| 摘要 |
PURPOSE:To keep the quality of communication in very excellent way, by detecting the change in line impedance of two-wire and adjusting the impedance of a side tone preventing element so that the side tone prevention is balanced. CONSTITUTION:An amplifier A1 inverts a detected voltage, resistors R11, R12 sets the gain of an amplifier A3, and a resistor R13 applies a control voltage to a transistor(TR) Q1. When a line impedance is increased, a voltage V5 across a resistor R2 is made small as a terminal current decreases to increase the output of the amplifier A3, i.e., the potential of a gate G of the TRQ1 and to decrease a resistance value between a drain D of the TRQ1 and a source S, allowing to suppress the increase in an unnecessary signal v'2. When the line impedance is decreased, a resistance value between the drain D and the source S of the TRQ1 is increased with inverse operation, the increase in the signal v'2 is suppressed, allowing to obtain a desired characteristic. |
