| 摘要 |
<p>PURPOSE:To surely supply a required DC current to a subscriber terminal equipment against a line resistance change over a wide range by applying constant current feeding at a region where the line resistance is comparatively low and bringing the characteristic to the constant voltage characteristic when the line resistance is large. CONSTITUTION:A voltage drop across resistors 2, 3 is extracted by operational amplifiers 6, 7 in opposite polarity and combined by the 1st differential amplifier 8. The output of the differential amplifier 8 is fed to a non-inverting input of the 2nd differential amplifier 10 via an LPF 9 and the 1st reference voltage Vr1 is fed to the non-inverting input. An output voltage of the differential amplifier 10 is fed to the control input of DC current supply sources 4, 5 directly and via an operational amplifier 11 whose gain is -1. Further, a line voltage E0 between telephone line L1 and L2 is detected through an operational amplifier 12 and an LPF 13 and when the voltage reaches the 2nd reference voltage or below being lower by the minute quantity from the power supply voltage VB, the gain of a variable gain amplifier 15 is changed depending on the voltage E0, then the 1st reference voltage Vr1 is increased and the feeding current IL is controlled so as to be depending on the voltage E0.</p> |
