Hybrid transformer

The primary use of a voiceband hybrid transformer 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 uncontrollable feedback oscillation (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.

A different kind of hybrid is used in telephone handsets to convert the four wires of the transmitter (earpiece) and receiver (microphone) to the 2-wire line connection.

It does not produce a high voltage, but like the high-voltage variety, it is a step-up transformer in order to impedance match the low-impedance carbon button transmitter to the higher impedance parts of the system.

The simple induction coil later evolved into a form of hybrid as a sidetone reduction measure, or volume of microphone output that was fed back to the earpiece.