Talk:Schmitt trigger

message: should it also be noted that it is also a regenerative comparator? Excuse me if I am not correct, but it appears to be the same thing.
:Yes, it's the same thing. Though a google search for "regenerative comparator" returns just 263 hits versus 364,000 for "schmitt trigger" and 25,400 for "schmidt trigger" so it's not a term used very much at all, and to me sounds like technobabble. I don't think we should encourage the use of this wordy term for such a common circuit by giving it a wikipedia endorsement. Graham 12:00, 15 March 2006 (UTC)
thread_title:  regenerative comparator? 

message: Could someone please better explain the purpose of resistor R4 in the figure at http://en.wikipedia.org/wiki/Image:Opampschmitt_realistic_xcircuit.png . The current explanation is "R4 has to limit the current due to the offset voltage on the input of the amplifier". I do not understand why R4 is needed at all. Thanks.
In an actual comparator, especially with bipolar transistor inputs, a small bias current is often required by both inputs.  If the small bias current was zero, then R4 would not be needed.  Normally, the bias current needed for each input is about the same, and changes such as temperature cause both input bias currents to increase or decrease together.  Note that if the positive input has a small bias current, that current arrives through resistors R1 and R2, causing a small additional voltage drop in those resistors.  By adding R4, we try to create the equivalent voltage drop at the negative input.  The goal is that if the voltage drop is the same for both inputs, the voltage drops cancel each other out.  Thevenin says that the value of R4 in ohms is equal to the computed value of R1 and R2 as if they were hooked up in parallel.  —Preceding unsigned comment added by 99.169.132.203 (talk) 04:50, 28 April 2009 (UTC)
thread_title:  inadequate explanation 

message: The idea of the Schmitt trigger is also used in digital implementations, e.g., to detect transients in a digitally sampled signal, e.g., for audio onset detection.  Does a "Schmitt trigger" have to be an analog circuit implementation as the article says?  Is there another name for this general idea (i.e., the logic of having a high and low threshold, etc.) when implemented in some other way?  
MusicScience 18:55, 9 October 2007 (UTC)
thread_title:  Digital/Software implementation? 

message: Is it possible to explain where this device is used in a common application we might recognize? A switch on an electronic device perhaps?Landroo (talk) 19:53, 5 April 2009 (UTC)
replies: text: See the updated Applications section. —TedPavlic (talk) 14:14, 22 June 2009 (UTC)
thread_title:  Applications? 

message: I would like to discuss the text below about Schmitt trigger with an output taken from the emitter instead from the collector.
"This non-inverting Schmitt trigger can be turned into a inverting trigger and another resistor saved in the process, by replacing Rc2 with a short connection, and connecting Vout to the emitter of T2 instead of its collector. In this case however, a larger value of resistance should be used for RE as it now serves as the pull-down resistor on the output, lowering the voltage on the output when the output should be low, instead of a serving as only a small resistance which is only intended to develop a small voltage across it that actually adds to the output voltage when it should be at a digital low."
Why Rc2 is replaced with a short connection as it is needed to determine the high threshold? What will be the high threshold then? Why RE has to have "a larger value of resistance"? Conversely, a pull-down resistor has to have high enough resistance to "pull" the node downwards. Circuit dreamer (talk, contribs, email) 15:41, 9 November 2010 (UTC)
thread_title:  Schmitt trigger with emitter output 

message: I have copied the text below from Talk:Hysteresis to move here the so interesting discussion about the relation between the two great phenomena. Circuit dreamer (talk, contribs, email) 15:39, 13 November 2010 (UTC)
thread_title:  About the relation between hysterisis and positive feedback 

message: This recent rewrite is very poor.  Poor grammar, misspellings, and incorrect assumptions throughout.  Zen-in (talk) 16:30, 14 November 2010 (UTC)
replies: text: Thank you for the "encouragement"; now, I am "inspired" to continue... You are welcome, as a native speaker, to correct the "poor grammar and misspellings", and, as a specialist, to correct the "incorrect assumptions" throughout the article where you can find them... Circuit dreamer (talk, contribs, email) 16:52, 14 November 2010 (UTC)
thread_title: Rewrite 2

