The patent badge is an abbreviated version of the USPTO patent document. The patent badge does contain a link to the full patent document.
The patent badge is an abbreviated version of the USPTO patent document. The patent badge covers the following: Patent number, Date patent was issued, Date patent was filed, Title of the patent, Applicant, Inventor, Assignee, Attorney firm, Primary examiner, Assistant examiner, CPCs, and Abstract. The patent badge does contain a link to the full patent document (in Adobe Acrobat format, aka pdf). To download or print any patent click here.
Patent No.:
Date of Patent:
Jul. 15, 1986
Filed:
Dec. 27, 1983
NEC Corporation, Tokyo, JP;
Abstract
In a subscriber line interface circuit of a telephone switching system, a differential amplifier (1) is coupled through the line terminals (T, R) of a two-wire subscriber line for detecting a voltage difference developed thereacross. First and second resistances (8, 9) of equal values are coupled at their first ends to the line terminals, respectively. Tip and ring drivers (2, 3) are connected to the other end of first and second resistances for driving the load impedance of the subscriber line. First, second and third feedback loops (f.sub.1, f.sub.2, f.sub.3) are provided, the first feedback loop being coupled from an output of differential amplifier (1) to the input of the drivers (2, 3) vary the impedance characteristics of the first and second resistances (8, 9) so that a high impedance is synthesized across each of the first and second resistances. The second and third feedback loops are also coupled from the output of differential amplifier (1) to the drivers (2, 3), each of the second and third feedback loops having a different passband frequency characteristic and having the effect of reducing the synthesized impedances to a predetermined value.