| 摘要 |
The invention relates to a centrally powered telephone station with piezoelectric microphone and amplifier for direct listening and hands free talking, an impedance transformer providing a small, stable feed current and a low alternating-current resistance with respect to microphone output. The impedance transformer is low-noise and can be connected to all supply voltages from 2V to 100V. The object of the invention is to be seen as that of connecting the impedance transformer to the subscriber station via only two wires of the handset cord. This is achieved (according to the figure) by predetermining a certain operating point for the transistor (T) of the impedance transformer by certain resistor arrangement and resistor rating (R1, R2, R3) and by being able to keep the alternating-current resistance sufficiently small with respect to the input resistance of the wires (A1, A2) used of the handset cord by a corresponding arrangement of a capacitor (C2) and a resistor (R3) . <IMAGE>
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