GB846080A - Circuit arrangement for the switching of electric voltages with the aid of transistors - Google Patents

Abstract

846,080. Transistor switching circuits. STANDARD TELEPHONES & CABLES Ltd. April 24, 1959 [April 26, 1958],No. 14153/59. Class 40(6). A transistor switching circuit comprises two series connected transistors to the base of one a controlling voltage is applied and the base of the other is connected to a constant voltage source the base voltage of this transistor being prevented from exceeding a safe predetermined level. Fig. 2 shows a circuit for switching a voltage U3 of - 160 volts to a load R L comprising two series connected switching transistors T2, T3. The switching control voltage U 1 is applied via transistor T 1 so that transistor T2 conducts only when transistor T 1 is rendered non-conductive by the control voltage. The conduction of transistor T 2 causes base current to flow in transistor T 3 which therefore conducts so that current is switched through the load R L . 'With the values of voltages shown in Fig. 2 the base potential of transistor T 3 when transistor T 2 is blocked is limited to +5 volts. In the modification shown in Fig. 3 when transistor T 3 is blocked the path through resistor R6 from the base of T 3 is blocked since diode D1 is cut off. When via transistor T 1 , transistor T 2 and thus transistor T 3 are caused to conduct the voltage at the emitter of transistor T 3 changes from - 80 to 0 volts so that diode D 1 conducts and transistor T 3 is also conducting. The voltage at the base of transistor T 3 is determined by the voltage divider R 9 , R 10 and is limited with the values shown to approximately + 6 volts.