel sensor cell array. More particularly, the invention pertains to an output amplifier for an active pixel sensor cell array, the amplifier having a two-stage design that reduces fixed pattern noise in the image data output from the array. 
     DESCRIPTION OF THE RELATED ART 
     Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting photons incident at individual pixel sensor cells (of a pixel sensor cell array) into electrical signals indicative of the intensity of light energy incident at each cell. In general, a CCD uses a photogate to convert light energy incident at a cell into an electrical charge, and a series of electrodes to transfer the charge collected at the photogate to an output sense node. 
     Although CCDs have many strengths, including high sensitivity and fill-factor, CCDs also suffer from a number of weaknesses. These weaknesses include limited readout rates and dynamic range limitations, and notably, the difficulty in integrating CCDs with CMOS-based microprocessors. 
     To overcome the limitations of CCD-based imaging circuits, imaging circuits have been developed which use active pixel sensor cells to convert pixels of light energy into electrical signals. An active pixel sensor cell typically includes a conventional photodiode and a number of transistors which provide amplification, readout control, and reset control in addition to producing the electrical signal output from the cell. 
     FIG. 1 is an example of two identical CMOS active pixel sensor cells ( 10  and  11 ) having conventional design, connected along a column of an active pixel sensor cell array, and circuitry  21  for use in reading all cells connected along the column. 
     As shown in FIG. 1, cell  10  includes photodiode d 1  (connected as shown between ground and Node  3 ), and reset transistor N 1 . Transistor N 1  is an NMOS transistor whose drain is connected to a power supply node (Node  1 ) maintained at potential V cc , whose source is connected to Node  3 , and whose gate is connected to Node  2 . The gate of transistor N 1  is controlled (in a manner to be described below) by a RESET voltage supplied to Node  2 . 
     Cell  10  also includes buffer transistor N 2  and row select transistor N 3 , each of which is an NMOS transistor. Transistor N 2  has a drain connected to Node  1 , a source connected to Node  4 , and a gate connected to Node  3 . Transistor N 3  has a drain connected to Node  4 , a source connected to Node  6 , and a gate connected to Node  5 . The gate of transistor N 3  is controlled (in a manner to be described below) by a ROW SELECT voltage supplied to Node  5 . 
     As shown in FIG. 1, circuitry  20  includes detection and calculation circuit  21  whose input terminal is connected to Node  6 . Circuit  21  includes a sense amplifier which outputs digital data indicative of light intensity incident at each selected cell along the column in response to voltages at Node  6  during a sampling period when each such cell is selected. Circuit  21  typically also implements correlated double sampling (“CDS”) or another post-processing method on the digital data output from the sense amplifier. 
     In normal operation, circuit  21  receives a sequence of voltages at Node  6  (which node is common to all cells connected along the column), with each pair of consecutive voltages being indicative of light intensity incident (during a sampling period) at a different one of the cells along the column. 
     Circuitry  20  also includes NMOS transistor N 6  (whose drain is connected to Node  6  and whose source is connected to ground) and a current mirror (comprising current source I 1  and NMOS transistors N 4  and N 5  connected as shown) which provides the necessary load for reading out the cells. Transistor N 5  of the current mirror preferably sinks no more than a small current (from Node  6  to ground), since fixed pattern noise resulting from mismatches in the channel lengths of the buffer transistors in the cells will increase with increasing current sunk by the current mirror. 
     The gate of transistor N 6  (at Node  8 ) is controlled by a Column Reset signal. Use of a column reset transistor such as transistor N 6  is described in U.S. patent application Ser. No. 08/871,519 entitled “Active Pixel Sensor Cell that Reduces Noise in the Photo Information Extracted from the Cell,” filed on Jun. 9, 1997, naming Richard B. Merrill as inventor and assigned to the assignee of the present application. 
