| 摘要 |
<p>PURPOSE:To make small the size of a communication current supply circuit and to make easy circuit integration, by constituting the communication current supply circuit without using any inductive element. CONSTITUTION:Telephone sets are connected to terminals 201, 204 and an exchanging circuit 1 is to terminals 200, 203 and the communication current is given from a power supply circuit 206. The output of a differential amplifier 208 which detects the output of the power supply circuit 206 passes through a low pass filter having the cut-off frequency of 300Hz or lower and it is added with the output of a differential amplifier 210 detecting a voltage across a resistor 207 and compared with a reference voltage Es at a comparison circuit 213. The comparison circuit 213 controls the power supply circuit 206 so that the output of an addition circuit 211 can be constant. Where a transfer function from the input of the comparison circuit 213 to the output of the power supply circuit 206 is taken as A, es is an audio signal voltage, is is a current flowing to the resistor 207, and Rs is the resistance value of the resistor 207, es/is=(1+A)Rs is obtained and a greater impedance can be obtained by making greater the transfer function A.</p> |
