CIRCUIT FOR CONNECTING POWER SUPPLY WITH TELEPHONE LOOP

摘要

PURPOSE: To realize inexpensive battery power supply to a telephone line card by driving the third winding of a transformer by means of a second amplifier which receives the output of a first amplifier that generates an output voltage proportionally to the current of a telephone loop. CONSTITUTION: An electric current from a ground flows to the junction of resistors 102 and 103 through a winding 121 of a three-winding transformer and further flows to a telephone loop 143 through a terminal 141. The current from the loop 143 flows to the junction of resistors 101 and 106 through a terminal 142 and further flows to a terminal 128 through the winding 122 of the transformer. A differential amplifier 124 outputs the signal transmitted from the loop 143, and another differential amplifier 125 drives a winding 123 of the transformer with a current proportional to the current of the signal transmitted from the loop 143 by driving an NPN transistor 126. Consequently, automatic magnetic flux cancelling action is produced in the transformer without using any auxiliary winding and inexpensive power supply can be made to a telephone line card from a battery with the action of the amplifier 125.