     Briefly, in operation of the FIG. 1 array, transistor N 6  is used as a switch to place a defined voltage (ground potential) on Node  6  before circuit  21  reads one of the cells (e.g., cell  10 ). Preferably, the gate of transistor N 6  is pulsed with a high level of column reset voltage “COLUMN RESET” prior to each pulsing of the row select voltage ROW SELECT. By pulsing the column select voltage COLUMN RESET just prior to each pulsing of the row select voltage ROW SELECT, the voltage at Node  6  is pulled to zero (ground potential) just prior to reading of the relevant one of the cells. When the voltage on Node  6  is set to zero immediately prior to pulsing the row select voltage, resulting noise (in the data determined by circuit  21 ) is reduced substantially. For example, in one implementation of FIG. 1, the noise is reduced from approximately 15 mV (in the case that N 6  remains “off” at all times) to approximately one millivolt. 
     Also in accordance with the teaching of U.S. patent application Ser. No. 08/871,519 entitled “Active Pixel Sensor Cell that Reduces Noise in the Photo Information Extracted from the Cell,” filed Jun. 9, 1997, switch transistor N 6  is optionally replaced by a switch transistor whose channel terminals are connected between Node  6  and power supply Node  1 , and whose gate is coupled to receive the column select voltage COLUMN RESET. By pulsing the voltage COLUMN RESET just before each pulsing of the row select voltage, the switch transistor pulls up the voltage at Node  6  to voltage Vcc just prior to reading of each cell. This technique also reduces noise in the data determined by circuit  21 . 
     The operation of sampling (reading) each cell (e.g., cell  10 ) begins by briefly pulsing the gate of the cell&#39;s reset transistor N 1  with a high level of reset voltage “RESET.” This high level of the reset voltage (typically equal to Vcc, where Vcc is typically 5 volts) resets the voltage on photodiode d 1  to an initial integration voltage to begin an image collection cycle. 
     Immediately after assertion of such pulse of the voltage signal “RESET,” the initial integration voltage on photodiode d 1  (the voltage at Node  3 ) is V ini =VRESET−V TN1 −V CLOCK , where V TN1  is the threshold voltage of transistor N 1 , VRESET is the high level of the voltage signal “RESET,” and V CLOCK  represents reset noise from the pulsed reset voltage (assumed to be constant). Similarly, the initial integration voltage at Node  4  is VRESET−V TN1 −V CLOCK −V TN2 , where V TN2  is the threshold voltage of buffer transistor N 2  (functioning as a source follower). 
     After the reset voltage has been pulsed and the voltage on photodiode d 1  (the voltage at Node  3 ) has been reset, the gate of transistor N 3  is pulsed with a high level of row select voltage signal “ROW SELECT.” The high level of the row select voltage causes the voltage at Node  4 , which represents the initial integration voltage of the cycle, to appear at Node  6 . Detection and calculation circuit  21  then amplifies, digitizes, and stores the value of the initial integration voltage as it appears at Node  6 . 
     Next, for a selected time period, photons are allowed to strike photodiode d 1 , thereby creating electron-hole pairs. Photodiode d 1  is designed to limit recombination between the newly formed electron-hole pairs. 
     As a result, the photogenerated holes are attracted to the ground terminal of photodiode d 1 , while the photogenerated electrons are attracted to the positive terminal of photodiode d 1 , each additional electron reducing the voltage at Node  3 . At the end of this image collection cycle, a final integration voltage will be present at Node  3 . The final integration voltage is V f =V ini −V S =VRESET−V TN1 −V CLOCK −V S , where V S  represents the change in voltage (at Node  3 ) due to the absorbed photons. Similarly, the final integration voltage at Node  4  is VRESET−V TN1 −V CLOCK −V TN2 −V S . 
     At the end of the image collection cycle, the gate of transistor N 3  is again pulsed with a high level of row select voltage signal “ROW SELECT” to cause the voltage at Node  4 , which represents the final integration voltage of the cycle, to appear at Node  6 . Detection and calculation circuit  21  amplifies and digitizes the value of the final integration voltage as it appears at Node  6 , and generates data indicative of the number of photons that have been collected during the image collection cycle by calculating the difference (V S ) between the digitized final integration voltage taken at the end of the cycle and the digitized stored initial integration voltage taken at the start of the cycle. 
