GB598601A - Variable gain electron multiplier - Google Patents

Abstract

598,601. Valve amplifying circuits. FARNSWORTH TELEVISION & RADIO CORPORATION. April 24, 1945, No. 10336. Convention date, Jan. 14, 1944. [Class 40 (v)] In an electron multiplier the output is limited by switching the utilisation circuit to a lower stage electrode when the output reaches a critical value, the switching means being under the control of current from a higher stage or collector electrode. In Fig. 2, the emission from photo-electric screen 12 is amplified by an eleven-stage electron multiplier in which collector 32 is normally connected to the utilisation circuit 61, 63 via the back contacts of three similar relays 48, 49, 51. The operating coils of these relays serve as load impedances respectively of secondary emitters 31, 29, 28, which may be substituted successively as collectors by operation of their associated relays. When the current to the electrode 31 reaches the operating value of relay 48, owing to increased strength of input signal, contacts 54, 56 change over to transfer the collector 32 to cathode potential and connect electrode 31 as collector for the utilisation circuit via back contacts 53, 52. Similarly, relay 49 operates when the current to electrode 29 reaches the critical value transferring electrode 31 to cathode potential and connecting electrode 29 as collector for the utilisation circuit. Relay 51 acts similarly under the influence of electrode 28 and the reverse sequence occurs for decreasing signal strength. In a modification, Fig. 3, the series connected relays 68, 69, 71 operate at successively higher anode current values from valve 72 determined by the grid potential built up across resistor 55 by current from collector 32. The secondary emitters 31, 29, 28 are not used as collectors and the phase inverting stage 72 is required for coupling the collector electrode to the utilisation circuit 78, 63 to preserve phase continuity during switching. Current to the secondary emitters builds up potentials across load impedances 65, 66, 67 which may be connected selectively to the utilisation circuit by operation of the associated relay when the required current density is received at collector electrode 32.