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HELP!Oscillator design

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Full Member level 2
Apr 4, 2012
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I need to design a 3 stage RC oscillator.the schematic is composed of 3 amplifiers with an RC negative reaction. Can someone explain how should i think the design,from where should i start?

Are you sure you're asked to design a 3 stage oscillator composed by 3 RC+amplifier sections instead of an amplifier with a negative feedback coming from a 3 stage RC network ?
This last is the simple "phase shift oscillator" in which each RC stage shifts the phase by 60 degrees for a total of 180 degrees. Since the amplifier is configured as inverting (i.e. you get the source of feedback from the collector of the transistor) the total shift will be 360 degrees that is theoretically perfect in phase positive feedback.
However even adding other two inverting amplifier it should works, since the additiional phase shift is 180+180=360, but it seems to me a not necessary complication of the circuit.
By googling "phase shift oscillator" you'll find many informations.
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You also could use the oscillator as shown in Fig. 1a.
However, Fig. 1c is not a bad choice.

Remark: For my opinion the author`s comment to Fig. 1a (This circuit has the disadvantage of having no independent control on the oscillation condition) is not correct.
Indeed, the frequency can be controlled with one element only - and the oscillation condition fulfills automatically. That is a unique feature of this oscillator type.
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After studying the profound article, what are you particularly missing?

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