open loop gain,how many is enough? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 7th May 2006, 02:33 PM   #1
Leolabs is offline Leolabs  Malaysia
diyAudio Member
 
Leolabs's Avatar
 
Join Date: Dec 2004
Location: Bukit Mertajam
Send a message via MSN to Leolabs
Question open loop gain,how many is enough?

How many??1000??100000??
  Reply With Quote
Old 7th May 2006, 02:36 PM   #2
Did it Himself
diyAudio Member
 
richie00boy's Avatar
 
Join Date: Nov 2003
Location: Gloucestershire, England, UK
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.
__________________
www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more.
  Reply With Quote
Old 7th May 2006, 03:57 PM   #3
diyAudio Moderator Emeritus
 
lumanauw's Avatar
 
Join Date: Oct 2002
Location: Bandung
Send a message via Yahoo to lumanauw
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.
  Reply With Quote
Old 7th May 2006, 03:57 PM   #4
diyAudio Member
 
darkfenriz's Avatar
 
Join Date: Jun 2004
Location: Warsaw
Default Re: open loop gain,how many is enough?

Quote:
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.

kind regards
Adam
  Reply With Quote
Old 7th May 2006, 06:45 PM   #5
diyAudio Member
 
traderbam's Avatar
 
Join Date: Jan 2002
Location: Earth
Quote:
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.
  Reply With Quote
Old 7th May 2006, 07:38 PM   #6
MikeB is offline MikeB  Germany
diyAudio Member
 
MikeB's Avatar
 
Join Date: Apr 2004
Location: GŁtersloh
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.

Mike
  Reply With Quote
Old 7th May 2006, 09:08 PM   #7
diyAudio Member
 
myhrrhleine's Avatar
 
Join Date: Jan 2006
Location: Avalon Island
Default Re: open loop gain,how many is enough?

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

What kind of circuit?
Preamp?
Poweramp?
Opamp?
  Reply With Quote
Old 8th May 2006, 01:59 AM   #8
Leolabs is offline Leolabs  Malaysia
diyAudio Member
 
Leolabs's Avatar
 
Join Date: Dec 2004
Location: Bukit Mertajam
Send a message via MSN to Leolabs
How about something like my Lizi???

Workable Lizi final
  Reply With Quote
Old 8th May 2006, 11:21 PM   #9
diyAudio Member
 
Join Date: May 2005
Location: Pretoria, South Africa
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.

Regards.
  Reply With Quote
Old 9th May 2006, 07:46 AM   #10
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Open loop gain, phase margin etc ceharden Solid State 67 22nd November 2011 02:16 PM
Gain Blockwith Variable Loop Gain Bonsai Solid State 17 11th July 2008 02:40 PM
Open Loop Gain Measurement? Fotios Need Help! fotios Solid State 21 22nd April 2008 12:02 AM
Opamp with open loop gain of 1,000,000,000,000,000. :-P Circlotron Solid State 9 26th June 2003 11:32 AM
Open loop gain (High or low) ? JensRasmussen Solid State 19 16th September 2002 09:26 PM


New To Site? Need Help?

All times are GMT. The time now is 07:18 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2