Abstract:
A photocell system includes a current control circuit that provides an offset voltage. Each photocell in a photocell array includes an opto-electrical converter receives the offset voltage such that the opto-electrical converter establishes a DC operating point. In one embodiment, each photocell includes the current control circuit. In an alternate embodiment, a single current control circuit provides the offset voltage for the entire photocell array.

Description:
BACKGROUND  
         [0001]    High gain photo cells based on the work of Baumgartner, et al. in U.S. Pat. No. 5,769,384, “Low differential light level photoreceptors”, and improved by Knee, et al. in U.S. Pat. No. 6,104,020, “Electronic shutter for a low differential light level photo-receiver cell”, both assigned to Agilent Technologies, Inc., have been extensively used in the area of optical navigation. The photocells have an optically biased PNP bipolar transistor that serves as both the light to current converter as well as the first gain stage in the signal processing system. FIG. 1 illustrates a prior art photocell. The PNP transistor  40  is shown with the parasitic base-collector capacitor  38  formed by the reversed bias base-collector diode  34 .  
           [0002]    [0002]FIG. 2 (prior art) illustrates an emitter-base diode that must be forward biased to place the transistor in the forward active region. When light is sensed, there is a large DC component and a small AC component that establishes the operating point of the transistor. The AC component modulates the current in the emitter due to the high current gain at this operating point. The disadvantage of this circuit for pulsed light operation is that the signal current is consumed in reestablishing the forward bias on the base-emitter diode and is not available for integration on the signal capacitor  30  until the operating point of the PNP in the forward region is reestablished. The only current available to re-bias the transistor is the optically generated current corresponding to the input signal.  
           [0003]    [0003]FIG. 3 present the integrated voltage on capacitor  30  versus time for DC and pulsed light conditions. Assuming an initial DC voltage on the capacitor  30  of Vref. For the DC case, there is a linear ramp down in time. For the pulsed case, there is a time T rb  that is needed to rebias the transistor before the slope on the discharge rate is the same as the DC slope. During T rb , the current generated from the input signal is used to rebias the PNP transistor  40  and is not available for processing by the downstream circuitry. The amount of charge that is needed to rebias the PNP transistor  40  back to its operating point is to the first order constant. Therefore, the time Trb is inversely proportional to the signal current generated in the base. In an application, e.g. optical mouse, the light is provided by driving a light emitting diode (LED) to illuminate the surface. The need to preflash the LED to reestablish the bias point is a large power consuming function. In battery application, this is a major limitation on the time between battery replacements.  
         SUMMARY  
         [0004]    The present invention improves the pulsed light response of PNP transistor based photodetector by keeping the DC operating point established when no light is present. This is achieved by placing a current source from the base node of the PNP during but not limited to periods of no light thereby keeping the transistor in the forward bias region.  
           [0005]    A photocell system includes a current control circuit that provides an bias voltage. Each photocell in a photocell array includes an opto-electrical converter receives the bias voltage such that the opto-electrical converter establishes a DC operating point. In one embodiment, each photocell includes the current control circuit. In an alternate embodiment, a single current control circuit provides the bias voltage for the entire photocell array. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 illustrates a photocell of the prior art.  
         [0007]    [0007]FIG. 2 illustrates an emitter-based diode of the prior art.  
         [0008]    [0008]FIG. 3 shows the integrated voltage on a capacitor versus time for DC and pulsed light conditions.  
         [0009]    [0009]FIG. 4 illustrates a block diagram of a photoelement circuit according to the present invention.  
         [0010]    [0010]FIG. 5 illustrates a schematic diagram of the current source shown in FIG. 4.  
         [0011]    [0011]FIG. 6 illustrates another schematic diagram of the current source shown in FIG. 4. 
     
    
     DETAILED DESCRIPTION  
       [0012]    The present invention improves the pulsed light response of high gain pixels in with reduced image degradation and with greater energy efficiency as compared to utilizing the light to rebias the pixels. This improved PNP based photodetector has the DC operating point maintained when no light is present. This is achieved by placing a current source from the base node to AGND when the light is off. The magnitude of the current source should be close to the magnitude of the base current that flows when the photocell has optical stimulus to minimize the readjustment period.  
