| Author |
Message |
achilles.wang
Joined: 02 Sep 2004
Posts: 14
|
 21 Sep 2004 8:17 what is the difference between loop gain and close loop gain |
|
|
|
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!
|
|
| Back to top |
|
 |
Question
Joined: 11 Aug 2004
Posts: 76
|
| 21 Sep 2004 10:18 what is the difference between loop gain and close loop gain |
|
|
|
| You can get the answer from the feedback chapter in the circuit design books.
|
|
| Back to top |
|
|
wky
Joined: 23 May 2003
Posts: 163
Helped: 82
|
| 21 Sep 2004 11:16 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)
|
|
| Back to top |
|
|
Amuro
Joined: 15 Mar 2002
Posts: 293
Helped: 3
|
| 21 Sep 2004 11:56 Re: what is the difference between loop gain and close loop |
|
|
|
| wky wrote: |
Loop Gain = AB (also called open loop gain)
Closed Loop Gain = A/(1+AB) |
The open-loop gain is A not AB, while B is feedback factor.
|
|
| Back to top |
|
|
dreamcard
Joined: 11 Sep 2004
Posts: 51
|
| 21 Sep 2004 11:59 what is the difference between loop gain and close loop gain |
|
|
|
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
|
|
| Back to top |
|
|
achilles.wang
Joined: 02 Sep 2004
Posts: 14
|
| 22 Sep 2004 4:08 what is the difference between loop gain and close loop gain |
|
|
|
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,
|
|
| Back to top |
|
|
Sprint Zagato
Joined: 23 Oct 2003
Posts: 7
|
| 27 Sep 2004 8:03 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 
|
|
| Back to top |
|
|
layes
Joined: 27 Sep 2004
Posts: 57
Helped: 2
|
| 27 Sep 2004 8:39 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.
|
|
| Back to top |
|
|
Regnum
Joined: 17 Jun 2004
Posts: 264
Helped: 12
Location: Hurlingham
|
| 27 Sep 2004 8:43 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
|
|
| Back to top |
|
|
achilles.wang
Joined: 02 Sep 2004
Posts: 14
|
| 29 Sep 2004 9:34 what is the difference between loop gain and close loop gain |
|
|
|
| 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,
|
|
| Back to top |
|
|
xdlg
Joined: 12 May 2003
Posts: 2
|
| 08 Oct 2004 13:15 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 .
|
|
| Back to top |
|
|
drasta
Joined: 15 Sep 2004
Posts: 26
|
| 13 Oct 2004 12:51 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.
|
|
| Back to top |
|
|
