The effect to output impedance from positive feedback

[bhl777]

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!

 

[LvW]

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).

 

[bhl777]

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!

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).

 

[LvW]

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!

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".