What are the pros and the cons of those 3 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 12th June 2010, 04:55 PM   #1
tessier is offline tessier  Canada
diyAudio Member
 
Join Date: May 2008
Default What are the pros and the cons of those 3 vas ?

Hi

Between the beta enhance vas, the buffered vas and the cascoded vas, what are the pros and the cons of those 3 vas ?

Thanx

Paul
Attached Images
File Type: gif vas beta enhanced-2.gif (2.4 KB, 650 views)
File Type: gif Vas-Buffer-3.gif (9.6 KB, 646 views)
File Type: gif vas cascoded-4.gif (1.9 KB, 639 views)
  Reply With Quote
Old 12th June 2010, 06:21 PM   #2
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
Hi

Some of my ........................

The beta enhanced has a much higher input Z and your input stage likes this very much but it adds another stage within the closed loop. The buffered VAS allows you to use a small signal transistor for the VAS which tends to have better gain linearity and higher GBW. The cascode VAS requires extra voltage for the supply rails in order to get around the lost cascode voltage. However cascode has wonderful benefits. For starters it allows you to use a lower Vce small signal device for the current amplifier and this brings out all kinds of possible components. Lower Vce transistors tend to have higher Hfe, much faster GBW and lower noise, and it does not add an extra stage in the CL. Also having a constant Vce not only will linearize the AC load line, it inhibits the Early effect. There are also benefits to having the voltage amplifier being common base instead of common emitter. Certainly as far a quality is concerned, cascode is the best way to go for sure, but like I said, it needs a boosted supply voltage to get rail to rail swing at the output stage and this adds complexity to the overall circuit. There are always trade-offs.

As for my amp, I use a voltage doubling circuit that then looses several volts in a series-shunt voltage regulator for the low current VAS and input stage supply placing it about 10V greater than the output stage rail. It's not a terrible addition to complexity but is well worth the benefits.
__________________
All the trouble I've ever been in started out as fun......

Last edited by CBS240; 12th June 2010 at 06:26 PM.
  Reply With Quote
Old 12th June 2010, 06:40 PM   #3
Bigun is offline Bigun  Canada
diyAudio Member
 
Bigun's Avatar
 
Join Date: Jan 2009
Blog Entries: 2
the middle option looks a bit like a Sziklai - offers some promise of good linearity.
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
  Reply With Quote
Old 12th June 2010, 11:23 PM   #4
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
....or a common emitter driving a common collector (emitter follower) judging from where the Miller cap is placed.
__________________
All the trouble I've ever been in started out as fun......
  Reply With Quote
Old 13th June 2010, 12:53 AM   #5
Bigun is offline Bigun  Canada
diyAudio Member
 
Bigun's Avatar
 
Join Date: Jan 2009
Blog Entries: 2
yup, but perhaps all we need is to add some emitter degeneration to the first device and fold the collector of the 2nd device back above the resistor to create that local feedback loop. Cdom offers some scope for experimentation, perhaps 2-pole ?
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
  Reply With Quote
Old 13th June 2010, 10:41 AM   #6
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
If Cascode is adopted, it must drive a high impedance load, either a buffer or a 3stage output.
__________________
regards Andrew T.
  Reply With Quote
Old 13th June 2010, 01:02 PM   #7
balaboo is offline balaboo  United States
diyAudio Member
 
Join Date: Jun 2004
Location: South Carolina
Andrew is absolutely correct - a small common-emitter stage coupled to CFP emitter-follower is the way to go (think Baxandall super-pair).
  Reply With Quote
Old 13th June 2010, 06:52 PM   #8
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
If the amplifying transistor has Vce of 5V and a med pwr device is used for the cascode transistor, 40-50mA bias could be used. Do you need a VAS with more current than that? A small signal TO-92, even with Vceo less than 25V could easily be the current amplifying transistor, perhaps with Hfe greater than 300. This would negate the need for the extra current gain stage and still only require ~150uA of DC bias from the input stage. You could even use J-fets. The biggest issue is the extra boosted voltage supply required to make up the cascode voltage difference. This may be the reason to go with one of the other solutions but cascode really is hard to beat in terms of performance.
__________________
All the trouble I've ever been in started out as fun......

Last edited by CBS240; 13th June 2010 at 07:00 PM.
  Reply With Quote
Old 28th September 2010, 06:45 AM   #9
diyAudio Member
 
Join Date: Apr 2007
Hello

There is also the VAS with a collector buffering, like in the image.

I never try this one, do we connect the collector of the buffer transistor to the upper rail ?

Anyone did try this one, any amps who use this VAS ?

Thank

Bye

Gaetan
Attached Images
File Type: jpg vas buffer- colector buffering.jpg (26.4 KB, 489 views)
  Reply With Quote
Old 28th September 2010, 08:50 AM   #10
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
D. Self recommends it for good performance.
He chooses not to use it in his published designs because it uses more components and takes up more PCB space than the simple EF before the VAS.
__________________
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
what are the pros and cons of valve amps? pkgum Tubes / Valves 24 26th January 2014 05:34 AM
Cons / Pros Borat Tubes / Valves 34 1st September 2009 12:52 PM
2 way + active sub in same cabinet pros and cons? FE3T Multi-Way 6 12th September 2008 10:34 PM
Pros and Cons of this technique? G Tubes / Valves 35 23rd October 2003 05:37 PM
tweeter offset: pros and cons gary f Multi-Way 6 15th August 2003 07:57 PM


New To Site? Need Help?

All times are GMT. The time now is 02:31 PM.


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