| 摘要 |
An output stage of a subscriber line interface circuit comprises two amplifiers, a first (111) one of which is configured to operate as an inverting amplifier (negative amplification) and a second (110) one as a non-inverting amplifier (positive amplification). Each amplifier has an output connected to a respective wire (a, b) in a subscriber line. A common reference voltage (Vref) and a common control voltage (Vc) are supplied to each amplifier (110, 111). The reference voltage (Vref) is used for setting the average voltage of the subscriber line. The control voltage (Vc) level determines the offset of the amplifier output voltages (Vb, Va) from the average voltage. The difference between the amplifier output voltages provides the subscriber line supply voltage which, together with subscriber loop resistance, determines loop current (Iloop). Loop current is measured (R1, R2, 117, 118) separately from each subscriber line wire and the measurement results are summed, a possible common mode interference being thereby cancelled. If loop current is too high, a control member (126) changes the control voltage, with a predetermined time constant, to a direction which reduces the amplifier output voltages (Va, Vb). The voltage difference between the subscriber line wires (a, b) thus becomes smaller and the loop current increases. When the measured loop current reaches a desired value, the control voltage (Vc) produced by the control member (126) sets at the voltage level reached. As a result, a very simple and inexpensive subscriber line power supply and loop current control is obtained. |
