RadioBanter - View Single Post

September 14th 05, 06:03 PM

On 14 Sep 2005 06:48:38 -0700, wrote:

Hello,

ha scritto:

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.

Yes, I understand it. What isn't clear to me is why the ARF488 is
specified
for 60 MHz and has input capacitance of the one of an IRF840.
On QEX of may/jun 1999 you can find a 300W 50 MHz amplifier with
ARF488 push-pull.

I'll check that one out. I do not have QEX back to '99 so it may take
a while to get it.

I suspect the ARF488 is a RF rated varient of the IRFs and may offer
other characteristice that are more complient at VHF. I cannot find
any data or even a reference to ARF488.

I am a 6m op that love to play on that band so curiousity always wins.

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.

no problem, I can even use 100V or more.

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

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.

I should have a couple of 4cx150 around, but this amplifier should
sit right under the antenna in the same box with a receive
preamplifier.
I was trying to avoid a big tube ampli. And also I was trying to
recycle
"normal" components, which is also a good way to learn something.

73

Francesco IS0FKQ

One of those 4cx150s will do over 100W CW and the PS for that power
is small. Like you say something different is always a draw.

Allison
KB1GMX