Patent Publication Number: US-7223957-B2

Title: Sensor including circuitry for recovering time-varying information and removing DC offsets

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
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   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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   FIELD OF THE INVENTION 
   The present invention relates to sensors and, more particularly, relates to circuits that process and amplify incoming signals such as pulse signals received from sensing devices. 
   BACKGROUND OF THE INVENTION 
   A variety of types of sensors exist for application in a wide variety of situations. Among these sensors are, for example, photodetectors/photosensors, infrared sensors, laser sensors, microwave sensors, proximity sensors, ultrasonic sensors, inductive sensors, magnetic sensors, among others. Many of these sensors operate by sensing/receiving analog signal inputs. The sensors in turn typically process these analog signal inputs in various ways. 
   In particular with respect to photodetectors, for example, such devices are employed in a wide variety of applications for a wide variety of purposes. In some embodiments, a light signal is provided by a light emitting device at one position and a photodetector is employed at another position to detect whether that light signal has been interrupted or not, either because the light signal is being turned on and off or because something has cut or interrupted the light path between the light emitting device and the photosensitive device. Photodetectors implemented in this manner can be utilized in a variety of applications such as industrial conveyor systems, in which it typically is necessary to detect whether items being conveyed have passed into or left a given region along the conveyor system, or in industrial systems that are designed to determine whether particular conditions are or are not met (e.g., light curtains). 
   In many applications, information is conveyed from a light emitting device to a photodetector by rapidly switching or pulsing the light emitting device on and off so. Depending upon the circumstance, this pulsed signal can take the form of a square wave, the form of an AC (or effectively-AC) signal, or some other form. Based upon the frequency of the pulsing, the duration of the pulses, the magnitude of the pulses, the duty cycle, and a variety of other factors (e.g., possibly, the color of the light being transmitted), a variety of information can be transmitted to the photodetector. The coding of this information can involve, for example, amplitude-modulation, frequency-modulation, phase-modulation, polarity-modulation. 
   Due to the many uses of photodetector circuits, such circuits have become ubiquitous. To reduce the circuits&#39; size and cost, the circuits have increasingly been implemented in the form of integrated circuits rather than out of discrete components. Despite such size and cost improvements, however, conventional photodetector circuits nevertheless suffer from certain inadequacies. First, to the extent that the pulsed or AC information received by the photodetector contains information that is of interest, it is necessary that the AC information be recoverable. Yet conventional recovery circuits, such as conventional rectification or peak detection circuits, typically utilize diodes or transistors that have significant forward-conductive voltage drops (e.g., 0.7 Volts) across them. Consequently, the resulting signals output by those recovery circuits include an undesirable offset. Further, to the extent that such recovery circuits provide an output signal that represents both the positive (e.g., positive with respect to a neutral level of the AC signal) and the negative (e.g., negative with respect to the neutral level) swings of the received signal, discontinuities are created at the cross-over points between the positive and negative portions of the output signal as a result of the forward-conductive voltage drops. 
   Additionally, regardless of the aforementioned issues relating to the forward-conductive voltage drops within recovery circuits, conventional photodetectors have additional inadequacies. In particular, it is common that the AC signals received by photodetectors include a DC offset. This offset, which can be magnified during propagation within the photodetector circuit, can significantly distort the resulting output signal. Although some conventional photodetector circuits employ DC offset removal circuitry to address this problem, conventional removal circuitry typically involves the use of bypassing or decoupling capacitors that are too large for practical implementation on integrated circuits. Consequently, conventional photodetector circuits having DC offset removal circuitry, when implemented on integrated circuits, typically require discrete capacitors coupled to the integrated circuits. The use of these discrete capacitors increases manufacturing costs and can impact robustness. 
   Further, to the extent that any DC offset may have been introduced into the signal received by the photodetector circuit itself rather than introduced as part of the input to the photodetector circuit, conventional DC offset removal circuitry fails to eliminate such DC offsets. Thus, even though conventional DC offset removal circuitry does ameliorate the DC offset problem (albeit through the use of discrete capacitors), such conventional circuitry cannot by its nature eliminate all DC offsets. 
   Still another disadvantage associated with conventional photodetector circuits generally is that it can be relatively difficult in practice for technicians to calibrate the circuits. Photodetector circuits commonly are implemented in situations where it is important that the circuits be capable of differentiating between high and low levels of light corresponding effectively to “on” or “off”. During setup of the photodetector circuits, the circuits are exposed to levels of light intended to be representative of levels that are likely to be experienced in practice, and the gain or amplification of the circuits is then adjusted/calibrated so as to arrive at an output signal that is representative of the light exposure. The calibration process should result in an amplification level that provides a strong output signal but at the same time does not excessively exaggerate unwanted signal components, particularly noise. 
