Homework Help: Help with voltage threshold calculations

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.

 

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.

 

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

 

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

 

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

 

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