US Patent 7167556 Circuit and method for stimulation of a telephone apparatus

An electric device (202, 301) for connecting an analogue data transfer device (202) by means of a control unit (203) to a digital transfer system comprises means for connecting to a data transfer device via a twin cable (204). In addition, it comprises a current amplifier arrangement (214, 314, 340) for feeding a certain current to the twin cable, a first current switching device (219) for switching a first control current to the current amplifier arrangement, and a second current switching device (220) for switching a second control current to the current amplifier arrangement. The current switching devices (219, 220) are preferably current mirrors.