jfets for preamp - 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 November 2008, 08:25 AM   #1
diyAudio Member
 
Join Date: Apr 2007
Default jfets for preamp

Hi,
Does anyone knows any good audio Complementary jfets (n-p channel) capable of operating with voltages (VDS >= +- 30 Volt). These will be used in the first stage of a preamp.

Thanks
  Reply With Quote
Old 21st November 2008, 09:01 AM   #2
diyAudio Member
 
brutepuppy's Avatar
 
Join Date: Jun 2008
Send a message via Yahoo to brutepuppy
I believe the PASS "B1" LSK170 ultra low noise is good up to 40 volts.
__________________
  Reply With Quote
Old 21st November 2008, 09:03 AM   #3
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
the best overall pair is Toshiba 2SK170BL / 2SJ74BL
.. they are reasonably in price, can be found and are good for audio = lownoise
... probably the most used .. ever
they are rated SK170 =40 Volt and 2SJ74 =25 Volt
so you will better have to cascode with some higher voltage bipolar transistor
Another pair is:
2SK246 =40V and 2SJ103 =50V

The problem is often to find the P-JFET transistor.

See for example this good supplier:
http://www.ampslab.com/components_fets.htm
__________________
lineup
  Reply With Quote
Old 21st November 2008, 09:11 AM   #4
diyAudio Member
 
Join Date: Apr 2007
Quote:
they are rated SK170 =40 Volt and 2SJ74 =25 Volt
so you will better have to cascode with some higher voltage bipolar transistor
Hi thanks, for the reply. Can you give an example how to cascode with BJTS
  Reply With Quote
Old 21st November 2008, 09:35 AM   #5
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
Click the image to open in full size.
http://diyaudioprojects.com/Solid/Je...ga-Le-Monstre/

In the above, you see 2k resistors divide 12 Volt into two 6 Volt parts.
At the 2SC1775 Emitter is like 5.3 Volt
and so the voltage across the 2SK170 is only like ~5 Volt.

The current from 2SK170 just passes trough the 2SC1775 and gets to the 1k resistor.
In this circuit 2SC1775 and 2SA872 cascodes the complementary JFET pair.

If you use 2 equal resistors, for example 5k, to divide 30 Volt
than each JFET will have like 14-15 Volt drain-source.

Cascoding with bipoar transistors is very often used for to help JFETs.
Not only because they can not take the high voltage
but also because they work best at a certain lower voltage.

In the Hiraga above, the reason is not high voltage (12Volt)
for cascoding .. it is because the JFETs perform better = at 5-6 Volt.


Lineup
__________________
lineup
  Reply With Quote
Old 21st November 2008, 10:19 AM   #6
diyAudio Member
 
Join Date: Apr 2007
Thanks for pointing out this schematic, I have though a question. Why in the schematic we take the output out of the BJT collector and not from the jfet drain as we would do if we didn't cascade them and use the BJT just for consuming power? If we take the utput out of the BJT collector, then we are not take advantage of the quality of the jfets (my opinion).
  Reply With Quote
Old 21st November 2008, 10:44 AM   #7
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
It is true that some minimal quality will be lost
as the current will have to pass the BJT, as any component will degrade signal a bit.

But in this case, the designer has figured out, that the gain in quality of operating the JFET at the lower voltage chosen
gives a better total result,
than if operating the JFET at higher voltage would have done.
Even counted for the current now has to pass another BJT.

2SK170 works not at best, when the drain-source voltage get above 10 Volt something
It can work, for sure, upto 40 Volt .. but there are some parameters that will be less good.
I do not know exactly WHAT, but some other may tell us the correct reason.
__________________
lineup
  Reply With Quote
Old 21st November 2008, 10:49 AM   #8
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
The cascode does a double function.
It limits the voltage on the input transistor.
It also operates as a common base amplifier.
We take the output from the collector of the common base amplifier.
You can also use a Jfet as the common gate amplifier with similar results. See Borbely for examples.
There is a third advantage to using the cascode loading of the input transistor; namely, it keeps the input transistor at near constant voltage and this results in almost no change in capacitance of the input device resulting in elimination of much of the non-linearity that device capacitance causes.
__________________
regards Andrew T.
  Reply With Quote
Old 21st November 2008, 10:56 AM   #9
leapcat is offline leapcat  United States
diyAudio Member
 
Join Date: Nov 2007
Quote:
Originally posted by kzeprf22
Thanks for pointing out this schematic, I have though a question. Why in the schematic we take the output out of the BJT collector and not from the jfet drain as we would do if we didn't cascade them and use the BJT just for consuming power? If we take the utput out of the BJT collector, then we are not take advantage of the quality of the jfets (my opinion).
Because the BJT's are just on. The JFETS still control the signal.
(IMHO)
  Reply With Quote
Old 21st November 2008, 10:58 AM   #10
diyAudio Member
 
jacco vermeulen's Avatar
 
Join Date: Oct 2004
Location: At the sea front, Rotterdam or Curaçao
Send a message via Yahoo to jacco vermeulen
Quote:
Originally posted by lineup
the JFETs perform better = at 5-6 Volt.
http://www.diyaudio.com/forums/showt...72#post1657272
__________________
The buck stops Here
  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
Matching Jfets NickC Solid State 13 23rd December 2013 11:18 PM
SiC JFETS revintage Pass Labs 15 27th August 2009 02:43 PM
JFETs for 5ma CCS? PhopsonNY Parts 7 24th December 2008 03:55 AM
Matched JFETs suzyj Solid State 8 22nd August 2007 02:10 AM
got a bag of 30 J201 JFETs, what can I do with them? bikehorn Solid State 2 2nd June 2007 01:18 AM


New To Site? Need Help?

All times are GMT. The time now is 11:20 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