enter image description here

Inverting Schmitt Trigger was rigged up in the lab using uA 741. The values of R1 and R2 is taken as 10K and Vsat at 12 V and Vin at 1 V sinusoidal input.The oscillation from +Vsat to -Vsat occured at around at aroud 11.6 V input(increased from 1 V gradually) which is expected.

But when I increased the input more I noticed the positive part of the input was clipped a little (I guess it was some kind of loading at the input) due to which I noticed a small increase in the -Vsat part, just a small notch like.

the input and output waveforms observed in the lab enter image description here

I changed the op-amp but the effect was same.

What might be the reason for this effect. What test circuit will help me know more about this effect.

  • \$\begingroup\$ You avoid many problems like this if you actually use a comparator instead of an op amp. \$\endgroup\$ – Scott Seidman Aug 9 '16 at 16:03

I'm assuming you have +/- 15 V power supplies and for that level of supply the input common mode range is between +/- 12 V and +/- 13 V.

Any signal outside these limits and the performance guarantees in the data sheet are out of the window. You appear to be injecting a peak to peak voltage of over 30 V (according to your o-scope).

Now I don't mind if you blow the 741 up (and all the rest in the universe because they are really crappy come to think of it) but you might care so, respect the information in the DS or you may damage it.

The DS says the absolute maximum input voltage is +/- 15V. It also says this: -

7.3.1 Overload Protection - The LM741 features overload protection circuitry on the input and output. This prevents possible circuit damage to the device.

This probably means that inputs approaching the "out-of-bounds" area will be somewhat clipped.

  • \$\begingroup\$ This effect was noticed well within the voltage limits of +/- 15 V also. If I have Vsat at +/- 12V then at around 13-14 V range of the input I noticed this effect. And once the +/- Vsat is set, no matter what the input is the output is limited to Vsat. So op-amp looks safe. \$\endgroup\$ – turtle Aug 9 '16 at 14:51
  • \$\begingroup\$ Might be due to the overload protection circuit. While trying with few variations to the circuit and changing the voltage levels, all the combinations resulted in only positive part of the input getting clipped. So that got me thinking! Like anything in particular is responsible for this effect. \$\endgroup\$ – turtle Aug 9 '16 at 15:09

Consider what is happening to the internal bias network as your inverting input approaches the positive rail:

enter image description here

  • \$\begingroup\$ Yes thanks for pointing that out. And you had also mentioned about reducing the frequency. If I do that , one thing noticed was, delay in switching between +Vsat and - Vsat. Is there anything I should be looking for. \$\endgroup\$ – turtle Aug 10 '16 at 13:14

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