need help with the schmitt trigger

Discussion in 'Electronic Basics' started by andrew james, Nov 18, 2004.

1. andrew jamesGuest

im a sophomore electrical engineering student and i need some help in
understanding the concept of the comparator and the schmitt trigger. could
anyone please explain its working briefly...

cheers..
andrew..

2. Jonathan KirwanGuest

My copy of Abraham I. Pressman's "Switching and Linear Power Supply, Power
Converter Design" is a first edition, I believe. (There are quite probably
later versions.) However, in my copy there is a great explanation of much of
what you want on pages 166 and forward. I wasn't able to grasp things so easily
from the Art of Electronics on this subject, but found this one much better.
Also, working out the rough details on paper isn't so hard.

How about describing what you think you know and what you just cannot get? It's
a lot easier to answer specific questions that to write a treatise on the
subject, you know.

Jon

3. Mark FergersonGuest

Thanks for the slap upside the head (mine, that is). I
was _that close_ to launching into an analogy about flexible
levers with detents...

Mark L. Fergerson

4. peterkenGuest

in short:
- a comparator compares one voltage level to another, and indicates which is
higher by switching it's output digitally as a function of which is higher,
using ALWAYS the comparision of both inputs
- a schmitt trigger will do identically, BUT the switching of the output is
only done if and only if a specific difference between the inputs exist
(called hysteresis)

this means a comparator is more sensitive to very small disturbances (like
noise) in input (thus sometimes switching needlessly), while a schmitt
trigger will ignore these very small differences and only switch at a
relevant (or even predefined) difference

5. John PopelishGuest

A comparator is essentially a 1 bit analog to digital converter. It
compares one voltage to another, and its output tells which is more
positive (the output is high if the + input is more positive). The
output state is indeterminate for inputs that are equal.

A schmitt trigger is similar in many ways, except that it has only one
input that is compared to an internal reference voltage. And that
reference is altered by the current state of the output so that when
the last comparison indicated that the input was more positive than
the reference, the reference charges to a more negative value (makes
the decision stronger). Likewise, when the comparison indicates that
the input is more negative than the reference, the reference changes
to a more positive value. The effect is that once a decision has been
made that the input has crossed through the reference voltage, the
input has to reverse direction a bit before the decision can be
reversed. It eliminates the possibility that a steady input can sit
at equality with the reference with the output being in an
indeterminate state for an arbitrary amount of time. It forces hard,
clean output state transitions.

Any comparator can be configured to act as a schmitt trigger by adding
some positive feedback components.

6. Jonathan KirwanGuest

Great overview. But the OP claims to be a "sophomore electrical engineering
student." At the university where I taught computer science, I also had a
number of electrical engineering students in every class I taught (usually 4-6,
out of say 55-65 students, total.) Bright folks, and the classes they were
pulling down in their sophomore (2nd) year were substantial and "difficult." I
desperately hope the OP isn't at the very basic level of understanding where
your answers were targeted. If so, that school isn't pacing their students
anywhere near fast enough. In my reply, I had assumed the OP was interested in
analyzing the analog details of a basic schmitt trigger, for example. But if

Jon

7. Bill BowdenGuest

There is a Schmitt trigger oscillator made from descrete
parts (resistors, caps, 3 transistors) and a short description
of how it works at this address:

http://ourworld.compuserve.com/homepages/bill_bowden/page13.htm#schmitt.gif

-Bill

8. dBGuest

The comparator has one threshold.
When the input is higher than the threshold the output is in one
state.
When the input is lower than the threshold the output is in the
opposite state.

The Schmitt trigger has two thresholds.
When the input is higher than the upper threshold the output is in one
state.
In order for the output to change to the other state, the input must
then go below the lower threshold.

You should, by now, have guessed that for the output to go to the
other state again the input must go higher than the upper threshold
again.

