RadioBanter - View Single Post

September 14th 05, 02:18 PM

On 14 Sep 2005 01:24:16 -0700, wrote:

ha scritto:

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.

Reading from the ARF488 datasheet (this mosfet is made for RF
amplification
up to 60 MHz or so) I see input capacitance of 1400 pf typical, and
150 pf output capacitance typical.
The IRF840 datasheet reports 1300 pf and 200 pf in/out capacitance
at the same frequency of the ARF488 (1 MHz). So what else affect
high frequency performance of these devices? Is the 50 pf difference
in output capacitance the "big" problem? Reverse transfer capacitance
of the ARF488 is 65 pf against the 18 pf of the IRF840.

Yes that is true. However that input capacitance is close to what I'd
use to bypass a 50mhz circuit. It's near RF dead short unless you
try to absorb it into the feed network. Even then it's a very low Z.

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.

not so encouraging...

All I can say it try the most likely candidate. They are designed for
switching power at a few hundred KHZ.

Best results will require a 24V or maybe a 40-50V power source.

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.

what do you suggest on the bipolar side to obtain at least 100W CW
on 50 MHz? Monitor line output transistors are any good?
Thanks and 73

Francesco IS0FKQ

MRF492 is a good 70w device I've had good success with. If you can
afford it MRF141 RF power fets or MRF150 but they are expensive.
There are a number of 2SCxxxx part numbers that also would fit the
need but experience with them is limited.

If all else failes a Valve such as a 4cx250 with 800-900V on the plate
would do 100 or more watts out.

Allison