A Compact, High-Performance Source Follower Module - 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 2010, 03:23 PM   #1
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
Default A Compact, High-Performance Source Follower Module

Thanks to the JFET noise measurements by Scott Wurcer, we know that the famous 2SK170 is not the lowest noise JFET available. In fact, both the 2SK369 and especially the underrated 2SK372 is significantly better.

More FET noise measurements (for EUVL)

The 2SK372 has a TO-92 equivalent, the 2SK363, which according to datasheet has a higher power rating. However, this is only true if the JFET is used free standing, i.e. dissipating to ambient by free convection. As we shall see, the smaller package of the 2SK372 has a sizable advantage when used as a coupled pair and together with a heat sink.

For the old workhorse, the current source loaded JFET source follower, recently reincarnated as the Pass B1, the following characteristics of the active device would be desirable:

1) high transconductance (for low output impedance),
2) low thermal resistance between the 2 devices (for best thermal tracking and low DC offset),
3) high Idss (for better linearity and current driving capability),
4) well matched Idss (for low DC offset at null signal),
5) low Cgd (for high bandwidth).

The 2SK389 in V grade is undoubtedly the best solution for criteria 2 to 5, only if you can still get them. Compared to the 2SK170 in TO92, 2SK372 scores on at least two counts. Its transconductance is about 2x that of the 2SK170, and its smaller package size means that when glued face to face with silver epoxy, the 2 silicon dies have a smaller physical distance between them and hence better thermal coupling. High Idss can be obtained by using the V grade (on average 20mA), but that also means higher dissipation, and hence the necessity of a heat sink so that reliability does not suffer. The 2SK372 does have a higher Cgd, but with 15pF still gives a 10MHz bandwidth with 1k ohm source impedance, so no real issue.

For thermal coupling, the most obvious is to glue the 2 devices face to face at their flat faces. This also provides a convenient connection for the source pin of the top device (the follower) to the drain pin of the bottom device (the current source). The gate pin of the bottom device can also be shorted to the source pin directly at the package (see photo), leaving only 4 external connections to the outer world -- +Vs, -Vs, input and output.

The thermal coupling can however be further increased by wrapping the outer surfaces of the coupled devices with a metal ring of sufficient thickness. This increases the gluing surface of the devices (the glue and the JFET package encapsulation are the highest resistance in the thermal coupling path), and hence reduce thermal resistance. Aluminium has a thermal conductivity > 10x that of silver epoxy, so that one can treat the thick aluminium ring as a thermal short circuit.

It is only natural that we make a heat sink out of the ring. We have tested 2 designs, a rectangular one measuring 10x10x4mm, and a cylindrical design of 8mm diameter and 6mm high.

The rectangular version is glued with 2x 2SK372V of Idss 21.82mA and 22.37mA respectively (these are my matching rejects and is used here for thermal design verification). Using supply rails of +/-9V, the steady state heat sink temperature was measured at 44C, and the output DC offset at -6.5mV. The output DC meets expectation, and it shows that even a 0.3mA mismatch will cause measurable DC offset (even more for 2SK170s with its lower transconductance). The thermal resistance of the heat sink is calculated to be 57C/W, not bad at all for such a small heat sink. Ambient temperature was 20C. But more importantly, the temperature of the JFET die, calculated to be about 70C, is low enough now to ensure reliability even when we are dissipating about 200mW per device. The DC offset remains constant to 0.1mV from cold start to steady state, confirming the very effective thermal coupling of the 2 devices.

The cylindrical version has even better performance, though quite a bit more difficult to manufacture. Using 2x 2SK372V with Idss of 21.82mA and 22.37mA, the DC offset was measured at -8.7mV, and the steady state sink temperature at 48C. This corresponds to a thermal resistance of 48C/W for the cylindrical heat sink.

All in all, the solutions shown here provide a compact, high-performance source follower which can find many useful applications such as Sallen Key filter buffers, CD IV converter output buffers, etc., at a very moderate cost. There is IMHO no good reason not to use these instead of connecting opamps as unity gain buffers.


