MOSFET output stage

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Radiation hardness Safe operating area Bandwidth

"MooseFET"

Rumors aside MOSFETs are not really easier to operate in parallel. You still need emitter / source resistors.

Amazing how so many MOOSE like idiots have no idea there are TWO kinds !!!!!!!!!!!!!!

The kind known as " lateral " share current just fine when in parallel in LINEAR applications with no source ballast resistors.

Septic Tank Imbecile.

..... Phil

ut

You may not have claimed it mattered but you sure implied it. That aside since we are talking about the output section of an amplifier, this sort of offset has no effect at all on the quality of the op-amp.

If you look for an op-amp with an extremely low offset voltage, you will find that it uses MOSFETs to obtain that extreme low offset.

the

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Beware: Mosfets like the STW55NM60 have a decreasing threshold voltage for increasing temperature. This means that biasing them to a low idle current isn't so easy.

"MooseFET" Vladimir Vassilevsky

Beware: Mosfets like the STW55NM60 have a decreasing threshold voltage for increasing temperature.

Equates to a VERY positive drain current tempco.

Many times more severe than a BJT.

..... Phil

er... yes...e.g.

"This was the amplifier pro sound companies were waiting for; many buy up to

Right now I use a few, one of which is a studiomaster AX2500 (750+750), which I did not design. I also have a Behringer 1280S mixer/amp, and a Carlsbro (600+600) PA..and a.etc...etc...

Kevin Aylward

Quite right. In hindsight, I could've been more clear about this not being relevant here, I just didn't want any "You're wrong because MOSFETs have lower offset"-type responses, so that's why I mentioned it...

Boundary condition.

Boundary condition.

AFAIK, BJTs still hold the world record for cutoff frequency.

Offset? Two BJTs of the same part number will have delta-Vbe well below a tenth of a volt. A similar pair of mosfets will have Vgs-th and transfer curves that can differ by a volt or more. And the mosfet thresholds will change with time a lot more then the bjt's.

Mosfets have zero storage/desaturation time, far better SOAR specs, and are easier to drive.

John

A lot of bits have been spilled on this thread so far, but I think it is the same question as "Why do tubes sound better than transistors?" or "How many angels can dance on the head of a pin?"

First of all, it implies a truth that may not objectively be the case. But tubes, BJTs and MOSFETs all have very different characteristics and engineering trade-offs. I think it has been reasonably well established that it is possible to make excellent sounding amplifiers with any of those technologies.

In the end, I think it boils down to the experience and skill of the engineer in working with a given technology, and how that impacts the economics of producing the end product using said technology.

For example, I think we no longer use vacuum tubes for mainstream audio power amplifiers because they:

1. Require an additional power supply and heat to operate.
2. Are bulky compared to their solid-state alternatives.
3. Are not available in complimentary pairs (e.g., P-channel/N-channel)
4. Have output impedance that usually requires an output transformer.

None of these are insurmountable obstacles, but they do increase the size, cost, power consumption and heat load for a given output power.

Similar, but less obvious considerations affect the choice of BJTs and MOSFETs. They have different characteristics, and the skilled engineer will exploit them for optimal effect -- sonic quality, economy, reliability and so on. The marketing department will define the requirements for the product based on the target market, and the engineer will attempt to design a product that fits within those constraints. In all that I have read here, I have not seen anything that would consistently make me select MOSFETs over BJTs. But I might have a preference based on a *given set* of product requirements.

No. Google "widest bandwidth transistor" or some such.

John

"Chronic Philharmonic"

You must not confuse switching types with laterals.

You must not confuse switching types with laterals.

You must not confuse switching types with laterals.

You must not confuse switching types with laterals.

Wot an posturing old wind bag you are.

...... Phil

All that yields is your post. ;)

Okay, sillyness aside, without the quotes it does indeed return some results. But I couldn't find a definitive answer either way. I do remember reading about a BJT that had an ft of 500 GHz a couple of years ago, which was a new record at the time. It's indeed possible that things have changed since then, although I'd bet that BJTs can't be far behind.

Of course this is all a whole different game than audio, but frankly I think both BJTs and MOSFETs offer sufficient bandwith in audio applications these days.

The really fast parts are compound semiconductor heterojunctions, Phemts and HBT's. The fastest silicon stuff, in the 150 GHz range, is mostly CMOS. Some next-gen Bluetooth type stuff will be cmos at 60 GHz.

As far as discretes that you can actually buy and do stuff with in real life, the fastest bipolars are the SiGe parts, in the 45-60 GHz Ft sort of range, and the fastest fets are things like the NEC Phemts. In real life, the phemts are faster, because Ft is where a transistor has a beta of 1, which is semi-useless for most applications, especially wideband stuff.

True, none of this is relevant for audio. Anything will work for audio.

John

Did you use MOSFET on the output stage, and why?

-- Rich

"RichD" "Kevin Aylward"

For technical reasons, that have been stated here and are completely beyond your infinitesimal comprehension.

You damn troll.

..... Phil