| 摘要 |
PURPOSE:To prevent previously the breakdown of an amplifier due to an excessive input by converting a part of the output signal into a DC signal and controlling the input bias or the input gate voltage of the amplifier when the level of said DC signal exceeds the set value. CONSTITUTION:An input signal P1 is supplied to a power amplifying circuit 2 via an amplifying circuit 1 of the preceding stage and delivered through an output terminal P2. The amplitude signal P2 proportional to the output signal is extracted out of the circuit 2 and converted into a DC signal P3 by a wave detecting circuit 3. This signal P3 is amplified by an amplifying circuit 4 and supplied to a threshold value gate circuit 5. When the level of this supplied signal of the circuit 5 exceeds the threshold value set by a constant voltage diode ZD, i.e., the signal P2 has an extremely high level, the signal is delivered to the juncture between resistances R10 and R11. Then the inverted signal is supplied to the gates of FETQ1 and Q2 from an operational amplifier A2, and the gate voltage is suddenly reduced. This can prevent previously the breakdown of an amplifier owing to the escessive input of an FET. |
