Help with FET's - 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 25th March 2003, 02:47 PM   #1
diyAudio Member
 
Join Date: Jan 2002
Default Help with FET's

I need simple help and recommendations.

I want a simple FET stage... not a DA or anything... that has a voltage gain of about 10 and that can be run from a +40v supply.

It will have an unbypassed source resistor to lower gain and lower distortion.

Should I look for FETs that have a lower inherent gain, or just get one and increase my source resistor to lower gain?

Recommendations on which FET's to use (ya I know.. the 2SK170)..

Running anywhere from 1-5mA is good. Will be driving a NPN follower with a current gain of 400. BC549.

Thanks for ANY specific recommendations.

  Reply With Quote
Old 25th March 2003, 03:32 PM   #2
Banned
 
Join Date: Feb 2002
Location: As far from the NOSsers as possible
Default Why 40 volts????

Most good JFETs don't like that much voltage across them. Sounds like time for one of them cascode things.............

Jocko
  Reply With Quote
Old 25th March 2003, 03:34 PM   #3
diyAudio Member
 
Join Date: Jan 2002
Default Yeah...

Cascode... was another idea too....

Ideas for good types? And should I bias the cascode up to over 5v or so?

Thanks!
  Reply With Quote
Old 25th March 2003, 03:39 PM   #4
Banned
 
Join Date: Feb 2002
Location: As far from the NOSsers as possible
I would run the JFET with at least 5 V, probably 10 V. Too low, and they sound like crud. Too high, and they get hot and, in theory, noisy.

BTW..................

If I get one more member ask me via e-mail why cascodes have more gain, I'm gonna rant.

They don't have more gain, just more bandwidth. And for all practical (engineering) purposes, they are a single gain element; as their response is very close to that of a single pole stage.

Jocko
  Reply With Quote
Old 25th March 2003, 03:45 PM   #5
diyAudio Member
 
Join Date: Jan 2002
Default Where to connect?

So for 12v... connect between upper FET gate and lower FET source?

Could I use zeners or diodes?

Should I just lower the supply voltage to 28v?

What FETS should I use.. and should I really cascode them?

TXS!
  Reply With Quote
Old 25th March 2003, 06:30 PM   #6
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Important question: what's the voltage level you want to amplify? I'd give you a different answer for 1 volt than 1 millivolt.
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  Reply With Quote
Old 25th March 2003, 06:37 PM   #7
diyAudio Member
 
Join Date: Jan 2002
Default Line Stage

Its for a pre-amp.

So... 1v is what I want to amplify.

50K volume pot b4 FET.

So... yeah it does limit my choices....

Looking for low distortion, simple 1 gain stage, voltage gain of 10, running from 24v to 36v power supply (SLA batteries).

Cascode or not.... doesn't matter really...

BC549 emitter follower on output.

does that help?
  Reply With Quote
Old 25th March 2003, 06:51 PM   #8
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Then a plain vanilla 2N5566 or the like should work just fine. You could cascode, but you probably don't need to; since it's a matched pair, you can parallel the two devices. Run it with 1.5 ma or so in cascode, 3 ma or so in parallel.
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  Reply With Quote
Old 25th March 2003, 07:02 PM   #9
diyAudio Member
 
Join Date: Jan 2002
Default temp drift?

Looking at those FETs....

there is wide variation in the graphs when temp is changed.

The best place to run them at looks like 4mA each.

Also noise level seems high.....
  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



New To Site? Need Help?

All times are GMT. The time now is 02:42 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