Power amp output - BJT or MosFET?

[Bill Fitzpatrick]

Assuming good class AB design practices do BJT output stages offer more or less quality than MosFET output stages?

[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...

[slowhands]

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.

[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

[sreten]

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.

[Bill Fitzpatrick]

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?

[sreten]

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.

[Nelson Pass]

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.

[millwood]

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.

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.

[gmarsh]

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.