Power amp output - BJT or MosFET? - 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 20th April 2004, 10:58 PM   #1
diyAudio Member
 
Bill Fitzpatrick's Avatar
 
Join Date: Jun 2001
Location: Eugene, OR
Default Power amp output - BJT or MosFET?

Assuming good class AB design practices do BJT output stages offer more or less quality than MosFET output stages?
  Reply With Quote
Old 20th April 2004, 11:53 PM   #2
lucpes is offline lucpes  Europe
diyAudio Member
 
Join Date: Dec 2003
Location: Iasi
Send a message via Yahoo to lucpes
MOSFETS vs BJTs

More like a question of sorts like "Are blondes better than brunettes?" - I guess that depending on quality of 'implementation' and due to their different nature one 'category' could be 'better' or the other way round. No cookbook unfortunately on women either . Sorry for being kind of sarcastic, just my 2c...
  Reply With Quote
Old 21st April 2004, 07:07 AM   #3
diyAudio Member
 
slowhands's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
This is just my opinion, for what it is worth.

Short answer:

Class B: For low to medium power output (<100 watt RMS), BJTs are the best tradeoff for most applications. For higher output, use MOSFETs -- primarily because they will have higher reliability at high power outputs. MOSFETS will cost more, and require a more complex circuit with more front end gain to achieve similar levels of output distortion.

Class A: I'd always use MOSFETs, simply because the heat so compromises BJT SOA that they are not reliable for significant power outputs (say above 50 watts, absent heroic cooling).

Notice I said tradeoff. There are no perfect devices, and you may value certain device characteristics more or less than I do. For instance, I value reliability most highly; cost is secondary since I am not trying to make a profit. Of course cost is the primary factor in most consumer audio equipment, and they almost never use MOSFETs because the bipolar solution is cheaper.

I have some fairly old BJT amps that still work perfectly (including a 1967 JBL SE400), but I also own several high power BJT amps that failed catastrophically (Phase Linear 400), either catching fire or exploding capacitors or both. The MOSFET amps that have failed were higher power output (Haflers), but only blew fuses and were far simpler to repair; I blame the failure on clogged cooling fans - poor housekeeping.

No doubt many will take exception, but remember these are based on my own personal evaluation, assigning my own weights to different characteristics to make the conclusion. You may assign different weights to these parameters and come up with a different and perfectly valid conclusion for yourself.
  Reply With Quote
Old 22nd April 2004, 05:36 PM   #4
diyAudio Moderator Emeritus
 
lumanauw's Avatar
 
Join Date: Oct 2002
Location: Bandung
Send a message via Yahoo to lumanauw
This is what I learn from Mr. Pass
For cold amp (class AB or B) use bipolars
For hot amp (class A or single ended) use vertical mosfet
If you build something between, use lateral mosfets
  Reply With Quote
Old 22nd April 2004, 06:22 PM   #5
sreten is online now sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Default Re: Power amp output - BJT or MosFET?

Quote:
Originally posted by Bill Fitzpatrick
Assuming good class AB design practices do BJT output stages offer more or less quality than MosFET output stages?
On one criteria, output stage crossover distortion its no contest is favour of BJT's.

As c/o distortion is the thorniest problem in aB design using BJT's is a good idea.

sreten.

Note :
in my book class aB = class B + optimal bias, class
AB = enriched to work in class A up to a few watts.
  Reply With Quote
Old 22nd April 2004, 06:37 PM   #6
diyAudio Member
 
Bill Fitzpatrick's Avatar
 
Join Date: Jun 2001
Location: Eugene, OR
Default Re: Re: Power amp output - BJT or MosFET?

Quote:
Originally posted by sreten


On one criteria, output stage crossover distortion its no contest is favour of BJT's.

As c/o distortion is the thorniest problem in aB design using BJT's is a good idea.

sreten.
OK. All other things being equal - which of course they never are - is the difference audible?
  Reply With Quote
Old 22nd April 2004, 07:02 PM   #7
sreten is online now sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Default Re: Re: Re: Power amp output - BJT or MosFET?

Quote:
Originally posted by Bill Fitzpatrick


OK. All other things being equal - which of course they never are - is the difference audible?

IMO absolutely. I can certainly hear the difference between a
class A design (does not have c/o distortion) and an aB design,
and I believe I know what a poorly biased amplifier sounds like,
which is very similar to Mosfets in a standard aB output stage.

(I had a tuned to the nines aB Mosfet amplifier for ten years)

The subject is covered in great detail in D.Selfs books, but his
concentration on THD rather than weighted harmonic distortion
does not lead him to the "optimum" solution IMO.

sreten.
  Reply With Quote
Old 22nd April 2004, 07:11 PM   #8
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Quote:
Originally posted by lumanauw
This is what I learn from Mr. Pass
For cold amp (class AB or B) use bipolars
For hot amp (class A or single ended) use vertical mosfet
If you build something between, use lateral mosfets
Actually, I think it's Charles who teaches the use of laterals.
  Reply With Quote
Old 22nd April 2004, 07:24 PM   #9
Account Disabled
 
Join Date: Apr 2003
Location: US
I cannot hear mosfets from bjts but I can hear a poorly biased class b to a well biased class b. I cannot hear class a from class b tho.

Quote:
Originally posted by Nelson Pass
Actually, I think it's Charles who teaches the use of laterals.

that's correct. But I am not sure if I am convinced about Charles' notion that laterals sound better than vertical mosfets.
  Reply With Quote
Old 22nd April 2004, 07:43 PM   #10
gmarsh is offline gmarsh  Canada
diyAudio Member
 
gmarsh's Avatar
 
Join Date: Apr 2004
Location: Halifax, NS, Canada
From personal experience, a properly designed BJT amp (with good thermal feedback) will bench better than a properly designed MOSFET amp on the Audio Precision gear.

This is no insult to MOSFET amps though - their performance is very close to that of BJTs, and both can pull off outstanding numbers that'd make me wonder what substance you're on if you can hear a difference between the two... Of course, if people can hear a difference between speaker cables, they can probably hear a difference between output device types... well, I won't get into that

For reliability, you can't beat MOSFETs.

As for vertical vs lateral FETs, I've only had experience designing amps with laterals, and I've always associated vertical MOS with hard switching power supply type applications instead of audio applications.
  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
Best MOSFET for output stages millwood Solid State 151 11th July 2014 01:23 PM
Lateral MOSFET, How do I calculate power output synonymous Solid State 4 18th December 2007 06:54 AM
mosfet output conversion engel dela pena Solid State 15 13th October 2007 10:16 AM
power output calculations, rated power and required power output metebalci Tubes / Valves 7 22nd February 2004 05:49 PM
output mosfet driver lazy Solid State 5 30th March 2002 04:13 AM


New To Site? Need Help?

All times are GMT. The time now is 12:35 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