| 摘要 |
<p>PURPOSE:To increase an AC load of a telephone set sufficiently and to suppress the variance in low frequencies for a power supply voltage, by using a constant current source device supplying the regulated power supply in DC manner to the power supply circuit of a subscriber circuit. CONSTITUTION:Since a cut-off frequency of a low pass filter 23 is sufficiently lower than the audio frequency, if a bias voltage VF has a voltage change in higher frequency than the cut-off frequency of the low pass filter VF, although an output voltage VREF' of an amplifying circuit A4 changes, the output voltage of the low pass filter 23, i.e., a reference voltage VREF is not changed. Thus, this filter circuit has no voltage regulating action to signals of audio frequency, and an AC impedance viewed from a telephone set SUB is increased. On the other hand, when a bias voltage VF has a stationary error with the change in the power supply voltage, i.e., the output voltage of a DC-DC converter 24, the value of a bias voltage VP is corrected through voltage regulation.</p> |
