Telephone voice frequency keyboard interface - has twin supply bridges, amplifier, line circuit, control circuit and impedance change circuit interconnecting amplifier and control circuit

摘要

The appts. has a power supply circuit comprising two supply rectifier bridges having the same negative polarity, on amplifier, on impedance charge circuit, a transmitter control circuit and line circuit. The control circuit is connected to the positive and negative terminals of the first bridge and comprises a first adjustable voltage regulator and series resistance. The line circuit connected to the same bridge terminals comprises a second voltage regulator, line transitor and series line resistance. A negative terminal of the first regulator is connected to the line transistor base. The impedance change circuit comprises a transistor whose emitter-collector is connected between a control input of the first regulator and the first bridge negative terminal. The impedance charge circuit controls the charge in impedance of the interface circuit so that it presents a low impedance in conversation mode and high impedance in dialling mode.