| 摘要 |
PURPOSE:To attain individual calling between indoor sets while keeping the number of indoor wirings to 4-wire by forming an amplifier for indoor talking as differential input type and connecting respectively an input terminal and an output terminal of this amplifier to each vertically opposite angle of a balanced resistance bridge circuit so as to prevent howling. CONSTITUTION:The amplifier circuit for door phone talking consists of a differential amplifier 4, a hybrid 5 and an input circuit 18 or the like, and the bridge circuit comprising an equivalent resistor Ri toward a sound line a' from the secondary winding 132 of a sound transformer 13 and resistors R1-R3, the vertically opposite angles (s), (t) are connected respectively to an inverting and a non-inverting input terminal of the differential amplifier 4, and the other vertically opposite angle (u) is connected to an outdoor line (e), i.e., an output terminal of the differential amplifier 4, and the vertically opposite angle (v) is connected to a common line (d) in the other vertically opposite angles (u), (v). The secondary winding 132 of the sound transformer becomes an input to the differential amplifier 4. The condition of R1XR3=R2XRi is established among the resistors R1, R2, R3 and Ri. Thus, a signal from the door phone D or an output of the differential amplifier 4 does not appear between the vertically opposite angles (s) and (t). Then, the generation of howling and oscillation is prevented. |
