Wien bridge oscillator output voltage

hi
what is the output voltage for this wien oscillator circuit ?
is it V0=3Vd/(2-(r2+r3/r1)) ?
but i have a negative V0 !!!!???

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Hi, simply build this circuit and measure Vout voltage.


I long time ago use this method

Vp = Vn = Vout/3

Vout/(3R1) = ( Vout - Vd - (Vout/3) )/R2

Vout = ( 3 • R1 • Vd )/( 2 • R1 - R2 )

Where:
Vd - diode forward voltage drop
Vp - voltage at non-inverting input
Vn - voltage at inverting input

But I never check this equation in real life.

To the Ineffable All,

Tektronics published a tech-note many years ago about looking at a sine wave with an O-scope. They said the distortion has to be about 3% before it can be discerned by a human eye. Far better to use a distortion meter or square wave analysis.

Ratch

"Ineffable", hee,hee.
If somebody allows a Wien Bridge oscillator to have output clipping or compresses its output with diodes then of course it will be distorted.

thanks for all
so if i want to control the amplitude ,,, is R2 must be variable ???
why the current doesn't flow through R3 ???
could you explain more about the above circuit

Adam2014,

Are you asking if charge flows from the output through R1 and R2 and R3? Yes, current exists through all three of those resistors. The larger R3 is, the more the OP-amp will amplify. It has to amplify enough to sustain oscillation, but not so much that it will make the output larger than the voltage sources of the OP-amp can accomodate. Therefore, when the output becomes too large, the diodes start to conduct and partially short out R3. This reduces the amplification but still sustains oscillation. Any questions about the theory?

Ratch

thanks Ratchit
if the current flow through R3 , why it didn't included in the current relation you wrote above

Vout/(3R1) = ( Vout - Vd - (Vout/3) )/R2

what the effect of source voltage Vcc on the oscillation ?

Adam2014

Look again. I did not write that equation.

You might enjoy having this sweet thing explain nonlinear feedback to you instead of us old burnouts. Try it, you'll like it.


Ratch

Adam2014 said:

what the effect of source voltage Vcc on the oscillation ?

Click to expand...

The datasheet for the opamp you use will show its maximum peak-to-peak output voltage vs the supply voltages with the load resistance you have.
You must adjust resistor values in the circuit so that the output level does not produce clipping when the supply voltages are at their minimum and the distortion from the diodes is acceptable.

sorry
but it's right ???

thanks
audioguru

about post#2
if the current flow through R3 , why it didn't included in the current relation you wrote above

Vout/(3R1) = ( Vout - Vd - (Vout/3) )/R2

for who can help me
is the relation on pose #2 right ?
if the current flow through R3 , why it didn't include in the current equation

Vout/(3R1) = ( Vout - Vd - (Vout/3) )/R2

Hi Adam,


The approximation for Vout is:

Vout=3*Vd*R1/(2*R1-R2)

but there are restrictions. I think Jony was the first to mention this equation.

R3 is not in the equation because it is not a significant gain factor. It's just there to make the diode curves a little softer so we get less distortion due to any clipping action. It is in parallel to the diodes so the diodes do most of the control and R3 just helps the diodes do it in a less abrupt way. If on the other hand we had cared to include the diode equation itself in the formula for Vout, then we would certainly have to include R3 in the analysis because we would need it to show exactly how the diode curve changes because of it's value. In the approximation however the diode (either one) is viewed as a voltage source, not a resistance, so we dont have to consider R3's value because it has no effect on the voltage source. Of course R3 can not be too low though, or it will swamp the diode action and we will no longer have any gain control.

The equation is developed from knowing the transfer function for the positive feedback loop and the negative feedback loop. When the circuit oscillates it oscillates at w=1/RC and that means at the non inverting terminal we get a voltage that is Vout/3 and it is in phase with Vout as well. For a perfect circuit, the negative feedback loop would produce Vout/3 at the inverting terminal and this is the basic idea behind the equations, except for the fact that we have diodes (a voltage source) in the negative feedback loop as well. The diode voltage subtracts from the output and that in combination with a tiny offset in the op amp itself produces a non zero output voltage that rises with time until the diodes start to conduct.

The restriction is that R2 can not be too large or there wont be enough feedback and the output will keep rising until the op amp output saturates, and that will cause intense distortion. So R2 has to be kept high enough to allow oscillation to start but still low enough to prevent the output from going too high. This brings us to the schematic which does not look right because R2 looks too large. It may have to be decreased to maybe 15k or 17k or something like that, but you can use the equation to investigate this further.

BTW the diode approximate voltage drop for this kind of circuit may be closer to 0.5v than to the usual 0.7v sometimes associated with a regular si diode.

Please let us know how you make out with it.

