Hi!
i know how to analyze the Inverting Operational Amplifie that is shown here:

the analyze is by simple rouls like: I(in)=I(out)(no courrent getin in the amplifier) and I get an inverse wave from the entrance

but the next Schmitt trigger i dont understend

sudntly the ruls are difference and i dont know how to analyze this circuit(my output is a squre wave and not a nverse wave from the input).

my question is what is the diffrent if the V+ is up or down and how i analys the Schmitt trigger amp circuit.

 

One circuit has negative feedback, and is stable and linear.
One circuit has positive feedback, and is unstable and non-linear.

BTW the rules don't change, but you do have to do two analyses. One with Vout at the positive rail and one with Vout at the negative rail. That is where the hysteresis comes from.

 

With negative feedback, the feedback is the opposite polarity to the input so the op amp settles into a stable state with the input current through Rin equal to the feedback current through Rf.

With positive feedback, the feedback is the same polarity as the input, thus the output keeps rising or falling (depending upon the initial input polarity) until it reaches one of the rails (maximum voltage).
This gives the square wave output.

 

The "rules" for a non-inverting Schmitt trigger circuit are exactly the same as the rules for a non-inverting linear amplifier. The problem is that the feedback tends to drive the amplifier into saturation, but saturation is a possibility with an on-inverting circuit also if the component choices dictate it.

Assume a perfect opamp has an open loop gain of 1 million (120 dB), powered by +/-12 V. Without R2, when the input is an -0.000001 V, the output will be at - 1.0 V. Same for positive voltages: +10 uV in gets you +10 V out. But when the input is+20 uV, the output tries to get to +20 V and saturates. Now the circuit is operating in a non-linear mode, and the opamp is effectively a non-inverting buffer. For any inputs outside of +/-10 uV, the output is saturated either high or low, essentially a digital component.

Now connect R2. Since the output impedance is much lower than the value of R2 (good design practice), the non-inv input now sees the middle of a voltage divider between +12 V or -12 Vout and Vin. As Vin crosses GND, the voltage at the non-inv input pin snaps between two different values. With Ohm's Law you can make those two values whatever you want.

ak

 

One circuit has negative feedback, and is stable and linear.
One circuit has positive feedback, and is unstable and non-linear.

BTW the rules don't change, but you do have to do two analyses. One with Vout at the positive rail and one with Vout at the negative rail. That is where the hysteresis comes from.

Quite so - the Schmitt trigger is traditionally made with a comparator, but an op-amp is sufficient in all but the most exacting applications.

 

In short, a Schmitt trigger has hysteresis.

https://en.m.wikipedia.org/wiki/Hysteresis

 

In short, a Schmitt trigger has hysteresis.

https://en.m.wikipedia.org/wiki/Hysteresis

The TS might need that fleshed out to explain that hysteresis is caused by a smidge of positive feedback.

An input trend is accentuated by that small positive feedback and causes the output to flip state like a bistable.