   A common conventional practice for conducting this calibration is for a technician to hold down a button for a specific period of time during the calibration process to, where the period of time determines the eventual amount of gain. For example, by holding down the button for an amount of time lower than a threshold, the amplification might be set to one level and, by holding down the button for an amount of time higher than the threshold, the amplification might be set to a second, different level. While this procedure has been used in practice, the procedure has proven to be somewhat unreliable, since the amount of gain is dependent upon the skill of the technician performing the adjustment, for example, upon the ability of the technician to hold down the button for an appropriate amount of time. As a result, it is sometimes if not often difficult to achieve consistency in the calibration of photodetectors, particularly insofar as calibrations can be performed differently by different technicians. 
   In view of the above, it would be advantageous if a new photodetector could be developed that addressed one or more of the inadequacies associated with conventional photodetectors. In particular, it would be advantageous if a new photodetector circuit could have an AC recovery circuit that successfully recovered AC information from an introduced signal without introducing significant distortions into that information due to diode-type voltage drops within the AC recovery circuit. It also would be advantageous if a new photodetector circuit could be designed that was capable of lessening or entirely eliminating DC offsets introduced to the photodetector circuit in the signals input thereto, where such DC offset removal circuitry could be more easily implemented on integrated circuits without the use of large, discrete capacitor components. It further would be advantageous if such DC offset removal circuitry not only served to reduce or eliminate DC offsets introduced by the signals input to the photodetector circuits, but also served to reduce or eliminate additional DC offsets introduced by internal operation of the photodetector circuits themselves. It additionally would be advantageous if the calibration process of photodetector circuits could be improved to reduce the difficulty with which technicians perform the process and improve the repeatability of the calibration process. It would likewise be advantageous if similar deficiencies to those discussed above with respect to photodetectors found in other types of sensors could similarly be ameliorated or eliminated. 
   BRIEF SUMMARY OF THE INVENTION 
   The present inventor has recognized the desirability of an improved amplifier circuit, for use in photodetectors and other devices as well, in which a time-varying portion of an input or amplified signal could be extracted without the introduction of substantial discontinuities into the resulting output signal due to the extraction process, and/or in which the resulting output signal did not have significant distortion due to DC offsets. The inventor has further recognized that, in at least some embodiments, an improved amplifier circuit that avoided the introduction of such discontinuities could be implemented by including a current splitter as the extraction circuit, where the current splitter was formed from the combination of an operational amplifier and a pair of matched, inverted MOSFETs. The inventor has additionally recognized that, in at least some of these or other embodiments, an improved amplifier circuit providing an output signal without significant DC offsets could be implemented, entirely on an integrated circuit, by incorporating an additional feedback circuit between the input and output of the main amplifier of the amplifier circuit. 
   In particular, the present invention relates to a sensor circuit that includes a first amplifier that receives an input signal at least indirectly from a sensing device and provides an amplified signal based upon the input signal, where the amplified signal at least includes a time-varying signal component, and a recovery circuit that receives the amplified signal and provides an output signal indicative of at least a portion of the time-varying signal component of the amplified signal. The recovery circuit includes a current splitting component that operates to generate negative and positive signals indicative respectively of negative and positive portions of the time-varying signal component, and the negative and positive signals are generated in a manner such that there are no substantial discontinuities between those signals at crossover points between those signals. 
   Further, the present invention relates to an amplification circuit that includes a first amplifier that receives an input signal at least indirectly from another device and provides an amplified signal based upon the input signal, where each the amplified signal at least includes a time-varying signal component. The amplification circuit additionally includes at least one of means for recovering at least a portion of the time-varying signal component, and means for removing a DC offset that is at least one of provided with the input signal and introduced by the means for recovering. 
