An active termination for a transmission line comprising a reference impedance, a terminating impedance and a control circuit. The reference and terminating impedances are identical circuits made on the same integrated circuit in close proximity to one another. Both impedances are made of an active and a passive resistor in series. The active resistor is a CMOS transistor operated as a voltage controlled resistor. A control circuit senses the impedance of the reference impedance and generates a control signal to change the impedance of the reference and terminating impedances to be equal to the impedance of the transmission line. An alternate embodiment of the invention comprises an active resistor and a passive resistor in series to form a terminating impedance network. A control circuit senses the voltage on the transmission line and adjusts the active resistor to terminate the transmission line with the correct value of resistance.
