MOSFET output stage

[Kevin Aylward]

**Non-sequitur. If you're saying that MOSFET outputs sound better,
they don't.

As a device, operating at typical bias currents (say) 10-50mA, MOSFETs
exhibit VASTLY more THD than BJTs. Only when bias currents are
elevated (around 0.5A - 1A) do MOSFETs exhibit THD characteristics
which are almost as good as BJTs.

MOSFETs are very tough, have an exceptional ability to deliver high
power, high frequency audio (and RF), but distortion is very high.
They require lots of Global NFB in order to operate linearly.

MOSFETs do not sound better than BJTs. At best, they can sound as
good. All things being equal.

Ahmm.... welll....here we go...

Well, I like mosfet outputs because they are easier to design with, imo.
Bipolars, often need an equivelent of 3 stage darlingtons. This makes it a
tad harder to stabilse the feedback loop because of each stage pole.

Basically, you only need about 6 low current transistors, or so, to achieve
silly distortion and bandwidth figures, with mosfets.

As far as "sounds better", that's all moot. Any competently designed amp
should have thd, imd below audibility.

Anyone that claims that a general purpose PA amp, sounds bad or not good, if
it has thd and imd < 0.005% and slew rates of 100V/us, is pretty much
delusional. Roll on the £200 oxygen free mains cable I say....

Kevin Aylward

www.anasoft.co.uk
SuperSpice

 

[Jorden Verwer]

Offset is IRRELEVANT to output devices you complete MORON !

I know that, and I never claimed otherwise. Offset is a form of noise, in a
sense. And like noise, it is caused almost completely by the input
transistors. I'm well aware of all that.

Do do you know what a 'closed loop' means ?

Yes.

 

[Eeyore]

Ahmm.... welll....here we go...

Well, I like mosfet outputs because they are easier to design with, imo.
Bipolars, often need an equivelent of 3 stage darlingtons. This makes it a
tad harder to stabilse the feedback loop because of each stage pole.

Basically, you only need about 6 low current transistors, or so, to achieve
silly distortion and bandwidth figures, with mosfets.

As far as "sounds better", that's all moot. Any competently designed amp
should have thd, imd below audibility.

Seems to be a few that miss that though. Esp those Chinese copies of copies.

Anyone that claims that a general purpose PA amp, sounds bad or not good, if
it has thd and imd < 0.005% and slew rates of 100V/us, is pretty much
delusional. Roll on the £200 oxygen free mains cable I say....

Why stop at £200 ? Oh !
http://www.asa.org.uk/asa/adjudications/Public/TF_ADJ_44177.htm

Kevin, you'd be just the guy to do a harmonic analysis of the distortion
spectrum of a properly biased bipolar vs lateral mosfet amp wouldn't you ? Any
chance ?

Graham

 

[Eeyore]

I know that, and I never claimed otherwise.

Yes you did.

BUGGER OFF YOU IMBECILE.

 

[Eeyore]

Offset is a form of noise, in a sense.

Every one a cracker. Maybe he hasn't heard of DC vs AC analysis ?

Graham

 

[Jorden Verwer]

Every one a cracker. Maybe he hasn't heard of DC vs AC analysis ?

Let me put it this way...

Noise-like phenomena, in increasing order of bandwith:
Offset ==> 1/f noise ==> white noise

 

[RichD]

As a driver ?

Now if you said as an output stage it might make sense.

There's a difference?

 

[Eeyore]

Let me put it this way...

Noise-like phenomena, in increasing order of bandwith:
Offset ==> 1/f noise ==> white noise

Noise is AC
Offset is DC

End of story.

 

[Eeyore]

There's a difference?

Damn right there is.

Graham

 

[RichD]

Well, I like mosfet outputs because they are easier to design with, imo.
Bipolars, often need an equivelent of 3 stage darlingtons. This makes it a
tad harder to stabilse the feedback loop because of each stage pole.

Basically, you only need about 6 low current transistors, or so, to achieve
silly distortion and bandwidth figures, with mosfets.

As far as "sounds better", that's all moot. Any competently designed amp
should have thd, imd below audibility.

Anyone that claims that a general purpose PA amp, sounds bad or not good, if
it has thd and imd < 0.005% and slew rates of 100V/us, is pretty much
delusional.

Do you have any experience designing audio amps?
Which did you use?

 

[Spehro Pefhany]

Noise is AC
Offset is DC

End of story.

Everything is AC when you get right down to it.

Best regards,
Spehro Pefhany

 

[Jorden Verwer]

Noise is AC
Offset is DC

End of story.

Not at all. You're oversimplifying. If offset were truly DC, it wouldn't
drift. And it does drift. Likewise, white noise also has an (infinitesimal)
component at DC. It isn't all as black and white as you claim it to be. I
realize this is a very theoretical approach, but I stand by my claim that
offset and noise are conceptually similar.

 

[Eeyore]

Do you have any experience designing audio amps?

LMAO ! Yes, Kevin had indeed had as have I. Amps that sold commercially into the
pro-audio market. I can't even estimate how many tens of thousands in my case.

Graham

 

[Eeyore]

Everything is AC when you get right down to it.

I call it moving DC. It never will stay still. ;~)

Graham

 

[Eeyore]

Not at all. You're oversimplifying. If offset were truly DC, it wouldn't
drift.

Drift is a different phenomenon unrelated to noise. Usually thermally caused.

Graham

 

[Jorden Verwer]

Drift is a different phenomenon unrelated to noise. Usually thermally

caused.
I meant drift of the offset. Of course drift is different from noise.
But offset really does have a lot in common with noise, when you think about
it.

Also, noise is often thermally caused as well.

 

[Eeyore]

caused.
I meant drift of the offset. Of course drift is different from noise.
But offset really does have a lot in common with noise, when you think about
it.

Not at all in any remote way shape or form.

Also, noise is often thermally caused as well.

Do you think you're telling me something I don't already know. I keep
Boltzmann's Constant in my head for that pirpose.

Graham

 

[Tony]

Exactly. I believe that even the very worst designers were generally able to get a
half-decent result with MOSFETs, whereas bipolars require a little more skill. So it may
well be that MOSFETs DO sound better in car audio, consumer-grade home theatre and cheap
"pro" sound systems, where the standards of the products and the designers are low.
Tony

 

[Jorden Verwer]

Not at all in any remote way shape or form.

Don't you at least agree there are many similarities between 1/f noise and
offset?

 

[Phil Allison]

"Vladimir Vassilevsky"

However there are few special cases when a FET output stage has an
advantage:

1) With FETs, it is simpler to control bias current, because of the
negative dependency from the temperature.

** Completely wrong.

The tempco of typical (ie switching) fets is strongly positive - the gate
threshold voltage drops by about 6mV per degree C.

Bias compensation is non trivial and crucial to get right or thermal runaway
is likely.



...... Phil