Low loop current switch latch circuit

Details Low loop current switch latch circuit Low loop current switch latch circuit

1981-12-28

1984-03-27

Pellinen, A. D.

179 81B, H04B 336

Patent

active

044396376

ABSTRACT:
A low loop current switch latch circuit (10) monitors a residual current which is derived from a telephone line and is proportional to the current drawn by a telephone subscriber circuit. Bipolar transistors (20, 26) are connected to produce a proportional current to the residual current. A current mirror circuit includes a master transistor (30) and a slave transistor (32). The proportional current is drawn through the master transistor (30). The current produced by the slave transistor (32) is provided to a node (33). A constant power source (36) provides a constant current to a node (39). A pair of cross-coupled transistors (40, 46) are connected to the nodes (33, 39) such that the cross-coupled transistors are set to first and second states as a function of the current derived from the slave transistor (32). A bias current transistor (52) is activated by the cross-coupled transistors (40, 46) to draw a preset biased current from an amplifier (60), thereby changing the amplifier from a first gain to a second gain condition. The circuit (10) therefor sets the gain of the amplifier (60) through switching action utilizing hysteresis, the switching being a function of the amplitude of the residual current passed through the transistor (20).

REFERENCES:
patent: 3781481 (1973-12-01), Shaffer et al.

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