how to find the values of Phase-shift Oscillator?

[LvW]

Re: Oscillato

Hello YASIR,

there is an error in your circuit: R2 has to be connected directly to the inverting input rather than to ground. In your circuit the signal gets clipped due to a gain which is to high. Try it.

 

[Audioguru]

Oscillato

The opamp clips because the simple circuit does not have anything to limit the output level.
Also, the RC networks are highpass which increases distortion. If they are lowpass then they reduce distortion.

 

[yasir_ali86]

Re: Oscillato

Thanks to all for fantastic advices (especially Karesz) Thanks a lot.
By connecting R2 in series with inverting input the output is going 0.

Now After All these, i still have Questions that what about output Voltage? the circuit itself is consuming much power. as when i applied 6VDc, The outpput is 3.8vAc. is there Any trick that i can increase the output voltage?
Moreover that an OpAmp require dual power Supply. so after conecting dual 6+6(12)V, i'm having only 3.8 at it's output.

 

[Audioguru]

Oscillato

With a 12V total power supply, the max clipped output of most simple opamps that have no load is 10V p-p which is 3.54V RMS. Your opamp is a little bit better than most.

 

[yasir_ali86]

Re: Oscillato

Hey! look at this brothers, I've Completed my circuit with Single power Supply .
I'm very Happy from yesterday becuase i am trying making an Oscillator from 6 Months but yesterday i Just made it, Because of you all people made me realized That i've to it. Thanks a lot all brothers.
Today i'll bring the componenets from Market because On Friday all the Things all Off & today All the market will open.

Here is my circuit

thanks

 

[LvW]

Re: Oscillato

Hi yasir,

may I give you one general advice?

When a circuit does (or does not) what you want or expect - try to find out the reason! That's the best and most efficient way to learn.

1.) Example: The circuit which was first proposed by you. I gave you a hint concerning R2. Moreover, the correct circuitry was given by AUDIOGURU on February 1st.
Why do you ignore these hints? Instead your answer was:
By connecting R2 in series with inverting input the output is going 0.

Did you ask yourself - why? Did you compare your circuit with the classical phase shift topology?

In your simulation (with R2 in series) the output was zero because each well designed oscillator normally needs a "kick" to start oscillation. For example, when you give on of the capacitors an intial voltage of some Millivolts or if you ramp the supply voltages (like in reality) you will see that the circuit works properly!

2.) I am afraid, with your last circuit you will be not very happy. It looks vry strange. At first, it needs a battery (without ground connection) and it consumes more power than necessary.
At second, it`s oscillation frequency and amplitude most probably will depend on the opamp type.

Added after 3 hours:

Addendum (later): The opamp will be operated strongly in its non-linear region (overdrive). I wonder how the circuit will behave in reality (hardware).

 

[yasir_ali86]

Re: Oscillato

Hey Lvw! I did not ignore your hint but I did not understand That, is there Any other type of same phase shift Oscillator?. And i've read a book "Electronics Devices By Thomas Floyd". And the Design is Same there as I constructed mine.
Is there Any differance between both Circuits?
Does connecting R2 in series or parallel with inverting input of OpAmp effect the Frequency,phase,output or any other stability?

 

[Audioguru]

Re: Oscillato

Here is a phase-shift oscillator that uses lowpass filters for low distortion. It has an output buffer opamp. It uses a single supply voltage.

 

[LvW]

Re: Oscillato

Hey Lvw! I did not ignore your hint but I did not understand That, is there Any other type of same phase shift Oscillator?. And i've read a book "Electronics Devices By Thomas Floyd". And the Design is Same there as I constructed mine.
Is there Any differance between both Circuits?
Does connecting R2 in series or parallel with inverting input of OpAmp effect the Frequency,phase,output or any other stability?

The circuit as given by AUDIOGURU (Feb. 1st) has R2 in series between the high pass feedback network and the inverting input of the opamp. This is one version of several other known alternatives of the phase shift oscillator.
And, of course, there is a difference between the classical and your circuit.
But to understand the explanation requires some knowledge of at least two areas:
* behaviour of real opamps (input impedance, slew rate, linearity, current in the power lines,...)
* loop gain, condition of oscillation (Barkhausen criterion), amplitude control mechanism.

