PATENT ABSTRACT
The amount of gain applied to a photodetector such as a photomultiplier tube (PMT) is limited to an amount that does not cause the applied PMT bias voltage to overdrive, hence damage, the PMT. Techniques for limiting the PMT gain are implemented in a way that does not interfere with the precision with which the PMT gain may be established (by selection of a reference level) below that limited level.

PATENT DESCRIPTION
FIELD OF THE INVENTION 
   This invention relates to automatic gain control in photodetectors, such as those used in a variety of electro-optical applications. 
   BACKGROUND AND SUMMARY OF THE INVENTION 
   Circuits that employ an automatic gain control for controlling the gain of a photomultiplier tube (PMT) or similar photodetector are known in the prior art. 
   For example, the prior art circuit shown in  FIG. 1  employs an operational amplifier (op-amp)  10  in a integrating configuration, where resistor R and capacitor C set the integration time constant as the product R*C seconds. The filtered DC portion of the PMT output signal  12  (hereafter referred to as “PMT output”) is compared to a pre-selected DC reference  14 . The output of the op-amp  10  is applied to the bias control  16  of the PMT, thereby to drive the PMT at a level such that the PMT output matches the DC reference. For example, if the level of the PMT output reaching the op-amp  10  is below the DC reference  14 , the op-amp will provide to the PMT bias control  16  a signal for driving the PMT at a relatively higher level until the PMT output matches the DC reference. 
   A controller (not shown) monitors and conditions the signals directed to and from the op-amp  10 . For example, in instances where the DC reference is user-selected, the system provides a user interface for receiving the selected input from the user, which input the controller converts as necessary to a corresponding DC reference level applied to the op-amp  10 . 
   It is important that, despite the selected DC reference, the PMT gain be limited to an amount that does not cause the applied bias voltage to overdrive, hence damage, the PMT. Moreover, such techniques for limiting the PMT gain should not interfere with the precision with which the PMT output may be established (by selection of the DC reference) below that limited level. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  a diagram of a prior art automatic gain control circuit for a photomultiplier tube (PMT). 
       FIG. 2  is a diagram of a preferred embodiment of an automatic gain control for a PMT, including a mechanism for limiting the gain to a level less than that which would otherwise lead to overdriving, hence damaging, the PMT. 
       FIG. 3  is a flow diagram for illustrating an aspect of the invention that ensures precise selection of the DC reference voltage level. 
   

   DETAILED DESCRIPTION 
     FIG. 2  is a diagram of a preferred embodiment of an automatic gain control for a PMT, including a mechanism for limiting the gain to a level less than that which would otherwise lead to overdriving, hence damaging, the PMT. The circuit of  FIG. 2  includes an operational amplifier (op-amp)  20  in an integrating configuration. The filtered DC portion of the PMT output signal  22  (hereafter referred to as “PMT output”) is applied to the negative input of the op-amp  20 . 
   The selected DC reference voltage  24  is applied to the positive input of the op-amp  20 . In this preferred configuration, however, a Zener diode  26  is interconnected between the op-amp  20  and DC reference  24 , as shown in  FIG. 2 , with its anode terminal grounded. The Zener diode breakdown or Zener voltage is selected to establish the upper limit of voltage (here, the upper limit of the DC reference) that can be applied to the op-amp  20 , hence limiting the bias voltage that can be applied to the PMT via the PMT bias control  30 . In one embodiment the Zener voltage is 1.8 volts. Thus, if the DC reference  24  is selected to be above that amount, the Zener diode becomes conductive and shunts the DC reference voltage to ground, thereby preventing the undesirably high voltage from appearing at the positive input of the op-amp  20 . 
   A resistor  28  (preferably 61.9 k Ohms in this embodiment) is located between the DC reference and the Zener diode  26  for protecting that diode from current levels that may damage the Zener diode itself. 
   In the preferred embodiment, the Zener voltage is relatively low. One can observe that, for such a low-voltage limit, the Zener diode will become conductive at voltage levels below the limit. Put another way, the Zener diode may become “leaky” at voltage levels approaching the established Zener voltage and thus prevent the application of the correct, selected DC reference voltage from reaching the op-amp, even though the selected DC reference voltage is less than the (Zener voltage) upper limit. The consequent lack of precision in applying the particularly selected DC reference voltage to the op-amp  20  for creating the sought-after gain of the PMT is unacceptable in many applications. 
   Accordingly, as another aspect of the present invention, there is provided a technique that compensates for the voltage drop attributable to a “leaky” Zener diode effect explained above. To this end, voltage measurements are taken at the positive input of the op-amp and correlated to the selected DC reference voltage level. Numerous such measurements are taken at suitable increments of selected DC reference levels. The correlated data is stored as a look-up table for use by the controller for compensating for losses caused by the leaky Zener diode. This is explained in more detail next, with reference to the flow diagram of  FIG. 3 . 
   An implementation of the present invention may provide a user interface  32  whereby the user may select the desired PMT gain represented by the DC reference  36 . It is noteworthy here that although a user-selected DC reference signal is discussed for this embodiment, it is also contemplated that other means for establishing the desired DC reference may be used, including automated methods based, for example, on changes in environmental conditions of the photodetectors of interest. In either case, the protection of the photodetectors and compensation for leaky Zener diodes is desired. 
   Once the user selects a DC reference value, the look-up table is consulted  34  to find the correlating voltage that is actually applied to the op-amp  20  as a result of the leaky Zener diode effect mentioned above. In instances where the actual and selected levels are unequal, the controller adjusts the DC reference  34  by the difference indicated in the look-up table so that the level applied to the op-amp  20  will match what the user selected for the desired gain of the PMT. 
   As noted, it is contemplated that the present invention is useful for automatic gain control of other types of photodetectors, such as avalanche photo-diodes. Also, other mechanisms for limiting the voltage applied to the op-amp may be used. For example, a resistor could be used in lieu of the Zener diode, with a corresponding look-up table developed and stored for use as discussed above.