   Additionally, the present invention relates to a method of amplifying a time-varying signal. The method includes receiving the time-varying signal at an input port of a first amplifier, and outputting an amplified time-varying signal an output port of the first amplifier. Further, the method includes at least one of extracting at least a portion of time-varying information within the amplified time-varying signal by way of a current-splitting circuit having a second amplifier and two complementary transistor devices, where an input terminal of the second amplifier is provided at least indirectly with the amplified time-varying signal and a signal indicative of an extracted portion of the time-varying information is output by at least one of the transistor devices, and removing a DC offset by way of a DC offset removal circuit having a third amplifier and a capacitor coupled between input and output terminals of the third amplifier, where the output terminal of the third amplifier is coupled at least indirectly to the input port of the first amplifier. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an exemplary photodetector; 
       FIG. 2  shows two exemplary sections of a modular conveyor system that each employ the exemplary photodetector of  FIG. 1  in one exemplary application of that photodetector; 
       FIG. 3  shows an exemplary photodetector circuit that is capable of recovering AC information from a received input signal, and that can be implemented in the photodetector of  FIG. 1 ; 
       FIG. 4  shows an exemplary waveform showing that positive and negative portions of the recovered AC information in the photodetector circuit of  FIG. 3  have aligned cross-over points so as to avoid significant discontinuities in the overall output signal; 
       FIG. 5  shows an additional exemplary photodetector circuit similar to that of  FIG. 3  except insofar as the photodetector circuit of  FIG. 4  also includes DC offset removal circuitry; 
       FIG. 6  shows steps of an exemplary calibration process capable of being implemented on certain embodiments of the photodetector circuits of  FIGS. 1–3  and  5 ; and 
       FIG. 7  shows steps of an additional exemplary procedure capable of being implemented on certain embodiments of the photodetector circuits of  FIGS. 1–3  and  5 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , an exemplary photodetector  10  is shown in schematic form. The photodetector  10  has a housing  12  that supports at least one photosensitive device  14  such as a photosensitive semiconductor device. Also inside the housing  12  is circuitry  11  as well as possibly a power supply (not shown). In preferred embodiments, the circuitry  11  is primarily or exclusively implemented on one or more integrated circuits, for example, application specific integrated circuits (ASICs) or other microchips, albeit in other embodiments the circuitry could be formed from discrete components as well. 
   Depending upon the embodiment, the circuitry  11  can perform any of a variety of functions including, for example, control functions relating to the control and operation of the photosensitive device  12  and processing of the signals received therefrom. Also, as discussed in further detail below, the circuitry  11  is capable of certain signal processing functions such as amplification of signals received from the photosensitive device  12 . The portion(s) of the circuitry performing control functions can also control such signal processing functions in particular. 
   The photodetector  10  can supply one or more output signals at an output terminal  16  as well as, in some embodiments, receive input signals at one or more input terminals. For example, the photodetector  10  can as shown in  FIG. 1  have one or more buttons such as a gain-set button  15  and light/dark (L/D) button  13 , which are discussed in more detail below. In alternate embodiments, communication of the photodetector  10  with operators/technicians and/or outside machines and other entities can be achieved by way of wireless communications devices including, for example, RFID devices. 
   In the embodiment shown, the photodetector  10  also in particular includes three light emitting diode outputs that serve as indicator lights to operators/technicians who may be installing or operating the photodetector, namely, a “power” LED  17 , a “set” LED  18 , and an “on” LED  19 . In alternate embodiments, none of the LEDs  17 – 19  need be present, or one or more of those LEDs or other LEDs, or other output indicator devices (e.g., devices capable of providing sounds such as “beeps” to an operator/technician), can be provided. 
   Turning to  FIG. 2 , the photodetector  10  of  FIG. 1  can be implemented in a variety of situations and in relation to a variety of applications. One exemplary application of the photodetector  10  is in a conveyor system  20  such as those commonly employed in industrial/manufacturing environments. As shown, the conveyor system  20  includes multiple conveyor sections or modules, such as a first conveyor section  22  and a second conveyor section  24 . Further as shown, the first and second conveyor sections  22  and  24  respectively include pluralities of rollers  26  and  28 , respectively, as well as first and second light sources  30  and  32 , respectively, and also first and second photodetectors  34  and  36 , respectively (each of which could be the photodetector  10  of  FIG. 1 ). 
   The light sources  30  and  32 , each of which has its own photoemission or light emitting device  38  (e.g., a conventional light bulb or a laser), are capable of being turned on and off in a controlled manner. Indeed, depending upon the embodiment, the intensity of the light emitted by the light sources  30 ,  32  can be varied continuously and/or controlled in a time-varying manner. For example, the intensity of the light emitted by the light sources  30 ,  32  could be controlled to take the form of a square wave, a sine wave, a triangular wave, or a pulsed signal of high, medium, low, or variable duty cycle. Further, depending upon the presence or absence of one or more objects on the rollers  26 ,  28  between the respective pairs of light emitting devices  30 ,  32  and photo detectors  34 ,  36 , the paths of light between the light emitting devices  38  and the photodetectors  34 ,  36  could be uninterrupted or interrupted, as the case may be. 
