At a test, I was given the following circuit and i was told that it will oscillate and it was required to mention the condition and frequency of oscillation : enter image description here

I tried thinking in many ways but i always conclude that it won't oscillate !

so I tried simulating it using Cadence OrCad using different combinations of R and C values, but the circuit didn't oscillate.

Could you please help me ?

  • \$\begingroup\$ You didn't bother to state why you don't think it will oscillate, or what values of R and C you used. \$\endgroup\$ – Bort Apr 8 '17 at 14:19
  • \$\begingroup\$ I had no problem making it oscillate in LTspice. I don't know OrCAD, but I needed to step the power supplies of the op-amps. \$\endgroup\$ – Renan Apr 8 '17 at 14:44
  • \$\begingroup\$ @Bort i don't think it will oscillate because the phase shift across the loop isn't zero. i tried using R=10k C=10nF, R=10k C=100uF, R=1k C=10nF, R=100k C=10nF, \$\endgroup\$ – Loksza Apr 8 '17 at 15:10
  • \$\begingroup\$ @Renan what values for R,C and the supply are you using ? \$\endgroup\$ – Loksza Apr 8 '17 at 15:11
  • \$\begingroup\$ @Loksza +/- 12 V, R = 1k, C = 10 nF. I put my schematic here: dropbox.com/s/cabibyh7reppziv/q.asc?dl=0 \$\endgroup\$ – Renan Apr 8 '17 at 16:13

They made a mistake. Its a negative feedback loop.

in fact I would urge you to challenge your Prof.

If you put a sweep generator on the U1 + input instead of ground each op amp becomes a unity gain + voltage follower and since the output is fed back to the Vin- is the same as the input there is NO feedback current (zilch , nada , nothing) , hence RC does NOTHING. and with no sweep the output is same as Vin+ on U1 .. meaning 0V.

Beware if you do, he may invite to be a post grad student for your intelligent question. and if he doesn't understand refuse his offer. (ROTFL)

  • \$\begingroup\$ I happen to miss the unity gain in U2; can you elaborate? \$\endgroup\$ – Marcus Müller Apr 8 '17 at 14:29
  • \$\begingroup\$ Opps must have been a hair on my screen in the feedback loop \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 8 '17 at 14:30
  • \$\begingroup\$ Even if each OA has a phase margin of 60 deg the output will always be 0V with a transient instability added with LPF feedback .\ If you FM sweep Vin1(+) and look at Vout2 it will be a high gain BPF between HPF and loop BW of OAs. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 8 '17 at 14:40
  • \$\begingroup\$ That's exactly what i thought . Now i'm relieved, thank you for the explanation. \$\endgroup\$ – Loksza Apr 8 '17 at 14:41
  • 1
    \$\begingroup\$ Now go ask Prof why he made a BandPass high gain filter an OSC question on test? with no assumptions on phase margin. If U2 was unity gain like the hair on my screen across output to Vin2(-) then my original answer is valid unity gain thruout. but open loop just becomes a BW limited BPF. But for Ideal Op Amps Vout 1= Vout2 =0V \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 8 '17 at 14:42

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.