In an electronic inductor circuit, an operational amplifier drives the base of
the electronic inductor transistor, and receives negative feedback from the emitter
of the transistor. The transistor and operational amplifier combine to form a voltage-controlled,
current source (VCCS) with respect to the loop current. A voltage divider connected
across the rectified Tip and Ring voltage (or another node of the circuit at an
equivalent voltage) provides a DC reference to the positive input of the operational
amplifier, so that the line current automatically increases with an increase in
line voltage. A capacitor couples the transmit signal driver to the positive input
of the operational amplifier. This electronic inductor circuit can be driven using
a low voltage supply and provides sufficient linearity for high-speed modem applications.