Single FET line stage - 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 2nd July 2004, 10:43 PM   #1
wleediy is offline wleediy  Singapore
diyAudio Member
 
Join Date: May 2004
Location: Singapore
Default Single FET line stage

Hi all,

I am interested to build a single FET lien stage wiht no input capacitor, and gain of just larger then 1. Any diagram that I can share?
I will use this with a simple filkered/no op amp DAC output and as a line stage in general.

tks

William lee
  Reply With Quote
Old 3rd July 2004, 02:44 AM   #2
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
As far as a gain just larger than one, I can't help you. Otherwise, I would suggest a current source loaded source follower (2 FETs), which would have a gain just slightly less than one. Two pieces of 2SK170 (low current drive) or PN4293 (high current drive) would do the trick. The line amp I'm currently playing with has a common source stage with source degeneration, followed by a current source loaded source follower. Overall gain is 14. Harmonic distprtion is flat at 0.4% from 20-20k, and the distortion is overwhelmingly 2nd harmonic. I intend to put this line amp directly behing a 10k ganged cermet linear potentiometer with suitable shunt resistor to have it approximate an audio taper pot.
  Reply With Quote
Old 3rd July 2004, 04:11 AM   #3
azira is offline azira  United States
diyAudio Member
 
Join Date: Dec 2003
Location: Near Seattle
JFET ok?

http://www.till.com/articles/GuitarPreamp/

You didn't say no output capacitor although I suspect if you don't want an input cap, you might not want an output cap which would make this design process significantly more challenging...
--
Danny
  Reply With Quote
Old 3rd July 2004, 04:15 AM   #4
azira is offline azira  United States
diyAudio Member
 
Join Date: Dec 2003
Location: Near Seattle
Here's something I just put together in Orcad. You'll probably want to tweak it yourself though. No "signal" caps, the gate and drain are direct coupled to ground. The bias drain resistor is pretty touchy though. The source resistor sets the standing current to around 5mA. The main tradeoff is with the sizing, both drain and source resistors can be scaled somewhat, by making them larger it'll make the bias easier to finetune but you sacrifice damping factor. If you actually try it let me know, I just did it in simulation to see if it would work.
--
Danny
  Reply With Quote
Old 3rd July 2004, 04:16 AM   #5
azira is offline azira  United States
diyAudio Member
 
Join Date: Dec 2003
Location: Near Seattle
sorry, forgot the image.
Attached Images
File Type: jpg se-dc-fet.jpg (34.4 KB, 1053 views)
  Reply With Quote
Old 3rd July 2004, 06:58 AM   #6
wleediy is offline wleediy  Singapore
diyAudio Member
 
Join Date: May 2004
Location: Singapore
Default Single FET

hi all, Tks for both advises.

1. I will to build a single FET version first, ten try the current source to see how it can efect the sound. wrenchone can you kindly provide the diagram and FET selection process?

2, For the MOSTFET amp which device do you use, Azira? I have only twice done the FET pre, and recently mostly with tubes. So my knowledge about JFET is not good not to mention MOSFET. But the circuit look very interesting.
I hope to a JFET amp just as a utility amp, which I can use as a small and convinient pre that I can even carry around. the output cap is the price I am willing to pay, and I have lot of good cap in my parts boxes.

The MOSFET is interesting experiment that can teach me on the device...

Tks again and best rgds

William Lee
  Reply With Quote
Old 3rd July 2004, 03:02 PM   #7
azira is offline azira  United States
diyAudio Member
 
Join Date: Dec 2003
Location: Near Seattle
If you knowledge is limited and you're looking to learn, I suggest that you don't tie both hands and a foot behind your back by disregarding signal caps. The reason for this is that your loading becomes a factor in your dc biasing. You would be best to try building the JFET line stage, it fits your requirements and is a proven design.

The MOSFET in my circuit can be anything, it's not important, I think I might have picked an IRFP240. That's a power FET and you don't want a power FET so a different device might do much better, perhaps a small signal fet with higher transconductance. I really do not recommend building that circuit unless you can analyze it, it is very likely not to work as is. It was more of an answer to the challenge "can a single stage FET design be direct coupled on input and output?" It seems the answer is yes.

