MOS FETs, BJTs or both? - diyAudio
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Old 2nd October 2001, 07:56 PM   #1
DarkOne is offline DarkOne  Slovakia
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I'm going to build power amp and I don't know which one can be better. I've designed 3 amps. First with BJT transistors, second with HEXFETs and third where signal goes trough MOS and cascode transistors were BJTs. All are designed without overall feedback.
I don't know which one to build, because I don't have enough money to build all of them so I have to choose one (the best).

Amp with MosFets I designed after I've seen Nelson Pass's patent for X-series. My design is similar but I use cascode transistors, symetrical input stage(both P & N), it don't have so many current sources and output stage is biased for class A to 6Amps.

My BJT amp is similar to well known LC Audio - The End Millenium but with more output transistors,with cascode transistors and output transistors biased for class A.

The third one was something between first and second.

Can someone help me to choose which one will sound best. I think that the best is with MOSFETs because has much less count of gain stages than others.

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Old 3rd October 2001, 11:57 AM   #2
JBL is offline JBL  Canada
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It's really depend on you. Mosfet got more distortion than bjt na di think they are a little bit less efficient.
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Old 4th October 2001, 05:51 AM   #3
hifiZen is offline hifiZen  Canada
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There's a lot to consider when choosing output devices. Personally, I prefer mosfets over bipolars as output devices... they have a higher input impedence so the driver stage doesn't have to work as hard (but be wary of their gate capacitance). This means the circuit can usually be simpler. MOSFETs also have a higher bandwidth than BJTs, and aren't subject to as severe thermal runaway problems as you can get with bipolars. One major downside is that the efficiency can be significantly lower due to their large Vgs. This efficiency problem is by no means insurmountable, but can drive up the cost of power supplies, and the heat penalty you pay. The lateral mosfets are better in this respect than generic mosfets like IR's HEXFETs. But, lateral mosfets like those from Hitachi or Semelab are also quite a bit more expensive than cheap, readily available hexfets...

So, you need to prioritize what you want out of your amp and go from there...
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Old 4th October 2001, 03:30 PM   #4
DarkOne is offline DarkOne  Slovakia
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I know about advatages and disadvantages of mosfets and bjts. But I've read some articles about mosfet amps and everywhere was written that mosfets do not have good lower bass. This part of music is important for me, of course I need detailed and clear mids and highs too. Lower bass was the reason why I as you. I'm not hanged on measurements of distortion or similar parameters, so it doesn't mean anything if amp has THH 0.2%, I know about lower and higher harmonic frequencies --> 2nd or 3rd harmonic frequencies are not bad as higher ones.
So I just don't know which one has better bass, because mids and highs are as good with bjt as with mosfet.

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Old 5th October 2001, 01:14 AM   #5
JBL is offline JBL  Canada
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This is not true or is only by the design. Mosfet amp are verry popular for subwoofer amp and car amp. They are very simple. The only thing is a bit more distortion but if you need the power of MOSFET you can forget the distortion problem they rule for high power.(and free from second breakdown .
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