Abstract: A power transistor in the final stage of an IC amplifier feeding a reactive load, such as a loudspeaker, has an emitter resistor connected across the input of a monitoring transistor by way of a diode, the monitoring transistor being part of a protective circuit which reduces the input signal to the power amplifier in the event of an overload. To retard the response of the protective circuit, for the purpose of preventing signal distortions in the event of brief power surges, the diode and the monitoring transistor are so disposed on the silicon chip of the amplifier that a thermal wave from the overheating power transistor will first strike the diode, thereby reducing its resistance to compensate for an increased voltage drop across the emitter resistor, and will reach the monitoring transistor with a certain delay; if the overload persists, the resulting increase in the conductivity of the monitoring transistor re-establishes the full sensitivity of the protective circuit.

Abstract: A protective device limiting the current flow through a power transistor 11 in a final stage of an integrated circuit comprises, as part of that circuit, a normally nonconductive shunt transistor 31 which is turned on by a monitoring transistor 22 whenever the collector current I and the collector-emitter voltage V.sub.CE of the power transistor reach values beyond a protective curve 52 plotted on a current/voltage diagram. The base-emitter circuit of the monitoring transistor 22 is connected across an output resistor R.sub.2, in series with the power transistor 11, via a diode 21 of logarithmic conduction characteristic, that diode being also connected across the power transistor in series with a voltage-sensing network. This network includes a voltage divider R.sub.3, R.sub.5 in series with a Zener diode 33, the voltage divider being connected in parallel with a switching transistor 35 whose base is tied to a tap of the divider.