| 摘要 |
PURPOSE:To avoid the malfunction of the AGC by cutting off the route between the AGC detection amplifier circuit and the AGC time constant circuit when the excessive signal arrives and then giving the control to secure increment of the time constant of the AGC time constant circuit. CONSTITUTION:The input signal level becomes excessive with the noise mixture into the input signal, and the base potential of transistor TrQ8 lowers less than noise comparison reference voltage E2 which is set at the joint between resistances 48 and 49. And thus TrQ8 is changed from ON to OFF, and vice versa with TrQ7 each. With TrQ8 turned off, the current path passing the common emitter of TrQ4 Q5 forming the differential pair is cut off along with TrQ6 turned off. On the other hand, the current of constant current source I2 is flowed to resistance 39 to lower the base voltage of TrQ9 constituting time constant control circuit 31 since TrQ7 is turned on. As a result, TrQ9 is turned on and then TrQ10 is turned on, and thus TrQ11 is turned off. Accordingly, only resistance 44 forms the time constant circuit, thus increasing the time constant. |
