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sikhjumn
Joined: 06 Jul 2009
Posts: 2
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 07 Nov 2009 8:35 The importance of loop gain |
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I'm confused about the usage of loop gain of Op Amp.
Can anyone tell me the importance of loop gain,e.g. open loop gain&close loop gain?
Thx a lot.
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LvW
Joined: 07 May 2008
Posts: 1465
Helped: 242
Location: Germany
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07 Nov 2009 10:30 Re: The importance of loop gain |
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| sikhjumn wrote: |
I'm confused about the usage of loop gain of Op Amp.
Can anyone tell me the importance of loop gain,e.g. open loop gain&close loop gain?
Thx a lot. |
Yes, sometimes one is confused, but it is important to know the difference.
There are three important gain parameters:
1.) open loop gain: gain of an amplifier without feedback (Aol)
2.) closed loop gain : gain of an amplifier with feedback (Acl<Aol)
3.) loop gain: gain of all components of the (open) loop. Example: opamp with resistive feedback: loop gain L= Aol*F (F=feedback factor R1/(R1+R2).
___________
The loop gain L is the most important parameter for stability investigations.
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sikhjumn
Joined: 06 Jul 2009
Posts: 2
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07 Nov 2009 10:52 Re: The importance of loop gain |
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Thx your answer.
Maybe...Are there any practical example for loop gain?
Or any example that the loop gain directly affect the results?
While i search the loop gain through internet, the example always based on mathematics. However, I need more examples baesd on macro views, or any product based on the principle of loop gain.
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LvW
Joined: 07 May 2008
Posts: 1465
Helped: 242
Location: Germany
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07 Nov 2009 15:22 Re: The importance of loop gain |
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| sikhjumn wrote: |
Maybe...Are there any practical example for loop gain?
Or any example that the loop gain directly affect the results?
While i search the loop gain through internet, the example always based on mathematics. However, I need more examples baesd on macro views, or any product based on the principle of loop gain. |
* One typical example was given in my first answer
* based on the classical feedback scheme, Blacks formula shows the importance of the loop gain L(jw):
Acl=Aol/(1-Aol*F)=Aol/(1-L)
(Notation as in my first answer)
* The sign of L determines if you have positive or negative feedback, and don`t forget: L=L(jw), which means that L decreases (and its phase changes!) with rising frequencies
* Special case: If there is one frequency with |L|=1 (0dB)
and phase of L is 360 deg, the circuit design for amplification will become unstable.
*Recommendation: Try to become familiar with stability criteria; the you will see the importance of loop gain.
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fencl
Joined: 17 Jun 2008
Posts: 19
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09 Nov 2009 5:41 Re: The importance of loop gain |
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Yes, as LvW said ,loop gain is most important in some case. such sc circuit for adc.
but, a litter confused me is the simple way of ac analysis for loop gain and phase?.since i've known how to do for Aol and Acl. can you help for this,LvW
ths a lot.
| LvW wrote: |
| sikhjumn wrote: |
Maybe...Are there any practical example for loop gain?
Or any example that the loop gain directly affect the results?
While i search the loop gain through internet, the example always based on mathematics. However, I need more examples baesd on macro views, or any product based on the principle of loop gain. |
* One typical example was given in my first answer
* based on the classical feedback scheme, Blacks formula shows the importance of the loop gain L(jw):
Acl=Aol/(1-Aol*F)=Aol/(1-L)
(Notation as in my first answer)
* The sign of L determines if you have positive or negative feedback, and don`t forget: L=L(jw), which means that L decreases (and its phase changes!) with rising frequencies
* Special case: If there is one frequency with |L|=1 (0dB)
and phase of L is 360 deg, the circuit design for amplification will become unstable.
*Recommendation: Try to become familiar with stability criteria; the you will see the importance of loop gain. |
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LvW
Joined: 07 May 2008
Posts: 1465
Helped: 242
Location: Germany
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09 Nov 2009 8:06 Re: The importance of loop gain |
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| fencl wrote: |
.............
but, a litter confused me is the simple way of ac analysis for loop gain and phase?.since i've known how to do for Aol and Acl. can you help for this,LvW
ths a lot.
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You ask for a way to measure/simulate loop gain?
This subject has been discussed intensively in this forum before.
For example, have a look at "similar topics" at the bottom of this page.
LvW
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fencl
Joined: 17 Jun 2008
Posts: 19
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09 Nov 2009 12:24 Re: The importance of loop gain |
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[quote="LvW"]
| fencl wrote: |
............
You ask for a way to measure/simulate loop gain?
This subject has been discussed intensively in this forum before.
For example, have a look at "similar topics" at the bottom of this page.
LvW |
oh, i've got it ,thx!
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fencl
Joined: 17 Jun 2008
Posts: 19
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13 Nov 2009 3:07 Re: The importance of loop gain |
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Hi, LvW,
since i follow the loop gain simulation way.i've got singal-end output gain&phase ,it works well. but when i try to the similar way for fullydiff-opamp. it become strange.
usually i needed is sc circuit in adc or fullydiff in pga.so i still can't use it for identification.
have you simulate for fullydiff before? can you share some experience?
Regards!
| LvW wrote: |
| fencl wrote: |
.............
but, a litter confused me is the simple way of ac analysis for loop gain and phase?.since i've known how to do for Aol and Acl. can you help for this,LvW
ths a lot.
|
You ask for a way to measure/simulate loop gain?
This subject has been discussed intensively in this forum before.
For example, have a look at "similar topics" at the bottom of this page.
LvW |
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