Matching of telephone receiver to line - uses bridge circuit to provide optimum coupling to various line lengths

摘要

A bridge circuit is used to provide line matching of a telephone receiver microphone-ear piece. The microphone (M) of the receiver handset is coupled to an amplifier (MV) whose input is connected in one of the bridges diagonals. The ear-piece (9) is connected to another amplifier (HV) in the other diagonal. Two adjacent bridge arms have a pair of series connected resistors the midpoints of the pairs being connected by a non-linear variable resistor (V). The line circuit (L) and its lumped equivalent (N) form the remaining two bridge arms. The circuit provides variable attenuation for different live lengths.