Telephone handset coupling circuit - has transistor and transformer linking unidirectional input and output circuits to bidirectional line without acoustic feedback

摘要

The circuit includes a unidirectional input receiving the microphone input signal. This also contains an automatic gain control circuit and a coupling capacitor. A second unidirectional circuit provides the output to the loudspeaker via a gain control circuit. A two wire bidirectional link is connected to one winding of a transformer. A transistor has its base connected to the input circuit, and a resistance bridge has its two terminals connected via coupling capacitors to the other terminals of the transistor. The final transistor terminal is also connected to the bidirectional link. The intermediate terminal of the resistor bridge is connected to the input of the unidirectional output circuit. The circuit is intended to avoid the problem of the microphone signal being re-transmitted by the loudspeaker, i.e. the Larsen phenomenon.