what is the difference between loop gain and close loop gain

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achilles.wang

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in some feedback circuits, we often use loop gain and close loop gain.Would you please tell me the difference between them? and sometimes, we try our best to keep the loop gain constant, why?
thanks for every suggestion!
 

You can get the answer from the feedback chapter in the circuit design books.
 
Re: what is the difference between loop gain and close loop

Loop Gain = AB (also called open loop gain)

Closed Loop Gain = A/(1+AB)
 

Re: what is the difference between loop gain and close loop

wky said:
Loop Gain = AB (also called open loop gain)

Closed Loop Gain = A/(1+AB)
Click to expand...

The open-loop gain is A not AB, while B is feedback factor.
 

Generally speeking we use negative feedback to form a closed loop due to the reason of system stability and bandwidth.

Loop gain is one of the characteristics of a system,
if it changes,the system becomes time-variant,which brings much trouble to the procedure of modeling.

For further investigation,you can look them up in some analog electronic books or control theory textbooks
 

Thanks for all of you.
i can draw this conclusion:the open-loop gain is A, B is feedback factor,loop gain is AB and the close loop gain is A/(1+AB). is is right?
we use output voltage as the negative feedback signal when we want to keep output voltage constant in the switching power supply design. it is easy to calculate B, but how to compute the loop gain?
In others' design, they often use all kinds of compensations to get a good gain_phase for loop stability, how to make it ?
best regards,
 

Re: what is the difference between loop gain and close loop

Loop gain is enough to determine phase margin or gain margin.
As to " how to ", it's depends on what kind of system you are designing.
Generally, you can do as following:
1. find the "loop" in the system.
2. Choose a point to cut th eloop and injecting "ac source" into the loop.
beware of not to disturbing all the bias in the circuit when you are doing this.
3. Check the bode plot of gain and phase and determine where to put zero or pole in the system
4. Doing compensator design according to 3

Well, I think you have better get a book for detail syudy becasue there are many tricks in doing 1 to 4
 

    minhchau

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Re: what is the difference between loop gain and close loop

You can get the answer from the feedback chapter in the circuit design books.
 

Re: what is the difference between loop gain and close loop

See chapter 8 of "Switchmode Power Supply Handbook" by Keith Billings

PM me if the book isn't here, I'll upload it.

Regards
 
Thanks for all of you. in fact, i have many books about feedback. but i dont know how to use these theories in my design. for example, i design a flyback DC-DC converter. i just make the output voltage as the feedback signal. how to compensate to make its performance better? and Dear Regnum,i am glad to see this chapter as you said. thans for your kindness,
 

Re: what is the difference between loop gain and close loop

You should learn some about frequent compensation, i think it 's good for you .
 

Re: what is the difference between loop gain and close loop

Loop gain is gain of the feedback circuits and close loop gain is overal gain of the circuit you analyse.
 

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