Hi All,

I tried to simulate RC phase shift Oscillator with copy book circuit and i was not able to generate sinusoidal wave even with permutation & combination.
I can not figure out the problem.
Could any body correct and explain why this copy book circuit not working ?

I used 3RC stage to give 180deg phase shift.
Attached is the simulation file.

 

Typically a lag phase-shift network, as show below, is used, not a lead network.
Note that the slew rate of the op amp your are using is too low, so its output looks like a sawtooth.
And use the UIC option for the transient simulation to help start the oscillation.

 

Typically a lag phase-shift network, as show below, is used, not a lead network.
Note that the slew rate of the op amp your are using is too low, so its output looks like a sawtooth.
And use the UIC option for the transient simulation to help start the oscillation.

View attachment 208307

I tried with fastest opamp from LT Spice and still find it is not working.
I am not sure how much output voltage amplitude should be for this circuit ?

 

Most circuit i found on google is ?

 

Hello,

Have a look here:
http://www.play-hookey.com/oscillators/audio/phase_shift_oscillator.html

Bertus

 

Hello,

Have a look here:
http://www.play-hookey.com/oscillators/audio/phase_shift_oscillator.html

Bertus

Hi
Above Circuit also did not work in simulation.
The output of this circuit is saturating.

 

Your post #3 circuit simulates ok if you increase R3 to 560k or more.

 

Your post #3 circuit simulates ok if you increase R3 to 560k or more.

Many thanks to your reply.
I changed opamp and now i used faster opamp. Yes, some how it worked.
I still do not agree with simulation results why because ?

1. R3 = 29*R5 = 29*10K = 290K (used 560K much far)
2. Calculated frequency = 6.5KHz and it is comming 22.7KHz
3. Amplitude is 1.6V peak no formula for this ?

 

Here is what Texas Instruments say about it:

 

3. Amplitude is 1.6V peak no formula for this ?

It's basically the op amp saturated output signal voltage times the attenuation of the phase-shift circuit at the oscillating frequency.
Unfortunately that's not a trivial calculation.

 

Here is what Texas Instruments say about it:

Thanks

 

Most older sinewave generators use a Jfet fed with rectified output signal for amplitude stabilization. Then the opamp does not saturate resulting in very low distortion. Temperature affects the output level a little.
I made a VERY amplitude-stable sinewave generator that has very low distortion by first making a digital stepped sinewave then filtering it with a switched-capacitor lowpass filter IC.

 

I should follow along like a lab day at school, but I need to fix my DMM. Thanks OP I'll bookmark this for later