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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 ?

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  • \$\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
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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)

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  • \$\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
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    \$\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

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