Good luck!
--
Danny
  Reply With Quote
Old 3rd July 2004, 04:14 PM   #8
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
Here are a couple of circuits that cheat and use two JFETs, though you could substitute a source resistor for the bottom FET in the circuit on the left and get away with it, though most likely with higher distortion. The circuit on the right uses feedback from the top FET's drain to mudulate the bottom FET - sort of a Scottman's push-pull. It is similar to the White follower tube circuit. You will of cource need bypass caps on the 30V supply - the type and value will depend on the regulator you use to ge the 30V.
Attached Images
File Type: gif followers.gif (7.6 KB, 957 views)
  Reply With Quote
Old 3rd July 2004, 04:27 PM   #9
wleediy is offline wleediy  Singapore
diyAudio Member
 
Join Date: May 2004
Location: Singapore
Default Tks and any gain...

Tks again to both of you. The MOSFET line stage looked interesting except will it be stable as far as DC offset is concerned? I think I will try teh single FET line stage first. I have J 202. But the last time I try I could not get any gain above 1.... I was using similar circuitry.
I have just bought some 2SK type, I think 2SK 170 and 389 bi-Fet, will take a look what I have got.

Tks again and best rgds

William Lee
  Reply With Quote
Old 3rd July 2004, 07:37 PM   #10
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
If you are using a common source stage as a line amplifer, you have 1) an inverting stage 2) not necessarily a low output impedance 3) can have difficulty getting a large symmetrical output voltage swing with a gain near unity, depending on your VCC and quiescent operating current. If all you are trying to do is to buffer your input sources, a source follower of some sort may be the best choice, even though the gain may be a little less than unity. With the source follower, you can have a reasonable output voltage swing and a low output impedance, with no phase inversion.

For the line stage I'm developing for my new preamp, I wanted some voltage gain so that I could compensate for a possible weak signal source. I also wanted the amplifier quiescent point to be at 1/2 Vcc for maximum possible signal swing. Some juggling with PSpice led to a stage with a gain of 14. This was a little higher than I wanted, but no big deal, since this stage will follow a volume control. What I ended up with was a common source amplifier with source degeneration feeding a source follower with current source loading. I used the "FETWhite" output circuit to get a little more output drive capability. It measures very nicely, and will be paired with a discrete JFET open loop RIAA amp built around a similar gain block. I'll have to wait until I can get the whole mess into a box before I can hear it. It will be interesting so see if I can notice any effect from the phase inversion.
I used the PN4393 devices for the line amplifier because thay are relatively low noise, have a high breakdown voltage (40V), and also can be selected for high Idss for good output current capability. They are also medium gain devices, so they don't require as much degeneration to trim the gain to the desired value. They are also easy to find. I am currently using matched output devices(within 0.1 ma Idss) selected for about 22-23ma Idss. Idss matching seems to be sufficient for decent results in this application. Output stage quiescent current is about 8ma. If you were fussy, you could set up an 8ma current source (or whatever other reasonable quiescent current you choose) and select devices based on the Vgs at this operating current. This will work even better. I used the simpler method because I had just moved and couldn't find my FET matching jig... The input device was selected to properly set the quiescent operating point, and if I remember correctly, was around 11ma Idss.

J202s have a pretty low Idss range and relatively low gain, and won't supply a lot of drive current. A better choice would be the PN4393 or 2SK170V, if you can find them. The RIAA amp used 2SK170 input devices, with a PN4393 output stage.
  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
Aikido line stage - plus phono line stage cjkpkg Tubes / Valves 1 22nd November 2008 09:50 PM
Single Stage Line Amp (Diff Amp) Tube? AudioAngel Tubes / Valves 9 28th November 2006 11:38 AM
Line stage preamp, based on SY's "Heretical Unity gain line stage" Danko Tubes / Valves 23 18th July 2005 04:07 AM
Single Stage Inverting or no? azira Tubes / Valves 7 14th December 2004 03:56 PM
Anyone play with the Zen Single-Stage Single-Ended Class A? Rino odorico Pass Labs 13 29th August 2002 09:01 AM


New To Site? Need Help?

All times are GMT. The time now is 03:56 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