Requirements for cascode VAS transistors - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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 30th January 2017, 02:25 PM   #1
diyAudio Member
 
Join Date: May 2016
Location: Hanoi
Default Requirements for cascode VAS transistors

Hello!
I want to build cascode VAS stage for my referrence amplifier projects since it is the lowest THD topology I know (I didn't say lowest of all, just in what I know).
My question is: What are requirements in the parameters for VAS transistors, cascode transistors, buffer transistors ( the last seem be not need to ask ) to achieve good result (minimize THD and optimize freq response, open-loop gain...)
I have read that: transistors for VAS stage should have low collector-base capacitance, but didn't mention any limit; and should have enough gain-bandwidth, but didn't mention the lower limit...
If what I have read are right, so why people use sth like MJE340/350 (in may design such as: Apex SR-xxx....) or 2N1711 (in JLH-1969) or 2N3439 (Leach as an example)? Those transistors seems have pretty high Cbc (except MJE340/350 which I can't find the mention about Cbc) and low Ft (like 2N1711 and MJE).
Sth like BF469/470 or 2SC3503/A1381 which have really low Cbc and high Ft seems much less popular (or which I have seen is just like a fish in the ocean?).
Attached Images
File Type: png Cascode.png (5.4 KB, 332 views)
  Reply With Quote
Old 30th January 2017, 07:59 PM   #2
jaycee is offline jaycee  United Kingdom
diyAudio Member
 
Join Date: Apr 2004
Location: Norwich, UK
Medium power, high voltage (must be able to take the rail-rail voltage), capable of at least 100mA to be comfortable, 100MHz fT or better, and as low a Cob as possible.

Fairchild KSA1381/KSC3503 are the main ones I know of still in production. If you're OK with surface mount try Diodes Inc DZT5401/DZT5551. Unfortunately most of the best devices have disappeared thanks to CRT's becoming obsolete.
  Reply With Quote
Old 30th January 2017, 08:00 PM   #3
jaycee is offline jaycee  United Kingdom
diyAudio Member
 
Join Date: Apr 2004
Location: Norwich, UK
The main reason MJE340/350 gets used so often is that they are easy to get. The BF469/BF470 are discontinued but their equivalent in surface mount are BF722/BF723
  Reply With Quote
Old 30th January 2017, 09:15 PM   #4
bwaslo is offline bwaslo  United States
diyAudio Member
 
bwaslo's Avatar
 
Join Date: May 2006
Location: Portland, Oregon!
Blog Entries: 2
I've been making some power amplifiers that are very similar in essence to the topology you show (but with a feed inductor instead of the current source). I've had very good luck using a DN2530 depletion MOSFET for the input device, running at about 10mA -- low noise, good linearity, very high input impedance, simple self bias using just a source resistor. Its input is sensitive to static and over-voltage, though, so I add some low leakage clamp diodes (two BC556B connected E-B-B-E, collectors open) from gate to source. Also it's drain to gate capacitance is VERY voltage sensitive, but the cascode arrangement takes care of that issue nicely.

If you want to make the distortion very low, add a PNP transistor to the MOSFET to form a FET-BJT Sziklai pair.

For the cascode transistor, I use BC556B. Actually, two of them in a darlington arrangement, as that assures all the input MOSFET's signal current passes through to the cascode's output (shows in the distortion a little). This transistor is only good to about 65V though. For higher voltages, I'd look at the KSA1381 too.

For the buffer, I add a mosfet stage (ZVP3310) before the BJT (which is KSA1381 and 2SC5200 in a Sziklai pair) to minimize loading on the cascode output. That probably depends on how much current the input and buffer are running.

Just a data point of a combination that seems to work really well, I'm sure there are lots of other choices. Hope this helps.
__________________
[W9MJE] Horn spreadsheet SynergyCalc/; SmallSyns SmallSyns;
Crossover design Xsim; Depot diffusor super-easy diffusors
  Reply With Quote
Old 31st January 2017, 12:47 AM   #5
diyAudio Member
 
Join Date: May 2016
Location: Hanoi
Quote:
Originally Posted by jaycee View Post
Medium power, high voltage (must be able to take the rail-rail voltage), capable of at least 100mA to be comfortable, 100MHz fT or better, and as low a Cob as possible.

Fairchild KSA1381/KSC3503 are the main ones I know of still in production. If you're OK with surface mount try Diodes Inc DZT5401/DZT5551. Unfortunately most of the best devices have disappeared thanks to CRT's becoming obsolete.
"DZT5401"? So go for 2N5401 as through hole devices?
  Reply With Quote
Old 31st January 2017, 01:33 AM   #6
diyAudio Member
 
Join Date: May 2016
Location: Hanoi
Quote:
Originally Posted by bwaslo View Post
I've been making some power amplifiers that are very similar in essence to the topology you show (but with a feed inductor instead of the current source). I've had very good luck using a DN2530 depletion MOSFET for the input device, running at about 10mA -- low noise, good linearity, very high input impedance, simple self bias using just a source resistor. Its input is sensitive to static and over-voltage, though, so I add some low leakage clamp diodes (two BC556B connected E-B-B-E, collectors open) from gate to source. Also it's drain to gate capacitance is VERY voltage sensitive, but the cascode arrangement takes care of that issue nicely.

