| 摘要 |
PURPOSE:To suppress mismatching loss with a line impedance and a DC power loss lower and to improve the talking quality by adopting a constant voltage circuit having an active element whose input terminals are shunted in terms of AC for a bias circuit of a transmission amplifier circuit. CONSTITUTION:A line current amplified by a DC current amplifier circuit 80 flows to a reception amplifier 72 in a reception circuit 70 and the amplifier is driven. In this case, a DC resistance R80 of the circuit 80 is expressed as R80=R79+ECE/Ic76 when the collector current Ic76 of a transistor (TR) 76 is maximum. The DC power loss in the DC current amplifier circuit 80 consists of voltage drop component by the resistor R79 and saturated voltage component of the TR 76. The AC impedance ZT of an AC equivalent circuit is expressed nearly as a parallel combined impedance of R81, R57', Z73 and R77, the AC impedance ZT depends substantially on the termination resistor R81 and the effect of a power supply circuit is very less. Thus, the DC characteristic is improved and the side tone prevention characteristic is improved. |
