Cascode input and VAS amplifier design, will it work :) - 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 21st January 2011, 02:39 PM   #1
diyAudio Member
 
Join Date: Dec 2007
Default Cascode input and VAS amplifier design, will it work :)

Hi,

12 years ago I build an bipolar version of the Elektor Electronics Cresendo amplifier, and it sounded great. This design used a Cascode VAS, which I think was the main reason for good THD performance for this amp.

Now I have made a new design, which is based on the Cresendo input stage, and Cascode VAS. The new thing I want to try is to use Cascode in the input stage, together with active current mirrors.

Im thinking of using Analog Devices SSM2220 PNP and SSM2212 NPN match transistors for the input stage and the Cascode input stage.

Has anyone experience with the combination of Cascode input stage + current mirrors ?

I have attached a preliminary schematic of my design.
(it may have some errors, as I have not build it yet )

Any feedback is appreaciated !

Regards
Flemming Bach
Denmark
Attached Files
File Type: zip Design1 (Schematic).zip (430.1 KB, 314 views)
  Reply With Quote
Old 22nd January 2011, 01:08 AM   #2
djoffe is offline djoffe  United States
diyAudio Member
 
Join Date: Oct 2008
Default A problem...

I believe you won't get a predictable quiescent current in the Q27-Q28-Q29-Q30 stage. You will have two dueling very high gain circuits, for the pull down and pull up...this is a version of the same issue that faces the Randy Slone circuit discussed at length elsewhere on the forum...I fear this is kind of a fatal flaw, without some surgery...
  Reply With Quote
Old 22nd January 2011, 05:41 AM   #3
diyAudio Member
 
Workhorse's Avatar
 
Join Date: Dec 2004
Current mirrors at both collector loads of differential will cause instability of idle operating point...................Fatal Mistake.
  Reply With Quote
Old 22nd January 2011, 07:48 AM   #4
alexcp is offline alexcp  United States
diyAudio Member
 
alexcp's Avatar
 
Join Date: Jun 2009
Location: Florida
Blog Entries: 37
Bob Cordell in his recent book, Designing Audio Power Amplifiers, descibes a stabilized current mirror - see the attached schematic - which should work with a complementary differential input stage.
current mirror.JPG
Also, a cascode VAS has a high output impedance, so you need a high input impedance output stage. The one shown in your schematic, a simple emitter follower, would overload your VAS and degrade its performance. If you want to stick with BJTs, you'll probably need a triple emitter follower to keep VAS performance high.

Last edited by alexcp; 22nd January 2011 at 07:53 AM.
  Reply With Quote
Old 22nd January 2011, 10:00 AM   #5
diyAudio Member
 
Lazy Cat's Avatar
 
Join Date: Sep 2007
Because LTP input diferential amp is voltage orientated circuit with high output impedance, it is natural to load it with high impedance interlink amplifying stage in "keep it simple" manner, like mosfet transistor or similar. In this way of solution you have nice transfer from pure voltage to pure current.
Attached Images
File Type: jpg P-CAD EDA - [VACA2000].JPG (527.1 KB, 473 views)
  Reply With Quote
Old 22nd January 2011, 11:04 AM   #6
Piersma is offline Piersma  Netherlands
diyAudio Member
 
Piersma's Avatar
 
Join Date: Jan 2003
Location: Netherlands
Default Current mirror loading

Contributions by Glenn Kleinsmidt and fab.
Please take a look:

Lateral mosfet w Normal or Diamond buffer?

My new mini Class A amp.

regards,
Piersma
  Reply With Quote
Old 22nd January 2011, 03:25 PM   #7
diyAudio Member
 
Join Date: Dec 2007
Hi,

Thanks for feedback!

I was also suspicious of the DC behavior of the current mirror in the LTP.

Looking at Bob Cordellīs circuit in figure 8, on page 9:
http://www.cordellaudio.com/papers/MOSFET_Power_Amp.pdf

I have made a new circuit based on Bobīs design.
I think Bobīs explain how it works very well, so I wont repeat it here.

Any comments about using this concept ?

Is the Diodes (D2 and D3) in Bobīs design a good thing to have ?

To Alexcp: Yes you are right, my circuits needs some extra emitter follower so the VAS is not loaded.

BR
Flemming Bach
Attached Images
File Type: jpg NewCircuit1.jpg (156.9 KB, 418 views)
  Reply With Quote
Old 23rd January 2011, 11:22 AM   #8
diyAudio Member
 
Join Date: Dec 2007
Hi,

I found that the Technics SEA3 amplifier, does use a current mirror in the LTP inputstage, which also happens to be connected to a Cascode couplede to the JFET input stage. I dont see that they do anything special to bias/balance the upper current mirror. But again they also do not have any source resistors in the JFET input stage, and they use dual transistors which are properbly pretty equal matched.

@Workhorse: Any comments on the current mirror in the LTP, that you mentioned was fatal ?

But Robert Cordells circuit of how to bias the current mirror does seem to a smart trick

BR
Flemming
Attached Images
File Type: jpg Technics-SEA3.jpg (424.5 KB, 376 views)
  Reply With Quote
Old 23rd January 2011, 03:44 PM   #9
diyAudio Member
 
Join Date: Jun 2005
Location: Zürich
Quote:
I found that the Technics SEA3 amplifier, does use a current mirror in the LTP inputstage, which also happens to be connected to a Cascode couplede to the JFET input stage. I dont see that they do anything special to bias/balance the upper current mirror.
The problem has nothing to do with the cascode, it is related to the complementary push-pull second stage which needs some sort of bias arrangement. In the schematic you show there is no complementary push-pull but rather a differential second stage (some call this push-pull as well, but its surely not complementary).

Samuel
  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
My folded-cascode VAS linearity investigation GK Solid State 96 19th July 2010 10:01 PM
RMI-FC100 MKII, single ended folded cascode VAS roender Solid State 27 9th July 2010 10:56 PM
Amplifier with 220 input voltage, will it work in US? usmcjlp Solid State 5 15th April 2010 06:19 AM
Cascode VAS miller compensation Rafael L Solid State 9 5th March 2009 01:06 AM
Cascode VAS biasing ? sam9 Solid State 8 25th June 2004 10:11 PM


New To Site? Need Help?

All times are GMT. The time now is 04:29 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright Đ1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2