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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.
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.
Hybrid coil circuit diagrams
Hybrids are realized using transformers. Two versions of transformer hybrids were used, the single transformer version providing unbalanced outputs with one end grounded, and the double transformer version providing balanced ports.
Single transformer hybrid
For use in 2-wire repeaters, the single transformer version suffices, since amplifiers in the repeaters have grounded inputs and outputs. X, Y, and Z share a common ground. As shown at left, signal into W, the 2-wire port, will appear at X and Z. But since Y is bridged from center of coil to center of X and Z, no signal appears. Signal into X will appear at W and Y. But signal at Z is the difference of what appears at Y and, through the transformer coil, at W, which is zero. Similar reasoning proves both pairs, W & Y, X & Z, are conjugates.
Double transformer hybrid
When both the 2-wire and the 4-wire circuits must be balanced, double transformer hybrids are used, as shown at right. Signal into port W splits between X and Z, but due to reversed connection to the windings, cancel at port Y. Signal into port X goes to W and Y. But due to reversed connection to ports W and Y, Z gets no signal. Thus the pairs, W & Y, X & Z, are conjugates.
Hybrids are used in telephones (see telephone hybrid) to reduce the sidetone, or volume of microphone 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, the transformer version of the hybrid has been replaced by resistor networks and compact IC versions, which uses integrated circuit 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.
- Douglas Rice (2008) Modelling Telephony Hybrids as 2 × 2 Matrices