Re: blast from past

John Larkin <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Wed, 02 Jul 2008 13:43:33 -0400, John Popelish <jpopelish@xxxxxxxx>
wrote:

Have you considered spreading the heat into two die and
eliminating the heat sink and letting you go with a much
smaller (and faster) die size? Perhaps a pair of BFG31 in
parallel.

http://www.nxp.com/acrobat_download/datasheets/BFG31_2.pdf

That's pretty radical. But I still have to get rid of the heat. The
prototype board uses a SOT-223 version of the 2907, and there's not
quite enough pcb copper to keep the case temp below 100C, and there's
already 10 pF of copper pour on the bottom side. I had to add a copper
flag to get it down to 80C

ftp://66.117.156.8/Copper.jpg

So if I use two transistors, I still have to get rid of the same
amount of heat. The nice thing about a leaded part is that it sticks
up, into the air flow - we're adding a servoed fan to the 8-channel
production unit - and air has an Er = 1!

Fast stuff is nasty: it gets real hot, but everything has to be really
close together, and stray C must be kept down, so cooling is usually a
pain. It would be nice to make boards out of BeO or AlN: both have
incredible ratios of thermal conductivity to dielectric constant. Or
diamond.

John

It's not clear what you are trying to accomplish.

You are not going to get far using descretes.

For 8pF:

Xc = 1 / (2 * pi * 8e-12 * 1e9) = 19.89 ohms reactance

Not much of a current source.

Wideband drivers are available:

http://www.hittite.com/products/view.html/view/HMC-AUH232

So it can be done.

You could review your goals and find a different way to drive whatever you
are trying to drive.

Or break out the wallet and find a commercial IC that does what you need to
do.

.

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