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Old 7th May 2006, 02:33 PM   #1
Leolabs is offline Leolabs  Malaysia
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Question open loop gain,how many is enough?

How many??1000??100000??
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Old 7th May 2006, 02:36 PM   #2
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There is no right answer or magic number. It depends entirely on the design and implementation. Some might say it should be as high as possible to give more feedback, others will say it leads to instability and sounds bad.
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Old 7th May 2006, 03:57 PM   #3
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The part which needs fixing the most is the output stage. Building full classA allows you to have low OL gain (merit=wide bandwith), cause there's not much to fix for output stage. If you build low biased classAB (especially mosfet) without EC, you will need quite some OL gain (drawback=low bandwith) to be burned in feedback process.
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Old 7th May 2006, 03:57 PM   #4
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Default Re: open loop gain,how many is enough?

Originally posted by Leolabs
How many??1000??100000??

1000 means loop gain of more than 30 for closed loop of 30 (typical amp's gain). This is low feedback (~30dB) that means you need to care much about open loop linearity and quite an idle current of output stage (say, >70mA for bipolars, >250mA for mosfets).

100000 will give about 3000 of loop gain (~70dB feedback) that implies heavy lag compensation inside low-mid frequencies, fight with slew rating and typical problems with op-amp types of circuit. So open loop gain at 20kHz will be probably not way higher than 1000. Measures very low THD at 1kHz, which is in taste for some.

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Old 7th May 2006, 06:45 PM   #5
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How many??1000??100000??
At what frequency?

Once you take the brave step of wanting to use feedback you might as well use as much as you possibly can. That is, as much open loop gain as possible whilst maintaining an acceptable stability margin.

Now, a rule of thumb is to decide how much phase margin you want...find the frequency at which the open loop response has this phase, then make the gain at this frequency the closed loop gain (using simple feedback). If you design the open loop response to have a single LF pole then you can enjoy good margin as well as very high OL gain at low frequencies. This is what standard op-amps do.

Example: a typical power amp with 200W output devices may have a open loop phase shift of 135 deg at, say, 1MHz (assuming a resistive load). If the gain were 30 at 1MHz you could use a simple resistor divider for the feedback signal to give a closed-loop gain of 30. The feedback factor will be 1 at 1MHz and will double for every halving of frequency until you reach the first pole frequency, normally set by a miller cap arrangement, and will usually be less than 1kHz. If it were 1kHz then the feedback factor here would be 1000. The factor at 20kHz would be 50.

It is possible to have higher feedback in the audio band by using phase lead compensation in the feedback resistor network.

There are those who believe that the first pole should be outside the audio band...above 20kHz. If you do this then in the example I gave you can have at most a feedback factor of 50. But I disagree with this approach...there are many reasons why feedback can degrade the sound but once you solve these then the more the better and the fact that the pole is in the audio band doesn't matter.

Because of circuit limitations it is often found that many circuits sound better with a little feedback but no more...say 14dB. Much more than this and degradation may set in.
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Old 7th May 2006, 07:38 PM   #6
MikeB is offline MikeB  Germany
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Hi Leolabs, a number from my symasym is an open loop gain of ~27.000, i choosed that one as it was the maximum i got stable without loosing more bandwidth. The gain is still above 10.000 at 20khz.
You can have olg of 1.000.000, but only at low frequencies...

If you ask like that, i suggest olg of ~10000 over the whole audioband, this seems a reasonable number to me.

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Old 7th May 2006, 09:08 PM   #7
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Default Re: open loop gain,how many is enough?

Originally posted by Leolabs
How many??1000??100000??

What kind of circuit?
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Old 8th May 2006, 01:59 AM   #8
Leolabs is offline Leolabs  Malaysia
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How about something like my Lizi???

Workable Lizi final
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Old 8th May 2006, 11:21 PM   #9
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You would by now have gathered that an answer to your question depends very much on the application and the final gain desired.

A bit that I might add, is that there is a disadvantage in having to shift the dominant pole into the audio band for stability reasons, as a consequence of too high a loop gain. That is that high order harmonic distortion becomes accentuated. (I presume that you are aware that high order harmonic distortion is usually the number one deteriorating factor in semi-conductor designs.)

For a power amplifier it should not be necessary to go to exorbitant amounts of NFB and thus loop gain. As an illustration, a 80W design of mine gets down to 0,007% of 3rd harmonic distortion at 85% output power, with all other harmonics below the noise floor (< 0,0015%) for all audio frequencies. For that only 29 dB of global NFB was required. The open-loop bandwidth is 50 KHz. For any significantly more NFB the high-order distortion rises and the NFB stability margin deteriorates - remember that a loudspeaker load can be an alien thing.

Thus for power amplifiers I see no reason to go to an open loop gain of higher than say 30x the final required gain. For pre-amplifiers it is different - here there is really no simple answer to your question.

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Old 9th May 2006, 07:46 AM   #10
AndrewT is offline AndrewT  Scotland
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this thread is shaping up to become quite a design tutorial if you all keep contributing.

Good luck lads, I'm listening.
regards Andrew T.
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