Input signal and amp bias - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

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
Reply
 
Thread Tools Search this Thread
Old 9th September 2011, 07:22 PM   #1
diyAudio Member
 
vdi_nenna's Avatar
 
Join Date: Oct 2000
Location: PA, USA
Blog Entries: 1
Send a message via Yahoo to vdi_nenna
Default Input signal and amp bias

Can someone please explain how an input signal and a bias voltage can be feed to the gate of a Mosfet without disturbing the input signal?

Taking from the original Zen as an example, page 3 states the following:

This constant 2 amp current is fed to Q1. Resistor R8 and potentiometer
P1 form a DC feedback loop which operates the gate of Q1 at about 4
volts and places the Drain potential of Q2 at the midway point of the
power supply, or about 17 volts. Input signal passes through C6 and R5
to the gate of Q1, and output signal passes through C3 and C4 in
parallel to the loudspeaker.
R9 and R2 are there to bleed off DC, but are
not particularly essential. Z1 is essential to insure that an input transient
cannot exceed the 20 V gate rating of the Mosfet.


The 4 volt bias is needed to turn on the Mosfet. If there is a 1v audio signal at the input, do the voltages not interact or crash or mess each other up?

In the case of the Zen, DC bias is placed at the gate because of the type of amp- a common Source V+I gain?

This is one of those audio amp building mysteries I'm trying to understand.

Thanks in advance,

Vince
Attached Images
File Type: jpg Zencircuitbias.jpg (59.9 KB, 106 views)
__________________
I build, therefore I am.
  Reply With Quote
Old 9th September 2011, 08:03 PM   #2
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
you have cap there - between DC bias voltage and input AC voltage

just think of DC bias voltage/potential as AC GND

so - input signal is riding around DC bias potential without disturbing it

in most amps/stages GND is just state of mind , nothing else ...

"real" GND is "just" overall reference potential
__________________
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM
  Reply With Quote
Old 9th September 2011, 08:13 PM   #3
diyAudio Member
 
vdi_nenna's Avatar
 
Join Date: Oct 2000
Location: PA, USA
Blog Entries: 1
Send a message via Yahoo to vdi_nenna
Quote:
just think of DC bias voltage/potential as AC GND
so - input signal is riding around DC bias potential without disturbing it
I though it was something like this, but I removed it from my post, because I was speculating. I need to better understand the relationship between AC and DC in audio signals?

Thanks ZM.

Vince
__________________
I build, therefore I am.
  Reply With Quote
Old 9th September 2011, 08:54 PM   #4
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
there is no DC (hopefully) in audio signal ;

so - take DC values as steady state elements (circuit biasing etc.) , while AC values are dynamic
__________________
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM
  Reply With Quote
Old 9th September 2011, 09:54 PM   #5
ra7 is offline ra7  United States
diyAudio Member
 
Join Date: Feb 2009
Blog Entries: 1
DC is 0 hertz, AC is x Hz. They don't interfere with each other. However, the way I understand it, when amplification happens, the input V at some frequency is impressed upon a much larger V which is at DC. What remains is the much larger V, but now it is transformed into an AC V.
  Reply With Quote
Old 9th September 2011, 10:23 PM   #6
diyAudio Member
 
vdi_nenna's Avatar
 
Join Date: Oct 2000
Location: PA, USA
Blog Entries: 1
Send a message via Yahoo to vdi_nenna
Quote:
What remains is the much larger V, but now it is transformed into an AC V
Is that varying DC?
__________________
I build, therefore I am.
  Reply With Quote
Old 9th September 2011, 10:28 PM   #7
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
in a way

AC is varying DC in - say - output node
it's simple - just reread few times those basic Papa's pdfs ;

or - linked article in Jfet basics thread ; it's started yesterday , I think
__________________
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM
  Reply With Quote
Old 9th September 2011, 10:53 PM   #8
ra7 is offline ra7  United States
diyAudio Member
 
Join Date: Feb 2009
Blog Entries: 1
Yup... exactly what ZM said. Output node can be the drain of the FET. When the FET receives an AC signal at the gate, the signal modulates or impresses itself on the 0 Hz (flat) signal at the drain of the FET. But because the drain is at much higher potential, you get gain or an amplified signal.

I'm sure its much more complex than that, but I find this way easier to understand.
  Reply With Quote
Old 9th September 2011, 11:13 PM   #9
diyAudio Member
 
vdi_nenna's Avatar
 
Join Date: Oct 2000
Location: PA, USA
Blog Entries: 1
Send a message via Yahoo to vdi_nenna
The DIY Opamp artical has a good explanation, but I need to re-read over and over.
__________________
I build, therefore I am.
  Reply With Quote
Old 9th September 2011, 11:27 PM   #10
diyAudio Member
 
Join Date: Sep 2004
Location: Minneapolis, Minnesota, USA
Think of it as an AC signal riding on a DC level. For example, if the input DC bias voltage is 10 volts , and the AC input signal is 1 volt peak to peak, the resulting voltage at the amp input will be from 9 volts to 11 volts.

Mike
  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
Quiet amp hums with signal source or input directly tied to star earth?? bigwill Tubes / Valves 51 13th June 2010 05:50 PM
Alpine amp plays fine but needs lots of signal input? spooney Car Audio 2 20th January 2007 08:20 AM
CDP output signal too strong for AMP input?? Florian Digital Source 12 4th August 2005 12:41 AM
Holton's N-Channel amp Exhibits Severe Cross-Conduction at high Input signal. Workhorse Solid State 106 13th November 2004 07:04 AM
amp with single input can't amplify pulse signal,please help me thanh Solid State 2 26th October 2004 02:56 PM


New To Site? Need Help?

All times are GMT. The time now is 11:35 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