Abstract: Four bipolar transistors are arranged in a bridge configuration and are interconnected with two additional bipolar transistors to provide an efficient polarity guard circuit. The circuit has two output terminals, one of which has a positive polarity regardless of the polarity across two input terminals. The two additional transistors prevent collector to base current from flowing through any of the bridge transistors which may be biased off.

Abstract: A circuit for use with telephony systems or the like for protecting these systems, which may be comprised of integrated circuits, from high transient voltages. The circuit includes a silicon controller rectifier (SCR) having the cathode and anode coupled between an input of the telephony system and earth potential. The gate of the SCR is coupled via a resistor to the cathode thereof and is also returned through a reversed-biased diode to a source of bias potential whereby negative voltage transients are shorted to earth potential such that the integrated circuits as well as the bias potential source are not damaged.