| 摘要 |
<p>Circuit apparatus for supplying a loop current to a two conductor loop telephone line includes first and second differential amplifiers each having the same gain factor. The first amplifier is coupled to one side of the line via a series resistance, while the second amplifier is coupled to the other side of the line via a series resistance of the same magnitude. A feedback amplifier has one input terminal coupled to one side of the line and another input terminal coupled to the other side of the line. The inputs of the feedback amplifier are further coupled to the outputs of the above differential amplifers. The output of the feedback amplifier is coupled to the input of a variable gain amplifier which has its output coupled to an input terminal of the first differential amplifier. The first differential amplifier receives a voltage reference source at its other input which source controls the magnitude of the current supplied to the line circuit. To accommodate different system characteristics, the gain of the gain controllable amplifier is varied which effectively varies the equivalent feed resistor for the subscriber line. Hence one may vary the gain of the amplifer to achieve a wide variation in feed resistance while employing a minimum number of components and a fixed reference voltage.</p> |
