Need good current sink JFET or MOSFET? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 2005, 03:58 PM   #1
rlaury is offline rlaury  United States
diyAudio Member
 
Join Date: Sep 2005
Default Need good current sink JFET or MOSFET?

Hi Group:
I'm looking for a good current sink to use with a 6DJ8 long tail phase inv. I've run test using an NPN transistors, but would like to try a FET. I'm not sure if I see benefits between each. My main concern is saftey (relibility) linearity, and noise. Also what about some of the 3 terminal current sinks. Should I consider them also?
Anyone have some FET part numbers I should consider?
Any response apprecaited.

Thanks!

RonL
  Reply With Quote
Old 21st November 2005, 06:03 PM   #2
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
JFETs will work fine.

The so called "current diodes" are merely jfets strapped to their IDSS value, selected from a given run for the specific value.

You can take any jfet and make a nicely adjustable current "sink".

Afaik, the only thing to worry about is that you don't exceed the JFET's breakdown voltage.

They are generally less noisy than most transistors... much less noisy than most any tube.

_-_-bear
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  Reply With Quote
Old 21st November 2005, 08:15 PM   #3
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
I'm looking for a good current sink to use with a 6DJ8 long tail phase inv. I've run test using an NPN transistors, but would like to try a FET. I'm not sure if I see benefits between each. My main concern is saftey (relibility) linearity, and noise. Also what about some of the 3 terminal current sinks. Should I consider them also?
For a differential phase splitter, best to stick with the BJTs. Since the CCS is in the tail, whatever it does is common mode, and therefore rejected by the amp's CMRR. Since BJTs are higher gain devices than JFETs, they make better current sources. The supplied current is steadier, and more importantly, the effective impedance higher. See This Design for how I implemented the same type of phase splitter. In testing this out, the voltage balance was excellent at an output of 40Vp-p. As for safety, simply use high voltage, small signal transistors, such as the MPSA42 (Fairchild) that I used which I got from Rat Shack in a 15 pack of transistors anonymous.

As for the IC current sources, can you find one that'll work at the design current? If so, then try 'em, but I'm not sure that these will provide any advantage (other than convenience) in this application.

"The so called "current diodes" are merely jfets strapped to their IDSS value, selected from a given run for the specific value"

I avoid those two-legged devils like the plague. Yuck!
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 21st November 2005, 09:12 PM   #4
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
It's very much a "horses for courses" issue. If you're using 6DJ8, then you're probably wanting to pass somewhere between 4mA and 20mA. The most important question is, "How many volts can you afford to drop across your CCS?" If you have enough voltage, then a cascode of bipolar transistors will do very nicely and probably be cheaper than FETS. It will certainly free you from FET device tolerances.
__________________
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 22nd November 2005, 03:42 AM   #5
hacknet is offline hacknet  Singapore
diyAudio Member
 
Join Date: Nov 2002
Location: sg
i was just curious, what is a good solution if my cathode voltage is low, say around 1v. is it still feasible to use a pair of BJTs?
  Reply With Quote
Old 22nd November 2005, 04:06 AM   #6
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
afaik transistors can operate with zero collector volts, but you have to bias the base emmiter with 0.6 volts for it to conduct current. tight call....
  Reply With Quote
Old 22nd November 2005, 06:06 AM   #7
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
A transistor operating at 0v collector would be switched hard ON and could not control the current. You need at least 15v to work with. If your LTP is direct-coupled to the plate of the preceding stage, that's easy but from what you say it is not like that.

Another way of getting a higher cathode-to-ground voltage is to raise the DC voltage of the 'grounded grid' of the LTP using a high resistance potential divider from B+. (e.g. 1Meg and 100k would give you 1/11 of B+). Ground the grid through a cap of 0.22uF or so. Connect the other grid to it through a 1Meg resistor and use a decoupling cap as input to the stage.
  Reply With Quote
Old 22nd November 2005, 06:14 AM   #8
rlaury is offline rlaury  United States
diyAudio Member
 
Join Date: Sep 2005
Thanks everyone. Good food for thought.
I'm planning on using a -60 tail voltage from the bias supply.
This will insure there will never be 0 volts.
  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
BUF420M Current Sink audiowize Tubes / Valves 2 13th September 2008 03:16 AM
Aleph 2 input/current source mosfet heat sink sugano Pass Labs 2 20th October 2006 01:57 AM
Pentode current sink ABJ Tubes / Valves 14 23rd January 2005 09:56 PM
Using Ixys current regulator as current sink. G Solid State 3 16th November 2002 10:23 AM
Help with Current Sink RichardJones Pass Labs 1 31st January 2002 04:52 PM


New To Site? Need Help?

All times are GMT. The time now is 08:11 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