Stabilising wienbridge sine wave using FET

[duke37]

This is getting personal, please desist.

As has been pointed out, the voltage of the R3/FET divider is connected wrongly.
The 'top' of the divider should be connected to the op-amp output and the tap goes to the amp input.
Resistance to the op-amp input is neither here or there with a fet input at low frequencies.

As shown the gain is one, not enough for oscillation which needs three.

 

[LvW]

Personally, I think you need to read up on ‘Wien-Bridge Oscillators’ before you give out any further information.

Thank you for your kind advice.
Just as a background information: I have written a textbook on filters and oscillators.
A lack of knowlege is never a problem, if one is able to accept some comments/corrections.
Do you have a good feeling while saying that you understand the circuit?

I must admit that I don`t like questioners (beginners) producing such sentences as quoted above.

 

[GeoffC]

This is getting personal, please desist.

As has been pointed out, the voltage of the R3/FET divider is connected wrongly.
The 'top' of the divider should be connected to the op-amp output and the tap goes to the amp input.
Resistance to the op-amp input is neither here or there with a fet input at low frequencies.

As shown the gain is one, not enough for oscillation which needs three.

Sorry, but I felt like school boy that mess up his homework. So I am Sorry.

I see where I have gone wrong now, and have rectified the error, and connected R3 to the output. The divider of the FET & R3 connected to the inverted input. I beg-to-differ with value R3. I need to have a frequency near as possible to 50KHz. I chose C1 & R1 that created an impedance that would match a preferred resistor value. (4.7K). In the past I find that the frequency holds to the theatrical calculation when the resistor matches the impedance. I’m really hoping to oscillate a Laser Diode, if that is possible. – Something I have not done.

Enclosed/Attached New Circuit – Once again - Sorry

Regards, GeoffC.

 

[GeoffC]

Here are the relevant posts

https://www.electronicspoint.com/th...-bridge-oscillator.260338/page-2#post-1573956

(and thanks to @The Electrician for pointing me to it)

Hi,

Thanks for the link, I have down loaded the pdf Journal and printed out the first two pages. I am going to have difficulty with the Maths on the second page. – Many thanks, GeoffC

 

[duke37]

Did the dog eat your homework? That story did not work for me.

Now the circuit is changed, does it work?

 

[LvW]

To: LvW
Sorry, for my outburst, I do apologise – Like you said, the resistor R3 was incorrectly placed. I have now reconnected R3 to the output. The divider of the FET & R3 connected to the inverted input. I wish to maintain the value of R3 at 4.7KΩ, as the calculated frequency holds, when the resistor is matched to the impedance.
Enclosed/Attached New Circuit – Once again I apologise.
Regards, GeoffC.

OK - accepted.

 

[GeoffC]

Thank you for your kind advice.
Just as a background information: I have written a textbook on filters and oscillators.
A lack of knowlege is never a problem, if one is able to accept some comments/corrections.
Do you have a good feeling while saying that you understand the circuit?

I must admit that I don`t like questioners (beginners) producing such sentences as quoted above.

To: LvW

What do I know of Wien-Bridge Oscillator? - In depth very little - but a reasonable amount for a hobby. I have built them before, but have used diode back to back to control voltage output. Also, Phase shift using a TL074 Quod Op-Amps, which to my mind was a very successful sine-wave. Each op-amp for each phase and the 4th as a buffer. Regards, GeoffC

 

[GeoffC]

Did the dog eat your homework? That story did not work for me.

Now the circuit is changed, does it work?

Well, it should do, it is still on the computer screen,

 

[LvW]

To: LvW

What do I know of Wien-Bridge Oscillator? - In depth very little - but a reasonable amount for a hobby. I have built them before, but have used diode back to back to control voltage output. Also, Phase shift using a TL074 Quod Op-Amps, which to my mind was a very successful sine-wave. Each op-amp for each phase and the 4th as a buffer. Regards, GeoffC

Yes - for my feeling, the WIEN type oscillator is very easy to understand. However, designing a good and effective amplitude stabilization mechanism could be an challenging task. Of course, a pair of diodes across the feedback resistor will "work" - however, the situation changes if we require a very low THD. But it is not an easy job to design an "optimum" FET stabilization circuitry.