Patrick
Attached Images
File Type: jpg 372 SF Sq 1.JPG (248.9 KB, 2660 views)
File Type: jpg 372 SF Sq 2.JPG (297.9 KB, 2569 views)
File Type: jpg 372 SF Rd 2.JPG (261.6 KB, 2492 views)
File Type: jpg 372 SF Rd 3.JPG (307.1 KB, 2414 views)

Last edited by EUVL; 18th October 2010 at 03:29 PM.
  Reply With Quote
Old 18th October 2010, 03:40 PM   #2
stoc005 is offline stoc005  United States
diyAudio Member
 
stoc005's Avatar
 
Join Date: Feb 2007
Location: Midwest in the USA
Has any of the Linear Systems parts been tested to see if they really are low noise as claimed? It seems that the LSK389 would be a viable part if so since you can buy them quite easily.
  Reply With Quote
Old 18th October 2010, 04:03 PM   #3
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
There is an article on noise of LSK389 :

http://www.audioxpress.com/magsdirx/.../colin2993.pdf

But if I remember correctly, someone some time ago posted a comment that JFETs from Linear Systems was not consistent in quality.
I have not use many of them, so no idea.

As someone who has worked in the semiconductor industry for over 25 years, I know how critical manufacturing processes are, and I simply do not believe a small company like Linear Systems would have the same resources at their disposal as Toshiba, or Philips for that matter. So I shall use Toshiba, Philips (NXP), Onsemi, .... , as far as I can.

You can say I am biased. I guess I am.


Patrick
  Reply With Quote
Old 18th October 2010, 05:18 PM   #4
qusp is offline qusp  Australia
diyAudio Member
 
qusp's Avatar
 
Join Date: Oct 2009
Location: Brisbane, Australia
yeah the main ones that interest me from linear are the small sot23 versions, more great and informative research Patrick, yet another reason for me to buy some of your sinks. hoping you still have some left, I have emailed you friend
  Reply With Quote
Old 18th October 2010, 05:23 PM   #5
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
I asked Linear Systems multiple time for SOT23.
I don't think they keep them in stock, which means you need to order a packaging lot (10k ??).


Patrick
  Reply With Quote
Old 18th October 2010, 07:10 PM   #6
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Quote:
Originally Posted by EUVL View Post
I asked Linear Systems multiple time for SOT23.
I don't think they keep them in stock, which means you need to order a packaging lot (10k ??).


Patrick
I will be measuring all the Linear System's parts after I build rev 2 of my test setup. I looked at one LSK170 and it was actually slightly worse but remember these measurements are higher res than reading an analog meter.
__________________
Silence is so accurate.
  Reply With Quote
Old 18th October 2010, 07:16 PM   #7
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
I hope to hear more details from you about your revision 2 (when finished), so that I can also repeat the tests and perform similar measurements on other JFETs.

Hopefully in the end we'll have a collection of measurement of a wide range of JFETs.
My sincere thanks for your work up to now.


Patrick
  Reply With Quote
Old 21st October 2010, 03:10 AM   #8
diyAudio Member
 
Join Date: Feb 2010
Location: Dallas Fort Worth area, Texas
Hi Patrick,

Do you have plans to produce either of these heatsinks for sale and public consumption?

Thanks,
Dave
  Reply With Quote
Old 21st October 2010, 04:15 AM   #9
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
If there is sufficient interest we shall make them available. Most likely only the square version, as the round version with the recess at the back etc. is quite a bit more machining, and cost more than double.

We shall start making them for a minimum demand of 100 pieces, and as before min 10pcs, over 25pcs no 5Euro surcharge, price probably around 2.50 to 3.00 Euros because of small batch. But we need to know quantities first.

(We shall change to Euro from now on as US dollar is a very unstable currency.)

If you volunteer to try 2SK372V and promised to report back, I have a few here to send you right now.


Patrick
  Reply With Quote
Old 21st October 2010, 07:35 PM   #10
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
Someone just placed an order for 50 pcs. Anyone else wants to join before I send drawings off for production ?

If you are in the US and want small quantities, you might consider group buying with him. I guess I can get that arranged.


Patrick
  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
Source follower and common source output stages slogan2112 Pass Labs 2 9th October 2008 08:19 PM
Aleph-type current source, but source follower this time tschrama Pass Labs 4 29th July 2005 12:55 PM
Compact Aleph 5 module OliverD Pass Labs 22 19th March 2004 09:12 AM
Source follower electret mike -> use with current source? capslock Solid State 1 28th February 2003 12:54 PM


New To Site? Need Help?

All times are GMT. The time now is 01:29 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