Schmitt triggers are commonly implemented using an operational amplifier or the more dedicated comparator. An open-loop op-amp and comparator may be considered as an analog-digital device having analog inputs and a digital output that extracts the sign of the voltage difference between its two inputs. The positive feedback is applied by adding a part of the output voltage to the input voltage in series or parallel manner. Due to the extremely high op-amp gain, the loop gain is also high enough and provides the avalanche-like process.
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... The effective voltage applied to the op-amp input is floating so, the op-amp must have a differential input ... the input source since the source is separated from the voltage divider output by the high op-amp input differential impedance ...