 

[GeoffC]

Yes - for my feeling, the WIEN type oscillator is very easy to understand. However, designing a good and effective amplitude stabilization mechanism could be an challenging task. Of course, a pair of diodes across the feedback resistor will "work" - however, the situation changes if we require a very low THD. But it is not an easy job to design an "optimum" FET stabilization circuitry.

To: LvW

Hi,

I think it best if I told you what I’m trying to achieve with the sine wave.

I need to have a frequency near to 50KHz, but not to wander higher or lower in frequency.

Frq= 1/(2*Pi*C*R) = 48,228·8 Hz. This frequency, together with voltage, p/p to be constant.

I chose [C-1nF & R-3·3KΩ] to give 4,666 Zs/ohm in the positive feed.

While a 4·7KΩ. resistor will be near as I can get to match the impedance at negative feedback.

The FET should (I hope) find its value from the preset settings, in negative feedback.

This in turn will oscillate the LED or (Laser Diode) at a constant level of light.

The light will travel through different thicknesses of optical flat sheets 20μ to 30μ thick.

The light received by a ‘Photo Diode’ - to a ‘turned circuit’ - to a ‘precision rectifier’.

These other two circuits here are not fully understood, - yet to be understood.

If the light intensity or frequency tends to alter in any way then all is lost.

I’m not sure how to measure the voltage output, may be a volt-meter or a voltage drop across a resistor. One idea came to mind was to input the voltage into a ‘ATtiny85 (Arduino) and read a PWM output. - This will need further investigation, and I’m not that good at programming them.

So, If you know any suitable circuit (Using Op-Amps) that can be used as -‘Precision Rectifier or turned circuits’ I’ll

will be of interest.

Regards, - GeoffC.

 

[duke37]

I see nothing in your requirements which need a sine wave and no indication of the frequency variation acceptable. A sine wave voltage into a led will not give a sine wave out.

To get 50kHz, divide a 4MHz crystal oscillator by 80 to get a square wave. Clip this at your preferred voltage.
After transmission, use a phase locked detector to eliminate noise.

I asked in #27 whether the circuit worked but did not get an answer.

 

[GeoffC]

I see nothing in your requirements which need a sine wave and no indication of the frequency variation acceptable. A sine wave voltage into a led will not give a sine wave out.

To get 50kHz, divide a 4MHz crystal oscillator by 80 to get a square wave. Clip this at your preferred voltage.
After transmission, use a phase locked detector to eliminate noise.

I asked in #27 whether the circuit worked but did not get an answer.

Duke37

Hi,
Does the sine wave circuit work?

It should do, - but it is still on paper (or Computer screen) I’ve not had time to put it on a ‘bread-board’ and when I know it all works, the circuit will be converted to a PBC.

I’m afraid you have rather thrown a spanner in my work, as I’m not too clever on PLL, and I need to read up on them ‘CD4046B’ If I have the right chip! Also I haven’t a clue on the connections. I am better with Op-Amps, sine waves and a little on filters. I need a little time to see things through, and I’ll come back to you. Regards, GeoffC.

 

[davenn]

OK guys
I have done a lot of post deletions in this thread to tidy things up

please keep it civil and play nice


Dave

 

[BobK]

You still haven't explained why you need a sine wave, and, as Duke pointed out, the output of the laser didoe will not be a sine wave with sine wave input. So, why a sine wave?

If you don't really need a sine wave, as Duke pointed out, (and as I did much earlier), a crystal oscillator plus divider will give you much more a much more accurate and stable frequency than your Wien bridge.

Bob

 

[GeoffC]

OK guys
I have done a lot of post deletions in this thread to tidy things up

please keep it civil and play nice

Dave

davenn

OK guys
I have done a lot of post deletions in this thread to tidy things up
please keep it civil and play nice
Dave

“I believe the appropriate metaphor here involves a river of excrement and a Native American water vessel without any means of propulsion.” ― Sheldon Cooper

davenn

Hello,
I have your message, OK, yes I am nice & civil – But how did the foot note get there? - (In blue)
Regards, GeoffC.

