| 摘要 |
PURPOSE:To increase the impedance of an AC output without increasing a DC resistance value of an amplifier for telephone set by adding an AC positive feedback loop to the amplifier circuit. CONSTITUTION:A capacitor 10 and a resistor 11 connected in series are provided between an output side of a voltage current converting circuit 9 and an inverting input terminal (voice signal input terminal) of an operational amplifier circuit 5. As to the AC impedance between terminals T1 and T2, capacitors C1, C2 and C3 are selected as values where the AC output impedance is sufficiently small and negligible, and an AC current iA flowing is expressed as iA=e/R4- e/RAXR3XK=eX(RA-R3XK)/(R4XRA), where eV is an AC voltage applied between the output terminals T1, and T2. Thus, the AC output impedance Zout is reduced to Zout=e/i=R4XRA/(RA-R3XR4XK)=R4/(1-R3XR4XK/RA), and when the resistance value RA of the resister 11 is set to e.g., RA KXR3X R4, the AC output impedance is increased without changing the resistor R4 of the resistor 1. |
