Miles Power: so the frequency derives from phase shift, ok. That means, w is the frequency, where every LP filter will have 60 degree phase shift, and with R1 I should set the right DC gain to start oscillations? But still, as I wrote, the frequency should change with R1. Why is that?
Michalh,
How come are you using 3 op amps for a phase-shift oscillator? Just use 1 op amp and let a RC network change the phase by 180°. The RC network attenuates the signal by -1/29, so the op amp has to have a gain of -29 for the circuit to oscillate. Ask if you have any questions.
View attachment 85080
Ratch
H again Ratch,
I think you have the frequency calculation right for the original circuit now, once simplified a little, however you can see from my diagram that it's not very much more difficult to work with especially if we make R1=R2=R3=R (an assumption all along) and also R4=R. It's just as simple as before just that we now have a slightly more complicated frequency calculation if we dont make R4=R also. With R4=R it's just a different constant in the frequency calculation.
Also, your 'new' circuit requires a resistor value of R5=29k not R5=28k. I think that was just a slight oversight though.
I have included a diagram with all the necessary corrections for both circuits.
As usual we assume that C1=C2=C3=C.
We always have to keep in mind that although these are theoretically sound circuits they need some sort of gain stabilization in the real world.
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