| 摘要 |
<p>A DC control circuit for limiting metallic and longitudinal currents flowing in a balanced telephone line, and maintaining a predetermined DC bias voltage on tip and ring leads of the balanced line. A negative feedback path is established for monitoring voltage on the tip lead, comparing the monitored voltage with a predetermined offset bias voltage and generating a DC bias signal for application to tip and ring driver circuits for application to the balanced line in response thereto. Longitudinal and metallic currents are detected in a further pair of differential amplifiers for generating further DC signals such that a voltage corresponding to the detected longitudinal current is generated and applied to the tip lead, whereby a first one of the further pair of differential amplifiers adjusts the generated DC bias signal for limiting the longitudinal current flowing in the tip and ring leads. The detected metallic current is compared with a predetermined threshold signal, and in the event the detected current is greater than the threshold signal, the second differential amplifier reduces the voltage difference between the tip and ring leads, thereby reducing the metallic current flowing therethrough.</p> |
