Various signal limiting techniques are known in the art, but some unique problems arise when limiting signals in a telephone system electronic line feed circuit of a telephone line interface unit. For example, the appearance of an extraneous voltage of large size compared to those normally occurring on a customer line can cause a line feed amplifier to be driven so hard to source or sink the current that physical integrity of the amplifier is in jeopardy.
The use of a common mode feedback arrangement to suppress longitudinal signals, which are often quite large, is taught by, for example, D. W. Aull et al. in "A High-Voltage IC for a Transformerless Trunk and Subscriber Line Interface," 1981 IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. SC-16, No. 4, Aug. 1981, pages 261-265. A common mode amplifier includes an output current limiting circuit. Separate thermal-shutdown circuits are provided.
A battery feed circuit with a differential operational amplifier connected to reduce common mode signals is shown in the I. K. Hetherington et al. U.S. Pat. No. 4,007,335.
Limiting arrangements of various types can be employed in the voice signal path, but limitations on their operation should be recognized. For example, Aull et al. use surge protection diode clamps; but those clamps come into play only when a large voltage is present. However, it is possible in circuits of this type for either the tip amplifier or the ring amplifier to operate within its permissible voltage range and still be called upon by its common mode feedback to produce a current that is large enough to burn out the amplifier. Likewise, the surge protection is at amplifier output points where the voltage level is comparatively high and similarly more difficult to deal with precisely. Thus, the activating threshold for the surge protection is often set somewhat lower than might otherwise be the case in order to allow a suitable safety margin. However, if the threshold is too close to the top of the voltage range required for quality signal reproduction, a large voice signal excursion may be clipped.
Aull et al. also provide a power-down/thermal shutdown arrangement that is responsive to a certain base-emitter voltage threshold in a temperature sensitive transistor to power down at least the tip and common mode amplifiers. However, in the short time required for this function to operate, a tip or ring amplifier could produce a sufficiently high current to burn itself out. The safety margin requirements to assure protection in the face of this time factor, and to take account of a certain amount of indefiniteness in the exact threshold current level which will produce power down, also consume a part of the amplifier operating range that would otherwise be available for desired signal amplification.
An R. S. Burwen U.S. Pat. No. 3,500,218 employs a network of resistors, diodes, and transistors to sense voltage at an amplifier output stage emitter to control a signal path limiting function.
Limiting diodes are connected across the input to an amplifier in a P. Schiff U.S. Pat. No. 3,624,533 to protect amplifier input transistors.
It is also known to perform limiting at least partly out of the voice signal path. Thus, the D. B. James U.S. Pat. No. 4,169,215 shows separation of an excessive signal component into a separate circuit where it is attenuated and inverted before being injected back into the path to cancel at least in part of the excessive signal.