message: Welcome, Oli! I am glad to see you again since Wikipedia is not so interesting without you:) My appeal to you is to be more constructive in your edits, to consider my edits before removing (not shoot first, ask questions later).
During the transition, an op-amp non-inverting Schmitt trigger is a typical Miller arrangement exhibiting negative resistance. IMO you are clever enough to realize this interesting fact. Let's consider the situation.
We have a resistive chain R1-R2 supplied by the input voltage from the left and by the output voltage from the right. Imagine the input voltage continuously increases. As the output voltage stays constant, the input current continuously increases as well and we have positive resistance. When the input voltage exceeds the high threshold, the output voltage begins changing vigorously in the same direction as the input voltage. The current begins decreasing vigorously although the input voltage continues increasing. Thus the input voltage increases while the current decreases; so, the circuit exhibit negative resistance. Look at the NIC on the right; a non-inverting Schmitt trigger is the same circuit where R1/(R1+R2) → 0. Regards, Circuit dreamer (talk, contribs, email) 17:02, 23 November 2010 (UTC)
replies: text: We've been through this sort of thing before.  The instantaneous behaviour at the moment of transition cannot reasonably be called "negative resistance".  Please provide a source if you disagree.  Oli Filth(talk|contribs) 21:48, 23 November 2010 (UTC)
thread_title:  Does the non-inverting Schmitt trigger exhibit negative resistance? 

message: Yes, it sounds paradoxical but there is a virtual ground (V+ = 0 V) at the non-inverting input (for a moment) in two situations at that: when the increasing input voltage becomes equal to the high threshold and when the decreasing input voltage becomes equal to the low threshold. In both the situations we have the same resistive chain as above (the two connected in series resistors R1-R2) supplied by two voltages with opposite polarities (see this picture). Here R1.Vs = -R2.Vin and the intermediate point is at zero voltage. In other words, the virtual ground represents the zero output voltage of the parallel resistive summer with weighted inputs composed by the two resistors R1 and R2. Circuit dreamer (talk, contribs, email) 17:59, 23 November 2010 (UTC)
replies: text: We've been through this before.  Virtual grounds don't exist in positive feedback situations.  Just because it's momentarily 0V at transitions does not make it a virtual ground.  I will be reverting this change, unless you can provide a source.  Oli Filth(talk|contribs) 21:45, 23 November 2010 (UTC)
thread_title:  Is there a virtual ground in a non-inverting Schmitt trigger? 

message: I'm inclined to remove the refimprove tag. It was place in 2009 with this edit. The article has been updated substantially since then, so I'm inclined to remove the refimprove tag unless there is something specific that is still lacking. Please discuss. 64.40.62.14 (talk) 04:21, 12 March 2012 (UTC)
replies: text: Removed tag per WP:BOLD. Please use  for any specific concerns. 64.40.62.84 (talk) 03:13, 13 March 2012 (UTC)
thread_title:  Removing refimprove tag 