If you want to make the distortion very low, add a PNP transistor to the MOSFET to form a FET-BJT Sziklai pair.

For the cascode transistor, I use BC556B. Actually, two of them in a darlington arrangement, as that assures all the input MOSFET's signal current passes through to the cascode's output (shows in the distortion a little). This transistor is only good to about 65V though. For higher voltages, I'd look at the KSA1381 too.

For the buffer, I add a mosfet stage (ZVP3310) before the BJT (which is KSA1381 and 2SC5200 in a Sziklai pair) to minimize loading on the cascode output. That probably depends on how much current the input and buffer are running.

Just a data point of a combination that seems to work really well, I'm sure there are lots of other choices. Hope this helps.
Thanks for very informative comment!
But in my place, I don't want to purchase any uncommon parts, since purchase parts seperately from American distributors will suck all of my small fund I want to collect my $$$ for sth really terrible like heatsink, transformers, filter caps... (these terrible thing can be custom-made or buy second-hand from my place)
And I don't love FET as much as BJT since they are very vulnerable, while I am a careless child I use a soldering gun and afraid strong electromagnetic spike can damage sth like FET. Don't ask me why I use soldering gun
  Reply With Quote
Old 31st January 2017, 02:23 AM   #7
diyAudio Member
 
Join Date: May 2016
Location: Hanoi
Quote:
Originally Posted by jaycee View Post
The main reason MJE340/350 gets used so often is that they are easy to get. The BF469/BF470 are discontinued but their equivalent in surface mount are BF722/BF723
Only because they are easy to get?
Poor BF469/470
  Reply With Quote
Old 31st January 2017, 02:28 AM   #8
diyAudio Member
 
Join Date: May 2016
Location: Hanoi
So If I have a transistor which have following parameters:
-Uce: 120V
-Ice: (cont.) 1A
-Pd: 0.8W
-Ft: min=50MHz
-Cbc: max=15pF
Can it be a VAS transistor?
  Reply With Quote
Old 31st January 2017, 03:29 AM   #9
diyAudio Member
 
de Ocampo's Avatar
 
Join Date: Nov 2010
Location: Ocean front, The Pacific
Quote:
Originally Posted by Nikolai Petrenko View Post
Only because they are easy to get?
Poor BF469/470
Hi,

Discontinued devices are very hard to get and if you can find one, the cost is way too high. IMO for VAS duty, a low cob high ft spec should be good enough. In this part of the world the TO-126 D669/B649 pair is a popular choice. If you want to look for the exotic type but very good device for VAS, grab an old CRT TV. Take those CRT video display drivers that can be found inside. Like this Sanyo device.
Attached Files
File Type: pdf C3600-Sanyo.pdf (40.8 KB, 23 views)
  Reply With Quote
Old 31st January 2017, 08:39 AM   #10
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally Posted by Nikolai Petrenko View Post
So If I have a transistor which have following parameters:
-Uce: 120V
-Ice: (cont.) 1A
-Pd: 0.8W
-Ft: min=50MHz
-Cbc: max=15pF
Can it be a VAS transistor?
Not a good VAS/TIS transistor.
It might make a good cascode transistor.
The cascode is very good at high frequencies relative to it's fT. So a low fT device will perform as well as needed (as a cascode) for a VAS/TIS that is apparently much faster judged by fT alone.
The VAS/TIS needs low power because it is protected by the cascode.
It needs adequate current, maybe 100mA to 200mA, not 1A
It needs speed, try fT>100MHz and needs to be biased to get close to this speed, so pick a VAS/TIS that gets near fT with a low Ic. There is no point in using an fT=300MHz but needs 150mA to get onto that part of the curve. look at 2sa1360 hits 100MHz @ 2mA and 2sa1381 hits 100MHz @ 5mA
Don't use TO-126 D669/B649. They make good driver transistors for the output stage.
__________________
regards Andrew T.
Sent from my desktop computer using a keyboard

Last edited by AndrewT; 31st January 2017 at 08:42 AM.
  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
cascode VAS phono amp hassome Analogue Source 2 7th February 2011 08:30 PM
Borbely cascode power requirements astouffer Solid State 5 11th December 2009 01:07 PM
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
Cascode cct minimises SOAR requirements of output bipolars. Circlotron Solid State 10 12th February 2003 11:13 AM


New To Site? Need Help?

All times are GMT. The time now is 03:15 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki