MOSFET output stage

[RichD]

Who do MOSFET sound better than bipolar, as an audio amp output
driver?

 

[[email protected]]

Who do MOSFET sound better than bipolar, as an audio amp output
driver?

MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower odd harmonic
distortin. on and on.

 

[Eeyore]

Who do MOSFET sound better than bipolar, as an audio amp output
driver?

As a driver ?

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

Graham

 

[Kevin McMurtrie]

They do? Seems like it's possible to design good amplifiers either
way.

--Damon

Exactly - zero difference in quality capabilities. It's usually a
matter of impedance matching. Silicon transistors have a fixed loss
around 0.5 volts. MOSFETs have a resistive loss inversely proportional
to their voltage rating. That usually makes MOSFETs less expensive for
low impedances and transistors less expensive for high impedances.

 

[Trevor Wilson]

Who do MOSFET sound better than bipolar, as an audio amp output
driver?

**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.

 

[Phil Allison]

"Trevor Wilson"
"RichD"

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

As a device,

** Same old red-herring BOLLOCKS from know nothing Wilson.

It is totally irrelevant how DEVICES behave on their own.

Only when they are properly configured and suitably biased in a
complimentary output stage can performances be compared.


operating at typical bias currents (say) 10-50mA, MOSFETs

exhibit VASTLY more THD than BJTs.

** UTTER BOLLOCKS !!!

When configured in typical amplifier topologies, THD is vanishingly small at
all power levels and with minimal bias current.

This is true for LATERAL mosfets as made by Hitachi and Semelab ( ie 2SK176,
BUZ905 etc ).

Switching mosfets ( ie IRFPxxx etc) are not the same.



...... Phil

 

[Kevin McMurtrie]

**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.

That's nonsense. MOSFETs have a lower voltage gain, more capacitive
load, and infinite DC current gain. If you swap your bipolar
transistors for MOSFETs and crank up the bias voltage, yeah, it will run
like crap because the local feedback gain is totally wrong. It might
even incapacitate every radio within two blocks of your home then
violently explode. The circuits to drive MOSFETs and bipolars are a
little different.

 

[Jorden Verwer]

Who do MOSFET sound better than bipolar, as an audio amp output
driver?

The device properties of BJTs are superior to those of MOSFETs in all
respects, except for offset - there MOSFETs have the advantage. Whether you
will actually hear this depends on many more factors.

 

[Eeyore]

Exactly - zero difference in quality capabilities. It's usually a
matter of impedance matching. Silicon transistors have a fixed loss
around 0.5 volts. MOSFETs have a resistive loss inversely proportional
to their voltage rating. That usually makes MOSFETs less expensive for
low impedances and transistors less expensive for high impedances.

TOTAL AND COMPLETE GARBAGE.

 

[Eeyore]

**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.

With the required amount of feedback (which isn't rocket science - just good
design) they have no trouble meeting VERY low THDs such as one I designed with
0.0008% THD @ 1 kHz.

They sound better because of the predominance of 2nd harmonic distortion and
almost complete absence of 3rd and ditto up the range AIUI with even and odd
harmonics.

GRaham

 

[Trevor Wilson]

That's nonsense.

**Er, nope. Here are the specs for a modern high power BJT:

http://www.toshiba.com/taec/components2/Datasheet_Sync//66/7890.pdf

Note the hFE vs. Ic. Particularly at elevated temps. It is almost a straight
line, from less than 10mA to several Amps. I call that spectacular
linearity.

Now, I draw your attention to a high power MOSFET:

http://www.magnatec-uk.com/pdf/magnatec/BUZ900.pdf

Note the characteristics of this device. They're pretty good, but as good as
a modern BJT.

Here is another, older, worse example:

http://www.datasheetarchive.com/pdf-datasheets/Datasheets-8/DSA-158541.html

Quite ordinary lineariy.

MOSFETs have a lower voltage gain, more capacitive

load, and infinite DC current gain. If you swap your bipolar
transistors for MOSFETs and crank up the bias voltage, yeah, it will run
like crap because the local feedback gain is totally wrong.

**I never suggested anything of the sort. I mentioned ONLY the intrinsic
linearity of the devices. MOSFETs are inferior to BJTs. For now. When
installed in an appropriate topology, it is likely that there will be little
audible, nor measurable difference between a MOSFET amp and a BJT amp.
Device linearity is another story.

