Patent ID: 7031462

Claim:
An interface circuit capable of connecting a modem to a telephone line, said interface circuit comprising: a voltage-controlled current source, said voltage controlled current source including an operational amplifier having a positive input capable of being connected to a transmit signal driver of said modem, said operational amplifier having an output configured to drive a base of an electronic inductor transistor, said electronic inductor transistor capable of being connected across a rectified tip and ring voltage of said telephone line, wherein said interface circuit is configured to linearly vary a line current of said telephone line; a second transistor, said second transistor having a base connected to a collector of said electronic inductor transistor; an impedance matching circuit connected between said positive input of said operational amplifier and a collector of said second transistor; wherein said second transistor provides increased isolation for said modem when said modem is on-hook, and wherein a negative input of said operational amplifier is connected to an emitter of said electronic inductor transistor.