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Electronics => Beginners => Topic started by: vargoal on February 08, 2015, 05:34:53 am

Title: Wien Bridge Oscillator
Post by: vargoal on February 08, 2015, 05:34:53 am
So I am trying to make a sine wave generator to replace my awful signal generator/power supply I got from school for free using a Wien Bridge Oscillator circuit. Anyway I built it up but it's not working and I was wondering if my op-amp has to be rail to rail? If anyone knows that would be great, also if you know of any tricks for a beginner building this circuit that would also be appreciated. For those of you who haven't heard of this here is a link http://www.electronics-tutorials.ws/oscillator/wien_bridge.html (http://www.electronics-tutorials.ws/oscillator/wien_bridge.html)
Title: Re: Wien Bridge Oscillator
Post by: T3sl4co1l on February 08, 2015, 06:15:21 am
You need bipolar supplies.

Tim
Title: Re: Wien Bridge Oscillator
Post by: AG6QR on February 08, 2015, 06:20:31 am
Your op amp does not have to be rail to rail.

In the resistive feedback loop to the negative terminal, the value of the gain is critical.  If the gain is too low, the oscillations will dampen to zero.  If it's too high, the oscillations will increase in amplitude until clipping and distortion.  Also notice that, in order to start oscillating initially, the gain has to be high, so that some random fluctuation will be amplified until it results in full scale oscillation.  When oscillation is established, the gain needs to be reduced in order to keep the amplitude steady and low in distortion.  To make a good sine wave oscillator with low distortion and a steady amplitude, some sort of active adjustment to the gain in that resistor network is usually used.  The classic approach is to use a light bulb, which increases its resistance as more current goes through it.  That's what Hewlett Packard did for their classic Wien Bridge audio oscillator.  There are other possible approaches, using PTC thermistors, diodes, and/or transistors.

If you're not seeing any oscillation, then I'd guess your resistor values in the negative feedback side might be resulting in insufficient gain.  Of course, the feedback on the positive side also has to be correct...

Here are a few info sources:

https://www.calvin.edu/~pribeiro/courses/engr332/Handouts/oscillators.pdf (https://www.calvin.edu/~pribeiro/courses/engr332/Handouts/oscillators.pdf)

http://www.zen22142.zen.co.uk/Design/wien_osc/Wien-Bridge%20Oscillator.htm (http://www.zen22142.zen.co.uk/Design/wien_osc/Wien-Bridge%20Oscillator.htm)

https://www.youtube.com/watch?v=vBsSASge7ls (https://www.youtube.com/watch?v=vBsSASge7ls)

Title: Re: Wien Bridge Oscillator
Post by: sdg on February 08, 2015, 08:35:29 am
You might want to take a look at an application note from Linear. Can't find the number, but the title is "Fidelity testing for A/D converters" by Jim Willliams & Guy Hoover. It has a very good description of a low-distortion Wien bridge sine generator with AGC and what is needed to make it work.
Title: Re: Wien Bridge Oscillator
Post by: dannyf on February 08, 2015, 12:35:35 pm
No need for r2r opamps or bipolar power rails.
Title: Re: Wien Bridge Oscillator
Post by: dannyf on February 08, 2015, 12:39:22 pm
If you post the schematic here it would be easier for others to help you.

Agc is helpful but not necessary. My prefered agc is led plus photoresistors.