OT: Self-Oscillation op amp Ckt

[Steven Donaldson]

I have an oscillation circuit that I would like to know the name of.  I'm told this is a "classic" op amp design but I can't find it in the Forest Mims books or anywhere I've looked online.  Can anyone identify it?

Thanks,

Steve

[Nick Sargeant]

It's a phase shift oscillator. It generates a sine wave at a frequency which gives a 180 degree phase shift output to input. With only two stages, it won't be very stable. I found a formula that says F= 1/(2*Pi*R*C*sqrt(2*number of stages)) .. where R is the value of the resistor between caps to ground, and C is the value of each capacitor - which ends up at about 160Hz with 0.01uF and 5K. 

[Steven Donaldson]

Interesting.  Thank you.  As drawin, simulation shows ~844Hz. witch is close to the built circuits.   This is in several test boxes I'm working on and I decided to change out for an LTC6902 square wave IC with a level shifting amp.  for my application, the square wave or a LPF'd square don't work as well as this circuit, possibly due to harmonic content.  FFT shows fairly linear drop-off compared to the harmonics of the square or LPF.

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[John Rehwinkel]

> Interesting. Thank you. As drawin, simulation shows ~844Hz. witch is close to the built circuits. This is in several test boxes I'm working on and I decided to change out for an LTC6902 square wave IC with a level shifting amp. for my application, the square wave or a LPF'd square don't work as well as this circuit, possibly due to harmonic content. FFT shows fairly linear drop-off compared to the harmonics of the square or LPF.

A relative of the phase shift oscillator is the Wein bridge oscillator, which includes a non-linear element to control gain so the active device(s) don't clip. These oscillators can give a good quality sine wave (low harmonics) with few parts. A Royer oscillator can also do so, but requires more specialty parts to build.

- John