Hybrid coil

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W and Y, X and Z are conjugate pairs

A hybrid coil (or bridge transformer, or sometimes hybrid) is a single 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. That is, a signal that arrives on one branch will be divided among the two adjacent branches and not appear on the opposite branch. In the schematic diagram, signal into W will split between X and Z and none to Y. Signal into X split to W and Y none to Z, and so forth. Correct operation requires matched characteristic impedance at all four ports.

Wrong (upper) and right way using hybrids

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.

Hybrids were also used in telephones to reduce the sidetone, or volume of micrphone output that was fed back to the earpiece. Without this, the phone user's own voice would be louder in the earpiece than the other party's. Such hybrids also had their windings so arranged as to act as an impedance matching transformer, matching the low-impedance carbon button transmitter to the higher impedance parts of the system. Today telephones use active transistorised electronics to do the job of the hybrid coil.

Radio-frequency hybrids are used to split radio signals, including television. The splitter divides the antenna signal to feed multiple receivers.