MOS FETs, BJTs or both?

DarkOne · 2nd October 2001, 07:56 PM

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.

Thanks

JBL · 3rd October 2001, 11:57 AM

It's really depend on you. Mosfet got more distortion than bjt na di think they are a little bit less efficient.

hifiZen · 4th October 2001, 05:51 AM

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...

DarkOne · 4th October 2001, 03:30 PM

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.

Thanks

JBL · 5th October 2001, 01:14 AM

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

.

4th October 2001, 05:51 AM	#3
hifiZen diyAudio Member Join Date: May 2001 Location: Mountain View, CA	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... __________________ - Chad.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
HiFI OP-AMPs, BJTs, and FETs how do I choose them?	777funk	Solid State	12	6th May 2009 11:35 AM
BJTs for VAS Stages	danville	Solid State	33	2nd July 2008 08:28 AM
Small signal FETs vs BJTs	rtarbell	Solid State	0	10th November 2005 02:43 AM
Toshiba JFETs BJTs Sanken BJTs	Praise!	Swap Meet	5	14th February 2004 06:45 AM
MOSFETS vs BJTs	azira	Solid State	17	2nd February 2004 07:02 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

2nd October 2001, 07:56 PM	#1
DarkOne diyAudio Member Join Date: Oct 2001 Location: Bratislava	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. Thanks

3rd October 2001, 11:57 AM	#2
JBL diyAudio Member Join Date: Aug 2001 Location: Canada, N.-B.	It's really depend on you. Mosfet got more distortion than bjt na di think they are a little bit less efficient.

4th October 2001, 03:30 PM	#4
DarkOne diyAudio Member Join Date: Oct 2001 Location: Bratislava	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. Thanks

5th October 2001, 01:14 AM	#5
JBL diyAudio Member Join Date: Aug 2001 Location: Canada, N.-B.	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 .