Re: blast from past

On Wed, 02 Jul 2008 22:01:01 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Thu, 03 Jul 2008 03:55:17 +0000, NoName <No@xxxxxxxxx> wrote:

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.

It's worse that that: the 1 GHz is a square wave.

I think I mentioned that we're putting a couple of ferrite beads in
series with the collector. That makes the very-fast-edge impedance
good. The very slow impedance is of course excellent, since the
transistor is in a closed loop, current sense resistor and opamp. It's
the middle that's tough. The generic problem is how to make a
super-wideband current source; bias tees have much the same issues,
how to make a DC-daylight hi-Z inductor.


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.

This circuit is supposed to make a logic level that has programmable
Vhigh and Vlow, up to 5 volts swing, 50 ohm output impedance, fast
(ballpark 150 ps) edges, and works from DC to 1 GHz. It's a "pin
driver" of sorts.

Dearly departed Gigabit Logic used to make a GaAs IC that did just
this; I still have a sample somewhere. It was about $200 a pop in the
early 90's. I don't know of any other (available) IC that can do
anything like this. The new SRS clock generator uses a Maxim laser
driver chip, surrounded by lots of stuff, to make logic outputs up to
2 GHz, but the p-p swing is low and it does have a good deal of
personality over frequency.

What does the Hittite chip cost? We've used their HMC465LP5, an 18 GHz
distributed amp, as an optical modulator driver, but it's about $185
and a brutal power hog; and we'd probably need two per channel to get
enough gain; about $3K for the amps alone! DC coupling would be a real
nuisance, too.

John

Maybe transistor in closed loop + bias tee (tapered inductor) + bead
?:-)

...Jim Thompson
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