Author Topic: Wien Bridge Oscillator  (Read 2694 times)

0 Members and 1 Guest are viewing this topic.

Offline vargoal

  • Regular Contributor
  • *
  • Posts: 52
  • Country: 00
Wien Bridge Oscillator
« on: February 08, 2015, 04:34:53 pm »
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
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 13089
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Wien Bridge Oscillator
« Reply #1 on: February 08, 2015, 05:15:21 pm »
You need bipolar supplies.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline AG6QR

  • Frequent Contributor
  • **
  • Posts: 779
  • Country: us
    • AG6QR Blog
Re: Wien Bridge Oscillator
« Reply #2 on: February 08, 2015, 05:20:31 pm »
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

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



 

Offline sdg

  • Regular Contributor
  • *
  • Posts: 67
  • Country: fr
Re: Wien Bridge Oscillator
« Reply #3 on: February 08, 2015, 07:35:29 pm »
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.
--
 -sdg
 

Offline dannyf

  • Super Contributor
  • ***
  • Posts: 8229
  • Country: 00
Re: Wien Bridge Oscillator
« Reply #4 on: February 08, 2015, 11:35:35 pm »
No need for r2r opamps or bipolar power rails.
================================
https://dannyelectronics.wordpress.com/
 

Offline dannyf

  • Super Contributor
  • ***
  • Posts: 8229
  • Country: 00
Re: Wien Bridge Oscillator
« Reply #5 on: February 08, 2015, 11:39:22 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.
================================
https://dannyelectronics.wordpress.com/
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf