Improvements in electron multiplier circuits

1,179,846. Gain control arrangements; photo-electric distance measuring. G. & E. BRADLEY Ltd. 14 July, 1967 [15 July, 1966], No. 31958/66. Headings H3Q and H4D. [Also in Division G1] In a variable-gain electron-multiplier circuit including an electron multiplier connected in series with a load, and with means for applying a varying voltage across the multiplier to vary its gain, means are provided for applying to the load a signal in opposition, and substantially equal, to an unwanted signal generated in the multiplier and applied to the load when the varying voltage is applied across the multiplier. The arrangement is described as applied to a laser distance measuring device in which reflected light pulses are amplified in photomultiplier 10, compensation for diminishing signal strength as distance increases being obtained by pulsing the anode current in the multiplier. Gain-controlling pulses are applied over a shaping network 16 to the grid of valve 15 to drive it conductive, producing a voltage drop across resistance 14 which is applied to the cathode of photo-multiplier 10 and also to the dynodes via a potentiometer network 24. The cathode circuit of valve 15 contains a further pulse-shaping network 17, and diodes 18, 20 connected between the anode and sources of fixed bias respectively determine the minimum and maximum voltage across the photo-multiplier. In such an arrangement an initial surge of current (22, Fig. 3, not shown) occurs in the multiplier such as to prevent measurement at near distances: the invention overcomes this by deriving a pulse from the dynode of the multiplier which is nearest to the anode and applying it via capacitor 25 to a transistor amplifier 26. This amplifies and inverts the pulse and applies it in opposition to the anode of the multiplier via capacitor 31 which is adjustable in order to obtain exact compensation. Alternatively, a pulse may be derived from the current to the dynode by connecting in series with it the primary of a ferrite cored transformer, the secondary of which is so connected to the anode as to apply a suitable compensatory pulse to it.