         [0013]    [0013]FIG. 4 illustrates a block diagram of a photoelement circuit according to the present invention. Incoming light  37  is converted into a current that is integrated during a sample period. The stored value is periodically read out, so that it is available for a next step in a processing sequence. At the start of an integration cycle, photocurrent that is generated by means of a photodiode  34  is amplified by a PNP transistor  36 . The photodiode and the transistor, along with a parasitic capacitance  38 , define a photoelement  40 . The amplified photocurrent discharges the integration capacitor  30  downwardly This change in voltage on the capcitor  30  represents the charge that will be processed by the later stages of circuitry.  
         [0014]    The photodiode  34  of the photoelement  40  generates a current in response to reception of light photons. The photodiode is connected to the base of the PNP transistor  36 . The reverse bias diode capacitance  38  is a parasitic capacitance that is quite large for large area collectors. The optical signal illuminating the photodiode causes a small current, for example about 0.1 nA, in the diode current source. Because of the low current level, amplification is needed to ensure that the optical variation signal, which is only approximately six percent of the constant photocurrent, will create sufficient voltage differences to be distinguished from noise.  
         [0015]    The PNP transistor  36  of the photoelement  40  amplifies the photocurrent. The amplification provided by the transistor allows use of an integration capacitor  30  that facilitates reproducibility from photoelement to photoelement. In the absence of amplification, the low current from the photodiode  34  would require a very small capacitor as an integrator in order to get a 1 volt swing, e.g. 10 pF. Because of the parasitics, this would be difficult to reproduce on an element-to-element basis. Changing the photoelement layout from a diode to a substrate PNP device is a convenient way of providing current amplification. A beta value of nineteen increases the output emitter current to  2  nA. Thus, an integration capacitor of 0.20 pF can be used. This facilitates reproducibility, but is not so large as to require excessive area.  
         [0016]    The present invention places a current source  70  from the base node of MOS transistor  50  to AGND. The magnitude of the current source is close to the magnitude of the base current that flows when the photocell has stimulus. This current source can be realized by a NFET with a gate voltage of Vlb. This “holding” current allows the base-emitter junction of the PNP transistor  36  to maintain a DC operating point without the need to use light.  
         [0017]    When the light signal is present, the current source  70  can be switched off by pulling Vlb to GND, so that the signal that is integrated on the capacitor  30  is only based on the actual light signal and not the holding current. Alternatively, the current source  70  can be kept on and holding current component of the signal integrated on the capacitor  30  is treated as a DC offset.  
         [0018]    A servo circuit  29  is formed by MOS transistors  50  and  52 . The MOS transistors form a bias point amplifier with a common gate stage for the output of the phototransistor  36 . MOS transistor  54  supplies the bias current via analog power supply line AVDD when the proper signal is received from bias voltage line PBB. To achieve proper transfer of the current generated in the photoelement  40  to the integration capacitor  30 , the photodiode reverse voltage (i.e., the transistor base voltage) must be kept at a substantially constant level. If the voltage at the base node  56  were to be allowed to shift, the photocurrent would be at least partially consumed in charging and discharging the diode capacitance  38 , rather than providing current to be amplified by the substrate PNP transistor  36 . The need to keep the base node fixed is the reason that a switchable current source was chosen to keep the PNP transistor  34  biased during no light. The high impedence of the current source allows this device to be present and not disturb the base node.  
         [0019]    The transistor base voltage at the node  56  is kept at a substantially fixed level by the three MOS transistors  50 ,  52  and  51 . While not critical to achieving the desired operation, in the embodiment of FIG. 3, the substantially fixed voltage level is approximately equal to the NMOS threshold level above AVDD at the collector node  58 . The MOS transistors operate as a negative feedback loop by means of transistor  52  operating as a source follower to the emitter node  62  of the PNP transistor  36 . Thus, the base voltage is controlled by the emitter voltage of the transistor. This is possible because the base voltage, i.e. the photodiode output, has a very high DC impedance level. The bias technique of emitter control operated effectively during testing. From the output perspective, transistor  52  is a common gate stage that has an added advantage of providing additional isolation of the emitter node  62  and the base node  56  of the transistor from the voltage swing of a node  64 .  
