Hybrid coil

A hybrid coil (or bridge transformer, or sometimes hybrid) is a transformer that has three windings, and which is designed to be configured as a circuit having four branches, (i.e. ports) that are conjugate in pairs.

A signal arriving on one branch is divided between the two adjacent branches but does not appear at the opposite branch. In the schematic diagram, the signal into W splits between X and Z, and no signal passes to Y. Similarly, signals into X split to W and Y with none to Z, etc.

Correct operation requires matched characteristic impedance at all four ports.

Explanation

The primary use of a voiceband hybrid coil is to convert between 2-wire and 4-wire operation in sequential sections of a communications circuit, for example in a four-wire terminating set. Such conversion was necessary when repeaters were introduced in a 2-wire circuit, a frequent practice at early 20th century telephony. Without hybrids, the output of one amplifier feeds directly into the input of the other, resulting in a howling situation (upper diagram). By using hybrids, the outputs and inputs are isolated, resulting in correct 2-wire repeater operation. Late in the century, this practice became rare but hybrids continued in use in line cards.