Solid state off hook phone line load

Abstract

A direct connect telephone line interface which includes a solid state off hook phone line load is disclosed. The solid state off hook phone line load utilizes an active semiconductor device coupleable between the phone lines, with the control input to the active semiconductor device being coupled to a bias circuit operative from the voltage across the phone lines to bias the active semiconductor device in saturation as a result of an answer voltage on the phone lines. The bias circuit is frequency selective so that the bias point of the active semiconductor device is substantially independent of information signals on the telephone line. Thus the active semiconductor device, in combination with any impedance in series therewith to set the desired answer load on the telephone lines, provides the desired load impedance on the lines to maintain the connection, but at the same time provides a very high impedance to the AC signals (communication signals) on the line so as to not attenuate the communication signals subsequently coupled to an isolation transformer. This allows the reduction in the size of the isolation transformer normally used in such interfaces without requiring a separate, relatively large holding coil.