Abstract: The invention is an electronic siren comprising in combination circuitry for generating one of a square wave siren and intelligible audio signal and a power output amplifier for audibly reproducing the signal. The amplifier includes transistor circuitry having base, emitter and collector means and a first circuit for coupling the signal-generating circuitry to the base means and one of said emitter and collector means for driving the transistors of the transistor circuitry into saturation in response to the siren signal. The first circuit includes circuitry for applying a dynamic biasing voltage derived from the siren signal to one of the emitter and collector means in phase with the voltage of the siren signal applied to the base means and of a plurality and opposition to the potential drop of the junction of the base means and the other of the emitter and collector means.

Abstract: This invention provides overload protection to power components such as power transistors. A pair of driver transistors is connected in push-pull configuration and is coupled through an auto transformer to a pair of power transistors also connected in push-pull configuration. An impedance-matching transformer couples the pair of power transistors to a load circuit, such as a speaker. A low turn winding is connected in series with the secondary winding of the impedance-matching transformer, and drives a high turn winding in a silicon controlled rectifier trigger circuit which is triggered "on" when an overload condition exists in the low turn winding. Upon conduction of the silicon controlled rectifier, a switching transistor is switched "on" and this, in turn, switches "off" a power disabling transistor which is connected directly to, and in power disabling relation to, the pair of driver transistors.