×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# Positive feedback evaluation

## Positive feedback evaluation

(OP)
Hello,

I´m trying to figure out the following issue:

If we have a feedback system:

Af = A /(1 + βA)

In the case that phase of βA = 180°, we have 3 cases:

1_ | βA| < 1 then, if we choose arbitrary βA = 0,5 (with phase 180°)  Af = A/(1-0,5) = 2*A ….. AMPLIFIER WITH POSITIVE FEEDBACK. STABLE AND REGENERATIVE.

2_| βA| = 1 (with phase 180°)  Af = infinite = A/(1-1) …. OSCILLATOR

3_ | βA| > 1 we choose arbitrary βA = 1,5 (with phase 180°)  Af = A/(1-1,5)= -2*A
According to nyquist we encircle point -1 what is the condition for instability. Now mathematically in equation Af=A/(1+ βA) I got a -2 value for Af module. Not an unstable value for Af?

How can I see the instability mathematically in equation: Af = A /(1 + βA)????

Thanks
Rodrigo

### RE: Positive feedback evaluation

you've just defined the stability boundary,

### RE: Positive feedback evaluation

(OP)
Ok. Then it is not a valid equation for the instability region?. In fact the equation has a discontinuity when BA cross the -1 point, it changes from +infinite to -infinite.

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!