In a Schmitt trigger the output vo is limited by Zener diodes.

In a Schmitt trigger the output v_o is limited by Zener diodes. Single V_s is connected to the negative input terminal (v_–) and v₊ = β v_o. The Hysteresis voltage is 1 volt. Triggering takes place for signals going through:

1. zero volt
2. 0 volt and + 1 volt
3. 0 volt and – 1 volt
4. + 0.5 volt and – 0.5 volt

In the schmitt trigger triggering is taken place from
+ V_sat β to – V_sat β
given that, V_H = 1. Also, we know

V_H =	2R₂ V_sat
	R₁ + R₂

or V_H = 2. β. V_sat		β =	R₂
			R₁ + R₂

or β V_sat =	1	= 0.5
	2

so triggering taken place + 0.5 V to – 0.5 V.
Hence alternative (D) is the correct choice.