sleepy man's question - 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 12th December 2004, 12:51 AM   #1
diyAudio Member
 
darkfenriz's Avatar
 
Join Date: Jun 2004
Location: Warsaw
Default sleepy man's question

well it is quite late in Poland and maybe my question is stupid,but:

1.we may imagine frequency compensation as a first order (RC) filter, don't we?
2. Also output devices are limiting bandwidth (filtering) the same way, mainly because capacitance (as mosfets do)
3.also small signal devices do but on higher freqs.
4.for this filter : for 6*border.freq.:
attenuation is something like -17dB
phase shift is something like 82 degrees
for 0.3*border freq.:
attenuation is less than half a dB
phase shift is about 16 degrees
5.assume the same border freqs. for output devices and freq. compensatiom-just for simplicity, and higher border for VAS
6.we get -35dB attenuation for phase shift of 180 degrees
7.negative fb. becomes positive
8.many ab class have open loop gain over 35dB
9.how the hell do they stay stable???
you may also consider it using Nyquist's stability criterion (looking at hodogram)
  Reply With Quote
Old 12th December 2004, 12:03 PM   #2
diyAudio Member
 
darkfenriz's Avatar
 
Join Date: Jun 2004
Location: Warsaw
i've waken up couple of hours ago, drunk coffee and still have no idea
  Reply With Quote
Old 12th December 2004, 12:25 PM   #3
MikeB is offline MikeB  Germany
diyAudio Member
 
MikeB's Avatar
 
Join Date: Apr 2004
Location: Gütersloh
To number 9:

That's what all the caps are good for, pushing down openloopgain
below 1 at this 180°-freq. That's why it is harder to stabilize large
feedback, but is nearly always possible.
When feedback get's positive, but gain is below 1, the oscillation
is damped to death.
I already made amp with OLG of 130db, was not too difficult to stabilize.
But is still bad idea, as the amp works with uV's, so any EMI get's
audible, unless you completely shield the amp...

Mike
  Reply With Quote
Old 12th December 2004, 03:31 PM   #4
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Default Re: sleepy man's question

Quote:
Originally posted by darkfenriz
[snip]9.how the hell do they stay stable???
you may also consider it using Nyquist's stability criterion (looking at hodogram)

Well, your assumption that all these corner frequencies are more or less the same is not correct. The trick is to have one so called dominant pole (the name speaks for itself) that rolls off before any of the others come into play. Often this is done with that famous miller cap in the Vas stage.

So, you get amps with an open loop corner frequency of a couple of kHz. Look at opamps, they have extremely high open loop gain but often have a dominant pole coming into play below 100Hz!

Jan Didden
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W. S. Maugham
Check out Linear Audio!
  Reply With Quote
Old 12th December 2004, 09:13 PM   #5
diyAudio Member
 
darkfenriz's Avatar
 
Join Date: Jun 2004
Location: Warsaw
oh thank you very much
now it seems much brighter to me
I guess thats one of reasons why we like very fast output devices
(much 'faster' than 20kHz)
I also think another solution is to filter OUTSIDE feedbackloop (like cap between collector of differential pair)
regards
  Reply With Quote
Old 13th December 2004, 10:28 AM   #6
MikeB is offline MikeB  Germany
diyAudio Member
 
MikeB's Avatar
 
Join Date: Apr 2004
Location: Gütersloh
Quote:
Originally posted by darkfenriz

I also think another solution is to filter OUTSIDE feedbackloop (like cap between collector of differential pair)
regards
This is still inside feedbackloop, but actually i do prefer a cap between
collectors of diffamp, but more like a RC. This seems far superior to
normals cdoms.

Mike
  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
do i REALLY need a crossover?? and an amp question. and an ohm question legendaryfrog Multi-Way 4 29th October 2006 02:39 AM
newbie with input tran. question and wiring question imo Solid State 0 18th January 2006 11:10 PM
I am sleepy again darkfenriz Solid State 2 10th January 2005 10:22 PM
Capacitor Question /question on cap TunaFish Tubes / Valves 14 12th January 2004 03:23 AM
old speaker question, bullet midrange question & link to speaker project wallijonn Multi-Way 10 5th November 2002 07:03 AM


New To Site? Need Help?

All times are GMT. The time now is 03:18 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