Sign in with
Sign up | Sign in
Your question

advice wanted BJT vs MFET 200 watt class a SE!

Tags:
Last response: in Home Audio
Share
June 16, 2008 3:42:58 AM

I decided to try you guys out because I saw the thread on debunking the urban myths about mosfets and I thought the responses were very good, very realistic, knowleagable (sp?). I am a member of numerous tube amp forums, but I doubt any of them would give me useful answers to the question I have.

I'm reasonably proficient at tube amps and op amp work, but I have a critical decision to make in my next amp....just for the fun of it, mind you:

I am going to build a 200 watt per channel pure Class A Single Ended amp, tube input and interstage, but I want the output stage to be either monster BJT or monster MOSFET - NPN or enhanced N channel, respectively - single ended, mind you, no phase splitting, no push pull - and the output device(s) will be op amp driven with full negative feedback and unity gain. All the voltage gain will be tube, the op amp will be a high output, very high slew rate device simply for the purpose of creating a very tight negative feedback loop for the output device, and the voltage gain will be unity...so the output stage is simply a voltage to current conversion.

But I'm really not sure which would be the better choice - BJT or MOSFET. The higher transconductance of BJT's has me leaning in that direction, since the negative feedback to the op amp is a critical part of the design, but - correct me if I'm wrong - it seems I can get monster wattage mosfets (digikey, mouser) much easier than BJT's. But this could just be that I'm not familiar yet with the hardware that's available for SS power devices.

Yes, I know it's inefficient and heat producing and all that, so please don't waste your time telling me all the reasons I shouldn't do this, I already know them ad nauseum. Like I said, just for the sheer fun of it. No, I don't want to be sensible and make a Class AB PP output, complementary or not.

I see in digikey I can get a mosfet with 1.2 KW power handling (yes, I know it will have to be massively heatsinked, perhaps even immersed in solvent with a heat pipe to a radiator) that was good for at least 10 MHz or something like that.

But I can't find any BJT's that go above 8 amps. I'm probably just not looking in the right place.

And I really don't want to parallel a **** of output devices, even though Nelson Pass has certainly demonstrated how well that approach works.

I would prefer to use one device per channel and really figure out how to cool it sufficiently.

And I understand that a monster MOSFET device is going to have massive gate capacitance that I'm going to have to drive the hell out of (although as a unity follower, perhaps not such a big deal, eh?) :kaola: 

Suggestions for output devices? Part numbers greatly appreciated, but info on BJT vs MOSFET also most appreciated, as well as other place to go other than digikey or mouser for the device I need!

Best, Charles

More about : advice wanted bjt mfet 200 watt class

June 16, 2008 8:49:01 AM

Hmmm, perhaps an electronics publication/forum would be more suitable for this question, but I'll give answers to a few of your questions.

(I'll try not to get too preachy/negative)
For a Class-A amplifier, 200W is a massive output, and considering their efficiency of ~50%, the heat needed to be dissipated will present a huge challenge. Typical Class-A semiconductor amps top out at 50W, let alone 200W or higher.

BJT will probably give you a better "sound", for want of a better word (lower distortion maybe?). MOSFETS are still quite good signal quality-wise, but are significantly more durable that BJT's, hence their popularity in Pro Audio equipment.

For cooling, I'm not sure about the die/tab size of these "monster' transistors you refer to, but assuming a fairly typical ~3x2cm component, that would require top quality heatsink compound, and a VERY efficient heatsink. Perhaps fan cooled, or water cooled. I would advise against using any sort of refrigerant or immersing the actual chip in liquid though.

Ways you can maximize heat transfer -
-Avoid thermal tape/silicon washers. Use heatsink compound. Mica can be used by manufacturing tolerances are terrible, and the thickness of the washers varies a lot.
-Use a fan on the heatsink
-After tapping the heatsink, run a drill bit lightly around the hole, to debur it. It is not visible, but the lip of the thread hole is raised slightly by metal deformation, reducing contact of the transistor with the heatsink.
-Avoid over tightening mounting screws, which can warp the transistor die by a minuscule amount (the base of the transistor lifts up ever so slightly, due to the pressure applied at the tab)
-Consider using a transistor clamp to apply even mounting pressure
-Clamp the transistors using a steel bar (which can be used as part of the HS itself).

1.2kW MOSFETS are typically a pipe dream. Well they're rated for 1.2kW, but certainly allow for de-rating (esp at 50*C+), then allow more headroom for peak power/surges.

Despite your reluctance to parallel, I would strongly suggest you do that (though it may defeat the point of a Class-A amplifier). Having more transistors will allow you to share the dissipated power, and subsequently, increase the surface area available to contact the heatsink.

Suggestions for suitable parts? Sorry, I mainly do class AB/PWM amplifiers/Pro Audio work so can't provide any suggestions.


June 17, 2008 2:13:57 AM

Thanks very much for the info, especially the tips on heatsinks. You're probably right, I'll have to parallel, but not knowing much about power BJT's I thought it would be worthwhile to ask, maybe I'd get lucky.

It does seem odd to me how large the MOFETS get...digikey lists some that are just flat HUGE...but nothing in BJT's that are even remotely comparable. 8 amp capacity is the largest NPN I can find in digikey. And I do remember reading that the wattage ratings on MOSFETs are vastly over-enthusiastic.

And yes, I was considering that 50-ish watts was probably as big as I could get in single ended class A; I just wanted the 200 watts for headroom, not for continuous duty...but to stay in Class A Single Ended, to get that headroom would require huge quiescent current.

Sorry if I've got the wrong forum. I thought this one was for SS hardware including audio, but perhaps not.
June 17, 2008 3:45:20 AM

Thanks very much for the info, especially the tips on heatsinks. You're probably right, I'll have to parallel, but not knowing much about power BJT's I thought it would be worthwhile to ask, maybe I'd get lucky.

It does seem odd to me how large the MOFETS get...digikey lists some that are just flat HUGE...but nothing in BJT's that are even remotely comparable. 8 amp capacity is the largest NPN I can find in digikey. And I do remember reading that the wattage ratings on MOSFETs are vastly over-enthusiastic.

And yes, I was considering that 50-ish watts was probably as big as I could get in single ended class A; I just wanted the 200 watts for headroom, not for continuous duty...but to stay in Class A Single Ended, to get that headroom would require huge quiescent current.

Sorry if I've got the wrong forum. I thought this one was for SS hardware including audio, but perhaps not.


oops. my bad. I wasn't using the search engine at digikey correctly. DUH!!!

at Mouser I found huge NPN transistors I didn't know existed.

Now I just have to find the right one, enough current, high enough hfe, high enough Ft, etc.

Still open to any experienced suggestions for high power, fast as *** NPN transistors, but I can figure it out now.

Thanks again.
!