JFET cascoded with tube gain calculation - 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 10th July 2010, 02:27 PM   #1
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Default JFET cascoded with tube gain calculation

How can the gain of JFET cascoded with tube, like in attached schematic, be calculated?
Attached Images
File Type: jpg Hybrid 1.jpg (13.8 KB, 365 views)
  Reply With Quote
Old 10th July 2010, 03:25 PM   #2
R.I.P.
 
Join Date: Dec 2001
Location: Schaffhausen Switzerland
Why fuss around with calculations? Build it and MEASURE! I very seldom caculate, but I do a LOT of measuring, and then I can be 100% certain.

Regards, Allen
  Reply With Quote
Old 10th July 2010, 03:31 PM   #3
diyAudio Member
 
Join Date: Apr 2006
Location: Minnesota
We need to see more of the circuit in order to do the calculation.
  Reply With Quote
Old 10th July 2010, 03:32 PM   #4
diyAudio Member
 
Join Date: Dec 2005
Location: Kuala Lumpur
It's just like a all jfet cascode, the gm is determined by the jfet.
The valve has very close to unity current gain
R2 gives series feedback
  Reply With Quote
Old 10th July 2010, 03:54 PM   #5
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Thanks, Allen and David.
  Reply With Quote
Old 10th July 2010, 03:55 PM   #6
diyAudio Member
 
Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
Quote:
Originally Posted by Allen Wright View Post
Why fuss around with calculations? Build it and MEASURE! I very seldom caculate, but I do a LOT of measuring, and then I can be 100% certain.

Regards, Allen
Hi Allen,

Although I generally agree with that, I do think there is a place for calculation, (modeling, simulation...)

Sometimes I mess around with calculations to predict how a circuit will behave before building it, mainly to get the circuit closer to it's desired operation before building. It helps in deciding which parts and part values to select and sometimes rules out impractical circuit approaches.

It's also very instructive in cases like this to work through the calculation as a way to teach the circuit theory.

When a question like this comes up, it's an indication that the poster doesn't fully understand the operation of the circuit (otherwise he or she could easily construct the formula themselves).

It's also not clear in this case whether Joshua is building this or studying for learning purposes. In any case, building it is definitely the only way to be sure, but maybe not always necessary. Maybe if 3 circuits are analyzed, only one of them need be built.

So I think a good response would be to either teach the circuit theory through example or refer the poster to some reference material that teaches about cascode circuits.

There is enough of the circuit shown to teach the stage gain analysis. Several clues have been given.

Who can teach the analysis? Can Joshua now demonstrate how to estimate the gain of a cascode amplifier stage?
  Reply With Quote
Old 10th July 2010, 04:10 PM   #7
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Hi Michael,
The question was not for studying purpose, I'm going to build a tape playback amp with the front end after Allen's phono stages, like in the schematic I posted (which is taken from Allen's website).

I admit I don't understand completely how cascoded stage work. Any explanation will be welcomed.
  Reply With Quote
Old 10th July 2010, 04:32 PM   #8
Gordy is offline Gordy  United Kingdom
diyAudio Member
 
Join Date: Nov 2006
Quote:
Originally Posted by Allen Wright View Post
Why fuss around with calculations?
Because people often have limited time, and therfore can not afford to build on the off-chance that it might be right.

Because people often have limited money, and therfore can not afford to buy a stock of components just to experiment.

Because people often have limited time and money, and therfore can not afford to have circuits explode because they were designed wrong.

Because calculating is a proven professional way of taking a step towards a successful design. (And I don't just mean in audio, but in any technical subject).

Because calculating goes hand-in-hand with understanding, and understanding often leads to improved design, easier modification, easier fault-finding, and a good chance of success, and hence ultimately leads to a sense of intellectual reward.


If you want to do it the other way then best of luck to you, however I would encourage folks to understand and calculate before 'buying and trying'.


  Reply With Quote
Old 30th May 2012, 11:55 AM   #9
diyAudio Member
 
Join Date: Dec 2006
Location: USA
Quote:
Originally Posted by Allen Wright View Post
Why fuss around with calculations? Build it and MEASURE! I very seldom caculate, but I do a LOT of measuring, and then I can be 100% certain.

Regards, Allen

I guess nobody in this forum knows exactly how to calculate the gain or to analyze the circuit. People here normally judge a circuit by ear. Except me, of course.
  Reply With Quote
Old 30th May 2012, 12:34 PM   #10
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by dia thien thai View Post
I guess nobody in this forum knows exactly how to calculate the gain or to analyze the circuit.
You guess incorrectly. A = gm of the FET times the AC load resistance of the tube. The effective gm of the FET is modified by the source degeneration resistor.
__________________
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

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
Tube Hybrid Amp Gain Calculation atiq19 Tubes / Valves 11 22nd November 2009 12:55 PM
Cascoded jfet buffer rbroer Solid State 22 1st July 2008 02:43 PM
Jfet Cascoded BOZ bobsinclar Pass Labs 26 21st September 2007 08:23 PM
Krazy JFET-cascoded Pass BOSOZ jh6you Pass Labs 60 17th March 2006 01:19 AM
Cascoded jfet in X-BOSOZ?? Magura Pass Labs 44 31st January 2006 09:00 PM


New To Site? Need Help?

All times are GMT. The time now is 12:07 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