     After the final integration voltage has been latched by detection and calculation circuit  21 , the reset voltage RESET is again pulsed to reset the voltage on photodiode d 1  to begin another image collection cycle. 
     One of the problems with active pixel sensor cells (e.g., cell  10  of FIG. 1) is that during typical operation, the reset voltage RESET and the row select voltage ROW SELECT have high levels for periods (typically about 30 msec) which are sufficiently long to introduce a substantial amount of 1/f noise into the cell. Such 1/f noise, which results from trapping and detrapping of surface charges, can be accurately modeled as variations in the threshold voltages of transistors N 1 , N 2 , and N 3 . Due to such noise, the number of photons which are absorbed by photodiode d 1  during an image collection cycle is more properly expressed as (VRESET−V TN1 −V CLOCK −V TN2 )−(VRESET−V TN1 −V CLOCK −V TN2 −V S −V α ), where V α  is a contribution due to variations in the threshold voltages of transistors N 1 , N 2 , and N 3  due to 1/f noise. Thus, the variations in the threshold voltages of transistors N 1 , N 2 , and N 3  add an error term V α  which erroneously yields V S +V α  as the value determining the number of absorbed photons, thereby limiting the accuracy of the cell. 
     In some applications (as explained in U.S. patent application Ser. No. 08/707,933, filed on Sep. 10, 1996, naming Richard B. Merrill and Kevin E. Brehmer as inventors and assigned to the assignee of the present application), it is desirable to choose Vcc to be substantially less than VRESET (the high level of the reset voltage RESET). For example, Vcc may be chosen to be 3.3 volts and VRESET may be chosen to be 5 volts. This forces reset transistor N 1  to operate in the linear region in which the high level of the reset voltage causes N 1  to pull the voltage at Node  3  up to V ini  in a manner subject to reduced variation due to changes in the threshold voltage of reset transistor N 1  due to 1/f noise. However, this technique does not eliminate fixed pattern noise due to systematic and random variation among the characteristics of cells of an active pixel sensor cell array. 
     Active pixel sensor cell arrays that use a conventional source follower amplifier in each cell (e.g., arrays of the type described with reference to FIG. 1) are subject to fixed pattern noise due to systematic and random variation between cells. Such fixed pattern noise is due to many different sources of gain variation that cannot easily be corrected with post processing techniques such as correlated double sampling. It has been proposed to implement a better amplifier within each cell (which would be less subject to such gain variation from cell to cell) by including a CMOS amplifier within each cell. Such a CMOS amplifier includes at least one PMOS transistor as a current source load for high gain (in addition to one or more NMOS transistors). Unfortunately, it is not currently possible to integrate a PMOS transistor into a single cell (of an active pixel sensor cell array) without increasing the cell size to an acceptable degree. 
     Conventional CCD imagers are typically subject to significantly less fixed pattern noise than are active pixel sensor cell arrays that include a conventional source follower output amplifier in each cell. It would be desirable to implement an active pixel sensor cell array that is subject to no more fixed pattern noise than a conventional CCD imager, without unacceptably increasing the cell size of such active pixel sensor cell array. 
     SUMMARY OF THE INVENTION 
     In a class of embodiments, the invention is an active pixel sensor cell array in which a two-stage amplifier amplifies the output of each cell of the array. For each column of cells of the array, one part of the two-stage amplifier for each cell is shared by all cells of the column, and another part of the two-stage amplifier for each cell is included within the cell itself. Preferably, the output amplification circuitry within each cell includes only NMOS transistors and does not include any PMOS transistor. In preferred embodiments, a differential amplifier within each cell is the primary stage of the cell&#39;s output amplifier, and PMOS load circuitry including a secondary output amplifier stage is shared by all cells of the column. Preferably, a switchable bias circuit is provided to assert a bias voltage (to the gate of at least one transistor of the load circuitry) whose level depends on the state of a bias control signal. The bias voltage undergoes a transition which rapidly turns off each such transistor of the load circuitry (to reduce power consumption by the array) in response to a transition of the bias control signal from a first level to a second level, and the bias voltage undergoes a transition which causes each such transistor to conduct a desired bias current (needed to amplify fully the photodiode output of each cell to be read) in response to a transition of the bias control signal from the second level to the first level. 