9. John FieldsGuest

---
A comparator is like a box with two inputs and an output switch which
can be either only on or off:

+-----+
+IN>----|+ |
| OUT|---->OUT
-IN>----|- |
+-----+

Its job is to make the output turn off when +IN is more positive than
-IN and turn on when -IN is more positive than +in.

Usually, the output switch is a transistor with its emitter connected
to ground, and when +IN is more positive than -IN the transistor is
turned off and its collector will be floating, but when -IN is more
positive than +IN the transistor will be turned on and the collector
will be connected to ground through the emitter:

Vcc
|
|
+-----+----+
| |
+IN>----|+\ C------->OUT
| >--B |
-IN>----|-/ E |
| | |
+-------|--+
|
|
GND

By connecting a resistor to the output of the comparator and making
+IN more positive than -IN, like this:

Vcc
|
+--------+ +V
| | |
| | [R2]
| +-----+----+ |
| | | |
+IN>-+--|+\ C--|---+---->OUT (+V)
| >--B |
-IN>-+--|-/ E |
| | | |
| +-------+--+
| |
+----------+
|
GND

The transistor will be turned off and OUT will be pulled up to +V
through R2.

However, by making -IN more positive than +IN, like this:

Vcc
|
+--------+ +V
| | |
| | [R2]
| +-----+----+ |
| | | |
+IN>--+-|--|+\ C--|---+---->OUT (GND)
| | | >--B |
-IN>--|-+--|-/ E |
| | | |
| +-------+--+
| |
+------------+
|
GND

The transistor will be turned on and the output will be pulled down to
ground through the collector-to-emitter junction of the transistor.

Notice that by using this convention, when +IN is more positive
(higher) than -IN the output will be high, (at +V) and when +IN is
less positive (lower) than -IN the output will be low (at GND)

That's how a comparator works. Hysteresis tomorrow...

BTW, Do you know how a voltage divider works?

10. John PopelishGuest

Thanks. You are thinking about the student's educational plan much
more than I do. I just try to figure out where their head is at the
moment and punt it from that point.

11. John FieldsGuest

---
Hysteresis...

Let's say that we have a see-saw which we can't see connected to a
switch, like this:

=================O================
|
+V--[R]--+---->OUT
|
|<--+
|
GND

and that when the see-saw is perfectly level we don't know whether the
switch is made or not, so OUT could be either +V or GND. However, we
do know that if the right end of the see-saw gets pushed down the
slightest bit the switch will be open and OUT will go to +V and if the
left end gets pushed down the least little bit the switch will close
and OUT will go to GND.

Unfortunately, with no weight on the ends, the see-saw stays perfectly
level and with the slightest breeze blowing on it the contacts will be
opening and closing randomly, so we won't know whether a weight on the
ends, or just noise (the wind) is causing the output to switch, .

The solution is to seal a marble in a tube and attach it to the
see-saw like this:

_________
|o |
=============|====O====|============
|
+V--[R]--+---->OUT
|
|<--+
|
GND

Now when a weight pushes the left end of the see-saw down, the marble
will roll to the left end of the tube, weighing down the left end of
the see-saw and the switch will stay made even if the weight is taken
off of the left end of the see-saw and the wind blows on it. If the
weight of the marble and its distance from the center of the see-saw
are determined properly, the wind won't be able to move the see-saw,
and the only thing that will open the switch is a weight on the right
end of the see-saw. When that weight is applied, the marble will roll
over to the right hand side and bias the switch against the wind the
same way it did on the left hand side. That's hysteresis.

Looking at our comparator model:

Vcc
|
|
+-----+----+
| |
+IN>----|+\ C------->OUT
| >--B |
-IN>----|-/ E |
| | |
+-------|--+
|
|
GND

and simplifying it to:

+V
|
[R]
|
|
+IN>----|+\ |
| >--+--->OUT
-IN>----|-/

we can liken +IN to the left end of the see-saw and -IN to the right
end.

Last installment tomorrow.

Maybe...