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Friday, September 24, 2010

Schmitt Trigger using Op-Amp

A Schmitt trigger circuit is a fast-operating voltage-level detector. When the input voltage arrives at the upper or lower trigger levels, the output changes rapidly. The circuit operates with almost any type of input waveform, and it gives a pulse-type output.The circuit of an op-amp Schmitt trigger circuit is shown in figure. 

The input voltage vin is applied to the inverting input terminal and the feedback voltage goes to the non-inverting terminal. This means the circuit uses positive voltage feedback instead of negative feedback, that is, in this circuit feedback voltage aids the input voltage rather than opposing it. For instance, assume the inverting input voltage to be slightly positive. This will produce a negative output voltage. The voltage divider feedsback a negative voltage to the non-inverting input, which results in a larger negative voltage. This feedsback more nega tive voltage until the circuit is driven into negative saturation. If the input voltage were, slightly negative instead of positive, the circuit would be driven into the positive saturation. This is the reason the circuit is also referred to as re generative comparator.