SWITCHING CIRCUIT

PURPOSE:To prevent unnecessary current loss and lower an output saturation voltage in a low load current area by controlling the base current of an output transistor (TR) according to the level of a load current. CONSTITUTION:The emitter of a TRQ4 is connected to the connection point of the base of a TRQ1 and a resistance R1, the collector of the TRQ4 is connected to the connection point of the emitter of the TRQ1 and the base of a TRQ2, and the base of the TRQ4 is connected to the collector of the TRQ2. Then, when a load current Ic is small, the saturation voltage VCE (sat) of the output TRQ2 is small, so the TRQ4 turns on by being biased forward between the emitter and base. When the current Ic is large, the voltage VCE (sat) of the TRQ2 rises and the TRQ4 turns off when said voltage becomes equal to the base-emitter voltage VBE of the TRQ1. Therefore, the current amplified by the TRQ1 becomes the base current of the TRQ2, so a large load current Ic is obtained.