   The conveyor system  20  represented by the pair of conveyor sections  22 ,  24  of  FIG. 2  is only intended to be exemplary of one application in which photodetectors such as the photo detector  10  could be implemented, and it should be understood that such photo detectors could also be employed in a wide variety of other industrial, residential, security, office, agricultural, construction, and other environments and applications. For example, the photodetector  10  could be employed in conjunction with a security system where interruption of light to the photodetector signifies the presence of an intruder, or in conjunction with a garage door opener of a residential home where interruption of light to the photodetector signifies the presence of an obstruction that might preclude proper closing of the garage door. 
   Referring to  FIG. 3 , a portion of the circuitry  11  of the photodetector  10  that is of particular interest is shown in simplified schematic form, as circuitry  40 . As discussed further below, the circuitry  40  in particular serves the purpose of allowing the photodetector  10  to amplify time-varying signals received from the photosensitive device  14 . It will be understood to those of ordinary skill in the art that additional components for additional purposes can be added to the circuitry  40  of  FIG. 3 ; nevertheless, the circuitry shown in  FIG. 3  is intended to show certain improvements to photodetector circuitry in accordance with at least some embodiments of the present invention. 
   As shown in  FIG. 3 , the circuitry  40  includes a first amplifier  42  that is capable of receiving input signals from the photosensitive device  14  (see  FIG. 1 ). In the embodiment shown, the first amplifier  42  is an operational amplifier in a shunt configuration that receives current signals from the photosensitive device  14 . In particular, a first terminal  44  of the first amplifier  42  receives current from the photosensitive device  14  and then that current is returned via a second, return terminal  46 . An output signal  48  is provided from the first amplifier  42  based upon the current flow into and out of terminals  44  and  46 , and this output signal  48  is provided then to a recovery circuit  50 , which as discussed below is intended to recover or extract all or a portion of the time-varying (or “AC”) component(s) of the output signal  48 . 
   As shown, the recovery circuitry  50  includes first and second complementary (or “balanced” or “mirrored”) metal-oxide-semiconductor field-effect-transistors (MOSFETs)  52  and  54 , where the first MOSFET  52  is a P-channel MOSFET and the second MOSFET  54  is an N-channel MOSFET. As shown, the output signal  48  is provided to a junction  51  to which each of the sources of the first and second MOSFETS  52 ,  54  is coupled. Additionally, the junction  51  is coupled to an inverting input of a second amplifier  56  that also is an operational amplifier. As shown, the non-inverting input of the second amplifier  56  is coupled to a reference voltage  58  while an output  60  of the second amplifier is coupled to a junction  62 , to which each of the gates of the first and second MOSFETs  52 ,  54  is coupled. 
   Further as shown, the current output from a drain  64  of the second MOSFET  54  is directed to a ground  66 , while the current output from a drain  68  of the first MOSFET  52  is provided to a third MOSFET  70 . The third MOSFET  70  is part of a current mirror circuit  72  formed by the combination of that third MOSFET along with a fourth MOSFET  74 . As shown, each of the third and fourth MOSFETs  70 ,  74  is a N-channel MOSFET, and the sources of both MOSFETs are coupled to a supply voltage  76 . Also, the gates of the third and fourth MOSFETs  70 ,  74 , are coupled to one another, and also the drain of the third MOSFET is coupled to its gate. A drain  78  of the fourth MOSFET  74  is coupled to a resistor  80 , which is coupled between the drain  78  and the ground  66 . The voltage across the resistor  80 , provided at an output terminal  82 , constitutes the output of the circuitry  50 . This voltage is determined by the current flowing through the resistor  80 , which in turn due to the functioning of the current mirror circuit  72  is equal to that of the current flowing out of the drain  68  toward the third MOSFET  70 . 
   The first and second MOSFETs  52  and  54  together with the second amplifier  56  operate as a current splitter circuit  49 . As such, the current flowing with respect to the drain  68  of the first MOSFET  52  is related to or representative of the positive portion of the time-varying output signal  48 , e.g., the portion of the time-varying output signal that is above a zero or neutral level of that output signal. Similarly, the current flowing with respect to the drain  64  of the second MOSFET  54  is related to or representative of the negative portion of the time-varying output signal  48 , e.g., the portion of the time-varying output signal that is below a zero or neutral level of that output signal. The neutral level is not necessarily a zero-voltage (or current) level, but rather typically can be understood as the level at which the area formed between the positive portion of the time-varying output signal and that level would be equal, over time, to the area formed between the negative portion of the time-varying output signal and that level. 
