| 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 is the subscriber line supply voltage. A polarity reversal circuit (128) supplies the control voltage (Vc) to the amplifiers (110, 111) at a first polarity or at a second, reversed polarity, depending on a state of a polarity control signal supplied by a telephone exchange. This offers a simple way of providing polarity reversal which is used in the telephone system for signalling transferred from an exchange to a subscriber terminal over a subscriber line. When the polarity of the control voltage is changed, the output voltage of the first amplifier (110) changes from operating voltage to ground and the output voltage of the second amplifier (111) from ground to operating voltage, or vice versa. |
