GB986850A - Improvements in or relating to electric circuits for terminating signal transmissionpaths - Google Patents

Abstract

986,850. Transistor switching circuits. GENERAL ELECTRIC CO. Ltd. April 1, 1963 [March 30, 1962], No. 12352/62. Heading H3T. In a transistor switching circuit the input impedance of a transistor connected in commonemitter configuration terminating a transmission or delay line is maintained constant by a Zener diode and a rectifier connected in series combination between the collector and base electrodes of the transistor. As shown in Fig. 1, transistor 9 is maintained conducting in the absence of uni-directional current pulses on coaxial cable 1 by the forward bias on its base 8 due to Zener diode 16, operating in its reverse biased low resistance region, connected in series with rectifier 17 between the collector and base electrodes of transistor. The value of the base current is such that the transistor is operating in the linear portion of its input characterized so that the correct impedance is connected across the coaxial cable and which is maintained constant by the negative feed-back over Zener diode 16 and rectifier 17. A current pulse on input terminal 2 causes the transistor to bottom and a current to pass from terminal 6 to a load. Rectifier 17 is now reverse biased and prevents current flowing therethrough. Rectifier 18 protects the transistor against reverse voltages applied across its base-emitter electrodes. In order that current may be supplied to the load between pulses applied to the base circuit the circuit of Fig. 2 is employed in which input line 19 is connected to terminal 6 and load is connected between output line 20 and earth.