   The recovery circuitry  50  shown in  FIG. 3  is specifically configured to provide the output  82  that is representative of the positive portion (e.g., above the neutral level) of the time-varying output signal  48  received from the amplifier  42 , but not the negative portion (e.g., below the neutral level). As such, the recovery circuitry  50  functions somewhat like a half-wave rectifier, with the information corresponding to the negative portion of the time-varying output signal  48  being shunted to ground via the drain  64 . Half-wave rectification is often of interest by itself insofar as, in many applications, only the pulsing on of the light source sending light to the photo detector is of interest, and further because typically the pulsing on of that light source occupies a relatively small proportion of the overall time of operation (e.g., the light transmission is a low duty cycle signal). 
   Nevertheless, because the first and second MOSFETs  52 ,  54 , are complementary, in alternate embodiments a current mirror circuit similar to the current mirror circuit  72  could be coupled to the drain  64  of the second MOSFET  54  so as to provide an additional output (not shown) representative of the current flowing with respect to the drain  64 , which in turn would be representative of the negative portion of the time-varying output signal  48 . (Such an additional current mirror circuit is largely shown in  FIG. 5 , albeit in that example the second mirror circuit is not being employed for the purpose of providing an additional output.) Thus, while the AC recovery circuitry  50  and the embodiment of  FIG. 3  operates similarly to a half-wave rectifier insofar as only the positive portion of the time-varying output signal  48  received from the amplifier  42  is reflected in the output of the circuitry  50 , the circuitry could readily be modified to operate as a full-wave rectifier. 
   The recovery circuit  50  shown in  FIG. 3  is advantageous in comparison with conventional recovery circuitry because it recovers the time-varying information provided from the amplifier  42  in a manner that is less distorted than is the case when conventional recovery circuitry is utilized. In particular, through the use of the first and second MOSFETs  52  and  54 , and the second amplifier  56 , the output current provided by the drain  68  (as well as at the drain  64 ) is not distorted due to the introduction of one or more diode-type forward biased voltage drops (e.g., 0.7 volt). Consequently, the output voltage at output terminal  82  is more closely reflective of the positive portion of the time-varying output signal  48  than would be the case if conventional recovery circuitry were employed. 
   Although  FIG. 3  shows the use of MOSFETs  52  and  54 , other transistor-type devices could also be employed. For example, in one alternate embodiment the first, P-channel MOSFET  52  could be replaced with a PNP bipolar junction transistor (BJT), while the second, N-channel MOSFET  54  could be replaced with a NPN BJT. In such an embodiment, the bases of the BJTs would be coupled to the junction  62 , while the emitters of the BJTs would be coupled to the junction  51  and the collectors of the BJTs would provide the output currents similar to the drains  68 ,  64 . In such embodiment, BJTs could also be employed in the current mirror circuit  72  as well. Further, in additional embodiments, a variety of different combinations of MOSFETs, BJTs, and other transistor or switching devices could be employed. 
     FIG. 4  shows in a graphical manner the advantageous, relatively undistorted output signals that are provided by the recovery circuitry  50  in accordance with the present design. In particular,  FIG. 4  shows both the positive and negative recovered information provided by the current splitter circuit  49 . As shown, the recovered positive time-varying information associated with the drain current at the drain  68  of the first MOSFET  52  (I d (M 1 )) meets up almost continuously with the negative recovered time-varying information associated with the drain current at the drain  64  of the second MOSFET  54  (I d (M 2 )), without any significant discontinuities between the two portions of the information at crossover points  65 . Thus, were the recovery circuitry  50  modified as discussed above to include not only the output terminal  82  representative of the positive time-varying information but also an additional output terminal representative of the negative time-varying information, those two types of information would be fully consistent with one another. 
   Referring to  FIG. 5 , another embodiment of a portion of the circuitry  11  of the photodetector  10  relating to amplification of received signals is shown as circuitry  140 . Although both circuitry  40  and  140  include the first amplifier  42 , the circuitry  140  in contrast to the circuitry  40  includes somewhat different recovery circuitry  150 . As shown, the recovery circuitry  150  like the recovery circuitry  50  receives the time-varying output signal  48  of the amplifier  42  and processes that signal by way of the current splitter circuit  49 . Also, similar to the recovery circuitry  50 , the recovery circuitry  150  further includes a current mirror circuit (in this case labeled circuit  172 ), that encompasses both the third MOSFET  70  and the fourth MOSFET  74 , the sources of which are coupled to the voltage supply  76 . Again, a voltage is output at the output terminal  82  that is representative of the current flowing out of the drain  68  of the first MOSFET  52 . 
