The effect to output impedance from positive feedback

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bhl777

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Hi all, it is well know the 4 different negative feedback's effect on output impedance from a lot of textbooks. I am curious if for a system with Zout, when there is a positive feedback path added, how can we judge the change of Zout? Thank you!
 

The influence of a small amount of positive feedback - as long as the system remains stable - can be found using the same formula as for negative feedback, however using a POSIVE sign for the loop gain. (Stability limit for unity loop gain).
Example: Voltage controlled feedback (as for operational amplifiers): Zout=Zol/(1-A,loop) with Zol=open loop output impedance and A,loop=Loop gain (positive).
 

Hi LvW, then if we have positive A,loop, and it is bigger than 1, how can we decicde Zout since it is a negative number? Thank you!


 

bhl3302 said:
Hi LvW, then if we have positive A,loop, and it is bigger than 1, how can we decicde Zout since it is a negative number? Thank you!
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That is the reason I wrote "a small amount of positive feedback - as long as the system remains stable".
A loop gain of unity is the stability limit. The positive loop gain must be below "1".
 
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