Stability of feedback systems

[shivamgarg]

Folks,

Howdy !!

I have a doubt relating to stability of a system. To check stability we always assume a FEEDBACK circuit. Why is this so?
If we stabilize the circuit for open loop shouldn't it be stable for circuit with feedback as well?

Thanks.

 

[doraemon]

Hello!

Of course it does: if there is no feedback, then you don't have much to control.
Example: input signal -> amp -> output signal. (well, there might be a closed loop in
the amp, but this is another story).
If you check for stability, it means that you have some kind of loop, the most usual
being input -> subtractor -> amp -> output and feedback to subtractor.

Dora

 

[crutschow]

It's feedback that cause instability in a system (when the feedback becomes positive at some frequency).
How would an open loop system be unstable?

 

[haykding]

Folks,

If we stabilize the circuit for open loop shouldn't it be stable for circuit with feedback as well?

Thanks.

What does it mean "stabilize the circuit for open loop" ? Isn't it stable in all cases ? Maybe you mean DC stability for example in the Bandgap circuit or beta multiplier ?

 

[crutschow]

What does it mean "stabilize the circuit for open loop" ? Isn't it stable in all cases ? Maybe you mean DC stability for example in the Bandgap circuit or beta multiplier ?

Good point.
Shivamgarg, are you referring to AC or DC stability since they are entirely different and often unrelated(?).