Schmitt Trigger

Theory

Op-amp Schmitt Trigger Circuit

 The simplest Schmitt Trigger circuit is implemented with a comparator with a positive feedback. Look at the following circuit:
Fig.1 Schmitt Trigger Circuit Diagram

When the non-inverting input (+) is higher than the inverting input (-), the comparator output switches to the POSITIVE voltage supply. On the contrary, the non-inverting input (+) is lower than the inverting input (-), the output switches to the NEGATIVE voltage supply.

The inverting input (-) is grounded, so someone would expect that the turn-on and off point would be the ground (0). The function of the ST comes from the feedback resistor RFB. When for example the output of the comparator is to the POSITIVE voltage supply, then the non-inverting input has through the RFB this voltage. The same happens when the output is to the NEGATIVE power supply.

The voltage needed to switch the output of the comparator must be above or below zero (ground), according to the POSITIVE and NEGATIVE power supply and according to the resistors RI and RFB. More specific, the formula to calculate the threshold voltage is:


So, if the output is to the POSITIVE voltage, the required negative voltage that must be applied to Vin is:

VINPUT < = - VTHRESHOLD

If the output is to the NEGATIVE voltage, the required positive voltage that must be applied to Vin is:
VINPUT > = VTHRESHOLD

The above circuit is non-inverting Schmitt Trigger. It can be easily converted into an inverting Schmitt Trigger as follows:


The same formula is used to calculate the threshold level. But the output of this circuit is inverted in comparison to the previous one. When the input voltage is above the High Threshold Level, the output goes LOW.

There are basically two symbols for the Schmitt Trigger. The symbol is a triangle with an input and an output, just like the one used for the non-inverting buffers. Inside there is the hysteresis symbol. Depending on the type of Schmitt Trigger, inverting or non-inverting (standard), the hysteresis curve sign differs.