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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Monostable and Bistable Multivibrators

Schmitt Trigger Circuit

The Schmitt trigger circuit is, in effect, a cathode-coupled version of the Eccles-Jordan bistable multivibrator. The triggering arrangements, however, are different. In the Schmitt trigger the output voltage takes up one or the other of its two stable levels depending upon whether the input voltage is above or below some chosen value. The circuit is usually designed so that the change-over between stable states occurs for dfferent values of input. For example, to change from the first stable state to the second, the input may have to rise through + 4V. To switch back to the first stable state the input may have to fall below + 1V (Fig 15a). The difference between the two triggering levels (3V in this example) is known as the 'backlash' or 'hysteresis' of the system.

For a bistable circuit which operates with this type of triggering input, a good square wave output is obtained for any input which passes through the required triggering points. This is illustrated in Fig 15b. In addition, the Schmitt trigger may be used to determine when the amplitude of a signal applied to it has exceeded a critical value (the 'trigger-on' level).


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