Hello there

Your antique 741 opamp does not oscillate because you did not "kick it" with a startup command in the simulation.

They will smoke in simulation as well as a breadboard

 

Yes you are right. But it still doesn't work. I also attached the diagram that I saw in a textbook and I don't realize what is wrong

I set up the circuit in thread #11, 5.png, on a breadboard today, using an ancient RCA 741 chip. I had to substitute a 2N5457 JFET for Q1, and used a 1K5 resistor for R3. With the 10K pot adjusted to 4K2, I had a rock solid 3 volts P-P un-distorted sinewave output, using a supply voltage anywhere from ±3.7V to ±15V.

 

Here is the output shown by LTspice for the 100k/47k version. Does yours still not work with those values?

View attachment 235520

Hello. can you help me get an amplitude between 5V and 10V peak to peak at the output of the oscillator? or can you explain to me how I could vary the amplitude between certain limits?

 

How good do you need the distortion to be?

 

How good do you need the distortion to be?

I think that the distortion is as small as possible, to obtain a sinusoidal signal at the output with amplitudes between 5-10V

 

You can stabilise the amplitude with a pair of back-to-back zeners across R9/R10. If R10 is adjusted so that the gain is just a tad more than 3, so it just starts, the distortion is still pretty good (though not as good as lamp-stabilised, or JFET stabilised) but the amplitude is predictable.
Wien bridges are a bit of a trade-off between complexity, amplitude predictability and distortion.

 

Wien Bridge 1 KHz Sinewave

View attachment 236054

A very hypothetical Wien Bridge! If R1 is 200k-1% and R2 is 100k+1% then its not an oscillator!

 

I left out the stabilizing network because the plain Wien Bridge is tedious enough at first. The circuit values may need fine trimming.
Further refining can be accomplished at a 2nd stage if necessary. The smaller jfets or the bead thermistor had success as stabilizer.
Having a buffer will also improve output and from there an attenuation is possible without affecting the resonant circuit. It was mentionsed before that some of the phase shift oscillators can lend their design for phase. Multisim shows a working simulation but without careful techniques and without the equipment to see the effect of how those small adjustments are reducing distortion is hit or miss.

 

View attachment 236061

And how do I vary the amplitude at the output?

 

And how do I vary the amplitude at the output?

Vary the supply voltage - because if the gain is >3 the output will keep increasing until it clips.
(By the way 201k/100k still isn't guaranteed to work for 1% tolerance, because it could be 199k/101k! Try 205k.)

 

View attachment 236061

I left out the stabilizing network because the plain Wien Bridge is tedious enough at first. The circuit values may need fine trimming.
Further refining can be accomplished at a 2nd stage if necessary. The smaller jfets or the bead thermistor had success as stabilizer.
Having a buffer will also improve output and from there an attenuation is possible without affecting the resonant circuit. It was mentionsed before that some of the phase shift oscillators can lend their design for phase. Multisim shows a working simulation but without careful techniques and without the equipment to see the effect of how those small adjustments are reducing distortion is hit or miss.

The stabilising networks definitely do respond to trimming. I tried the circuit in reply #11 again, this time with an NPC branded 2N3819 having an ID of 4mA @ Vg=0. With pots for RF and R3, suitably adjusted, I got 6 Volts peak to peak before distortion on the negative tip. The Fet shows -3 Volts on the gate at that point. Supply V+ 5.7, V- 6.2.

 

One approach is to simplify ( at least that was the intent ) and get more exact (great nice to see )however there are more methods after trimming the
frequency determining network hopefully it's <2% THD If you want to use AGC to compensate for amplitude gain you can read about that:
The slower startup 300 mS approaches and then goes past the frequency slightly before settling in simulation showing increasing amplitude.
I think the small bead 5K thermistor when carefully balanced would be effective in sensing when bridge goes out of balanced.
http://www.csun.edu/~ih20409/SCHOOL1/SCHOOL1/DISCRETE AND INT OSCILLATORS.pdf

 

Vary the supply voltage -

Don't think so. A stable Wein Bridge oscillator requires an amplitude stabilization servo. Whether an PTC resistor (light bulb) or a FET (both act as an amplitude-dependent resistance), the output amplitude is not directly related to the power supply voltage(s). If the oscillator is running correctly, varying the power supplies will affect only the amount of signal peak clipping.

ak

 

Don't think so. A stable Wein Bridge oscillator requires an amplitude stabilization servo. Whether an PTC resistor (light bulb) or a FET (both act as an amplitude-dependent resistance), the output amplitude is not directly related to the power supply voltage(s). If the oscillator is running correctly, varying the power supplies will affect only the amount of signal peak clipping.

ak

You’re right..

 

Don't think so. A stable Wein Bridge oscillator requires an amplitude stabilization servo. Whether an PTC resistor (light bulb) or a FET (both act as an amplitude-dependent resistance), the output amplitude is not directly related to the power supply voltage(s). If the oscillator is running correctly, varying the power supplies will affect only the amount of signal peak clipping.

ak

If we take the circuit in post #28, with the gain adjusted until it does oscillate, the amplitude will continue to increase until the gain reduces to exactly 3. That will happen at just as clipping starts, so the circuit stabilises at that amplitude.
Any Wien bridge made with amplifiers with symmetrical output stages will do that.
Once the signal stats to clip, the amplifier gain can't increase the amplitude any further.

 

And how do I vary the amplitude at the output?

The easiest way is to add another variable gain amplifier after the existing oscillator, that way you both avoid loading the output of the oscillator and can vary your desired signal it will!
Your circuit in post #11, and that I responded to in post #22, only needs amplifying by just over 3 to meet your needs.

 

Mariachi, are you forced to use the WIEN type oscillator?
There are other types which do not need a separate amplitude stabilization - because the amplitude is set by (varying) the supply voltages. However, these types need two or three opamps.

 

Mariachi, are you forced to use the WIEN type oscillator?
There are other types which do not need a separate amplitude stabilization - because the amplitude is set by (varying) the supply voltages. However, these types need two or three opamps.

Not exactly, but i want to make an oscillator that has 50kHz and amplitude output vary from 5V to 10V peak to peak. I don’t know how to make the output amplitude to vary in those limits

 

10V p/p at 50kHz is quite a challenge for many op-amps mentioned here. No chance with a OP281 or a 741.
What shape waveform did you want?

 

10V p/p at 50kHz is quite a challenge for many op-amps mentioned here. No chance with a OP281 or a 741.
What shape waveform did you want?

But with OPA621? I managed to obtain a 50kHz frequency with Wien bridge. But i don’t know how to change the amplitude from 5 to 10V