[Milsurplus] Insulated Gate Bipolar Transistors

[David Stinson]

In case anyone was interested-
Time for me to risk showing my considerable "ig-nurn-ss" again.
I did some research on using the Insulated Gate Bipolar Transistor
(IGBT) devices as a "Borg Implant" to plate modulate 
a  high-power AM transmitter, for us "po-folks" 
who can't afford hundreds of bucks for a kilowatt transformer.
I got the spec sheet on a tiny little device-

http://www.supertex.com/pdf/datasheets/GN2470.pdf

a Supertex GN2470- that can do 3.5 amps, 
CE breakdown 700 volts.  There are "hefty-er" 
ones out there, but this will do for discussion.
It has a low on-state voltage drop, so probably
won't even need heat sinking at our low currents.
And did I mention it's tiny?
This little booger is a quarter of an inch square 
and a tenth of an inch thick.

Now, I'm not the brightest candle on the cake-
I just barely get by.  But I think I understand what I'm 
looking at and hope you brighter lights can comment, 
and tell me where I'm wrong (or right). 
I promise you won't hurt my feelings ;-).
I'm not addressing the audio filtering requirements yet-
just the operating points.

The Transfer Characteristics curve looks a lot like that 
for a triode with -grid bias, and only has a small amount of "bow" 
at the bottom near cut-off.  I'm going to use some figures 
that are "above legal limit," but it's only for illustration
to see just how full of "hooo-weee" I am.
Let's say I want my big final to run, at full carrier, 
2000 volts at 1 amp.  I series eight or ten of these for a large
breakdown margin and hook all the gates together.  I can
get away with that because the input capacitance of one
of these is about 100 pFd, vs 2700 pFd 
for an IRF450 MOSFET;  I think .03 mFds across my 
audio chain would make for some "bass-ee" audio ;-).

I bias the things "full on" at 6 volts or more for tune-up,
then back the bias down to 4.5 volts, lowering the current 
to about 400 mils on transmit with no modulation.  
Drive the audio to it about 2 V PTP, and the current swings 
from near cut-off, up to 1 amp at about 6 volts.
The slight bow at the low end introduces some asymmetry,
but doesn't look too bad, considering.

This device takes more drive than 
the power MOSFET; about 2 V PTP for the 
swing I want vs. half a volt for something like
the IRF450, and the curve is straighter and steeper
for the MOSFET, but this is actually a "plus" on the
side of the IGBT, because getting the bias exactly right
on the MOSFET and keeping it there when you've
only got a half a volt to play with is pretty tough.
And getting a couple of volts drive to the IGBT thing
is easy- the output of any old audio amp with a 
few watts "umph" should do nicely.

What do you smart guys think?
73 D.S.