A dynamic current supply is connected between the emitter electrode of an output transistor and a point of reference potential, to progressively increase the magnitude of the flow of current therebetween, as the output signal at the emitter electrode approaches the point of reference potential. The dynamic current supply includes a current mirror, the output of which is in the path of and controls the flow of current. An MOS transistor, the conductivity of which is a function of the difference in potential between the operating voltage supplied to the output transistor and the output signal, supplies the input or control current to the current mirror.