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I am trying to build a Wien Bridge Oscillator in Multisim that has a frequency of 500kHz. I went through all of the necessary calculations to choose values for R and C. When I build the circuit in Multisim it says I have a frequency of 25 kHz and the oscilloscope does not show any oscillations, just a constant voltage of 5V. Any ideas of how to correct this issue? enter image description here

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    \$\begingroup\$ Simulating oscillators is tricky. The simulator is perfectly happy to maintain a metastable state that would never persist in real life. You'll need to find a way to give the circuit a "kick" to get it going. Sometimes this is as simple as starting with a nonzero voltage on one of the capacitors. \$\endgroup\$ – Dave Tweed Oct 21 '17 at 21:06
  • \$\begingroup\$ At 500 kHz, this op-amp hasn't much gain, and its output impedance approaches 100 ohms. Loading it with R5=50 ohms is asking too much. Your Wien RC product should increase R and decrease C. Even so, frequency will likely be off because the op-amp introduces phase shift at this high frequency. \$\endgroup\$ – glen_geek Oct 21 '17 at 21:44
  • \$\begingroup\$ You have connected resistors R3 and R4 the wrong way, the inverting input should be connected between R3 and R4, also R4 needs to be slightly higher than 20k to start oscillation. Also, the 68 ohm resistors are way too smal. \$\endgroup\$ – S.s. Nov 30 '17 at 3:12
  • \$\begingroup\$ Have you made sure you set the initial conditions to 0? I've made this mistake in MultiSim almost every single time. \$\endgroup\$ – Sean M Mar 4 '18 at 6:35
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Your feedback circuit is incorrect. You have the non-inverting input grounded.

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Figure 1. Move the op-amp inverting input to the mid-point of the feedback divider.

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  • \$\begingroup\$ Okay so I did that and know I have zero for peak-to-peak voltage and the frequency shows blank. Any other ideas? \$\endgroup\$ – user166481 Oct 21 '17 at 20:38
  • \$\begingroup\$ Try connecting the left side of R3 to your 0.833 V ref. instead of GND. The output needs to swing below the ref. \$\endgroup\$ – Transistor Oct 21 '17 at 20:41
  • \$\begingroup\$ Wein Bridge stability is poor for sine since it relies on gain matching for each R ratio to be perfect. With SIM OK but with real tolerances it relies on soft limiting to balance gain, so there are practical modifications needed for soft limiting \$\endgroup\$ – Sunnyskyguy EE75 Oct 21 '17 at 20:42
  • \$\begingroup\$ While we're waiting, your 5 V ground symbol is upside down - the symbol represents buried earthing plates so they appear to be in the sky. Convention is to invert the battery so that + is at the top and negative (and GND) at the bottom. It makes reading much more easy. \$\endgroup\$ – Transistor Oct 21 '17 at 20:46
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    \$\begingroup\$ Where did you get your 68 Ω resistor values? They look very, very small. Also as Dave Tweed said, you might need to inject a little voltage to get it to start. Also feedback gain has to be >= 3. You are just on three. Try making R4 a little bigger. \$\endgroup\$ – Transistor Oct 21 '17 at 21:20
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I did some tests. Unfortunately I have not Multisim, only some freeware and no models for high performance opamps. But I inserted a generic opamp model and did open-loop analysis of your circuit (=the suggested wiring corrections done).

The phase shift network was disconnected from the non-inv input of the opamp. A signal source was inserted and simulated the gain and phase shift of the signal path. I found that the total gain was a little too small and the phase shift was ok at 400% too high frequency. I increased the gain and made the 68 Ohm resistors much larger, 560 Ohm. After that the phase shift is 0 degrees at abot the wanted frequency and the total gain at that frequency is a little more than 0dB. There's still error, but at least it's a few kHz accurate.

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Now it's possible that it oscillates.As suggested in a commet, some initial voltage can be needed in the capacitors at least if there's only ideal components. In this case the oscillation started as wished.

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The 50 Ohm resistor pulls the output of my opamp onto its knees, but there's still enough gain for oscillation. The amplitude grows to the clip limit because there's no gain controll circuit. If one wants pure sinewave, there must be a circuit which rapidly reduces the gain when the right output amplitude is achieved. If one has loop gain 0dB exactly at the frequency where the loop phase shift is exactly 0 degrees, the amplitude stays stable and the waveform=sine. But that's another story.

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  • \$\begingroup\$ Great thank you! I will test that out and see what happens. I also have to make the 2nd harmonic of the output be -25 db below the 500Khz. Would it be possible to do so using a low pass filter? \$\endgroup\$ – user166481 Oct 21 '17 at 23:42
  • \$\begingroup\$ @user166481 If this is a homework and you are asked to make the oscillator to have low distortion, I will not recommend filtering, altough it would make the signal clean. You must find how steep filtering is needed in your case. Just talking about 2nd harmonic can also be a prank, because just that harmonic can be very weak. Consider to add some common gain reduction trick which affects over certain output level. You need much simpler filter, even no filter at all. \$\endgroup\$ – user287001 Oct 22 '17 at 0:46
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The Texas Instruments tutorial about Sinewave Oscillators has the correct schematic that you wrongly copied. Here it is:

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  • \$\begingroup\$ @Dave Tweed: Do you expect a full-wave sinusoid at he output of this opamp with single-supply? (Independent on a possible clipping effect). \$\endgroup\$ – LvW Feb 4 at 17:02
  • \$\begingroup\$ @LvW: No, probably not, given that the noninverting input is biased at 0.833 V. \$\endgroup\$ – Dave Tweed Feb 4 at 17:37
  • \$\begingroup\$ Yes - in this case, it works. Did I overlook this biasing or did you correct the drawing? \$\endgroup\$ – LvW Feb 5 at 7:56
  • \$\begingroup\$ @LvW: No, I didn't touch the drawing. I just fixed how it was formatted into the answer. \$\endgroup\$ – Dave Tweed Feb 5 at 21:27
  • \$\begingroup\$ Perhaps my eyes are getting older and older... \$\endgroup\$ – LvW Feb 6 at 9:45
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When you get the opamp biased properly and increase the values of the 68 ohm dead short resistors and 50 ohm dead short on the output (use 10k ohms), then an oscillator uses amplified noise to get started. But a simulation program has no noise so the oscillator does not start until you [b]kick it[/b] with a pulse on the opamp input.

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  • \$\begingroup\$ Such a "kick" is produced automatically by switchung on the supply voltages at t=0. \$\endgroup\$ – LvW Oct 22 '18 at 16:53

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