Source follower and common source output stages - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Pass Labs This forum is dedicated to Pass Labs discussion.

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
Thread Tools Search this Thread
Old 8th October 2008, 04:31 PM   #1
diyAudio Member
Join Date: Dec 2003
Location: austin, tx
Default Source follower and common source output stages

I have been trying to wrap my head around the performance of these two output stages. In particular how the Mosfet capacitance effect the frequency response and drive requirements. Looking at text book formulas the source follower has an input capacitance of Cin=Ggd+(1-Av)Cgs and for the common source it is Cin=Ggs+(1-Av)Cgd. From this I would assume the CS is more difficult to drive. But the comment below from Nelson himself leads me to believe otherwise.

Can someone help with an explanation? I am looking at amplifier designs similar to the Aleph, A75 and F5. One idea is a two stage amplifer with complementary diff input stage and complementary CS output stage.

  Reply With Quote
Old 8th October 2008, 05:43 PM   #2
The one and only
Nelson Pass's Avatar
Join Date: Mar 2001
No, it's consistent with those formulas. The difference is in the proportion
of drive voltage to output. The same amount of charge current goes into
the gate, but with a follower the voltage is much higher, so the Cgs looks
proportionately lower.

  Reply With Quote
Old 9th October 2008, 08:19 PM   #3
diyAudio Member
Join Date: Dec 2003
Location: austin, tx
It amazes me you can reply to all these messages. I am still having trouble understanding the frequency response and drive requirements. Is there recommended reading you would suggest?

Thanks again for your contribution to this hobby and all the personal time you put in.

  Reply With Quote


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
Common Source versus Common Drain output stages alaskanaudio Solid State 36 1st June 2014 02:35 AM
output impedance of source follower and load in mH? AudioGeek Solid State 2 18th November 2005 11:02 PM
Aleph-type current source, but source follower this time tschrama Pass Labs 4 29th July 2005 12:55 PM
Common-cathode triode vs source-follower Hexfet. Circlotron Analogue Source 14 22nd March 2003 07:35 AM
Source follower electret mike -> use with current source? capslock Solid State 1 28th February 2003 12:54 PM

New To Site? Need Help?

All times are GMT. The time now is 10:33 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2