RC Network Oscillators
Since there is no input, there is no initial current so start-up gain is open-loop. Thermal noise provides initial start-up voltage which quickly multiplies through the op-amp. Since noise is an AC quantity, the output signal is also AC. The frequency of oscillation will increase until Xc = R (resonance) at which it will stabilize. Once resonance is reached, gain will settle to equal one.
Phase Shift Oscillator
The inverting input introduces a 180° phase shift; the three RC networks produce another 180° phase shift providing positive feedback (60° each).
Gain must always be approximately 29.
Note that R and C could switch positions; however, that arrangement is know to provide a less "clean" signal.
Frequency of Oscillation = resonant frequency for a phase shift oscillator
Frequency of Oscillation = resonant frequency for a wien-bridge oscillator.
At resonance, the phase shift between the two RC networks = 0°. The gain must be approximately 3.