CFP For The VAS - 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 18th October 2006, 09:04 PM   #1
selim is offline selim  Turkey
diyAudio Member
 
Join Date: Apr 2003
Location: izmir turkey
Default CFP For The VAS

Hi All,

Most of the circuits I have seen here in this forum and elsewhere
utilize a single transistor for the VAS. Mr.Self, describing his
'Blameless Amplifier', maintains that it is best to use a darlington
connected pair for the VAS.I would like to hear your opinions
on swapping this darlington configuration with a Sziklai pair.

One downside I can think of is, in a darlington connection as in
Mr. Self's amps, the input transistor of the darlington pair
operates in an almost constant Vce because the collector is
tied to the ground.In a CFP configuration, both transistors
making up the VAS will see the rail-to-rail excursion, forcing
the first transistor to be a high voltage type as well.

Selim
  Reply With Quote
Old 18th October 2006, 09:38 PM   #2
peufeu is offline peufeu  France
diyAudio Member
 
Join Date: Mar 2001
Location: Lyon, France
Sziklai has advantages for voltage followers but the VAS is just a current amplifier, so using a darlington or a CFP is about the same.

View the input stage as a current source (it converts the error voltage into a current) if you have problems understanding this.

If you don't use a current mirror in the input stage, it's a bit less clear, though...
  Reply With Quote
Old 19th October 2006, 12:23 AM   #3
mikeks is offline mikeks  United Kingdom
Account Disabled
 
Join Date: Jun 2004
Location: Animal farm
Quote:
Originally posted by peufeu
......the VAS is just a current amplifier.......
Nyet. Transimpedance amplifier.
  Reply With Quote
Old 23rd April 2009, 11:51 AM   #4
diyAudio Member
 
Workhorse's Avatar
 
Join Date: Dec 2004
I think there is an advantage of using CFP in VAS and that is more linearity .
  Reply With Quote
Old 23rd April 2009, 04:50 PM   #5
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Self shows 6 variations of the VAS, all with his preferred Cdom.
a+b) two single transistor versions.
c+d) two same sex versions.
e+f) two versions with complementary transistors.
__________________
regards Andrew T.
  Reply With Quote
Old 23rd April 2009, 07:44 PM   #6
diyAudio Member
 
blueskynis's Avatar
 
Join Date: Aug 2007
Location: Niš
Send a message via Skype™ to blueskynis
Quote:
Originally posted by AndrewT
c+d) two same sex versions.
I just can't get it what are you saying. Is this a miss-type?


Cheers
__________________
Windows-free with Linux
  Reply With Quote
Old 23rd April 2009, 09:54 PM   #7
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
No, it is not a misquote. Andrew is very precise in his contributions.

It is a reference to both transistors being identical types, that is, both npn (fixed at the negative rail) or both pnp (fixed at the positive rail).

Cheers,

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com
  Reply With Quote
Old 24th April 2009, 12:10 AM   #8
forr is offline forr  France
diyAudio Member
 
forr's Avatar
 
Join Date: Dec 2004
Location: Next door
A VAS using CFP's :

http://pagesperso-orange.fr/francis....36_palouda.gif

Simulation shows it is very linear :

http://francis.audio.monsite.wanadoo.fr/page5.html

(chapter "SIMULATION DU DEUXIEME ETAGE DE L'AMPLIFICATEUR")
  Reply With Quote
Old 24th April 2009, 12:23 AM   #9
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Thanks Sebastien. I have used this precise VAS configuration in one of my amp designs.

Make each stage as linear as possible, then apply feedback, not much at all in this case..... there is a LOT of degeneration in there.

Output stage, another CFP, shows signs of very great design care. I assume it's Class AB, however, with some issues at switching.

There is nothing new under the sun.

Thank you for sharing this circuit, Sebastien,

Cheers,

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com
  Reply With Quote
Old 24th April 2009, 12:31 AM   #10
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
Hi

It depends on how you perceive the circuit. The best VAS I have ever built consists of a bipolar, constant power, differential bridge. If you look at the constant power part, it resembles a CFP that happens to be cascode. So I guess my vote goes towards the current feedback in that sense.
__________________
All the trouble I've ever been in started out as fun......
  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



New To Site? Need Help?

All times are GMT. The time now is 06:17 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