Electronic – How to determine low and high threshold levels of a Schmitt Trigger for a known input

The basic problem is how the positive feedback in your Schmitt trigger robs low output voltage. The feedback voltage is generated across RE, which is always added to the low level output voltage.

A Schmitt trigger is basically a amplifier with a little bit of positive DC feedback. Can you think of other ways of using two transistors to make positive gain? Perhaps some of those configurations allow for a feedback path that does not limit the output voltage swing.

Hint 1: Your exising amplifier topology can be used with a different feedback path.

Hint 2: The two transistors can be a mix of NPN and PNP. That may open your thinking to more amplifier topologies.

Such considerations simply don't apply to the normal applications of Schmitt triggers. Schmitts are generally used where the input slew rate is much less than the output slew rate. With a slow input, the trigger point is not clearly defined, being affected by other factors such as noise and feedback from the switching transients as the output changes. Once the trigger point is reached, the positive feedback pulls the threshold quickly to the other level, and oscillation is prevented.

However, it is entirely true that Schmitt triggers exhibit metastability, and this was demonstrated in 1977/1979 by a series of remarks in the IEEE Transactions on Computing, by E.G.Wormald and Thomas Chaney. See http://fpga-faq.org/FAQ_Pages/0017_Tell_me_about_metastables.htm for a discussion of the subject.

So basically, Schmitt triggers are not much use for fast signals. For slow signals, they are very effective, but in these cases the low frequencies make metastability much less of a problem, since high sampling rates don't make sense, and there is relatively more time for metastable conditions to resolve themselves.