c. Receiving circuit.
(1) When the circuit is used for receiving, its operation is slightly different from that described above. Suppose
that a voice current is being received from the line and enters the circuit through terminals L1 and L2 (fig. 46). This
current will pass through path 1, and will induce a voice voltage across the primary winding and across the transmitter, the
diaphragm of which is now at rest. The received current will not enter path 2 because of the high impedance of ringer
RG. Because of this condition, it might at first appear that no output would be obtained from the receiver unit. However,
this is not the case, because of inductive coupling between the windings of the induction coil. By transformer action, the
primary current produced by the received signal induces a voltage into the secondary circuit consisting of the secondary
winding, the receiver, the hookswitch contacts, the transmitter, and capacitor C. Voice current flowing through the
receiver unit provides reception of the incoming signal.
(2) Again, a booster, or regenerative action is produced by the in-phase connections of primary and secondary
windings of the induction coil, and these currents add in the common impedance presented by the transmitter.
d. Circuit efficiency. The over-all performance of the booster circuit is characterized by high efficiency. The
chief disadvantage of the circuit is the presence of a high level of sidetone.
a. Arrangement. A circuit that reduces the amount of sidetone produced in the receiver of its telephone set is the
sidetone-reduction circuit (fig. 47). It is like the booster circuit of figure 46, except that here the transmitter is connected
in series with the primary winding of the induction coil on the opposite side of the hookswitch contacts, as shown.