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oscillator circuit

I have constructed an oscillator circuit using the resistor and capacitor values shown in the diagram above (from the Proteus ISIS simulator).

I run the circuit and use a switch to trigger the oscillator. I get the following waveform.

output waveform

Using cursors in the oscilloscope I observed the time for each cycle of the waveform was around 7.6 microseconds which was considerably lower than the required frequency.

Could someone tell me why I'm not obtaining the required frequency? I understand that that it will never be exact, but this is a pretty large difference. Also, is there any way to obtain a more sinusoidal output?

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Why, exactly, did you delete the images from your question? –  Connor Wolf Jul 30 at 10:49
    
Rolled it back, so answers would make sense –  Scott Seidman Jul 30 at 12:36
    
Sorry, there was an error from the site I uploaded them to, so I thought I would reupload them from elsewhere –  user2802349 Jul 30 at 12:38

3 Answers 3

up vote 2 down vote accepted

As you can see, the signal is not a real sinus. I suppose the reason is a bad slew rate.

Therefore, in addition to increasing both of the resistor values (kohm values and nF capacitors are always good) you should select an opamp which has a sufficient slew rate.

For example, many opamps have a maximum large frequency bandwidth of app. 10 kHz only with a maximum slew rate of SR=0.5V/µs only (for example, the classical reference type 741).

I recommend for your application at least a value of SR=(2..5)V/µs.

As another hint: Make the gain somewhat larger than "3" (3.1...3.2) and use two antiparallel diodes across R3 for amplitude control. Then, you will get an acceptable signal quality.

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Regarding amplitude control: The method as proposed by George Herold (see his comment above) is an improvement regarding signal quality (using antiparallel diodes across one part of the feedback resistor only). And - yes - distortions (signal quality) are determined by the selected gain/amplitude control only. –  LvW Jul 29 at 15:53
    
Thanks so much! :) What difference would if make if I connected the diodes directly across a 2.1k resistor rather than split the resistance into two series resistors of 1.5k and 0.55k? –  user2802349 Jul 29 at 18:30
    
The diodes introduce distortions because of their non-linearities. However, this cannot be avoided because some non-linearities are necessary (amplitude dependent resistance). But to keep this non-linear influence as small as possible you should try to keep a large part of the feedback resistor "linear" resistive and only a smaller part amplitude-dependent. That´s the "secret" behind this method. –  LvW Jul 29 at 20:00

Your poor little op amp is trying to drive loads below 10 ohms, and it is not happy. Try multiplying R1 and R2 by 1000, and dividing C1 and C2 by 1000. Furthermore, you need to actually look at and understand your op amp data sheet. Take http://www.ti.com/lit/ds/symlink/ua741.pdf as a source. Look at figures 6 and 8. Do they tell you anything?

As a rough estimate, the open loop gain of an op amp should be 100 times the closed loop gain at that frequency. Since your closed loop gain is 3, this means the open loop gain should be around 300 at 20 kHz, which in turn says that your gain-bandwidth product should be around 6 MHz. It can be, oh, half that if you're not too picky about accuracy. Do you see this happening with a 741?

As for distortion, well, first you need to get the frequency response up. Then try the really obvious step: Wikipedia. http://en.wikipedia.org/wiki/Wien_bridge_oscillator.

In the future, please do even a minimal amount of research before you come asking for help.

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The mentioned factor of 100 (which I agree upon) gives an open-loop gain at 20 kHz of 3*100=300 (instead of 3000). I think, a 741 type can marginally manage it, except the slew rate. –  LvW Jul 29 at 14:03
    
Nice answer, I think you are being a bit conservative with your open loop gain requirements. I've done a nice Wien bridge at ~80kHz with 8 MHz GBW opamps. I've never tried pushing the frequency.. I'm thinking slew rate may bite me before GBW. –  George Herold Jul 29 at 14:09
    
@LvW - Ack! Thanks, edited. –  WhatRoughBeast Jul 29 at 14:13
    
@GeorgeHerold - Well, I usually think of Wein bridges for low distortion - there are simpler topologies for non-critical applications. And for low distortion you need the excess gain. –  WhatRoughBeast Jul 29 at 14:16
    
Well I guess I've always found distortion to be set by how you do the gain control. If you like Wien bridge osc's here's my simple control scheme. dropbox.com/s/lmso5zx69g2l1lc/DSCF0047.JPG third harmonic is ~60dB down. You can play with the R's and get less, (and higher amplitude to boot.) but this works fine with 1% R's. To be honest I don't have the tools to look for distortion above ~20 kHz. (Except for a 'scope FFT.) –  George Herold Jul 29 at 14:36

~3 ohms is way too low a value for use with non-ideal op-amp model.

Try increasing the resistors by a factor of perhaps 1000 to more like ~3K and reducing the capacitors from 2.2uF to 2.2nF.

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Thanks so much!:) –  user2802349 Jul 29 at 18:31

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