 

[yasir_ali86]

Re: Oscillato

Hey! Lvw!
I've designed phase shift With R2 in series with inverting input. but at first the output was going zero & then i made many changes. finally i got on one state where i connected a variable resistor Rf. at start i keep it 0% and i start simulation the output shows some constant dc volts when i just increase Rf to 1% the Sine wave starts. so my Question is that if i use it in a circuit every time, i'll have to do this? or it has a SOLUTION.?

my schematic is:

 

[LvW]

Re: Oscillato

From oscillator theory the opamp with feedback must have a gain of -29.
Remember the values of the first circuit: Rf=63.8 kohms and R2=2.2 kohms
with Rf/R2=29.
Now, with 1.3 kohms the ideal feedback resistor is Rf=37.7 kohms.
For a safe start-up I suggest to use Rf=39 kohms fixed. A variable pot is not a good solution.
For start-up give one of the capacitors an intial condition (IC=) of some millivolts.
Or, alternatively, use a time dependent power supply (rising pulse with period of 1 sec or so).

 

[yasir_ali86]

Re: Oscillato

I did the experiment of connecting Rf=39k the Oscillation took i think 5 minutes to start. very gradually increaseing from nV to V. & then i increased the value of Rf. i made it 200k then the oscillation stared in 5 Seconds but with some distortion.
Tell me What is the theory behind "The kick voltage"? what does it do and all. and what should i do for Rf?
Thanks

 

[LvW]

Re: Oscillato

I did the experiment of connecting Rf=39k the Oscillation took i think 5 minutes to start. very gradually increaseing from nV to V. & then i increased the value of Rf. i made it 200k then the oscillation stared in 5 Seconds but with some distortion.
Tell me What is the theory behind "The kick voltage"? what does it do and all. and what should i do for Rf?
Thanks

OK, some basics in short:

An oscillator is a selfstanding circuit without any input. It`s function is based on the principle that the active device (opamp) delivers a signal Vout which - after passing through the frequency selective feedback network - exactly is needed at the input of the amplifier to create Vout. Thus, the circuit creates its own input signal.
That`s the secret behind the requirement "loop gain=1".
Now, if you start oscillations there is not yet a signal within the circuit which could be amplified, unless you give an initial voltage at one capacitor.
Without such a initial voltage, in reality the oscillator can start working by amplifying the proper frequency contained in the noise floor - in simulation this task is taken over by "calculation uncertainties" (rounding errors). That`s the reason for the 5 minutes you have mentioned.
As far as the feedback resistor (that means: the gain) is concerned:
If the gain is much higher than the required one (in this case -29), the oscillator will start in a shorter time but will go into deep saturation (clipping of amplitudes). Rf=200 kOhms is too large!!!
Therefore, the gain should be only a bit higher than the calculated one.
Try the initial voltage principle!.

 

[Audioguru]

Oscillato

In the phase-shilt oscillator That I posted the first opamp goes into saturation and the three RC lowpass networks reduce its clipping distortion to almost nothing.

 

[LvW]

Re: Oscillato

In the phase-shilt oscillator That I posted the first opamp goes into saturation and the three RC lowpass networks reduce its clipping distortion to almost nothing.

Yes, that´s true. That is the advantage of a second amplifier. Of course, the RC lowpass oscillator by far is better than the high pass version.
Question:
In this context - what do you think is the "best" RC oscillator based on opamps
(regarding frequency stability/sensitivity, tuning capability, amplitude limiting mechanism,...) ?

 

[yasir_ali86]

Re: Oscillato

well..... this is my first Oscillator i ever made. but 2 phase shift oscillator i designed. the second one seems better (in which i used R2 in series with inverting input of OpAm) with compare with the first one.
can you give me complete detailed notes on these 2 or give me any link through which i can learn this Oscillator from all the Angles. I want to learn all the things about phase shift Oscillator. and also i want to compare both the types.
Thanks

 

[yasir_ali86]

Re: Oscillato

What happened? No one is replying me. :|

 

[LvW]

Re: Oscillato

What happened? No one is replying me. :|

For my opinion, there is nothing left to say.
On the 6th of February I did recommend to you first to study some basics about oscillator principles - especially some problems connected with loop gain and oscillation condition.
Without these basics, i believe, it makes not much sense to "learn all the things about phase shift Oscillator". This type of oscillator is only one out 10...20 other oscillator topologies.
I hope, you don't expect a complete lesson about oscillator theory within the frame of this forum.

 

[yasir_ali86]

Re: Oscillato

Hey LvW! i know what you are talking about but i think you did not read my last post. In my last post asked for the detailed notes or any link through which i can read this Oscillator from all every point of veiw.

 

[Audioguru]

Instead of asking for a handout in this forum, look in Google. That is where I found the first two circuits that I posted.