I am trying to work on a design project and part of the overall circuit includes an Inverting Bistable Multivibrator using an OpAmp.

I have designed the circuit which is supposed to have a Upper/Lower Trigger point at +/-2 volts. As the circuit contains positive feedback, it is bound to saturate the output at L+/- of OpAmp. However my output seems to follow input with some gain instead of saturating.

Could someone explain what is wrong in the circuit and how can the desired functionality be achieved. I do not want to use non inverting bistable multivibrator ( and I have verified the saturation using non inverting) so as to keep the overall circuit polarity negative. Any help would be much appreciated if I can get a square wave (Green Trace) instead of triangular wave.

Following is the design schematic and the scope capture.

enter image description here


1 Answer 1


Your title is misleading - there is no bistable multi-vibrator in the circuit.

The circuit diagram as drawn has the op-amp shown with positive feedback, in this configuration it is a Schmidt trigger, which would give the output you want.

However, you have constructed the circuit with the +/- inputs (pins 2 and 3) the wrong way round - giving negative feedback. In this configuration the op-amp acts as a linear amplifier with a gain of 2 (1 + Rf/Rg)

  • \$\begingroup\$ ...as a linear amplifier? Icy - do you realize that the opamp has positive feedback? \$\endgroup\$
    – LvW
    Oct 12, 2015 at 8:25
  • 1
    \$\begingroup\$ Yes, It is drawn on the schematic with positive feedback, but it is clear from the OP's description of the circuit operation - and his scope trace - that he has built the circuit with negative feedback - not as drawn! \$\endgroup\$
    – Icy
    Oct 12, 2015 at 8:34
  • \$\begingroup\$ I have edited my answer to make it clearer. \$\endgroup\$
    – Icy
    Oct 12, 2015 at 8:40

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