GB965726A - Improvements in or relating to transistor protection circuits - Google Patents

Abstract

965,726. Protective arrangements. MULLARD Ltd. Aug. 10, 1962, No. 30788/62. Heading H2H. [Also in Division H3] In a transistor protection circuit a control transistor T 1 , Fig. 1, is normally biased to be non-conducting, but when a fault exists in the circuit of the transistor to be protected and a rectified voltage V 2 , derived from the signal current through the protected transistor and applied to the control transistor T 1 , exceeds a rectified signal V 1 derived from the load resistance of the protected transistor and also applied to the control transistor, current flows through the collector of the control transistor, is smoothed and then employed to limit or cut-off the signal gain of at least one transistor stage preceding the protected transistor. The protection circuit of Fig. 1 may be used to protect the transistors of a class B power amplifier, Fig. 2. As shown, the signal across load R7 of preamplifier stage T 3 is fed to amplifier block A and then to the primary winding of input transformer I T of an output stage comprising push-pull connected transistors T 4 , T 5 to be protected. The voltage developed across resistive load R 13 is fed through diode D 1 and resistor R 1 to the base of control transistor T 1 to the emitter of which is applied a signal voltage developed across resistor R 11 . The valves of resistors R 1 , R 2 are selected so that transistor T 1 is normally non-conducting but when resistor R 13 is short-circuited and the voltage across R 11 exceeds the voltage across R 13 transistor T 1 conducts. Its collector current I c , smoothed by capacitor C 3 and amplified by transistor T 2 , is applied to the collector of transistor T 3 so that the voltage drop across R 7 will increase so as to limit the signal gain of transistor R 3 thereby reducing or cutting-off the signal applied to transistors T 4 , T 5 .