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?
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.
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.
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.
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.