Overload protection for switching transistor amplifier - incorporates additional transistor to act as overcurrent trip

摘要

The switching amplifier comprises an output transistor (15) controlled by a driver transistor (18). A stabilised power supply (14) is provided for the circuit. The additional transistor (19), acting as an overcurrent protector has its base connected to the main transistor output (17) through a protection diode (22). The collector-emitter path of this transistor (19) is connected between the main transistor base and the driver collector. In normal conducting state all transistors are on. If the load current rises above normal the saturation voltage (Usat) on the power transistor (15) rises. This is equivalent to a drop in potential between the output (17) and the protection and driver common point (26). The main transistor base current is thus diminished causing a further rise in voltage leading to a snap turn off. Capacitors (24, 23) restart the conductibility, and the circuit will remain on if the overload has been eliminated.