| 摘要 |
PURPOSE:To change and set optionally a DC resistance value and to exert hardly the influence upon the AC impedance by providing a terminal which feeds back a DC voltage appearing between input terminals of a power supply circuit to the power supply circuit and connecting a resistance between this provided terminal and a trunk line. CONSTITUTION:An applied voltage becomes a current source current i1 of the power supply circuit, and a current i2 determined by emitter area ratio 1:n of transistors TRs Q1 and Q2 is flowed to the TRQ2. The current flowed to a speaking circuit is determined by resistances R1 and R2 and TRs Q1 and Q2, and the DC voltage between terminals 3 and 5 is determined automatically. If a resistance RA is inserted between terminals 3 and 4, the current i1 flowed to the resistance R1 is limited by a current i3 and is reduced and the current i2 is reduced also in proportion to it because the current i3 is flowed to the resistance R2. Therefore, the current flowed to a telephone set is reduced by the resistance RA as the result. In this case, the AC impedance of an integrated circuit A is determined by the resistance R2 and the impedance due to an early effect of the TRQ2, and the AC impedance is hardly affected. |
