| 摘要 |
A differential pair of a first and a second transistor is used in a tone burst generator circuit for generating tone bursts for periodic pulse metering. The first transistor carries all of the current through the differential pair when no tone burst signal is to be generated. When a tone burst signal is to be generated, the base or gate voltage of the first transistor is reduced relative to the base or gate voltage of the second transistor, thereby causing the second transistor to conduct and the first transistor to be turned off. A continuous tone signal connected to a common current path through the differential pair is then coupled through the second transistor to a telephone connection in order to apply a tone burst signal to the connection. When a tone burst signal is to be decoupled from the connection, the base or gate voltage of the first transistor is raised to a level above that of the second transistor, again causing all of the current through the differential pair to flow through the first transistor, thereby turning off the second transistor. This decouples the continuous tone signal from the telephone connection. A RC delay circuit is employed to control the rise and fall times of the tone burst signal.
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