RadioBanter - View Single Post

September 13th 05, 06:52 PM

On 13 Sep 2005 07:38:33 -0700, wrote:

Hello,
I have lots of power mosfet (IRF640, 740, 840 and similars), I'd like
to try
to build a power amplifier for 50 MHz (even 100W CW is ok), what
parameters
of these mosfet can tell how high they can go in frequency? I've only
seen
some projects for HF bands with these kind of mosfets, so probably
there is
a good reason not to try to go higher. I think fall and rise time give
the
limit, but what else? Also, the datasheet of these devices don't have a
table of Zin and Zout, so how one is supposed to calculate them?
Thanks in advance for any hint.

73 de

IS0FKQ

Good luck. Most of those devices will exhibit gain to amazing
frequencies due the fact that fets have no real limits like junction
transistors. What they do have that limits them is real world things
like lead inductance, Input capacitance, output capacitance and Drain
to Gate Miller capacitance all of which make it difficult to get power
in and out of them as frequency increases.

The input capacitance for the IRF510 is 135pf, the higher power parts
can easily be upwards of 1275pf for the IFR640 (Xc of around 2ohms).
Output capacitance around 400pf and feedback capacitance of 100PF
also for the 640.

I have driven an IRF510 at 50mhz and gotten 4.5W out of it. To do so
I had to drive it as a 5 to 10ohm very reactive input, use a 24V
supply and load the output to around 10ohms, and bias it for 100ma
minimum. It was unstable thermally(tended to runaway toward device
burnout). The output spectrum (harmonics) and IMD were poor.
At 12V power out was nearly RF power input.

It was more effort than driving a 2SC1307 (old cb final) at 12V, more
gain and the IMD was better. At 12V the 1306 gave 6W carrier power
with good stability. A pair of 2sc1971 driven pushpull easily give
10-11W carrier at 12V and are fairly clean.

There are FETs characterized for RF power but they are not inexpensive
and corosponding bipolar devices are fairly cheap.

Allison
KB1GMX