     In preferred embodiments, the two-stage amplifier for each cell is an op amp, and the op amp for each cell comprises NMOS transistors (at least one of which is included in the cell itself) and at least one PMOS transistor, but no PMOS transistor is included within the cell itself. In some such preferred embodiments, at least one op amp is provided with capacitor feedback for increased gain. 
     Another aspect of the invention is an active pixel sensor cell including a differential amplifier (which asserts an amplified signal indicative of a sampled output voltage of the cell&#39;s photodiode) and load circuitry coupled to the cell. The load circuitry includes a secondary amplifier stage (which further amplifies the amplified signal produced within the cell and typically includes at least one PMOS transistor). Preferably, the differential amplifier includes no PMOS transistor. Also preferably, the differential amplifier is the first stage of an op amp, and the remainder of the op amp (including optional capacitor feedback circuitry) is included in the load circuitry. Thus, the remainder of the op amp is coupled to the cell but not included within the cell itself. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of two CMOS active pixel sensor cells (having conventional design) connected along a column of an active pixel sensor cell array, and circuitry for use in reading all cells connected along the column. 
     FIG. 2 is a schematic diagram of two CMOS active pixel sensor cells (which embody the invention) connected along a column of an active pixel sensor cell array, and circuitry which embodies the invention for use in reading all cells connected along the column. 
     FIG. 3 is a simplified schematic diagram of a variation on the FIG. 2 circuit, with only one cell connected along the column. 
     FIG. 4 is a schematic diagram of an alternative embodiment of the invention which is a variation on the FIG. 3 embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 is a schematic diagram of a portion of an active pixel sensor array which embodies the present invention, including active pixel sensor cells  100  and  101  connected along a column of the array, and circuitry (comprising transistors P 1 , P 2 , P 3 , P 4 , N 5 , N 7 , N 8 , N 9 , and N 10  and current sources I 1  and I 2 ) for use in reading cells  100 ,  101 , and all other cells connected along the column. Cell  100  has several structural similarities to cell  10  of FIG.  1 . Thus, the same reference numerals are used in FIGS. 1 and 2 to designate the structures which are common to both cells. In FIG. 2, transistors P 1 , P 2 , P 3 , and P 4  are PMOS transistors, and transistors N 7 , N 8 , N 9 , N 10 , and N 11  are NMOS transistors. 
     With reference to FIG. 2, the column along which cells  100  and  101  are connected has four column lines: line C 1  (to which the drains of N 1  and N 2  and the channel of P 2  are coupled); line C 2  (to which the drain of N 11  and the channel of P 1  are coupled); line C 3  (to which the gate of N 11 , the source of P 4 , and circuit  21  are coupled); and line C 4  (to which the source of N 3  and the drain of N 5  are coupled). In contrast, in the FIG. 1 array, the column along which cells  10  and  11  are connected has only two column lines (one to which the drain of N 1  is coupled and another to which circuit  21  is coupled). In an integrated circuit implementation, the two extra wires needed to fabricate the two extra column lines of the FIG. 2 array can be accommodated with a minimal amount of extra space if three-layer metal is used (so that the two extra wires can be implemented as a third metal layer overlaying either of the other two meal layers). 
     The dashed portions of lines C 1 -C 4  indicate that additional cells (identical to cells  100  and  101 ) will typically also be connected along the array column including cells  100  and  101 . Of course, it is contemplated that the array includes many additional columns (not shown). Thus, the signals ROW SELECT and RESET 2  are provided simultaneously to all the cells connected along the same row as is cell  100  (i.e., to one cell in each column of the array). Similarly, the signals ROW SELECT n  and RESET 2   n  (which have the same function as signals ROW SELECT and RESET 2 , respectively, but are asserted independently from signals ROW SELECT and RESET 2 ) are provided simultaneously to all the cells connected along the same row as is cell  101  (i.e., to one cell in each column of the array). If a separate detection and calculation circuit  21  is provided for each column, one cell from each column can be simultaneously read. 
