Line equalizer with differentially controlled complementary constant

Abstract

In a system employing half-span preemphasis of transmitted signals on a cable pair telephone line, equalization is accomplished in a circuit arrangement including first and second complementary constant-resistance networks having the input impedances thereof in the emitter and collector circuits, respectively, of a common emitter amplifier. These impedances control the feedback and gain of the amplifier such that their effects are additive in a particular manner in the collector-output circuit thereof. The terminating resistances of the networks are the resistances produced by a pair of differentially controlled thermistors. A DC error signal that is proportional to line length controls the current driving the thermistors and thus the resistances thereof. When the circuit arrangement is at half-span spacing, a difference circuit causes the terminating resistances of the thermistors to be the same values, which make the impedance characteristics of the networks such that the net gain of the amplifier is flat. When the circuit arrangement is located at other than half-span spacing, the terminating resistances presented by the thermistors are different values, which cause the impedance characteristics of the networks to vary in a prescribed manner. The effects of these networks are additive in the amplifier so as to make the overall loss of the system flat.