         [0020]    The electronic shutter  31  is formed from NMOS transistor  53  and PMOS transistor  55 . When a signal from input node SHUTTER is at a logic low, transistor  55  is in a conducting mode and transistor  53  is in a non-conducting mode. In this state, the servo circuit continues to discharge the integration capacitor  30 . In this mode, electronic shutter circuit  31  is in an “open” mode. When a high logic signal is received from node SHUTTER, then transistor  53  is in a conducting mode and transistor  55  is in a non-conducting mode. In this state the photo-generated emitter current from photoelement  40  is diverted from the integration capacitor  30  to the power supply line AVDD. In this mode, electronic shutter circuit  31  is in a closed mode and integration capacitor  31  is not integrating any photo-generated charge. Thus, to end an integration period, electronic shutter circuit configured into a “closed” mode. During the closed mode integration capacitor  30  holds its charge until it is read. In this period the light signal can be removed and the bias current is maintained by the current source  70 .  
         [0021]    At the end of a read period, a read switch  42  is turned “on” via a logic signal from line NRD to output the charge stored on the integration capacitor  30  to a transfer amplifier (not shown) via node OUT. The read switch may be a PMOS transistor that is controlled by a read control line NRD. In this manner, integration capacitor  30  holds its charge until it is read after the shutter circuit is in a closed mode until it is read via read transistor  42 . The operation of the transfer amplifier (not shown) pulls the node  64  back to the reset voltage. This accomplishes the transfer of the charge signal to transfer amplifier circuitry (not shown). At the conclusion of the transfer process, the read control line NRD is caused to return to a logic low and transistor  42  returns to a non-conducting state.  
         [0022]    [0022]FIG. 5 illustrates a schematic diagram of the current source  70  shown in FIG. 4. The current source  70  gate node includes an optional switching feature. This current source provides a global solution/control for an array of photocells.  
         [0023]    A first current mirror is formed by FETs P 1  and P 2 . FET P 1  has a width of M while FET P 2  has a width of N. The first current mirror connects between the positive supply rail VDD and a current source I ref . The current source I ref  is further connected to GND. A second current mirror is formed by FETs N 1  and N 2 . FET N 1  has a width of R while FET N 2  has a width of Q. The second current mirror connects to the drain of FET P 2  in the 1 st  mirror and GND. The source of FET N 2  provides the global bias voltage Vlb to the photocell array. The optional switch  72  while shown within the second current mirror could be placed anywhere within the current path.  
         [0024]    The current mirrors defined by P 1 :P 2  and N 1 :N 2  need not have a 1:1 ratio. The devices P 1 :P 2  and N 1 :NQ form a current multiplier if I ref  of Iref*(N*Q)/(M*R). The sample switch is optional depending on the mode of operation, e.g. switched or constant.  
         [0025]    In this implementation the current I lb  based on a DC I ref  is not tied to an optical input. The current may be fixed or programmable. The current source I ref  may be adjusted via digital hardware (not shown) based on images and algorithms to optimize the type of response desired from the array. Alternatively, the current source I ref  could be generated from an optical detector, e.g. a single diode detector in or near the array or from multiple detectors that are averaged together to get the average light level seen by the array.  
         [0026]    [0026]FIG. 6 illustrates an alternate embodiment for the present invention. In this embodiment, the current source is integrated into the individual photocell. This allows each DC operating point to be based on the last optical operating point for each pixel.  
         [0027]    In operation, the current through the PNP Q 1  is sensed via the PMOS mirror P 4 : P 3  and voltage on node CCN is sampled and held via N 7  onto node CCB. when the light is removed, the current source N 6  is turned on by transferring the sampled voltage on CCB to node CCa after N 9  is turned off. This allows the base node to remain biased to a base current of emitter current during sampling divided by the ratio of the mirrors P 4 :P 3  and N 5 :N 6 . The sample and hold circuit formed by N 7 , N 8 , and CCC is to keep the circuit from going into a positive feedback mode and locking to the rail.