The Mitel MT91610, with an external bipolar driver (Figure 4), provides an interface between a switching system and a subscriber loop. The functions provided by the MT91610 include battery feed, programmable constant current with constant voltage fold over for long loop, 2W to 4W conversion, offhook and dial pulse detection, direct balance ringing with built in ring tripping, unbalance detection, user definable line and network balance impedance's and gain, and power down and wake up. The device is fabricated as a CMOS circuit in a 36 pin QSOP package.