Short-circuit-proof transistor output stage, especially ignition output stage for motor vehicles

Short-circuit-proof transistor output stage which forms an ignition output stage for a motor vehicle, including a power transistor (10) having a collector-emitter path connected in series to a load (12);

• voltage detection means for detecting voltage drop across the load when a short circuit occurs in the load, and a blocking device including a first switching transistor (27) for turning off the power transistor when a voltage drop is detected by the detection means, said detection means comprising a second switching transistor (23) and a third switching transistor (25) of opposite conductivity type, a base-emitter path of said second switching transistor (23') being connected across said load (12), an emitter-collector path of said second switching transistor being connected in series with a base-emitter path of said third switching transistor (25) and a collector-emitter path of said third switching transistor being connected parallel to a base-emitter path of said first switching transistor so that during normal condition of the load the third switching transistor (25) is turned-on by a collector current of the second switching transistor (23) and blocks the first switching transistor (27), while in the event of a short-circuit the second and third switching transistors are blocked, the first switching transistor is turned on and the power transistor is blocked, and a transistor switching stage connected to a base of the power transistor (10) and including a fourth switching transistor (30) and a fifth switching transistor (31), a collector of the fifth switching transistor (31) being connected with a collector of the third switching transistor (25), the collector of the fifth switching transistor (31) being at positive potential when a drive signal is present for the power transistor (10) and the first switching transistor (27) leading away a base current for the power transistor (10) to earth only when both the collector of the third switching transistor (25) and the collector of the fifth switching transistor (35) are at the positive potential.