design of cmos schmitt trigger
You can use a non inverting input like one from CD4050.
Let's see how it works.
For begining let's suppose that the output of the gate is at low level, near ground.
The voltage V2 = ( R2 x V1 ) / ( R1 + R2 ).
Now raising the input voltage V1 from immediately bellow VT+.
When voltage V2 will cross beyond the transition voltage Vtr of the gate, the positive feedback that appears from the output to the input forced the output swinging to high level.
This happens very quickly due to the positive feedback.
Vtr = (( VT+ ) x R2 ) / ( R1 + R2 ) -> ( VT+ ) = (Vtr x ( R1 + R2)) / R2
When the input voltage V1 decrease, the output V3 goes to low level when
V2 = Vtr = (VT-) + ((VDD - (VT-)) x R2) / ( R1 + R2 )
We can get now
(VT-) = Vtr - ((VDD - Vtr) x R1 ) / R2
The hysteresis will be:
H = (VT+) - (VT-) = (VDD x R1) / R2
If Vtr = VDD / 2 then
(VT+) = Vtr x ( R1 + R2 ) / R2
(VT-) = Vtr x ( R2 - R1 ) / R2
The histeresis not depends upon transition voltage Vtr.
The resistance of the source signal is included on R1, too.