The effect to output impedance from positive feedback

Status
Not open for further replies.
B

bhl777

Full Member level 6
Joined
Sep 30, 2008
Messages
363
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,296
Visit site
Activity points
5,140
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!
Click to expand...

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".
 
Reactions: bhl777

    B

    bhl777

    Points: 2
    Helpful Answer Positive Rating
Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…