0
\$\begingroup\$

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

\$\endgroup\$
1
\$\begingroup\$

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)

\$\endgroup\$
  • \$\begingroup\$ ...as a linear amplifier? Icy - do you realize that the opamp has positive feedback? \$\endgroup\$ – LvW Oct 12 '15 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 '15 at 8:34
  • \$\begingroup\$ I have edited my answer to make it clearer. \$\endgroup\$ – Icy Oct 12 '15 at 8:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.