     In operation, the voltage (RESET 1 ) at the gate of PMOS transistor P 3  (Node  1 ) is pulsed low before each read of a cell (e.g., during a sequential read of all cells connected along the column, Node  1  is pulsed low, then cell  100  is read, then Node  1  is again pulsed low, and then cell  101  is read). In cell  100 , the step of applying a low voltage pulse to Node  1  resets column line C 1  to reference voltage Vref (Vref is typically 3 volts) in order to reduce variation (due to noise) in the voltage at Node  3  during subsequent resetting of photodiode d 1 . 
     Optionally also, a column reset pulse is asserted to the gate of a column reset transistor whose channel is connected to Node  6 , in order to reset Node  6  to a known voltage before each read of a cell by circuit  21  (as described in the above-referenced U.S. patent application entitled “Active Pixel Sensor Cell that Reduces Noise in the Photo Information Extracted from the Cell,” filed on Jun. 9, 1997). 
     After column line C 1 , and optionally also column line C 3  (and thus Node  6 ), have been reset, a read of a cell is performed. For specificity, the following description of such a read is with reference to cell  100 , although it applies (with obvious modifications) to a read of any cell of the array. 
     Initially, the gate (Node  2 ) of cell  100 &#39;s reset transistor N 1  is briefly pulsed with a high level of reset voltage “RESET 2 .” This high level of the reset voltage (typically equal to 5 volts) resets the voltage on photodiode d 1  to an initial integration voltage to begin an image collection cycle. 
     Immediately, after assertion of such pulse of reset voltage “RESET 2 ,” the initial integration voltage on photodiode d 1  (the voltage at Node  3 ) is V ini =VRESET−V TN1 −V CLOCK , where V TN1  is the threshold voltage of transistor N 1 , VRESET is the high level of the voltage signal “RESET 2 ,” and V CLOCK  represents reset noise from the pulsed reset voltage (assumed to be constant). 
     After the reset voltage has been pulsed and the voltage on photodiode d 1  (the voltage at Node  3 ) has been reset, the gate of transistor N 3  (Node  5 ) is pulsed with a high level of row select voltage signal “ROW SELECT.” In accordance with the invention, each sampled photodiode voltage (at Node  3 ) is amplified by a two-stage amplifier to produce an output voltage (at Node  6 ) which is detected and processed by detection and calculation circuit  21 . Specifically, circuit  21  amplifies, digitizes, and stores the value of the amplified initial integration voltage at Node  6 , then amplifies and digitizes the value of the amplified final integration voltage at Node  6 , and subtracts the former digital value from the latter digital value to generate data indicative of the number of photons incident at photodiode d 1  during the image collection cycle (between the two pulses of the “ROW SELECT” voltage). The difference value (V S ) indicative of the difference between the digitized final integration voltage and the digitized initial integration voltage preferably undergoes correlated double sampling (“CDS”) or other conventional post-processing in circuit  21 . 
     The first stage (sometimes referred to herein as the “primary” stage) of the two-stage amplifier comprises NMOS transistors N 2  and N 11  (connected as shown with their sources at Node  4 , the drain of N 2  coupled to column line C 1 , and the drain of N 11  connected to column line C 2  and thus to the common gate and drain of P 1 ) which form a differential amplifier whose tail current flows to ground through NMOS transistors N 3  and N 5 , and PMOS transistors P 1  and P 2  (connected as shown with their gates connected together and their sources held at supply voltage Vcc). Transistors P 1  and P 2  form matched loads for the differential pair N 2 , N 11 . Supply voltage Vcc is typically 5 volts. 
     The output of the first stage (the voltage at the drain of N 2 ) is asserted via column line C 1  to the gate of PMOS transistor P 4 . This output is further amplified in the second gain stage (sometimes referred to herein as the “secondary” stage) which consists of PMOS transistor P 4  and NMOS transistor N 10  connected as shown (with supply voltage Vcc applied to the source of transistor P 4 , the drain of P 4  connected to the drain of N 10 , and the source of N 10  connected to ground). The common drain of transistors P 4  and N 10  (Node  6 ) is the output of the secondary stage. 
