Coupler circuit for telephone networks and the like

Claim 1

A coupler circuit, for protecting a telephone set against voltage transients appearing on a telephone line connected to said telephone set, said coupler circuit having a first terminal for connection to the telephone line, a second terminal for connection to a line terminal of the telephone set, and a third terminal for connection to ground potential, said coupler circuit comprising in combination; a. a first transistor and a second transistor, said first and second transistors being complementary transistors, the collector electrode of the first transistor being connected to said first terminal, the emitter electrode of the second transistor being connected to the second terminal and the base electrode of the first transistor being connected to the collector electrode of the second transistor; b. a resistive means connecting the emitter electrode of the first transistor to the base electrode of the second transistor; c. a first breakdown diode means, having a first predetermined breakdown voltage, connecting the third terminal to the base electrode of the second transistor so as to clamp the base electrode of said second transistor at a voltage level, with respect to ground potential, corresponding to the first predetermined breakdown voltage; whereby when a signal voltage, of a magnitude greater than the first predetermined voltage, appears across the first and third terminals, the first resistive means and the collector emitter junctions of the first and second transistors absorb the portion of said signal voltage in excess of the first predetermined breakdown voltage so as to limit the voltage appearing across the second and third terminals at a level corresponding to the first predetermined breakdown voltage while minimizing the flow of signal current via the first and third terminals.