# rc phase shift oscillator

Discussion in 'General Electronics Chat' started by maxpayne, Nov 23, 2008.

1. ### maxpayne Thread Starter New Member

Nov 23, 2008
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0
why a fet rc phase shift oscillator?
fet doesnt bring any 180 degrees phase shift then what use of those three rc networks?
i'm simply a starter!

2. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,043
292
ANY class A amplifier with a common emitter, source or cathode, will have 180 degree phase shift (or close to it)

ERic

3. ### DickCappels Moderator

Aug 21, 2008
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To oscillate, you need to have 360 degrees phase shift.

With the RC phase shift network, the FET provides the first 180 degrees. Each RC stage can supply up to almost 90 degrees (so just two stages won't get you 90 degrees), so using three stages gets you a total of 180 derees for the phase shift network, with each stage contributing about 60 degrees.

FET 180 degrees
RC stage 60 degrees x 3 = 180 degreest

TOTAL = 360 degrees

4. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Gain tends to be critical in these circuits too.

5. ### maxpayne Thread Starter New Member

Nov 23, 2008
3
0
sorry i was wrong
fet really brings that 180 degrees phase shift and it is used for the rc phase shift oscillator
but how is it proved?(that fet brings a 180 degrees phase shift?

6. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,043
292

This is etremely simple. Model the FET and the load resistance as a simple voltage divider. The greater the FET current, the lower the voltage at the drain.

7. ### maxpayne Thread Starter New Member

Nov 23, 2008
3
0
it sparked to glow