 

[davenn]

davenn

Hello,
I have your message, OK, yes I am nice & civil – But how did the foot note get there? - (In blue)
Regards, GeoffC.

That's my signature ... go into your profile and you can add a signature as well
some use the feature some don't. that's been my signature for the last 2 -3 years
I change it occasionally if I find some quote that amuses me

my one from Sheldon Cooper of the Big Bang Theory TV show where he gives a more scientific description of a very old and well known saying



Dave

 

[GeoffC]

You still haven't explained why you need a sine wave, and, as Duke pointed out, the output of the laser didoe will not be a sine wave with sine wave input. So, why a sine wave?

If you don't really need a sine wave, as Duke pointed out, (and as I did much earlier), a crystal oscillator plus divider will give you much more a much more accurate and stable frequency than your Wien bridge.

Bob

To: BobK

Why a Sine wave – OK,Thiscould be boring. My electronic knowledge is somewhat limited, and I tend to be more at home with Op-Amps and sine waves. I also think a sine wave is more suited for the purpose intended. So, the light from the diode needs to be constant level. In the path line of light. There will be two polariser crossed. (light blocking). Between the polarisers a known lambda plate will be placed between them. This will open the light path to give a reference as to voltage setting. (A chance to calibrate) After which other lambda plate of different thicknesses can be placed to find (I hope) different voltage readings. The lambda plates will be between 20μ to 30μ thick. IF the circuit can be made very sensitive it might just work, as ones eye could never see the difference.

I think sine waves is more gentle in a sensitive circuit. Also, the sine wave from the photo diode can be amplified to increase the sensitivity. (I hope). - All this can be for nothing, but then again nothing ventured nothing gained. So this is why I want a - ‘Sine-Wave’.

Regards, GeoffC.

 

[davenn]

Why a Sine wave – OK,Thiscould be boring. My electronic knowledge is somewhat limited, and I tend to be more at home with Op-Amps and sine waves. I also think a sine wave is more suited for the purpose intended. So, the light from the diode needs to be constant level. In the path line of light. There will be two polariser crossed. (light blocking). Between the polarisers a known lambda plate will be placed between them. This will open the light path to give a reference as to voltage setting. (A chance to calibrate) After which other lambda plate of different thicknesses can be placed to find (I hope) different voltage readings. The lambda plates will be between 20μ to 30μ thick. IF the circuit can be made very sensitive it might just work, as ones eye could never see the difference.

I think sine waves is more gentle in a sensitive circuit. Also, the sine wave from the photo diode can be amplified to increase the sensitivity. (I hope). - All this can be for nothing, but then again nothing ventured nothing gained. So this is why I want a - ‘Sine-Wave’.

I suspect you are still not understanding the main reasonwhy the other guys are trying to tell you that a sine wave voltage to the LED is pointless

you just stated ....

So, the light from the diode needs to be constant level.

a sine wave (AC) voltage supply isn't going to achieve that, it's in contradiction to what you just stated

For a stable output from the LED ( laser diode ... whatever), it needs to be supplied with a stable DC voltage at a specific current

Also, I suspect they are trying to explain to you that, like any diode, if you supply an AC current to it,
it's going to 1/2 wave rectify it


Dave

 

[GeoffC]

I suspect you are still not understanding the main reasonwhy the other guys are trying to tell you that a sine wave voltage to the LED is pointless

you just stated ....

a sine wave (AC) voltage supply isn't going to achieve that, it's in contradiction to what you just stated

For a stable output from the LED ( laser diode ... whatever), it needs to be supplied with a stable DC voltage at a specific current

Also, I suspect they are trying to explain to you that, like any diode, if you supply an AC current to it,
it's going to 1/2 wave rectify it

Dave

To: davenn

I have an ideas about that. - Is to use three op-amp 1 Non-inverted 2 Inverted and 3 An Adder. It should work, I think, I hope.
Regards, GeoffC.

 

[davenn]

To: davenn

I have an ideas about that. - Is to use three op-amp 1 Non-inverted 2 Inverted and 3 An Adder. It should work, I think, I hope.
Regards, GeoffC.

and what are they are supposed to achieve ?