| 摘要 |
In a telephone line feeding arrangement, the line current is monitored by a multi-resistor network (RpRs) and the output of the network applied to the inputs of an operational amplifier (A3). The output of this amplifier, which is a voltage whose value depends on the line current, is compared by a comparator (A4) with a reference voltage (VR) to produce a control voltage for a variable voltage (VVS), the output of which provides line feeding voltage. In an alternative, the monitoring is effected by connecting the two line wires each via a resistor to one of the inputs of an operational amplifier (A6) whose output is a voltage dependent on line voltage. This voltage is applied to an input of another operational amplifier (A7) where it is in effect added to a reference voltage (VR). The output of this amplifier (A7) is used to control the variable voltage source (VVS), which in turn controls a constant current source (CCS) whose output provides line current. |
