" bravely wrote to "All" (14 Sep 05 12:18:39)
--- on the heady topic of " switching power mosfet as RF amplifiers"

no From:
no Xref: core-easynews rec.radio.amateur.homebrew:87858

no 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.

no Yes that is true. However that input capacitance is close to what I'd
no use to bypass a 50mhz circuit. It's near RF dead short unless you
no try to absorb it into the feed network. Even then it's a very low Z.


Can one drive them with a grounded gate using a bipolar transistor at
the source?

A*s*i*m*o*v

.... - Grains Of Salt. Take As Needed With Above Message.