Phase-Shift and Positive Feedback Oscillators

Author: N.H. Crowhurst

Phase-shift oscillator

The other two types of oscillators do not use inductors because inductors can lead to various difficulties. They are, for example, likely to pick up magnetic induction, which can cause hum, or to pick up unwanted audio. The first of these oscillators uses a progressive phase shift to take advantage of the oscillatory condition of a feedback amplifier. The combination of successive resistors and capacitors are arranged to produce the phase reversal with a gain of 1 at a frequency that can be specifically controlled. This frequency can be adjusted by varying either the capacitors or the resistors.

The third type of oscillator uses positive feedback. The positive feedback is sufficient to cause oscillation at just one frequency. An arrangement of resistors with capacitors in series produces a loss of low frequencies. A capacitor in parallel produces a loss of high frequencies. The correct combination produces a maximum transfer of the positive feedback at one frequency with no phase shift. The amplifier oscillates at this frequency because the feedback is smaller at all other frequencies.

Positive feedback oscillator