# Wien Bridge Oscillator Resonant Frequency cannot reached!

I am so bothered by the output frequency of my Wien Bridge oscillator because I cannot reached the resonant frequency which is given by f=1/2iπRC.

FOR EXAMPLE:

R = R1 = R2 = 1.5kΩ
C = C1 = C2 = 10nF
The resonant frequency is 10.61kHz
but the output frequency that I got (in my oscilloscope) is ~7.40kHz

I used a LM324 and OPA541AP op amps but the results are the same!

**Is there something wrong in my calculations? Or there should be some factors that I need to consider! **

• Try posting the full circuit diagram... It'll make it easier for us to help you.
– BobT
Commented Feb 23, 2017 at 17:13
• Please post a schematic, and use the tool. Commented Feb 23, 2017 at 17:15
• How are you controlling the gain? Does the output look like a sine wave? or are you hitting the power rails? Commented Feb 23, 2017 at 17:18
• the output is a sine wave!
– DC.
Commented Feb 23, 2017 at 17:31
• What supply rails are you using? Commented Feb 23, 2017 at 17:34

If you are using the LM324 device, it is no surprise that the result is not as expected. It is the limited slew rate that restrict the (sinusoidal) output and causes distortions (triangle form). At the same time additional phase shifts are effective which cause a much lower frequency than desired.

The minimum recommended supply voltage for both op amps is 10 volts. If you are below this then all bets are off.

If you do get the supply voltage correct you will find that as a basic circuit, its performance is totally sub standard. This is because you don't have anything to dynamically keep the gain at overall unity and the amplitude will drift all over and you will get clipping more often than not.

The sensible approach is to use a jfet as a gain control and there are dozens of circuits on the web that show this so, get the power supply voltage correct then apply jfet amplitude control.

• I also tried using 12V but it has the same output frequency!
– DC.
Commented Feb 23, 2017 at 17:52
• So why did you say 9 volts in your comment? Commented Feb 23, 2017 at 17:53
• because it is the minimum voltage that my signal is not clipped but I tried using higher voltages
– DC.
Commented Feb 23, 2017 at 17:54
• The basic Wien bridge oscillator will always clip the sine amplitude because there is no dynamic gain control. Try using a less theoretical circuit instead because you will never get consistent sinwaves from this without amplitude control. Commented Feb 23, 2017 at 17:58
• so if I I did not have the gain control I will not get the resonant frequency! oooh! Then what equetion can I use to calculate the output frequency (theoretically)?
– DC.
Commented Feb 23, 2017 at 18:03

The most obvious place to look is your RC network. What tolerance parts did you use? If you used 10% resistors and 20% capacitors, that would give you about a 30% error if you were unlucky enough to get all your parts out on the edge of the distribution.

disregard RC Wien bridge designs as they are very inaccurate and unstable for sine output.

If you want a sine wave with exact f. then all parts must be matched perfectly of 1% and use Rf just above 200k. It will start slowly and approach saturation. If it saturates quasi square, then output when saturated lowers the f because Zout rise with lack of linear feedback and lowers f.

We know Zout is Z/beta or loop gain feedback so it is normally near zero. then rises upto 500~ 1kohm or so for CMOS R2R out types.

Often the fix is to add zeners with high series r in feedback for soft limiting or similar so gain is reduced from -2.x to -2.0 for perfect sine on the inverting side.

x just needs to compensate for R tolerances.

• I rather think (and have experienced) that the WIEN oscillator is as good and exact as other RC oscillators (phase shift, double-integrator,...). More than that, for perfectly matched parts (R, C) the gain of the non-inv. opamp stage must be "3" (and not 2.0). In any case, it is recommended to cover mismatching and to design the gain somewhat larger (for example 3.2) - and to use an amplitude regulation mechanism (diodes or FET as a restance,...).
– LvW
Commented May 21, 2017 at 8:38