# Op-Amp RC Phase Shift Oscillator Simulation help needed

#### mishra87

Joined Jan 17, 2016
947
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.

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• 2.6 KB Views: 21

#### crutschow

Joined Mar 14, 2008
28,538
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.

#### mishra87

Joined Jan 17, 2016
947
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 ?

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#### mishra87

Joined Jan 17, 2016
947
Most circuit i found on google is ?

#### Alec_t

Joined Sep 17, 2013
12,469
Your post #3 circuit simulates ok if you increase R3 to 560k or more.

#### mishra87

Joined Jan 17, 2016
947
Your post #3 circuit simulates ok if you increase R3 to 560k or more.
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 ?

#### Audioguru again

Joined Oct 21, 2019
4,117
Here is what Texas Instruments say about it:

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#### crutschow

Joined Mar 14, 2008
28,538
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.

#### mishra87

Joined Jan 17, 2016
947

#### Audioguru again

Joined Oct 21, 2019
4,117
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.

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#### DarthVolta

Joined Jan 27, 2015
521
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