# Schmitt-trigger how to make a connection

## Main Question or Discussion Point

OK, i know i am confusing something here big time, so...that's why i'm posting.
In my book it says that regular inverter has both positive and negative threshold at 2.5V, while Schmitt-trigger has VT+ = 2.9V and VT- = 2.1V.
So my question is how to make a connection (if there is any or how are they related ) to logic Low (0-1.5V) and High (3.5-5.0V) as defined by Wakerly, the author of our Digital design book.

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berkeman
Mentor
That's a good question, EvLer. The input circuit for traditional CMOS devices is pretty symmetric, and hence the actual switching point is pretty close to mid-rail. The CMOS gate actually goes into a linear mode when the input is near mid-rail, so you generally don't want to keep a CMOS input near mid-rail for long, or the Idd for the part will go up a fair amount. There are times when you run a CMOS gate in linear mode, but those are tricks outside the scope of normal digital logic.

The 1.5V and 3.5V threasholds listed in the datasheets for CMOS logic's Vil and Vih are there to provide voltage margin, not to reflect the actual switching voltages. If you test and plot Vo versus Vi for a CMOS gate, you will see that the switchover of the output occurs pretty close to when the input voltage passes through half of Vdd. The Vih and Vil levels for regular CMOS logic are listed away from the mid-rail switching point so that the logic that drives the CMOS input is ensured to have a good wide drive capability, and to ensure that the CMOS gate is kept away from its linear (higher Idd) mode of operation.

Schmidt input gates have hysteresis feedback built in, so that the gate can be driven with a slow analog waveform and the gate won't stall in the linear mid-range of the supply. For example, if you want to have a slow R-C relaxation oscillator with a CMOS gate as the active component, you need to use a Schmidt trigger inverter. You would connect the output of the inverter through a resistor to the input of the gate, and connect a capacitor from the input to ground. The period of oscillation is close to the R-C time constant, with some adjustments.

Vih and Vil numbers make less sense to publish for Schmidt input gates, because what you care about is what the hysteresis voltage is, and the + and - trip voltages are generally centered around mid-rail (with some tolerance). If you are driving a Schmidt input gate with other logic, you still want to treat it as a CMOS input and drive it with good CMOS levels. If you are using the Schmidt input gate for a more analog function or to handle slowly-changing input waveforms, then you will generally treat it more as an analog input and use the trip voltages in your margin calculations.

Hope that helps. -Mike-

Aaaaaahhhhh, i see...that's definitely more and more understandable than what my TA "explained" :uhh:
thanks so much!
berkeman (Mike), you rock!

berkeman
Mentor
EvLer said:
Aaaaaahhhhh, i see...that's definitely more and more understandable than what my TA "explained" :uhh:
thanks so much!
berkeman (Mike), you rock!

You might have seen me post this suggestion in other threads, but I'll repeat it anyway. Check out "The Art of Electronics" by Horowitz and Hill. Your college technical library should have copies that you can look through to see what you think. If you have a little spare cash, I'd highly recommend buying a copy from Amazon.com or whatever (2nd edition or whatever is the latest). It's one of the best and most intuitive and practical beginning electronics books I've found. It covers everything from basic diode and transistor concepts, to opamps, complex analog circuits, digital logic, micoprocessors, and lots of other stuff. It also has cool sections at the end of each chapter about "Good Circuit Ideas" and "Bad Circuits". The "Bad Circuits" part has some obvious errors shown, and also some subtle hard errors to figure out. Great stuff.

I was already out of school and working as an R&D engineer when I came across the book. I read it cover-to-cover, and found lots of great gems throughout the book. I'd find myself saying stuff like, "Hey, I always wondered why that was true." and "Oops, that's a mistake I made a few times before I figured it out." and "Dang, I sure wish I'd had this book while I was learning basic electronics!"

berkeman said:
Check out "The Art of Electronics" by Horowitz and Hill. Your college technical library should have copies that you can look through to see what you think.
8 copies, all checked out must be good thanks! recalled it from someone...

schmitt trigger project.HELP ME!!

I want to design a schmitt trigger circuit by using op amp UA741 and BJT as a switch. The problem is when the input signals is decreased from 4v the output in HIGH state while when the input is increased to 8v the transition in LOW state. Is it Vut= 8v and and Vlt= 4v. Before I'm forgot the single input is 12v. Is it Vcc=12 v is Vref? Until now, I become more confused because many of book doesn't mention about Vcc in their calculation.

berkeman
Mentor
far8 said:
I want to design a schmitt trigger circuit by using op amp UA741 and BJT as a switch. The problem is when the input signals is decreased from 4v the output in HIGH state while when the input is increased to 8v the transition in LOW state. Is it Vut= 8v and and Vlt= 4v. Before I'm forgot the single input is 12v. Is it Vcc=12 v is Vref? Until now, I become more confused because many of book doesn't mention about Vcc in their calculation.
Vcc will come into the hysteresis calculation because when the output of the opamp is high, that voltage affects the positive feedback voltages, right? Although, with a 741, you are not going to get rail-to-rail output voltages, so you will need to check the datasheet to figure out what its output voltages will be. Also check the input and output voltage ranges compared to the rails. The 741 is an old-school opamp, and you will generally not be running the input and output near the rails.