Could anyone explain the Schmitt trigger op-amp to me? i find it a bit confusing

 

My guess is that you are talking about an op-amp circuit that is configured as a Schmitt-trigger comparator.

Look up what a Schmitt trigger is and see if that helps.

 

Schmitt trigger is a comparator version op amp.
Max.

 

Hello,

Here is the wiki page on what a Schmitt trigger does:
https://en.wikipedia.org/wiki/Schmitt_trigger

Bertus

 

In general, a Schmitt trigger is a circuit that has a small amount of positive-feedback that makes the trigger voltage for activating the circuit higher than the voltage for deactivating the circuit (trigger hysteresis).
That makes the circuit immune (prevents it from triggering or oscillating) to noise voltages smaller than the hysteresis value.

 

Look at LM311 etc, these op amps act as a Schmidt Trigger.
Max.

 

Look at LM311 etc, these op amps act as a Schmidt Trigger.

That's a comparator not an opamp, and it only acts as a Schmitt Trigger if you add some positive feedback.

 

..

 

@subatomic particle The LM311 comparitor acted as a Schmidt trigger for me used this way.

1. Schmidt Trigger: a bistable circuit in which the output increases to a steady maximum when the input rises above a certain threshold, and decreases almost to zero when the input voltage falls below another threshold.

Max.

 

@subatomic particle The LM311 comparitor acted as a Schmidt trigger for me used this way.

1. Schmidt Trigger: a bistable circuit in which the output increases to a steady maximum when the input rises above a certain threshold, and decreases almost to zero when the input voltage falls below another threshold.

View attachment 177919

Max.

Where did you get that definition from? It's unusually vague. Every definition I've read talks explicitly about feedback and hysteresis.

Although the definition hints at the possibility of hysteresis by describing two thresholds, it's somewhat unclear. Further, the illustration you attached shows no hysteresis at all.

I'm no expert, so I could easily be wrong, but I'll say this: Even if the definition and illustration are technically correct, they don't reflect what people expect when you discuss a Schmitt trigger.

 

Schmidt Trigger: a bistable circuit in which the output increases to a steady maximum when the input rises above a certain threshold, and decreases almost to zero when the input voltage falls below another threshold.

In your posted comparator circuit both thresholds are basically the same.
In a Schmitt Trigger circuit there is positive feedback so that the rising threshold is higher than the falling threshold.
Otherwise there would be no difference between a simple comparator circuit and a Schmitt Trigger circuit.

 

@subatomic particle The LM311 comparitor acted as a Schmidt trigger for me used this way.

1. Schmidt Trigger: a bistable circuit in which the output increases to a steady maximum when the input rises above a certain threshold, and decreases almost to zero when the input voltage falls below another threshold.

View attachment 177919

Max.

I've got a few problems with this definition.

First, it's not a Schmidt Trigger, it's a Schmitt Trigger, named after it's inventor, Otto Schmitt.

Second, just saying that the second change occurs at "another threshold" isn't enough. If the first threshold is for when the input signal is rising, then the second threshold (for the falling signal) must be at a lower value. This definition would settle for any threshold that was simply different.

Third, this definition requires that the output goes HI when the input signal is high. But that's not required and many Schmitt triggers behave the opposite. The classic op-amp implementation behaves this way because that's one of the simpler ways to implement the hysteresis.

Fourth, this definition insists that one of the output levels has to be zero. There's no such requirement. The outputs can be whatever two levels are desired. Bipolar outputs are quite common.

In addition, as others have noted, the figure is NOT for a Schmitt Trigger, even by this definition. The entire purpose of the Schmitt Trigger requires that the threshold move in the direction opposite the direction the input signal is changing (although an alternative is to augment the input signal to make it move further in the direction it was already moving while keeping the threshold the same).

 

Have a look at this link...
The Schmitt Trigger

 

Have a look at this link...
The Schmitt Trigger

There are also Scmiditt Trigger mechanisms used in digital circuits. For instance a 7414 TTL digital inverter embodies Scmidtt Trigger characteristics. These are very useful in circuits where there is noise on a digital line.

 

There are also Scmiditt Trigger mechanisms used in digital circuits. For instance a 7414 TTL

Also 74LS132 to name another.
Max.

 

Some CMOS Schmitt Trigger logic gates are the CD4093 NAND gate, and the CD40106 Hex Inverter.

The CD4093 is useful to configure as a Schmitt Trigger XOR function when building a low-frequency XOR pulse doubler circuit.