non inverting schmit trigger circuit not working

Thread Starter


Joined Apr 26, 2020
Hey guys,
I'm doing a deboucing circuit with a schmit trigger. I am simulating it in LTSPICE. Everything works fine when I use the model of the schmit trigger.
However I want to replace the schmit trigger with a non inverting op amp circuit.
The logic is the following : my RC circuit charges my capacitor.
1. Once the cap voltage reaches a threshold the schmitt trigger switches state.
2. When the voltage of the cap decreases the schmit trigger will eventually switch state again.
The step one (1.) works fine. However my schmit trigger output never goes low. When I look at the input in_+ I do see that the voltage at this pin never falls back so obviously the schmit trigger never changes state. The screenshot below shows the problems.
Is there something I am misunderstanding or is this the normal behaviour of this circuit ?
Thanks a lot for the help guys. I also linked the simualation files.



Joined Feb 24, 2006
Generally speaking, when working with single supply opamp circuits you should bias that amplifier at Vcc/2. Also inverting and non-inverting circuits are not interchangeable. An inverting configuration can have any gain that you can represent with a combination of resistors from near zero to near infinite. You cannot do this with a non-inverting configuration. In the noninverting configuration you CANNOT realize a gain of less than one. I'll take a look in my archive to see if I can find additional information. Take a look at how you are deriving the upper and lower threshold points of the Schmitt Trigger characteristic.


Joined Feb 24, 2006
Here are the basic calculations:
The lowest trace in dark green of V(vo) shows the upper and lower threshold points using a comparator with an open collector output. I would call this an inverting configuration. Notice the logic gate U2, following the comparator which provides an inversion. You could also use a buffer here if the inversion was not desired.


Joined Feb 24, 2006
Schmitt Triggers are normally implemented with devices that produce a digital output. Can you explain the reason for wanting to use an opamp?