Telephone Circuits

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Two-way operation is essential for satisfactory telephone service, and many of the problems of the industry arise from this fact. A simplified circuit for obtaining two-way operation is shown in the below figure. This is called a local-battery system because a separate battery is required at each end of the line, and in such a telephone system each subscriber must have a battery in his house equipment. The operation of the circuit is simple, since speaking into either microphone will set up voice currents in both receivers. The transformers isolate the direct current required for the operation of the microphone, and also improve the efficiency by stepping' up the voltage and reducing the line current, or more nearly matching the microphone impedance to that of the line.

Local - battery telephone circuit

Common-Battery Circuits

Common - battery telephone circuit

The obvious advantage of removing the batteries from the subscribers' premises to the central office led to the development of the common-battery system. This system uses one large battery to supply microphone current to all subscribers' sets. This led to a new difficulty, however, in that the voice currents of all circuits in use flow through the same battery, and because of its internal impedance there is a possibility that some of these currents will find their way into other circuits. This effect is known as cross talk, and it may be avoided by use of a transformer called the repeating coil.

A repeating coil and its manner of use are shown in the above figure. It may be recognized as essentially a one-to-one transformer with primary and secondary windings split at the battery. Other repeating coils, connecting other pairs of subscribers, may be tied in at the points BC, B'C', and when G, connected will offer extremely high impedance to the flow of voice current from one channel to the other, but practically none to the flow of voice current in its own channel.

The talking circuit of the subscriber's set is different in the case of the common-battery telephone. 'transformers T₁ and T₂ usually called induction coils, and capacitors C₁ and C₂, are connected as auto-transformers, that is, the lower portion of the winding serves as primary and also as pan of the secondary to step up the a.c. component of the microphone voltage.

Telephone systems require additional equipment for signaling the operator and subscribers, and for switching connections between subscribers. Their discussion is beyond the scope of this book.