   At the same time, the recovery circuitry  150  of  FIG. 5  differs from the circuitry of  FIG. 3  insofar as the recovery circuitry  150  includes DC offset removal circuitry  160  that is coupled to the input  46  of the amplifier  42 . The DC offset removal circuitry  160  includes a third amplifier  162  (also an operational amplifier) having a noninverting input that is coupled to a voltage reference  164 , and an inverting input that is coupled to a capacitor  166  that in turn is coupled to both the output of the third amplifier  162  and to a resistor  168 . The resistor  168  is coupled between the input  46  of the first amplifier  42  and the output of the third amplifier  162 , while the capacitor  166  is coupled between the inverting input of the amplifier  162  and its output. 
   In addition to being coupled to the input  46  of the first amplifier  42 , the DC offset removal circuitry  160  is also coupled to the remainder of the recovery circuitry  150  as follows. As shown, the current mirror  172  includes not just the third and fourth MOSFETs  70  and  74 , but also includes an additional fifth MOSFET  176  (all three MOSFETs being n-channel MOSFETs). The fifth MOSFET  176  is coupled with respect to the third MOSFET  70  and the supply voltage  76  in the same manner as the fourth MOSFET  74 , but has a drain  178  that (instead of being coupled to a resistor and providing a voltage output), is coupled to an additional current mirror circuit  180 . 
   As shown, the additional current mirror circuit  180  parallels the structure of the third and fifth MOSFETs  70 ,  176 , insofar as it has sixth and seventh MOSFETs  182  and  184  (in this case, p-channel MOSFETs), the sources of which are coupled to the ground  66  and the gates of which are coupled to one another as well as to the drain  64  of the second MOSFET  54 . Further as shown, the drain of the sixth MOSFET  184  is coupled specifically to the drain  178  of the fifth MOSFET,  176 . The additional current mirror circuit  180  thus parallels, in relation to the second MOSFET  54 , the current mirror circuit formed specifically by the third and fifth MOSFETs  70 ,  176  in relation to the first MOSFET  52 . 
   The purpose of the fifth MOSFET  176  and the additional current mirror circuit  180  is to allow reassembly of the positive and negative portions of the time-varying information. A junction  186  in particular links the drains  178 ,  184  of the fifth and sixth MOSFETs  176 ,  184 , to allow for the reassembly of the positive and negative portions of the recovered time-varying information that was previously split due to operation of the current splitter circuit  49 . The junction  186  is coupled as an input to the DC offset removal circuitry  160 , and in particular is coupled to the inverting input of the third amplifier  162  and is also coupled to the capacitor  166 . 
   The DC offset removal circuit  160 , in the configuration shown in  FIG. 5 , serves to remove two types of DC offsets that otherwise might be introduced into the output of the recovery circuit  150 . First, the DC offset removal circuitry  160  removes DC offsets that are provided directly to the inputs  44 ,  46  of the first amplifier  42 . For example, if the current provided to the input  44  of the amplifier  42  varied between 5 microamps and 15 microamps, and had an average value of 10 microamps, the DC offset removal circuitry  160  would tend to remove 10 microamps of current. 
   Second, the DC offset removal circuitry  160  serves to remove any additional DC offsets that are introduced due to the operation of the current splitter circuit  49 , as well as the various current mirror circuits  172 ,  180 . This is achieved because the input to the DC offset removal circuitry  160  is tied to the junction  186  between the current mirror circuits rather than directly to the output  48  of the first amplifier. However, in alternate embodiments, the input of the AC recovery circuit  160  could indeed be coupled directly to the output  48  of the amplifier  42 , although this is less preferred. In such event, the additional current mirror circuit  180 , as well as the fifth MOSFET  176 , would no longer be necessary. 
   Turning to  FIG. 6 , steps of an exemplary procedure for calibrating the photodetector  10  of  FIG. 1  are shown in an exemplary flow chart  200 . As shown, after starting at a step  202 , the photodetector  10  is presented with a light path by a technician or other person setting up the photodetector system in a step  204 . For example, the light emitting device  38  of  FIG. 2  corresponding to the photodetector might be turned on (this also presumes that there is no physical blockage in the light path between the light emitting device and the photodetector). Once the light path is presented at the step  204 , the technician then presses the gain-set button  15  of the photodetector to indicate that a normal or maximum level of light is being shined on the photodetector, at a step  206 . Subsequently, at a step  208 , the technician releases the gain-set button  15 . 
