Re: Low noise mosfet amplifier

Winfield Hill wrote:
Joerg wrote:
Winfield Hill wrote:

One more point, these are high-voltage MOSFETs
we're talking about, and when they're seeing
more than 20 to 25V drain-source. I haven't
seen the problem with say, 55V parts.
Question: What are your favorite HV-FETs du jour? I mean the range
of 800V plus. Finally I got the SPICE files from Infineon for their
SPD06N80 and its brethren. It's a bit like pulling teeth but this time I
received an answer within a week. Still waiting for another one from a
couple years ago ;-)

I have about 30 parts in my inventory that I cycle among,
depending on the task. Most of them have 1000 to 1200V
ratings. I usually use as small a part as possible, to keep
the capacitance down. The IRFBF20 is one part I use,
having purchased 100 some years back (small-die high-
voltage MOSFETs all-too-often become discontinued,
and lifetime buys are one tool to combat that problem).

Last year we made 160 amplifiers, each of which had
six 1.2kV MOSFETs (in series), I used Fairchild's
FQD2N100 in that design. Right now I'm working with
an elegant 1.5kV part by ST, the STP4N150.


Thanks, I look into the ST part. I've also checked IRFGB etc. but availability is one of the concerns since HV designs often have very long production lifetimes. And lifetime component buys are very frowned upon by the CFO. Also by the VP of Sales (what if we land a windfall account and then we don't have enough stuff to fill the order?).

Using small parts is smart, I also like to go as high in Rdson as I can without the dissipation increasing too much.


But that's not to say I don't have a good collection of 600
and 800V MOSFETs. There certainly are plenty of nice
ones to chose from in that region. Some would make
good choices for fast 600-volt 50-ohm pulse generators,
but in those designs I often end up with 12A 1kV parts.

While we are at it, any hot leads on BJT? Do you think there is a chance
the BUX85 might stick around another 10 years or so?

Sorry, I'm not much of a fan of large, slow, low-beta
high-voltage BJTs. I know they can be very useful in
fluorescent ballast designs, etc. I suppose the days
of color-TV HOTs are coming to an end, and obscure
companies, like Wing Shing Computer Components
in Hong Kong, making the 1.5kV 2SD870, etc., will
have to find something else to make. I see they're
selling NiMH battery packs and bluetooth headsets!


That's exactly what I am afraid will happen. H-flybacks going lalaland and only the slow ballast BJTs with poor current gain remain.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
.

Quantcast