A transistor push-pull output stage, particularly for pulse shapers having inverse feedback from the output to the input, as well as a high-resistance input and a low-resistance output is disclosed. In testing components of data technology, short circuits in the component should not lead to the destruction of the output stage of the testing apparatus. To this end, the invention provides that a field effect transistor is employed in each push-pull stage as the input transistor, the field effect transistor respectively driving the parallel connection of a plurality of output transistors by way of an emitter follower stage. The bases of the output transistors are driven by way of a constant current source which is switched off given overvoltage of the output. The transistors and the push-pull stages are complementary to one another. In addition, a structure is provided for the power supply which is driven as a function of the voltage shift and of the pulse basic voltage of the signal. The transistor push-pull output stage is particularly suited as the output stage for a pulse shaper in an automatic testing unit for data processing devices.

An intermediate amplifier device (20) with a signal amplification stage (21) in a transmission network in which signals can be transmitted from a central station (23) to a number of receivers (24) by way of a number of transmission lines, which are connected to one another in a tree structure, with a number of intermediate amplifier devices of the same type is characterized in that the intermediate amplifier device has a detection device (22) for detecting a malfunction (28) in the transmission line (27) connected to the intermediate amplifier device, a switching device (26) for coupling and uncoupling the transmission line (27) to or from the central station, as well as a delay element (25) with a delay time t.sub.i for the delayed reaction to a malfunction detected by the detection device (22) by means of a correspondingly delayed actuation of the switching device. As a result, in the event of a malfunction, only the last intermediate amplifier device before the malfunctioning line section as well as subsequent line parts possibly connected to it are affected, but not network sections that do not receive their signals by way of the affected intermediate amplifier device. Furthermore, the localization of the malfunction is made easier.

A general purpose, short circuit protector/controller circuit for drivers and amplifiers which is external to the internal circuit of the driver amplifier, which is arranged to drive a load such as electronic instrumentation. In this circuit, current sensing resistors are placed in both the positive and negative power supply input lines to the amplifier, and the voltage across each of the current sensing resistors is monitored independently for an overload condition. The voltage across each current sensing resistor is proportional to the current therethrough which drives the load, and is compared to a reference voltage, which is an equivalent representation of an overload or short circuit limit. When the sensed voltage across either resistor is greater than the reference voltage, the amplifier is tri-stated (disconnected from the load). In the preferred embodiment the amplifier is tri-stated for a period T.sub.OFF, protecting the amplifier, and is subsequently reconnected to the load for a period T.sub.ON. If the current limit violation persists, the above cycle is repeated.