| 摘要 |
PURPOSE:To nearly eliminate a sound signal loss by a call-current supplying circuit in a telephone switching unit, by connecting the input of a power amplifier, whose voltage amplification factor is not more than one, to a reference voltage via a resistance and then by sending the output to a subscriber's terminal via a different resistance. CONSTITUTION:Since sound signal V appearing at lead 30 is transmitted via capacitor 22 to power amplifier 20 whose voltage amplification factor is not more than one, denoting the output voltage of amplifier 20 as V1 and the sound signal voltage as V results in that when voltage V1+V is applied to lead 30, the input voltage of amplifier 20 is V1+V and its output voltage is also V1+V. Therefore, there is no difference in potential between outputs of lead 30 and amplifier 20 and the sound signal generated via resistance 21 has no loss. Similarly, the sound signal appearing at lead 31 has no loss caused by power amplifier 10. In addition, operationa amplifiers 14 and 24 are high in input impedance. When resistance values of resistance 13 and 23 are further increased, a loss through capacitors 12 and 22 can be ignored. Thus, a sound signal loss can nearly be eliminated with the miniature and lightweight constitution. |
