Positive feedback has an unusual effect on the circuit. It forces the reference voltage to have the same polarity as the output voltage, The reference. voltage is positive when the output voltage is high (+v_sat) and negative when the output is low (–v_sat).

In a Schmitt trigger, the voltages at which the output switches from +v_sat to –v_sat or vice versa are called upper trigger point (UTP) and lower trigger point (LTP). the difference between the two trip points is called hysteresis.

	Fig. 5

The hysteresis loop can be shifted to either side of zero point by connecting a voltage source as shown in fig. 5.

When V_O= +V_sat , the reference. Voltage (UTP) is given by

When V_O= -V_sat , the reference. Voltage (UTP) is given by

If V_R is positive the loop is shifted to right side; if V_R is negative, the loop is shifted to left side. The hysteresis voltage V_hys remains the same.

Non-inverting Schmitt trigger:

In this case, again the feedback is given at non-inverting terminal. The inverting terminal is grounded and the input voltage is connected to non-inverting input. Fig. 6, shows an non-inverting schmitt trigger circuit.

Fig. 6

To analyze the circuit behaviour, let us assume the output is negatively saturated. Then the feedback voltage is also negative (-V_sat). Then the feedback voltage is also negative. This feedback voltage will hold the output in negative saturation until the input voltage becomes positive enough to make voltage positive.

When v_in becomes positive and its magnitude is greater than (R₂ / R₁) V_sat, then the output switches to +V_sat. Therefore, the UTP at which the output switches to +V_sat, is given by

Simillarly, when the output is in positive saturation, feedback voltage is positive. To switch output states, the input voltage has to become negative enough to make. When it happens, the output changes to the negative state from positive saturation to negative saturation voltage negative.

When v_in becomes negative and its magnitude is greater than R₂ / R₁ v_sat, then the output switches to -v_sat. Therefore,

The difference of UTP and LTP gives the hysteresis of the Schmitt trigger.

In non inverting Schmitt trigger circuit, the β is defined as

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