A
plain old telephone system (POTS) device, for example but not limited to, a telephone,
facsimile machine, analog modem, caller identification (ID)
system, speaker phone,
cordless phone, etc., employs an on-board
transistor-based linear polarity guard to enable decoupling of a POTS channel from one or more xDSL channels on a telephone connection (subscriber loop associated with a public service
telephone network), while reducing undesirable interference, such as
intermodulation distortion imposed upon the POTS and xDSL channels. Transistors associated with the polarity guard are operated in the ohmic region, or in a nonsaturated linear mode, to achieve a substantially linear
transfer function through the transistors and through the overall linear polarity guard. Current that is input to the polarity guard and the
voltage drop across the transistors that conduct the current in the polarity guard exhibit a substantially
linear relationship, or linear VI
transfer function. Hence, the VI relationship between the overall output
voltage and overall input current of the polarity guard is substantially linear in the operating region of the polarity guard.