| 摘要 |
PURPOSE:To secure execution of the AGC action by increasing the AGC time constant as well as reducing the gain to the exessive signal each in case the pulse- type excessive signal like the noise arrives with the level higher than the prescribed value. CONSTITUTION:When the noise or the like arrives, and the level of the noise mixing in is within the level under which the emitter potential of transistor TrQ1 is lower than voltage E1 and then reaches voltage E2. And thus TrQ3 among TrQ2 and Q3 forming the differential pair is turned off, and the current flows to TrQ2 from current source I1. With increment of the current to TrQ2, the collector current of TrQ5 decreases. Then the base voltage of TrQ6 lowers down, and the collector current of TrQ4 decreases. As the sum of the collector current of TrQ4 and Q6 features the constant current, the charging electric charge of capacitor 42 flows into TrQ4. The discharge of capacitor 42 lowers the output voltage of time constant circuit 30 and then the gain of video intermediate frequency amplifier circuit 21, and then suppresses the pulse-type excessive signal. And parallel resistance 41 is cut off when the excessive signal arrives. |
