so I have to design a circuit which lights up 2 LEDs depending on the output voltage of the Non-Inverting Amplifier. I haven't connected the LEDs in the circuit below because it's not part of the problem I'm facing. The Reference voltages are 0.5Vp and 11Vp as shown in the image, I already designed them using comparator circuits with the voltage divider method.

When I first designed them I didn't use Hysteresis, instead I connected the feedback loop to VCC but that generated noise so I used Hysteresis instead. However, now I'm not sure how the output voltage of 26Vpp on the comparators is calculated. I've tried calculating but I can never get the right answer.

Can anybody help to show how the output voltage of 26Vpp is calculated?

enter image description here

  • \$\begingroup\$ Try read this electronics.stackexchange.com/questions/465430/… \$\endgroup\$
    – G36
    Nov 11, 2019 at 12:13
  • \$\begingroup\$ But the opamp output voltage has not thing to do with the use of hysteresis. But about this topic you can read here e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/05/08/… \$\endgroup\$
    – G36
    Nov 11, 2019 at 12:16
  • \$\begingroup\$ @G36 I tried to worked out values for R1,R2 and R3 like you showed but in the simulation it wouldn't trigger at the right voltage so wouldn't the circuit I'm using be different because the one in the article is using single supply but I'm using dual supply? \$\endgroup\$
    – Jason
    Nov 11, 2019 at 15:08
  • \$\begingroup\$ For PR4 the threshold point will be equal to \$14V*\frac{1k\Omega}{25k\Omega +1k\Omega } \approx 0.54V\$ and for the negative, it will be the same \$-0.54V\$. \$\endgroup\$
    – G36
    Nov 11, 2019 at 15:33
  • \$\begingroup\$ Ok I see what you mean now thank you for the helping me \$\endgroup\$
    – Jason
    Nov 11, 2019 at 16:33

1 Answer 1


The simple answer here is that in this case the supplied Vcc sets the maximum output swing of the opamps. Per the UA741CP spec sheet a Vcc of +-15v gives a typical maximum output swing of +-13v (or 26Vpp).

Since the UA741CP is not a rail to rail output opamp the saturation voltages of the output stage prevent seeing the full Vcc (+-15v) at the output.

For reference here is the AU741CP datasheet:
https://www.jameco.com/Jameco/Products/ProdDS/830522.pdf (See specification Vom).


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