Nonsaturating interface supply

US4731830A · kind A · utility

Assignee

• GTE Communication Systems Corporation · US

Inventor

• Robert G. Thomson · Westford, US

Key dates

• Filing date: Oct 4, 1985
• Grant date: Mar 15, 1988
• Expiry date: Oct 4, 2005

Classification

• Technology area (CPC H): Electricity
• CPC primary: H04M19/08
• WIPO field: Telecommunications
• WIPO sector: Electrical engineering

Abstract

An integrated circuit includes an interface circuit for coupling with a relative high-level VOICE circuit and a relatively low-level DIALER circuit to a subscriber line. A first shunt transistor is coupled across the telephone line and has its collector current forced by a current sink driven from the VOICE circuit. A second shunt transistor is coupled in a current mirror configuration to the first shunt transistor and supplies current to the DIALER circuitry. The interface circuit includes a saturation prevention circuit coupled to the current sink and to the second shunt transistor for sensing a tendency of the second shunt transistor toward saturation and for increasing the collector current load on that transistor in response. The saturation prevention circuit includes a voltage offset element and a sensing transistor arranged to form a loop with the collector-to-base junction of the second shunt transistor. As the collector-to-base junction of the shunt transistor approaches a forward-biased condition, the normally nonconducting sensing transistor is biased into conduction so that the collector load on the shunt transistor is increased.