message: This article is not easy to read and comes to conclusions that are not so obvious to the reader. This article should be improved. I have a series of questions and comments.
1. The third and the fifth figures from the top have captions that are way to long. The information should probably be part of the text.
2. Several sentences in the "Fundamental idea" section are written in italics. If they are quotes from something that was written, maybe by Schmitt, they should have quotes and it should be pointed out where they come from.
3. Under the "Initial state" section I read "It is approximately equal to the high threshold and may not be low enough to be a logical zero for next digital circuits."
Should that "next" be "following" if there is a stage to the right of the circuit?
4. Under the "Crossing up the high threshold" section I read "This avalanche-like process continues until Q1 becomes completely turned on (saturated) and Q2 turned off" and "Q2 becomes completely turned on (saturated) and the output voltage becomes low again".
Shouldn't Q1 and Q2 be past the saturation region or in the active/linear region if "completely turned on"?
5. Under the "Direct-coupled circuit" section I read "In this case, the common emitter voltage and Q1 collector voltage are not suitable for outputs".
What's the reason for the conclusion? Also, is this for the non-inverting variation of the circuit?
6. Under "Collector-base coupled circuit", what's the meaning of "VBE0 ∞ 0.65 V"? Why is "0" used? Is "∞" used for proportional or should it be a "="?
7. Under the "Op-amp implementations" section I read "An open-loop op-amp and comparator may be considered as an analog-digital device"
If that is the case, shouldn't the device be called an "open-loop op-amp-based comparator mixed-signal device"?
8. The "Non-inverting Schmitt trigger" section says "The output voltage V+ of the resistive summer". The "Inverting Schmitt trigger" section says "The output voltage V+ of the voltage divider is".
This is really confusing because, after simulating both non-inverting and inverting configurations of the Schmitt trigger, it is clear that, for the non-inverting circuit, the thresholds are V_\mathrm{\pm} = \pm\frac{R_1}{R_2}{V_s} and for the inverting circuit, the thresholds are V_\mathrm{\pm} = \pm\frac{R_1}{R_1+R_2} V_\mathrm{s}. If R1=5kΩ and R2=25kΩ then thresholds are respectively ±3V and ±2.5V.
Having said this, I do not understand what the wording about V+ in the non-inverting and inverting sections means (including "resistive summer" and "voltage divider" labels). To me the explanation is quite confusing. Also, I do not understand the meaning of V_\mathrm{+} = \frac{R_2}{R_1+R_2} \cdot V_\mathrm{in} + \frac{R_1}{R_1+R_2} \cdot V_\mathrm{s}.
I would suggest to revise the sections and use a single statement followed by V_\mathrm{\pm} = \pm\frac{R_1}{R_2}{V_s} for inverting and V_\mathrm{\pm} = \pm\frac{R_1}{R_1+R_2} V_\mathrm{s} for non-inverting. It would also be nice to specifically state that the hysteresis for non-inverting is V_\mathrm{H} = 2\frac{R_1}{R_2}{V_s} and inverting is V_\mathrm{H} = 2\frac{R_1}{R_1+R_2} V_\mathrm{s}, followed by the statement that says the hysteresis can be shifted by a DC voltage offset at the negative pin for non-inverting and the positive input pin for inverting.
9. Under the "Inverting Schmitt trigger" section I read "In contrast with the parallel version, this circuit does not impact on the input source since the source is separated from the voltage divider output by the high op-amp input differential impedance."
This sentense is unclear and needs to be revised.
10. The "Use as an oscillator" section says "A Schmitt trigger is a bistable multivibrator, and it can be used to implement another type of multivibrator, the relaxation oscillator."
This sentence seems to imply that the relaxation oscillator is a type of multivibrator when the truth is that the thing are actually reversed because a multivibrator is a type of relaxation oscillator.
ICE77 (talk) 03:36, 3 August 2015 (UTC)
thread_title:  Questions and comments 

message: It's referenced to Schmitt's article ... but it's not there. Only 'manuscript received' 26-11-1937, no mention of earlier years. Which sounds reasonable: the long-tailed pair was invented concurrently by Blumlein, Offner, Scmitt, Toennies in 1936-1937. There's certainly no prior work on the LTP published or patented before 1936 (and Schmitt's 1938 paper clearly describes the LTP, although in a different language). So the news of some graduate student inventing something that wasn't even conceived by the best engineers of the period - sounds highly unusual. Google books didn't quite help; some casually mention 1934 and squids, and some even disclose that 'we took it from wikipedia' . Vicious circle. 19:56, 15 July 2019 (UTC)
thread_title:  Sources for 1934?