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Effect of the use of Schmitt trigger (B) instead of a comparator (A)

In electronics a Schmitt trigger Schmitt trigger is a special type of comparator circuit. It was invented by American Otto Herbert Schmitt.

Operation

A Schmitt trigger output changes state when the voltage at its input exceeds a certain level, the output does not change back when the low input voltage, but the level of tension for change is a different, lower than the first. This effect is called hysteresis. This is the main difference with a normal comparator, which is a simple operational amplifier without feedback, and that its output depends only on the largest input.

The uses Schmitt trigger hysteresis to prevent noise that could overlap with the original signal and cause false changes in status if the reference and input levels are similar.

For its implementation is often used an operational amplifier positive feedback. The reference levels can be controlled by adjusting the resistors R1 and R2:

For example, if the initial trigger is activated, the output is in high state at a voltage Vout = + Vs, and the two resistors form a voltage divider between the output and input. The tension between the two resistors (entry +) is V +, which is compared with the input voltage - which assume 0V (in this case, the absence of negative feedback in the operational, the tension between the two inputs need not be the same). To produce a transition to the output, V + must descend and reach at least to 0V. In this case the input voltage is
At this point the output voltage changes Vout =- Vs For an argument equivalent to the condition we get to go from-Vs to + Vs:

This causes the circuit to create a band centered at zero, with trigger levels ± (R1 / R2) VS. The input signal must come from that band to get to change the output voltage.

If R1 or R2 is zero is infinity (open circuit) the band has a width of zero and a comparator circuit will work as normal.

To indicate that a logic gate is the Schmitt trigger type is placed on the inside of the hysteresis symbol:

Mario Dominguez Zambrano

EES Section: 02