   The pressing and releasing of the gain-set button  15  at the steps  206  and  208  provides the photodetector  10  with an indication that the normal or maximum level of light is being shined upon it. Given this to be the case, the photodetector  10  next at a step  210  resets its control gain to a minimum level, and further at a step  212  causes the photodetector to change its indication status by illuminating the “set” LED  18  and turning off the “power” LED  17  (see  FIG. 1 ). Then, at a step  214  the circuitry  11  within the photodetector circuit  10  automatically increments the gain of the amplifier circuitry (e.g., the gain of the first amplifier  42  discussed with reference to  FIGS. 3 and 5 ) until a comparator output level is high. In the present embodiment, the gain preferably is set to a level of 5 times the input signal (e.g., the input signal at terminals  44 , 46 ), although this is not necessary in other embodiments. 
   After incrementing the gain to a high level at the step  214 , the photodetector  10  at a step  216  causes the “set” LED  18  to flash in a noticeable manner, for example, at a rate of 6 hertz. When this is happening, the technician realizes that is appropriate to interrupt/end the shining of the light upon the photodetector  10  by breaking the light path or otherwise, at a step  218 . Upon discontinuing the shining of the light upon the photodetector  10 , the technician then at steps  219  and  220  respectively presses and releases the gain-set button  15  a second time, which results in illumination of the “set” LED  18  at a step  221 . Because the light has been discontinued, the signal now received at the photodetector is indicative of a low light level or a dark light level and consequently, the output of the photodetector should be at a low level. Nevertheless, in certain circumstances, the output of the photodetector  10  will not be at a low level despite discontinuing the light upon the photodetector. 
   In particular, noise can be at a sufficiently high level such that the amplification provided by the circuitry  11  (especially given that the gain is at a relatively high factor of 5) results in a large output signal notwithstanding the absence of light being provided to the photodetector. To avoid a situation where the photodetector outputs a large signal despite the absence of light upon the photodetector, at a step  222  in this circumstance (e.g., in a circumstance where the output signal exceeds a given threshold) decrements the gain until the comparator output of the photodetector is sufficiently low. For example, at the step  222 , the gain could be reduced by 20% of more. To the extent that the gain is not excessive, and does not need to be reduced, step  222  can be skipped. Finally, at a step  224 , the resulting gain-setting is stored in a memory portion of the circuitry  11  (or possibly in another location as well) and subsequently at a step  226  the “set” LED is turned off while the “power” LED is turned on, thus signifying the end of the procedure at a step  228  such that the photodetector can now be used in practice. 
   The steps of the procedure shown by the flow chart  200  can be implemented and practiced on the photodetector  10  in a variety of manners and by way of a variety of techniques. In certain embodiments, the procedure is implemented by way of programming on an application specific integrated circuit. Such programming can be implemented through the use of state diagrams as well as other programming languages. The particular procedure of the flow chart  200  is particularly advantageous in comparison with conventional photodetector systems that require a technician or other operator to carefully hold down a gain selecting button for a specific period of time in order to arrive at a particular gain. In contrast to such systems, the present procedure allows for effectively automatic setting of the gain that is accomplished with only two pushes of a button by the operator. 
   Referring to  FIG. 7 , an additional flow chart  230  is provided showing an additional exemplary procedure that can be performed by the photodetector  10 . This procedure can be implemented in combination with, or separately from, the procedure shown by the flow chart  200  of  FIG. 6 . As shown in  FIG. 7 , after starting at a step  232 , a technician or other operator will press the light/dark (L/D) button  13  at a step  234  and then release that button at a step  236 . The pressing and releasing of the button  13  causes the “set” LED to be turned on and the “power” LED to be turned off, at a step  240 . Then, a toggling of an output state of the circuitry  11  occurs at a step  238 . After the toggling of state at the step  238 , at a step  242 , the setting is stored in memory. Finally, at a step  244 , the “set” LED is turned off and the “power” LED is turned back on, at which point the procedure is ended at a step  246 . 
   The procedure of  FIG. 7  can be used to toggle a variety of output states (or other states) of the circuitry  11 . For example, in one embodiment, a state of the photodetector  10  can be changed from a light setting to a dark setting, or vice versa. When in the light setting, the photodetector provides an output that is indicative of the degree of brightness/intensity of the light or magnitude of the light received by the photodetector, while when in the opposite state the photodetector provides an indication of the darkness or relative absence of light received at the photodetector. 
