RC Oscillator : RC oscillator consists of resistor and capacitors .This is used to generate low or audio frequency signals. They have small size as compared to LC oscillator .
Principal of RC Oscillator : We know that single stage FET amplifier introduces 180° phase shift at amplified output signal. This give rise to negative feedback of amplifier. But for producing oscillations, we must have positive feedback of sufficient magnitude. This can be achieved if output voltage is feedback with the same phase as that of input signal. This can be achieved using phase shift network.
How it works : A fraction of output signal of a single stage amplifiers is passed through a phase shift network, before going back to input. This network give rise to another phase shift of 180° in addition to the phase shift of 180° introduced by the amplifier. Therefore , there is overall total phase of 360° which is same as 0°. This is done by phase shift oscillator principal .
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| Phase-Shift Oscillator |
Above figure shows the circuit of phase shift oscillator using field effect transistor ( FET ). The amplifier stage is self-biased by capacitor, by source resistor (Rs) and drain bias resistor ( Rd )
Voltage gain is given by ,
Av = gm * Rd . rd / ( Rd + rd )
Where , gm is transconductance of FET ,
Rd is drain resistance and rd is FET output resistance .
The feedback network consists of three identical RC section. Each section produces a phase shift of 60°. Therefore the net phase shift of feedback network is 180°. As amplifier stage also introduces a phase shift of 180°. therefore the total phase shift between the input and output is 360° or 0°. when circuit is energized by switching on the supply , circuit starts oscillations. The variation in gate current is amplified in the drain circuit . Then it is feedback to gate using phase shift network .
Circuit frequency of oscillations is given by ,
fo = 1 / 2š¯›‘RC√6
Application : Phase shift oscillator is used for generating fixed spot frequencies which done by switching of RC network. For variable frequency oscillator, capacitors are ganged and varied simultaneously .