Hello again,

I took a few minutes to play around with the circuit when the diode is modeled as a more true life diode rather than a voltage source. I came up with this nonlinear equation:

Vout*(2*R1-R2)/(3*R1)=Vt*N*log(((3*IS*R1+Vout)*R3+Vout*R2-2*Vout*R1)/(3*IS*R1*R3))

Note that this is now closer to the real circuit and includes R3 so we can see the effects on the output voltage Vout. This isnt that much help though because the real nature of R3 is to help reduce distortion by making the diode appear more linear. It takes away the range of control but decreases distortion. So this equation is mainly to see the effect of the output voltage change with R3. When we halve or double R3 we see a little difference but not that much really, so the approximation in the previous post is more recommended. Note that this equation is nonlinear too so it has to be solved using a numerical solver or some trial and error with a hand calculator or programmable calculator. The previous post equation does not need that much work so it is easier to use.

In the above equation the following values are:
R1,R2,R3 are the circuit resistors as noted in the schematic early in this thread,
Vt is approximately taken to be 0.026 around room temperature,
N and IS are spice parameters for the diode being used,
Vout is of course the output voltage peak of the oscillator.
Of course we also assume the usual assumptions for circuits like this in that the op amp response does not interfere too much with the circuit and neither does the parasitic parameters of the other components.

As mentioned, this does not show the true value of using R3 because it just shows how the output voltage changes and so only serves to help us calculate a more exact output voltage (given the correct diode parameters). It will not show the effect on the output distortion due to the addition of R3.
Generally the bigger R3 is the higher the distortion, but the more control the diode has over the amplitude stabilization. The smaller R3 is the less the distortion but then the diode has less control so with R3 too small it is possible that the oscillator will work for weeks and then we might see an increase in output voltage for example and it could even go unstable.

I made a Wien Bridge oscillator about 50 years ago. It used transistors in a circuit similar to an opamp and used an incandescent light bulb filament for controlling its output level.
The resistance of the filament was low when it was cold so the oscillator started and began increasing the output level. As the output level increased then the signal caused the filament to heat which increased its resistance. If the level got too high then the higher resistance of the filament reduced the gain and the level dropped to normal.

But the signal leveling action of the filament was very slow so when the frequency or range was changed then the amplitude would bounce and bounce and bounce and bounce then gradually level out. Years later I replaced the light bulb with a Jfet that controls the output level without bouncing.

Hi,

We could probably analyze that and see what was happening. First guess is that the bulb filament had minimal mass. A bulb with a thicker filament might have worked better, but then again it would have to be able to work with the available current from the output. Nice thing is if it is right then the feedback is almost purely Ohmic unlike with the diode.
The JFET sounds good too though, and is not subject to vibrational noise.
I've also read about LDR's being used.

Mr
&
audioguru
thank you very much ...
now I'm using TL 074/301/ti but I can't find any free datasheet .
you Have it ?

Adam2014 said:

now I'm using TL 074/301/ti but I can't find any free datasheet.
you Have it ?

Click to expand...

I get all my datasheets for free from www.datasheetarchive.com where you can select a certain manufacturer. I usually select the manufacturer who invented the device because their datasheet has more information on it than the copycats.
The TL071, TL072 and TL074 were invented by Texas Instruments. I have used them for most of my audio projects. The LM301 is completely different and was invented by National Semiconductor who is now owned by Texas Instruments.

Adam2014 said:

Mr
&
audioguru
thank you very much ...
now I'm using TL 074/301/ti but I can't find any free datasheet .
you Have it ?

Click to expand...

Hi,

These days we have the internet where we find all kinds of data sheets on a huge number of IC products. Count yourself lucky that you can do this today, because back when i was working in the field we had to rely on printed text so we had tons of books around as well as tons of magazines and articles. The TTL book we had was three inches thick just for that one book alone.

Today you can go to Google and type in the part number and come up with several links to the part data sheet. That's the way to do it today. If you think you will use the part often you can download the data sheet and store it on your computer under your "Parts/DataSheets" directory if you like.

thanks for your help
i design this circuit for change frequency between 25-50 Hz and amplitude between 2-10 volt
although i do the conditions (R2> 2R1)&(R2+R3/R1>2)
it delay about 100ms to start oscillation ???
when I use it in pwm just the sine-wave circuit (wien) didn't oscillate while the triangular do ???

R1=5k , R2=7.5K , R3=15K
for frequency R=300 ohm , R=330 ohm , C1=C2=10 uf

Your value for "R" is WAY too low. Then you are using POLARIZED (?) and inaccurate 10uF electrolytic capacitors?
Instead use 33k/5% for R and use 0.1uF (100nF) 5% film capacitors.

You say it didn't oscillate "when I use it in pwm" then what is its load?

Does the Wien Bridge oscillator have a +/- 15V supply?