Calculating voltage threshold with saturation voltage divider

[BigMan52]

Please can someone tell me if this calculation is correct? Please see attached question.
I have used the +/-3V as the saturation voltage and multiplied it by R2/R2+R3 to give me +/-1.5V.

 

[Jony130]

Is this a Homework ?
For the circuit in FIGURE 3(b) your answer is wrong.

Notice that you have additional +3V voltage source (V4) in your circuit. And this voltage source will have big influence on threshold voltage.

 

[BigMan52]

Sorry yes, I think I may have posted it in the wrong place. First time using this site. I will have a rethink and post it back in the correct section.
Thank you very much for your help.

 

[Jony130]

So now, can you show us the "right" answer? And my tip is: Find the voltage at non inverting input for two cases: one for Vout = +3V and Vout = -3V. And do not forget about V4.

 

[BigMan52]

Struggling to find the formula I should be using. I think this is a Schmitt Trigger and all calculations I have seen include a third feedback resistor which isn't here.

 

[Jony130]

OK. But are you able to find the Vx voltage for this circuit?

 

[BigMan52]

Would it be:
Vx = R1/R1+R2 x Vout = 0.5 x 3 = 1.5V?

 

[Jony130]

No, wrong again? Why do you keep ignoring the V4 source?

 

[BigMan52]

OK using basic ohms law:
I = 3/1000 = 3mA
So voltage dropped across the 1K resistor would be 9mV making Vx =2.991 V?

 

[Jony130]

OK, let us start again from the beginning.
The circuit look like this:

And notice that now V4 = 0V and R1 and R2 are connected in series.
The voltage at Va is equal to Vout = 3V, the Vb voltage is 0V because V4 = 0V.
So the voltage across (R1+R2) is Va - Vb = 3V - 0V = 3V and the current is I = 3V/(R1+R2) = 3V/2kΩ = 1.5mA therefore Vx = Vout - I*R1 = 3V - 1.5V = 1.5V
But in your circuit V4 is not 0V it is 3V so the situation looks like this (for Vout = +3V)

And when Vout = -3V

Can you solve it for Vx ?

And I highly recommend you to read this by starting at page 3
http://www.ittc.ku.edu/~jstiles/312/handouts/312_Introduction_package.pdf

 

[BigMan52]

OK thank you very much.
So here goes:
Vx when Vout is -3V:
Vb =V4 = 3V
Voltage across R1+R2 = Va-Vb = -3-3 = -6V
Current I = -6/2000 = -3mA
Vx = Vout - (I x R1) = -3 - -3 = 0V

Does that mean the upper threshold voltage for this op amp is +3V and the lower threshold voltage is 0V?

 

[Jony130]

Does that mean the upper threshold voltage for this op amp is +3V and the lower threshold voltage is 0V?

Yes exactly, this additional source (V4) will "shift" the threshold voltage up or down depend on V4 sign.

 

[BigMan52]

Brilliant, thank you very much for all your help!