     As in the FIG. 1 circuit, the gate of transistor N 3  (Node  5 ) is twice pulsed with a high level of row select voltage “ROW SELECT” to read a cell (a first time to assert an initial integration voltage at Node  6 , and a second time to assert a final integration voltage at Node  6 ). Each time the gate of transistor N 3  (Node  5 ) is pulsed with a high level of row select voltage “ROW SELECT” (assuming transistors N 10  and N 5  are “on” in response to a low voltage at Node  10  and transistor P 3  is off in response to a high voltage at Node  1 ), a partially amplified voltage indicative of the Node  3  voltage (the initial or final integration voltage on photodiode d 1 ) appears on column line C 1  and a fully amplified voltage (also indicative of the Node  3  voltage) appears at Node  6  (on column line C 3 ). 
     Between the assertion of the two pulses of the high level of voltage ROW SELECT at the gate of transistor N 3  (Node  5 ), photons are allowed to strike photodiode d 1 , thereby creating electron-hole pairs. The photogenerated holes are attracted to the ground terminal of photodiode d 1 , and the photogenerated electrons are attracted to the positive terminal of photodiode d 1 , each additional electron reducing the voltage at Node  3 . At the end of this image collection period, the following final integration voltage will be present at Node  3 : V f =V ini −V S =VRESET−V TN1 −V CLOCK −V S , where V S  represents the change in voltage (at Node  3 ) due to the absorbed photons. 
     After a read operation is performed on cell  100 , another cell (e.g., cell  101 ) can be read in essentially the same manner: the cell&#39;s first column line (e.g., line C 1 , for each cell connected along the column that includes cells  100  and  101 ) is initially reset to reference voltage Vref (optionally also the cell&#39;s output column line, e.g., line C 3  for each cell connected along the column that includes cells  100  and  101 , is reset), then the cell&#39;s photodiode is reset, and then two pulses of the high level of voltage ROW SELECT are asserted sequentially to the cell to cause two pulses of tail current to flow from the cell&#39;s differential pair to ground. In response to the two pulses of voltage ROW SELECT, detection and calculation circuit  21  generates data indicative of the number of photons incident at the cell&#39;s photodiode during the image collection period (the time period between the two pulses of the voltage ROW SELECT). 
     The described two-stage amplifier (whose primary stage is the differential amplifier comprised of P 1 , P 2 , N 2 , N 3 , N 11  and N 5 , and whose secondary stage comprises P 4  and N 10 ) can be implemented using conventional CMOS fabrication techniques to have a gain of 10,000 or more. Such a large gain effectively reduces typical error due to pixel gain to 0.01% or less, in the following sense. When many cells are read sequentially after each is exposed to identical incident light energy, the error in the output voltage at Node  6  due to systematic and random variation among the characteristics of the cells is not more than 0.01% of the average output voltage at Node  6  (averaged over all cells). 
     In accordance with the invention, a first part of the two-stage amplifier for each cell is implemented within the cell itself (e.g., elements N 1 , N 2 , N 3  and N 11  within cell  100 ) and another part of the two-stage amplifier is implemented outside the cell (e.g., elements P 1 , P 2 , P 4 , N 5  and N 10 ). The latter portion of the two-stage amplifier is shared by all the cells connected along a single column of an array. 
     Still with reference to FIG. 2, the function of the circuit comprising elements N 8 , N 9 , N 7 , I 1 , and I 2  is to turn off transistors N 5  and N 10  rapidly when desired and to bias them when desired to sink current from their drains. It is desirable to conserve power by turning transistors N 5  and N 10  off rapidly at times when it is not necessary to sink current through them. The channel of NMOS transistor N 7  is connected between supply voltage Vcc and Node  10 , and current sink I 2  is connected between Node  10  and ground. The gate of N 7  is connected to one channel terminal of NMOS transistor N 8 , and the other channel terminal of transistor N 8  is connected to the drain of NMOS transistor N 9 . The source of transistor N 9  is connected to ground and the gate of transistor N 9  is connected to Node  10 . Current source I 1  is connected between supply voltage Vcc and the drain of transistor N 9 . 
