Why are Emitter-resistors needed? - 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 4th February 2007, 09:53 AM   #1
diyAudio Member
 
Join Date: Jul 2004
Location: Sweden
Default Why are Emitter-resistors needed?

In BJT amps there's no question about it, but in MOSFETS? Are source-resistors really needed to prevent Iq from excursion? Even in parallell output, I believe in theory it shouldn't be needed. I've read about in the Wiki but I would like to see some non-theoretical evidence too it.
I'm gonna do some tests later this day about this, but I'd like to know if anyone has a comment about it
  Reply With Quote
Old 4th February 2007, 10:14 AM   #2
diyAudio Member
 
Join Date: Jul 2005
Location: North Derbyshire
I believe it depends on the particular devices?, FET's are supposed to have a negative temperature coefficient, so automatically prevent thermal runaway and have no problems in parallel. However, many high power audio devices don't have a negative temperature coefficient, so require resistors just like bipolar.
__________________
Nigel Goodwin
  Reply With Quote
Old 4th February 2007, 10:42 AM   #3
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
Lateral FETs have a -ve temp coef above about 500mA so are effectively self protecting when used in parallel.
Source resistors here help the circuit to stabilise the quiescent and dynamic currents.

Vertical FETS have a -ve temp coef above about 6A to 40A depending on model. At these kind of currents and at the voltages used in audio they are already destroyed before they get out of the +ve coefficient region.
Source resistors are mandatory for current sharing when driving the output.
At quiescent conditions, things are a bit different.
Very accurate matching can equalise individual Iq resulting in near identical junction and case temperatures when no output current is flowing.
__________________
regards Andrew T.
  Reply With Quote
Old 4th February 2007, 10:59 AM   #4
hacknet is offline hacknet  Singapore
diyAudio Member
 
Join Date: Nov 2002
Location: sg
i read somewhere that the emmitor resistors are required to prevent the devices self distructing on turn on due to the suddent surge in current.

is this true?
  Reply With Quote
Old 4th February 2007, 11:21 AM   #5
diyAudio Member
 
Join Date: Jul 2004
Location: Sweden
Hi,
Thanks for Your replies. Ok so the tempratur-coef can vary from devices. How about an old faithful IRF540/9540? I think Iv'e got these on my shelf so I shall try with these later. First single output and see if Iq rise when they are hot.
I had a look at the datasheet for IRF540 and the curve Temperature vs. Id is of interest but it seems like the current has to go very high to take these effects that we're talking about.
  Reply With Quote
Old 4th February 2007, 11:54 AM   #6
diyAudio Member
 
Join Date: Jul 2005
Location: North Derbyshire
If in doubt, add the resistors, they won't do any harm, and may well make the amp more reliable.
__________________
Nigel Goodwin
  Reply With Quote
Old 4th February 2007, 12:01 PM   #7
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Quote:
Originally posted by Fritzell
Hi,
Thanks for Your replies. Ok so the tempratur-coef can vary from devices. How about an old faithful IRF540/9540? I think Iv'e got these on my shelf so I shall try with these later. First single output and see if Iq rise when they are hot.
I had a look at the datasheet for IRF540 and the curve Temperature vs. Id is of interest but it seems like the current has to go very high to take these effects that we're talking about.
Yes they have a neg TC but only at currents higher than normally used in audio bias, so the effect is pretty useless unless you use them in heavy biased class A.

Another reason for the Re is to smoothen the cross-over region to avoid hard xover distortion (it is local feedback), and in case of multiple parallel devices, to help equal load sharing.

Jan Didden
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W. S. Maugham
Check out Linear Audio!
  Reply With Quote
Old 4th February 2007, 01:05 PM   #8
diyAudio Member
 
Join Date: Sep 2006
Hi all

actually there are competing mechanisms in mosfets.
But, I think you mean positive temperature coefficient when you said negative.
A positive temperature coefficient makes the resistance increase with temperature, a negative tempco decreases it.

In a Mosfet, the Vt has a negative tempco, while the channel resistance has a positive tempco. This means that for each fet there will be a temperature neutral point, which as people have said already, only works at relatively high current.

Because most fets are biased to a relatively low current (well, maybe 100 mA or so) to avoid crossover, the Vt is still usually in the negative tempco region, so needs a bias stabiliser.

Parallelling MOSFETS without local source resistors can be dangerous - especially at high frequencies and currents, the impedances need to be matched to make sure that one device does not hog the amps and destroy itself.

cheers
John
  Reply With Quote
Old 5th February 2007, 10:42 AM   #9
diyAudio Member
 
Join Date: Jul 2004
Location: Sweden
Aha I understand, thank you.
But Vbe multiplier won't be as critical? Iv'e seen many designs with just a potentiometer..
  Reply With Quote
Old 5th February 2007, 10:59 AM   #10
diyAudio Member
 
Piercarlo's Avatar
 
Join Date: Nov 2006
Quote:
Originally posted by Fritzell
Aha I understand, thank you.
But Vbe multiplier won't be as critical? Iv'e seen many designs with just a potentiometer..
Many designs are just good for french fries... Vbe multiplier is not well apted to mosfets biasing because mosfets havent not Vbe at all. But if nothing of better is available (as may be a dedicated biasing circuit as that provided by Linear Technology LT1166), a Vbe multiplier is surely more reliable of a raw potentiometer.

Hi
Piercarlo
  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
MPC70 0.27 ohm 2W emitter resistors Luke Swap Meet 2 22nd August 2009 11:47 PM
7w dual emitter resistors? JJGross Parts 3 1st July 2008 04:37 AM
Dual emitter resistors - where from? Stuey Parts 14 10th March 2007 06:58 AM
Emitter Resistors hawkfeather Solid State 15 29th January 2006 05:28 AM
Emitter Resistors EchoWars Solid State 16 16th July 2005 10:47 PM


New To Site? Need Help?

All times are GMT. The time now is 07:16 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