Phase Shift Oscillator


Posted on Feb 5, 2014

We select the so called phase shift oscillator as a first example as it exemplifies verysimply the principles set forth in the previous blog post. The circuit is drawn to show clearly the amplifier and feedback network. The circuit consists of a common source FET amplifier followed by a three section R-C phase shift network. The amplifier stage i


Phase Shift Oscillator
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s self-biased with a capacitor bypassed source resistor Rs and a drain bias resistance RD. The output of the last section is supplied back to the gate. If the loading of the phase-shift network on the amplifier can be assumed to be negligible, a phase shift of 180 ° between the amplified output voltage Vout and the input voltage Vin at the gate is produced by the amplifier itself. The three-section R-C phase shift network produces an additional phase shift, which is a function of frequency and equals 180 ° at some frequency of operation. At this frequency the total phase shift from the gate around the circuit and back to gate will be exactly zero. This particular frequency will be the one at which the circuit will oscillate provided that the magnitude of the amplification is sufficiently large. In a FET phase-shift oscillator voltage series feedback [that is, feedback voltage proportional to the output voltage Vout and supplied in series with the input signal at the gate is used. The frequency of the oscillator output depends upon the values of capacitors C and re ­sistors R used in the phase shift network. Using basic RC circuit analysis technique, it can be shown that the network phase shift is 180 ° when The frequency can be adjusted over a wide range if variable capacitors are used. As well as phase shifting, the R-C network attenuates the amplifier output. Network analysisshows that when the necessary phase shift of...




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