MOSFET output stage

RichD wrote in news:b78fc9c2-fe9c-444c-8ac5- snipped-for-privacy@p31g2000prf.googlegroups.com:

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

--Damon

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

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.

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.

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.

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.

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:

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

Here is another, older, worse example:

Quite ordinary lineariy.

MOSFETs have a lower voltage gain, more capacitive

It might

"Gareth Magennis"

Since it was a UTTERLY meaningless TROLL.

You pathetic fool.

.... Phil

Who do? That's voodoo!

Not in any relevant way for audio power amps.

Straw man. It is entirely practical to build power amps that are never clipped in actual use. If you don't like how your power amp clips? Get one with enough output so that it never clips.

Power amps aren't just devices, they are circuits. Circuit design can easily trump device characteristics.

In fact the sharpness of the clipping of a power amp relates to things like how much negative feedback it has, all things considered. If you have a circuit with lots of negative feedback, it is very likely to clip very sharply and ideally. If you have a circuit with less negative feedback, the clipping will be softer, but will occupy a larger proportion of the transfer characteristic.

If they need lots of power, get a powerful power amp. Forget about what's inside the box, worry about how the box works.

This whole "amp sounds better" stuff was fully debunked 30 years ago. Good power amps sound the same and they sound like a piece of wire with gain.

There are tons of power amps that can't be distinguished from a piece of wire with gain, while driving well-designed speakers.

There are quite a few amps that meet the same criteria while driving even the weirdest speaker load.

A really good power amp will destroy a poorly-designed speaker before it starts sounding bad, and really good power amps aren't all that unique.

On a sunny day (Wed, 17 Sep 2008 16:12:25 -0700 (PDT)) it happened RichD wrote in :

MoSfEtS zound batter becuaze thOze work wiz EleKtRONS, LikE TubeS. Traanzisters WOrk wiz HOLES, AND Thoze HoLES YoU wILL HeAr in Ze Muzick.

In most cases, this is fallacy, and the result is just the opposite. Reason: FETs have lower transconductance compared to BJTs. It is impossible to build a half bridge stage with an ideal transfer curve.

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

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Because you believe they will.

Cheers! Rich

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

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

-- Rich

"Vladimir Vassilevsky"

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

Ah, but since audiophools argue endlessly about cable and wire characteristics, you need to specify exactly what TYPE of wire with gain. :-) :-)

MrT.