Unlike a Wien-bridge oscillator, the phase-shift oscillator in Fig 1 starts up quickly. Also, the circuit does not require that you adjust several trimming resistors just to tune the oscillator to a given frequency. Experiments show that the circuit's total harmonic distortion (THD) measures approximately 0.5% or less.
Connecting the output of a bandpass stage to its input via a phase inverter realizes a phase-shift oscillator. In practice, a phase-shift oscillator also needs a limiter stage. Unfortunately, the limiter can distort the output sine wave.
Diodes D1 and D2 in the feedback loop of IC1A form a limiter that does not distort the output. The equation for the circuit is
R1 tunes the frequency of the oscillator.
To have a stable output voltage and minimum distortion, you must set the gain of the loop carefully. The gain needed for oscillation varies slightly with frequency; the circuit needs more gain for lower frequencies. You can leave R1 fixed to a value that yields the lowest frequency.
The circuit can generate a good-quality sine wave at low frequencies. In those cases, the values of capacitors C1 and C2 may be several microfarads. With such large capacitors, the startup time increases to several hundred milliseconds. You can avoid this delay by connecting diode D3. With D3 in place, startup always takes no more than a few milliseconds. D3 does not distort the characteristics of the circuit because D1 and D2 already limit the output to a few hundred millivolts, peak-to-peak. With D3 connected, the circuit needs a little more gain to develop a stable output.
EDN BBS/DI_SIG #1423contains a writeup of this circuit and a comprehensive Spice model.(DI #1423)
1. "Easily Tuned Sine Wave Oscillators," National Semiconductor, Linear Application Handbook 1986, pp 1118 to 1119, LB-16.