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HaaiFaai 20th November 2001 09:07 AM

Hello diy-ers

Browsing through the several webpages on the world wide web.
I saw many amplifier design concepts using Mosfet and bipolair.
What I found most of the time is that the designers using bipolair devices such as(the ring emitter) transistors from
various brands as Sanken/Motorola/Toshiba etc is that
they are suspicious in using Mosfets at all in their designs,telling that Mosfets are unreliable/more non lineair/prone to oscillation and therefor more useful in switching applications.
The designers of Mosfet amplifiers are blaming the bipolair of being hard to get them working with high base current and thermal unstable etc.
Who can give clarification and has experience on both devices?

[Edited by HaaiFaai on 11-20-2001 at 04:34 AM]

AudioFreak 20th November 2001 10:00 AM

both are partially true........

if the amps is highly efficient (ie. it doesnt produce much heat and therefore has a low bias current) bipolar transistors are usually better and more linear but you will need to protect them from secondary breakdown ........

if the amps is not so efficient (ie. it produces quite alot more heat and therefore has a somewhat higher bias current) mosfets are usually better and more linear and you will not need to protect them from secondary breakdown ........

mosfets get a bad wrap when people try to use them with very little bias current because in this situation they sound dreadful and can suffer from thermal creep......

when the bias current is measured in amps, mosfets are brilliant which is why in almost all the (solid state) highly biased class AB amps and the true class A amps use mosfets at the output ......

in short, each device has its place ..... neither is perfect ..... but if you dont care about how much power you waste, dont mind your amp doubling as a space heater and want the best quality sound possible use mosfets with as much bias current thru the output devices as you can afford.

Nelson Pass 20th November 2001 06:22 PM

AudioFreak pretty much nails it.

Myself, I've made good money designing both, for the
last 30 years, but I personally prefer Mosfet in
Class A mode.

HaaiFaai 22nd November 2001 12:24 PM

Thanks! another question..

I recently was browsing on the Hafler website and I saw that they are using (amongst others as well) a different kind of set up with grounded Mosfets (and QSC grounded transistors)in the output stage
Is this the way to go if you want to reduce the amount of active devices in the chain? Or is this only for the PA market?
I saw diagrams of Hafler,Chevin,QSC and others..

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