It might

even incapacitate every radio within two blocks of your home then
violently explode. The circuits to drive MOSFETs and bipolars are a
little different.

**Again, not in dispute. The intrinsic linearity, is what I refer to.

 

[Eeyore]

"Trevor Wilson"
"RichD"

** Same old red-herring BOLLOCKS from know nothing Wilson.

It is totally irrelevant how DEVICES behave on their own.

Only when they are properly configured and suitably biased in a
complimentary output stage can performances be compared.

operating at typical bias currents (say) 10-50mA, MOSFETs

** UTTER BOLLOCKS !!!

When configured in typical amplifier topologies, THD is vanishingly small at
all power levels and with minimal bias current.

This is true for LATERAL mosfets as made by Hitachi and Semelab ( ie 2SK176,
BUZ905 etc ).

Switching mosfets ( ie IRFPxxx etc) are not the same.

Yes.

Just for fun once I configured a lateral mosfet output stage with zero feedback.
Its THD at almost all power levels was ~ 1%. From that you can get a good idea
of the negative feedback required to obtain any THD you like.

Lateral mosfets, correctly biased also exhibit virtually NO crossover
distortion. This is all but impossible with bipolars due to their inherent
transfer characteristics unless operated in pure Class A.

Graham

 

[Eeyore]

That's nonsense. MOSFETs have a lower voltage gain

Mosfets are used as source followers in an audio output stage you inane idiot.

Voltage gain is NOT AN ISSUE, regardless of whether or not your statement is true.

Graham

 

[Eeyore]

That may be true, but only because of the negative feedback that Trevor
referred to. The basic device IS more nonlinear.

But when given suitable NFB, the transfer characteristic IS 'cleaner'. Very little
odd harmonic distortion (the nasty stuff) and very very low crossover distortion
(which makes a big difference).

I know, I've designed audio amps with both types. As in designed from the ground
up for commercial pro-audio sale.

Graham

 

[Eeyore]

The device properties of BJTs are superior to those of MOSFETs in all
respects,

How about SOA for one you UTTER MORON ?

Do you even know what SOA is ?

except for offset - there MOSFETs have the advantage. Whether you
will actually hear this depends on many more factors.

YET MORE INSANE BOLLOCKS

Is this all down to techs not knowing their chops ?

 

[Eeyore]

Well this is probably an urban myth/load of bollocks, but years ago lots of
people were talking about power Mosfets exhibiting different distortion
characteristics than BJT's in the same way thermionic valves distort with
differening harmonics and sound nicer.

Fact or crapology?

Fact. Different transfer characteristics. Fundamental device physics.

Graham

 

[Fred Bartoli]

Eeyore a écrit :

With the required amount of feedback (which isn't rocket science - just good
design) they have no trouble meeting VERY low THDs such as one I designed with
0.0008% THD @ 1 kHz.

They sound better because of the predominance of 2nd harmonic distortion and
almost complete absence of 3rd and ditto up the range AIUI with even and odd
harmonics.

Uhhh.

Please explain how you obtain even distortion products from a
symmetrical (suppose paired mosfets) push-pull.

Any symmetrical function produces odd (and no even) harmonics.

...unless you used single ended output stages.

 

[Phil Allison]

"Randy Yates"

"Phil Allison"

No it's not.

** Yes it is you pathetic, know nothing MORON,

That may be true, but only because of the negative feedback that Trevor
referred to.

** Bull.

The basic device IS more nonlinear.

** Irrelevant.

Can't you read - dickhead.



..... Phil

 

[Phil Allison]

"Gareth Magennis"

Well this is probably an urban myth/load of bollocks, but years ago lots
of people were talking about power Mosfets exhibiting different distortion
characteristics than BJT's in the same way thermionic valves distort with
differening harmonics and sound nicer.

Fact or crapology?

** Audiophool crapology - repeated parrot fashion, ad nauseam.

The THD residual of even the simplest topology, lateral mosfet audio
amplifiers is vanishingly small.

Sub the threshold of audibility.



..... Phil

 

[Phil Allison]

"Gareth Magennis"

Well in that case the OP's question is thus answered, as far as I can
tell.

** It can never be answered.

Since it was a UTTERLY meaningless TROLL.

You pathetic fool.




..... Phil