Wien Bridge Oscillator

Thread Starter

Wendy

Joined Mar 24, 2008
22,646
So I have been trying to get this circuit to work with no success. I started with R5 using 200 ohms and finally concluded that that was just too small of a resistance. So I am opening this project for general comment. Ideas? Suggestions?
Wien Bridge oscillator post.png

to start off I adjusted R5 to 600Ω/300Ω for a gain of two.
 

AlbertHall

Joined Jun 4, 2014
11,592
What are you using for R6?
What is the supply voltage?

U2b has to be able to supply enough voltage/current via R5 to heat the filament of the lamp.
 

Thread Starter

Wendy

Joined Mar 24, 2008
22,646
At this point the lamp is irrelevant it measures less than 1Ω cold. I could remove it entirely if it would help. 12 volts DC
 

Audioguru again

Joined Oct 21, 2019
3,821
Your circuit will work with a power amplifier like an LM386, it is in its datasheet.
For a low power opamp, a low power incandescent light bulb might not be found.
The gain for a Wien Bridge oscillator must be 3.

Pin 5 of the opamp connects to nothing.
R1a and R1b do not connect together.
There is no supply voltage.
 

Audioguru again

Joined Oct 21, 2019
3,821
Now R5 does not connect to pin 7 because it is cut away and instead it wrongly connects to pin 5.

Then when fixed and the gain is set to 3 or a little more does the oscillator work with no AGC because the light bulb needs too much current?
Can you see and hear and see on a scope the crossover distortion and hiss from the lousy old LM358?
 

KeithWalker

Joined Jul 10, 2017
2,033
This circuit works very well, using diodes to stabilize the feedback. LM358A is used to give a virtual ground center tap to the single supply.

WeinBridgeV2.jpg
 
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Thread Starter

Wendy

Joined Mar 24, 2008
22,646
OK so I will get rid of the light bulb, try this instead:

Wien Bridge oscillator 3.png

There are other approaches I can use to create an AGC function. Once I have oscillation I will tweak the design.
 
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KeithWalker

Joined Jul 10, 2017
2,033
That should work but the output may vary between clipped and nothing with temperature changes. The non-linearity in the signal diodes in my circuit stabilize the amount of feedback and avoid that problem..
 

Thread Starter

Wendy

Joined Mar 24, 2008
22,646
This I understand. I am hoping to come up with my own design. With the brain fog I no longer grok in fullness wiring that was once simple for me. Any Robert Heinlein fans here?
 

michael8

Joined Jan 11, 2015
230
Instead (or addition to) checking the output for AC, I'd check various points in the circut for DC voltage.

Since the circuit is single supply, I'd measure between a point and ground (not the virtual ground).

What is the voltage at Vcc? pin 3? pin 1? pin 7? Are these as "expected"?
 

Ian0

Joined Aug 7, 2020
3,752
I had been wondering about this idea for some time, so I thought I'd try it on SPICE to see how well it works.
wien.png
This is the Linsley-Hood two-op-amp Wien Bridge, made with rail-to-rail op-amps. The final output (from U2) is a clipped sinewave, but that output goes into the Wien network then it appears band-pass filtered, out of phase and at half the amplitude at the output of U1.
The band-pass filtering removes quite a lot of the distortion, so that the U1 output has 3rd harmonic distortion at 50dB below the fundamental (0.3%).
[Edit] SPICE seems to require me to add a bit of noise to make it start up - hence the Gaussian noise source.
 

Thread Starter

Wendy

Joined Mar 24, 2008
22,646
Next brain fart:

Wien Bridge oscillator 3.png

I was going to use a 2.7KΩ for R5 but all I had handy was a 2.0KΩ. Results will follow.
 
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