   Although the present discussion relates to certain embodiments of the present invention shown in  FIGS. 1–7 , the present invention is not limited to such embodiments. While the amplification, recovery and DC offset removal circuitry shown in and described with reference to  FIGS. 3 and 5 , as well as the various procedures shown in and described with reference to  FIGS. 6 and 7 , are described as being particularly useful in photodetectors such as the photodetector  10 , such circuitry/procedures are also applicable to other devices and in other circumstances and applications as well, where the functions provided by such circuitry and such procedures are of use. Indeed, the present invention is not intended to be limited to use in photodetectors, but rather is intended to be applicable to a wide variety of sensors including both photodetectors/photosensors as well as, for example, infrared sensors, laser sensors, microwave sensors, proximity sensors, ultrasonic sensors, inductive sensors, magnetic sensors, among others. It is in particular envisioned that the particular circuitry and procedures described above are applicable to a wide variety of sensors that receive signal inputs, particularly analog signal inputs, that are to be amplified and/or processed in one or more various manners, and/or that require setting/calibration of their gain characteristics and/or some other operational characteristics. 
   It is additionally envisioned that one or more aspects of the present invention are applicable to such various sensors in a variety of sensor applications in addition to the industrial conveyor system application discussed above. For example, sensors in accordance with one or more aspects of the present invention could be employed in other industrial applications (e.g., in conjunction with light curtains) as well as in relation to a variety of other residential, security, office, agricultural, construction, and other environments and applications. Additionally, the present inventive embodiments can readily be combined with various other electrical or other technologies in a variety of applications such as the conveyor application discussed above as well as many other applications not necessarily relating to conveyor systems or industrial/manufacturing systems. As mentioned above, signals to and from the circuitry discussed herein, whether or not used in relation to photodetector devices, can be wirelessly transmitted/received by way of a variety of devices known in the art including Bluetooth devices and RFID devices. Further, it is envisioned that aspects of the present invention could be employed in applications not limited to those involving sensors. For example, the above-discussed circuitry involving recovery of time-varying information and/or DC offset removal could be implemented in circuits used in motor controllers or motor drives. 
   Also, it is envisioned that numerous particular aspects of the embodiments of the invention discussed above could be varied depending upon the circumstance. In particular, the exact circuitry and steps of the flow charts shown herein are merely exemplary and can be modified in myriad ways. For example, while the recovery circuitry  150  of  FIG. 5  is described as including the DC offset removal circuitry  160 , the recovery circuitry need not be understood to include such circuitry. Indeed, the DC offset removal circuitry  160  of  FIG. 5  and the recovery circuitry  50  of  FIG. 3  can each be implemented alone as well as in combination with one another as shown in  FIG. 5 . Also for example, while in  FIG. 6  it is shown that the incrementing of the gain occurs prior to the decrementing of the gain, it is also possible that those steps could be reversed in their relative ordering in alternate embodiments. Further, while  FIG. 6  shows indications as being provided by the pressing and releasing of a push button, the present invention is intended to encompass embodiments in which operator indications (or indications from other machine control devices such as computerized controllers) are provided in other manners. Also, while  FIG. 6  relates to calibration of gain, the procedure of  FIG. 6  could also be employed in relation to the calibration or other setting of other characteristics of sensors or other devices, particularly where such characteristics can be varied along a range. 
   Further for example, as discussed above, the MOSFETs used in the circuitry shown herein could be replaced with other transistor or switching devices (e.g., BJTs) that provided the same or similar advantages. Also for example, the first amplifier  42  shown, which is a current-input device, could be replaced with a voltage-input amplifier such as the second and third amplifiers  56 , 162 . In the event that the amplifier  42  was a voltage-input device, the resistor  168  in some embodiments would not be necessary and/or, in certain embodiments the feedback provided to the amplifier from the DC offset removal circuitry could be provided via a different input to the amplifier rather than to one or both of the inputs  44 ,  46 . Further for example, while the output signal provided at the output terminal  82  is shown to be a voltage output signal, in other embodiments the output signal would be simply the current flowing out of the drain  78  of the MOSFET  74  (or comparable component). Additionally, while it is envisioned that the circuitry  11  would be implemented in the form of one or more integrated circuits (e.g., an application specific integrated circuits), the circuitry could also be implemented in other manners such as by way of discrete components or by way of software implemented on a computer or microprocessor. Additionally for example, some of the steps of the flow charts could be eliminated or reordered, and other steps could be added to the flow charts depending upon the embodiment. 
   It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.