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Why closed-loop control may cause stability problem?

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powersys

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One of the pros of open-loop control is it does not, by itself, cause stability problem. And one of the cons of closed-loop control may cause stability problem. Would someone pls explain why a closed-loop may cause stability problem? Thanks.
 

if any error somehow increases in the loop it causes oscilatons
 

because u know positive feedback is used to construct oscillators .... and the condition for that is that the loop gain =BA = 1 ... so total gain =A/(1-BA) = infinity so for zero input there will be output

but why should we worry about oscillation for negative feedback amplifiers :

this is because for any negative FB amplifier ... total gain = A/(1+BA)

so if the phase response ( phase of loop gain (BA) ) =-180 when the magnitude of BA is >1 .... the negative FB will turn into positive FB and oscillation takes place ....

i.e amplifier becomes unstable

that's why we must make sure that for mag(BA)>1

angle(BA) should be >-180 .... and (angle(BA)-(-180)) is called the phase margin should be >0
 

ahmed_nasr said:
because u know positive feedback is used to construct oscillators .... and the condition for that is that the loop gain =BA = 1 ... so total gain =A/(1-BA) = infinity so for zero input there will be output

but why should we worry about oscillation for negative feedback amplifiers :

this is because for any negative FB amplifier ... total gain = A/(1+BA)

so if the phase response ( phase of loop gain (BA) ) =-180 when the magnitude of BA is >1 .... the negative FB will turn into positive FB and oscillation takes place ....

i.e amplifier becomes unstable

that's why we must make sure that for mag(BA)>1

angle(BA) should be >-180 .... and (angle(BA)-(-180)) is called the phase margin should be >0
Click to expand...
Thanks for your explanation... but I still don't get the points... probably due to my limited knowledge...
 

Hope this will answer your question :)

**broken link removed**
 

with the frequency change the phase of the loop will change.... so it may not give you 360 degrees phase shift, whhich is basic requirement for stabilty..
 

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