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Dear baby_1
Hi
Did you forget the principles of oscillation ? of course it isn't possible . if you have negative feed back for all of the loop , the out put will be damped .
If you have a transistor with components connected from its collector circuit to its base circuit, its negative feedback, because when th collector signal goes positive, the base signal is going negative. If however you change the components so they introduce greater then 180 degrees phase shift and the transistor has a gain greater then 1. The circuit will oscillate, because at this frequency the transistor has positive feedback. So the feedback is always positive at the frequenncy of oscillations but can be negative feedback at other frequencies.
Frank
Hi again
It is enough to know that if the poles of each circuit be at right of s plane the circuit surely , will start the oscillations . ( or in js , axis )
In the other words , the most important thing is that the out put voltage of feed back network , be in phase , so the circuit will keep oscillations .
Don't confuse ! if you have negative feed back and then if you have a circuit to shift the out put phase , you can't say that it is negative feed back . it will consider as positive feed back .
An oscillator is circuit with no input signal that will generate an output. This happens because the circuit will feed itself, the output is also connected to some point that may be considered the input, so this is feedback. If the feedback is negative, then the output signal would "try" to cancel itself, so possible oscillations would fade away. Since you want sustained oscillations, the feedback must be positive.
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