why oscillate only at 180 degree when negtive feedback ?

[wangkes9]

Why the circuit must oscillate only at 180 degree when negtive feedback.

IF transfer function H(jw)=10 ,Angle (jW) =-179 and H=0.5 ,Angle (jw)=-180 , then the cuicuits will oscillates at the frequence W which phase shift is -179 I think?

 

[LvW]

Re: why oscillate only at 180 degree when negtive feedback

Why the circuit must oscillate only at 180 degree when negtive feedback.

IF transfer function H(jw)=10 ,Angle (jW) =-179 and H=0.5 ,Angle (jw)=-180 , then the cuicuits will oscillates at the frequence W which phase shift is -179 I think?

The circuit consists of 3 inverting stages - thus, the whole circuit is inverting (180 deg phase shift) .
When each stage contributes additionally 60 deg. because of "parasitic" phase shift, you arrive at 360 deg. which fulfilles the oscillation criterion.