| 摘要 |
First and second transistors of the same conductivity type have their emitter-collector paths serially disposed across a DC operating potential source, with the emitter electrode of the first transistor directly connected to the collector electrode of the second transistor and to an output terminal. First and second emitter-followers have inputs commonly connected via a resistor to a reference point. The output of the first emitter-follower directly drives the base electrode of the first transistor, while the base electrode of the second transistor is rendered responsive to the output of the second emitter-follower. A control transistor is commonly connected to the inputs of the emitter-followers so as to disable said emitter-followers and render both of said first and second transitors non-conductive when said control transistor is conducting, and to enable said emitter-followers and render both of said first and second transistors conducting when control transistor is cut off. A keying circuit is coupled to said control transistor for cutting off the control transistor only during predetermined keying periods. The selective cutoff of the control transistor effectively clamps the output terminal during each keying period to an output voltage level which is independent of the magnitude of said operating potential and determined in accordance with the potential at said reference point.
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