# Wien Bridge Oscillator

Discussion in 'Homework Help' started by sakas, Nov 29, 2011.

1. ### sakas Thread Starter New Member

Nov 29, 2011
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Hello,I am new to the group and working on something similar . I am trying to get my circuit to oscillate between 150Hz- 15KHz continuously variable and output a voltage between 10mV - 5V variable. Please what components would you suggest and will there be any modifications to the circuit.I currently have the basic circuit oscillating at 1.9KHz with output voltage at 23.2 volts. Thanks for your time.

Last edited: Nov 29, 2011
2. ### Audioguru New Member

Dec 20, 2007
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You need a dual potentiometer to adjust the frequency.
A logarithmic (audio taper) potentiometer at the output will adjust the level. You might need a buffer opamp following the output level adjust pot.

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3. ### Wendy Moderator

Mar 24, 2008
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Welcome to AAC!

A thread belongs to the OP (original poster). Trying to take over someone elses thread is called hijacking, which is not allowed at All About Circuits. I have therefore given you a thread of your very own.

This was split from http://forum.allaboutcircuits.com/showthread.php?t=61215

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4. ### sakas Thread Starter New Member

Nov 29, 2011
10
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Using your suggested circuit,I am hitting 10KHz at the moment with a dual 100k pot replacing R.I encounter distortion after 10kHz.Capacitors are 2 15nf

5. ### Audioguru New Member

Dec 20, 2007
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Which opamp? What supply voltage? What is the load? What limits and stabilizes the output level so the opamp does not clip?

Nov 29, 2011
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7. ### sakas Thread Starter New Member

Nov 29, 2011
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R2 is 140 ohms, bulb is 70 ohms

8. ### Audioguru New Member

Dec 20, 2007
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You do not show a part number for the opamp.
The load on the output of the opamp is 140 ohms plus 70 ohms = 210 ohms (plus the frequency-determining parts). Most opamps cannot drive a load lower than 1000 ohms.

Therefore you should use a lightbulb that uses much less power then its resistance will be much higher and the value for R2 can be much higher.

If your opamp is a lousy old (43 years old) 741 then it has trouble above 9kHz.

Using a lightbulb for amplitude control causes amplitude bouncing when it is turned on. also the amplitude will probably bounce when the frequency is changed.
The amplitude bounces because the lightbulb is slow to adjust its resistance with power through it.

9. ### sakas Thread Starter New Member

Nov 29, 2011
10
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Its an LM741 opamp
I dont know if its the lousy op amp, but moving forward if the jfet will do a better job,please what component and circuit will be needed and how does the jfet prevent the amplitude from jumping around like the bulb.
Thanks

10. ### sakas Thread Starter New Member

Nov 29, 2011
10
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the concept of loading is also new to me, considering the buffer amp you stated at the beginning

May 11, 2009
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12. ### Audioguru New Member

Dec 20, 2007
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The lousy old 741 opamp has poor high frequency response above 10kHz and
slew rate limiting above 9kHz. Its distortion is fairly high for an opamp and it is noisy (hiss).

The light bulb uses heating and cooling to slowly change its resistance which causes the signal to bounce up and down.

A Jfet is a voltage-controlled resistor and the voltage is made by recifying and filtering the output signal which is fast so there is no bouncing.

If you use a volume control at the output of your oscillator then it becomes a resistor in series with the load which is a voltage divider. Then adding a load changes the output level from the volume control. If a buffer opamp is added following the volume control its output level does not change when it gets a load (unless it is overloaded).

13. ### Wendy Moderator

Mar 24, 2008
20,772
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What AG means is you need a different op amp, the part you have selected is not suitable. A TL084 might work, or you may need a op amp with better specs.

Nowdays the 741 is used for low frequency/DC applications in teaching. Its specs are so low it is a good part for a teacher to use to show how the various limitations of an op amp work, since you run into them so quickly for a 741.

Since it was invented over 40 years ago literally hundreds/thousands of new part numbers have come out, with vastly improved specs.

Give hard specs, numbers that will not change for your oscillator, then ask AG what part number he would suggest. It might save you both some time.

14. ### sakas Thread Starter New Member

Nov 29, 2011
10
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Thanks for all the help so far.
This is the specification
Wien bridge oscillator.Freq between 150Hz- 15KHz continuously variable and output voltage between 10mV - 5V variable.

15. ### Audioguru New Member

Dec 20, 2007
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Your specification does not mention low distortion so get rid of the light bulb and let the output of the opamp clip like crazy.

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16. ### sakas Thread Starter New Member

Nov 29, 2011
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That i have noticed during these experiments. Van we add that to the spec LOW NOISE DISTORTION too and no bouncing amplitude

17. ### Wendy Moderator

Mar 24, 2008
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Actually you don't want variable output on the oscillator. That can be accomplished with a variable gain amplifier after the oscillator. You want a steady signal out of the oscillator, it will be a lot more stable that way.

18. ### sakas Thread Starter New Member

Nov 29, 2011
10
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Thank you very much, finally got the oscillator to meet the specification. I really appreciate.