The Hybrid Coil

Figure 5. Circuits for developing the principle of operation of the hybrid coil from the theory of the impedance bridge.

Fig. 5(b), no difference of potential will exist across the receiver terminals, and the bridge will still be balanced. Introducing the energy into the bridge circuit as in Fig. 5(c) does not alter the condition of balance.

The last bridge circuit has been redrawn in Fig. 6. In this figure, Z_w and Z_e represent the impedances of the lines west and east, respectively, and Z_i and Z₀ represent the impedances of the input and output circuits of the amplifying element of Fig. 4,

When an impulse comes in over line west of Fig. 6, it is impressed across the input Z_i of the element and the amplified signal is introduced on the line by the output coil Z₀ of the transformer (see also Fig. 4). This amplified impulse must not again introduce energy into the input circuit or continuous oscillations will be produced and the repeater will sing or howl. The action and requirements are similar for an impulse from line east. There are then three sources of voltage to consider: first, a voltage impressed at Z_w as in Fig. 6 (a) ; second, a voltage (not shown) impressed at Z_e; and third, a voltage impressed at Z₀ as in Fig. 6(b).

Figure 6. The hybrid coil (or bridge transformer, as it is sometimes called) is connected as shown, except that in practice a balanced hybrid coil with additional line windings would be used (Figs. 4, 7, and 8).

In Fig. 6(a) the voltage impressed in series with Z_w represents an incoming impulse from line west. Assume that Z_e is temporarily disconnected, leaving terminals 3-4 open; then the impressed voltage will cause a current flow as indicated by the arrows. If the value of Z₀ and the turns ratio are such that the impedance measured between terminals 1-2 (with the remainder of the circuit disconnected) equals Z_i, then the opposing voltage drop across the coil 1-2 will equal that across Z_i. The identical coil 2-3 is linked by the same flux as coil 1-2 and will therefore have a voltage induced in it equal to and in the same direction as the voltage across coil 1-2. This induced voltage will also be equal to the voltage drop Z_i. Thus, points 3 and 4 are at the same potential, and no current will flow through Z_e when line east is again connected. For the ideal conditions assumed, one-half of the incoming energy from line west is dissipated in Z₀, and the remainder is available in the input circuit Z_i for actuating the amplifying element. The action is similar for a signal voltage at Z_e instead of Z_w.

Fig. 6(b) represents the output of the amplifying element being induced into the telephone line. Coils 1-2 and 2-3 are connected in series aiding, and one-half of the total voltage will be induced across each. Currents will flow at a given instant as indicated by the arrows. The voltage drops across the equal impedances Z_e and Z_w will be the same, and thus no difference of potential will exist across Z_i and no current will flow through the amplifier input as indicated by the opposing current arrows at Z_i. Thus, the output circuit will not feed back into the input circuit and will not cause sustained oscillations.

The bridge transformer has other uses in communication and is also adapted to numerous measuring circuits. Thus, with a voltage impressed at Z₀, differences in impedances connected at Z_e and Z_w are readily detected by a tone heard in the headphones connected at Z_i. A mathematical treatment of the hybrid coil is given in reference 14.