The two circuits below illustrate generating low frequency sinewavesby shifting the phase of the signal through an RC network so thatoscillation occurs where the total phase shift is 360 degrees. The transistorcircuit on the right produces a reasonable sinewave at the collectorof the 3904 which is buffered by the JFET to yield a low impedanceoutput. The circuit gain is critical for low distortion and you may needto adjust the 500 ohm resistor to achieve a stable waveform with minimumdistortion. The transistor circuit is not recommended for practicalapplications due to the critical adjustments needed.
The op-amp based phase shift oscillator is much more stable than thesingle transistor version since the gain can be set higher thanneeded to sustain oscillation and the output is taken from theRC network which filters out most of the harmonic distortion.The sinewave output from the RC network is buffered and the amplituderestored by the second (top) op-amp which has gain of around 28dB. Frequencyis around 600 Hz for RC values shown (7.5K and 0.1uF) and canbe reduced by proportionally increasing the network resistors (7.5K).The 7.5K value at pin 2 of the op-amp controls the oscillator circuit gainand is selected so that the output at pin 1 is slightly clipped at thepositive and negative peaks. The sinewave output at pin 7 is about 5 voltsp-p using a 12 volt supply and appears very clean on a scope since theRC network filters out most all distortion occurring at pin 1.
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