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ccw27
Joined: 13 Oct 2004
Posts: 292
Helped: 8
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 08 Dec 2004 9:28 Open Loop and Closed Loop |
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Just a quick question,
What is the relationship between open loop unity gain frequency and closed loop 3dB bandwidth?
Thanks
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aryajur
Joined: 23 Oct 2004
Posts: 675
Helped: 69
Location: Sunnyvale, USA
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| 08 Dec 2004 10:43 Re: Open Loop and Closed Loop |
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| There is no exact general relationship. But for systems which have a dominant pole, the product of the closed loop gain and the 3dB bandwidth is approximately the open loop unity gain bandwidth. (can be proved for single pole systems)
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ccw27
Joined: 13 Oct 2004
Posts: 292
Helped: 8
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| 09 Dec 2004 3:02 Re: Open Loop and Closed Loop |
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Just want to clear some doubt. So normally when designing Op-amp alone, you check stability by connecting it as unity gain amplifier (β=1) and simulating the Loop Gain (βH), but βH=H. So for a differential Op-amp you can basically simulate Open Loop Gain with the output loading.
But what if you are designing a OP to be used for LPF and you know the feedback resistors and capacitors (on chip) to be used. Do you simulate Loop Gain or Open Loop Gain or first Open Loop Gain and then Loop Gain?
Thanks
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mike_bihan
Joined: 21 Mar 2002
Posts: 259
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| 09 Dec 2004 6:28 Open Loop and Closed Loop |
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Although there is some method called return ratio to calculate the loop gain by hand. Simulating the amplfifer's loop gain is hard to be obtained directly in hspice.
However, you can follow Paul Gray'e method to break the loop and simulate it. Refer to his text book and you will find it.
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Pipeline
Joined: 07 Dec 2004
Posts: 120
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| 09 Dec 2004 18:56 Re: Open Loop and Closed Loop |
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| But for current-feedback opamp, the -3dB frequency depends on the feedback resistor.
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amraldo
Joined: 29 Aug 2004
Posts: 262
Helped: 23
Location: egypt, cairo
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| 21 Dec 2004 9:51 Re: Open Loop and Closed Loop |
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"But what if you are designing a OP to be used for LPF and you know the feedback resistors and capacitors (on chip) to be used. Do you simulate Loop Gain or Open Loop Gain or first Open Loop Gain and then Loop Gain?
u must check stability in open & closed loop
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allenyyq
Joined: 17 Dec 2004
Posts: 2
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| 21 Dec 2004 10:31 Open Loop and Closed Loop |
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| It is not always so. In the voltage feedback, it's OK,but in the current feedback system,it's not correct
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segabird
Joined: 11 Nov 2004
Posts: 39
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| 22 Dec 2004 3:18 Re: Open Loop and Closed Loop |
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| allenyyq wrote: |
| It is not always so. In the voltage feedback, it's OK,but in the current feedback system,it's not correct |
This confused me and which is not correct in the current feedback system?
My opinion is that the 3db bandwidth of close loop system is BW/As.(Bw is the unit gain bandwidth of open loop system and As is the Gain of close loop system)
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layes2
Joined: 03 Dec 2004
Posts: 346
Helped: 5
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| 22 Dec 2004 4:52 Re: Open Loop and Closed Loop |
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the gain bandwith product is fu\
it is a constant
either in open loop or close loop
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segabird
Joined: 11 Nov 2004
Posts: 39
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| 23 Dec 2004 6:40 Re: Open Loop and Closed Loop |
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| layes2 wrote: |
the gain bandwith product is fu\
it is a constant
either in open loop or close loop |
This is correct.
But the 3db BW is different between open loop system and closed-loop system.
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PONDERBOY
Joined: 04 Jan 2004
Posts: 48
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| 24 Dec 2004 13:45 Re: Open Loop and Closed Loop |
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| there difference betwween current feedback and voltage feedback opamp
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qutang
Joined: 25 Oct 2004
Posts: 315
Helped: 6
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| 24 Dec 2004 14:03 Open Loop and Closed Loop |
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| I want to know a opamp is used to a close loop,the proportion of feedback resistor is low,&the whole gain of the circuit is low?
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allenyyq
Joined: 17 Dec 2004
Posts: 2
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| 27 Dec 2004 4:57 Re: Open Loop and Closed Loop |
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| layes2 wrote: |
the gain bandwith product is fu\
it is a constant
either in open loop or close loop |
It is not always so. In the voltage feedback, the gain bandwith product is constant,but in the current feedback system,it's not constant
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layes2
Joined: 03 Dec 2004
Posts: 346
Helped: 5
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| 14 Jan 2005 10:59 Re: Open Loop and Closed Loop |
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no relation ship
the gain *bandwith are the same
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FD357
Joined: 19 Jul 2003
Posts: 49
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| 14 Jan 2005 12:13 Open Loop and Closed Loop |
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| Current mode or voltage mode?
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Pipeline
Joined: 07 Dec 2004
Posts: 120
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| 15 Jan 2005 23:18 Re: Open Loop and Closed Loop |
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voltage-feedback:
gain*bw=constant
current-feedback:
-3dB bw=constant
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