     The level of the voltage “bias on” asserted to the gate of transistor N 8  controls the state of transistors N 5  and N 6  as follows. When “bias on” switches from a low level (e.g., ground potential) to a high level (e.g., 5 volts), elements I 1 , N 9 , N 8 , and N 7  function (with transistors N 10  and N 5 ) as a current mirror to cause Node  10  to rise rapidly to a level which biases transistors N 5  and N 10  to sink a level of current appropriate for reading a cell (a cell connected along the column comprising cells  100  and  101 ). When “bias on” switches from a high level (e.g., 5 volts) to a low level (e.g., ground potential) voltage, Node  10  is rapidly pulled down to ground potential, thereby rapidly turning off transistors N 5  and N 10 . 
     Optionally, elements I 1 , I 2 , N 7 , N 8 , and N 9  of the FIG. 2 embodiment (and variations thereon) are replaced by conventional bias circuitry such as the circuit comprising current source I 1  and transistor N 4  of FIG.  1 . 
     FIG. 3 is a simplified schematic diagram of a variation on the FIG. 2 circuit, with only one cell connected along the column. The FIG. 3 circuit is identical to that shown in FIG. 2 except that the column includes only one cell (cell  100 ). In FIG. 3, the op amp is implemented by elements P 1 , P 2 , N 2 , N 3 , N 11 , P 4 , N 5 , N 7 , N 8 , N 9 , I 1 , I 2 , and N 5  of FIG. 2 (connected as shown in FIG.  2 ). 
     FIG. 4 is a variation on the FIG. 3 embodiment, which includes capacitor feedback circuitry (capacitors CAP 1  and CAP 2 , and NMOS transistor N 12 ) for increasing the gain of the op amp. The other components of the FIG. 4 circuit are identical to the corresponding, identically identified components of the FIG. 3 circuit and the above description of them with reference to FIGS. 2 and 3 will not be repeated with reference to FIG.  4 . In the FIG. 4 circuit, capacitor CAP 1  is connected between output node  6  and the gate of transistor N 3  (shown in FIG. 2) of the op amp, and capacitor CAP 2  is connected between the gate of transistor N 3  and ground. As shown in FIG. 4, capacitors CAP 1  and CAP 2  provide an additional gain of [(CAP 1 +CAP 2 )/CAP 1 ] to the output of the cell. 
     In FIG. 4, the channel of transistor N 12  and capacitor CAP 1  are connected in parallel between output node  6  and the gate of transistor N 3  of the op amp. The gate of transistor N 12  receives control voltage V 3 , whose level is varied to switch transistor N 12  off and on at desired times during operation. 
     It should be understood that it will typically be desirable to implement any of the embodiments of the inventive cell array so that different gain is provided from cell to cell by the “in-cell” portions of the two-stage amplifiers (the amplifier portion within each cell of the array). For example, the FIG. 2 embodiment can be implemented with the characteristics of transistors N 2 , N 3 , and N 11  within cell  100  being different from the characteristics of the corresponding transistors within cell  101 , so that different “primary stage” gain provided at the gate of transistor P 4  when reading cell  100  than is provided at the gate of transistor P 4  when reading cell  101 . This may be desirable for example, when the output of cell  100  indicates intensity of blue light incident at cell  100 &#39;s photodiode and the output of cell  101  indicates intensity of red light incident at cell lolls photodiode (since a photodiode will typically have different response to red light than to blue light, and thus it is desirable to compensate for such different response by the amount of “primary stage” gain provided in the cell). 
     It should be understood that various alternatives to the embodiment of the invention described herein may be employed in practicing the invention. For example, the concepts of the present invention can readily be applied to a row of cells in an array of cells. With an array of cells, a latch/column sense amplifier and a current mirror are utilized with each column of cells. 
     Thus, it is intended that the following claims define the scope of the invention and that structures within the scope of these claims and their equivalents be covered thereby.