Schmitt Trigger:

If the input to a comparator contains noise, the output may be erractive when v_in is near a trip point. For instance, with a zero crossing, the output is low when v_in is positive and high when v_in is negative. If the input contains a noise voltage with a peak of 1mV or more, then the comparator will detect the zero crossing produced by the noise. Fig. 1, shows the output of zero crossing detection if the input contains noise.

Fig. 1	Figure 19.2

This can be avoided by using a Schmitt trigger, circuit which is basically a comparator with positive feedback. Fig. 2, shows an inverting Schmitt trigger circuit using OPAMP.

Because of the voltage divider circuit, there is a positive feedback voltage. When OPAMP is positively saturated, a positive voltage is feedback to the non-inverting input, this positive voltage holds the output in high stage. (v_in< v_f). When the output voltage is negatively saturated, a negative voltage feedback to the inverting input, holding the output in low state.

When the output is +V_sat then reference voltage V_ref is given by

If V_in is less than V_ref output will remain +V_sat.

When input v_in exceeds V_ref = +V_sat the output switches from +V_sat to –V_sat. Then the reference voltage is given by

The output will remain –V_sat as long as v_in > V_ref.

Fig. 3	Fig. 4

If v_in < V_ref i.e. v_in becomes more negative than –V_sat then again output switches to +V_sat and so on. The transfer characteristic of Schmitt trigger circuit is shown in fig. 3. The output is also shown in fig. 4 for a sinusoidal wave. If the input is different than sine even then the output will be determined in a same way.

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