19-09-2012, 12:02 PM
RC Phase Shift Oscillator Circuit.
RC Phase Shift.doc (Size: 77.5 KB / Downloads: 36)
Theory: -
An oscillator is a circuit, which generates ac output signal without giving any input ac signal. This circuit is usually applied for audio frequencies only. The basic requirement for an oscillator is positive feedback. The operation of the RC Phase Shift Oscillator can be explained as follows. The starting voltage is provided bynoise, which is produced due to random motion of electrons in resistors used in the circuit. The noise voltage contains almost all the sinusoidal frequencies. This low amplitude noise voltage gets amplified and appears at the output terminals. The amplified noise drives the feedback network which is the phase shift network. Because of this the feedback voltage is maximum at a particular frequency, which in turn represents the frequency of oscillation. Furthermore, the phase shift required for positive feedback is correct at this frequency only.
The Phase Shift Oscillator
The real problem was the assumption that the three RC phase shift segments remain independent of each other. In reality, they don't. They can't. The first segment is OK, as it is driven directly from the op amp. However, the second segment can't help but load the first, and the third segment loads the second. This means a different phase shift response and increased attenuation.
One way to keep this from happening is to add buffer amplifiers between the individual RC segments, as shown to the left. As shown here, these are unity-gain amplifiers. Their only purpose is to isolate the RC segments from each other. When this is done, the required gain for the original amplifier is reduced to very close to 8, and the operating frequency drops to appoximately the calculated value. The main error now comes from component tolerences.
A possible variation is to use inverting amplifiers at each step, and to set the gain of each amplifier to 2, with one amplifier being adjustable to accurately set the gain around the entire loop. If this is done correctly, each amplifier will output a sine wave of the same amplitude as the others, and they will be phase-shifted from each other by 120°. The circuit can then serve as a three-phase signal source.