How can we get maximum angle of 90 degrees in RC phase shift oscillators?

To achieve a maximum phase shift of 90 degrees in RC phase shift oscillators, you can use a combination of three RC stages in series, where each stage contributes 60 degrees of phase shift at the desired frequency. By carefully selecting the resistor and capacitor values to ensure that the total phase shift across the three stages reaches 180 degrees, you can then use a phase inverter, such as a transistor or an op-amp, to provide the additional 180 degrees needed for positive feedback. This setup ensures that the oscillator can sustain oscillations at the desired frequency.