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Old 21st April 2013, 04:51 AM   #1
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Question FET drivers for BJT outputs?

The OnSemi ThermalTrak outputs work well for my application but comparably excellent drivers are hard to find. For 6 output pairs the 2SA1930+complement is not quite adequate and the best seems to be the MJE15034/35.
This is barely faster than the outputs and it occurs that this could be a suitable spot for a fast FET, even if it is the reverse of a common layout.
I have little experience with FETs but expect there are smaller (TO220?) FETs with lower capacitance than a typical output FET.
Does anyone have recommendations?

David
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Old 21st April 2013, 07:27 AM   #2
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The FET don't have truly complementary pairs. Ciss and few specs different a lot.
Try IRF610 and IRF9610.
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Old 21st April 2013, 07:57 AM   #3
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by Dave Zan View Post
even if it is the reverse of a common layout
You're in good company. IIRC, John Curl does it that way in his power amp designs. Why not go over to his Blowtorch thread and ask for recommendations there?

Don't worry about OT - that thread's permanently OT. Actually, now's a good time to barge in with your question, IMO. They're just between arguments at the moment and this might start some useful discussion.
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Old 21st April 2013, 08:41 AM   #4
AndrewT is offline AndrewT  Scotland
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There are some prominent amplifier designers that claim that BJT output is the best way to go and then add that driving these with a FET stage is better yet.
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Old 21st April 2013, 08:49 AM   #5
godfrey is offline godfrey  South Africa
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They're just between arguments at the moment
My bad, they're at it again.

Go for it anyway, that thread needs a good technical question to get it unstuck.
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Old 21st April 2013, 09:27 AM   #6
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Originally Posted by Walkalone View Post
The FET don't have truly complementary pairs. Ciss and few specs different a lot. Try IRF610 and IRF9610.
Thanks, there are no real complements even in BJT and these look the best so far.

Quote:
Originally Posted by godfrey View Post
You're in good company. IIRC, John Curl does it that way in his power amp designs.
Quote:
Originally Posted by AndrewT View Post
There are some prominent amplifier designers that claim that BJT output is the best way to go and then add that driving these with a FET stage is better yet.
It makes sense as a circuit so I am pleased learn I am not the alone.

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Originally Posted by godfrey View Post
My bad, they're at it again.

Go for it anyway, that thread needs a good technical question to get it unstuck.
LOL! thanks for that, I really did!

Best wishes
David
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Old 21st April 2013, 09:39 AM   #7
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Quote:
Originally Posted by Walkalone View Post
The FET don't have truly complementary pairs. Ciss and few specs different a lot.
Try IRF610 and IRF9610.
Quote:
Originally Posted by Dave Zan View Post
Thanks, there are no real complements even in BJT and these look the best so far.
Use IRF710/IRF9610, they are better complements than IRF610/9610.

Or (4x)BD512/(3x)BD522 from ITT if you can find them...mind low Vds!
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Last edited by aparatusonitus; 21st April 2013 at 09:46 AM.
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Old 21st April 2013, 09:43 AM   #8
lineup is offline lineup  Sweden
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A substancial drawback by using MOSFET drivers
is of course the voltage drop across G-S.
It can be 3.5-4.5 Volts per halfwave.
This means you either need higher power supply or you will get less power out.
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Old 21st April 2013, 10:38 AM   #9
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Originally Posted by lineup View Post
A substancial drawback by using MOSFET drivers
is of course the voltage drop across G-S.
It can be 3.5-4.5 Volts per halfwave.
This means you either need higher power supply or you will get less power out.
It makes sense to me to have driver FETs on elevated supply rails.
But elevated rails are usually reserved for the VAS, so I am a bit out of step here too
Bart Locanthi's seminal T-circuit had two extra pairs of separate elevated rails for the driver and the pre-driver respectively so I have some company. Obviously a bit more expensive so not much done any more.
I plan a CFP pre-drive+driver to the EF output so only one pair of elevated rails will be needed.

Best wishes
David
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Old 21st April 2013, 11:05 AM   #10
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by Dave Zan View Post
driver FETs on elevated supply rails.
Mind the clipping behavior, you don't want to saturate the output devices.
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