On Wed, 19 May 2004 18:22:05 +0200 Angel Vilaseca <avilaseca@bluewin.ch>
writes:
> How about using a 15 V Zener instead of a hot-carrier diode?
Hi Angel,
Probably not. If the lower transistor gate drive goes away, then the
full supply will show up on the drain.
Were you planning to run linear? Or class C?
Single ended? or PP?
What power level are you contemplating?
The diode clamp to the +15 (or thereabouts) upper FET bias is just to
protect against the effects of stray L & C. With perfect parts,
wouldn't be necessary... and even with the parts we have it may be
superfluous.
>
> I think that the gate of the HV MOSFET should be grounded for AC.
> So in the end we have what the name of the thread says: A
> grounded-gate
> MOSFET
> :-).
Yes. Well bypassed, probably with multiple capacitors totalling at
least .1 uF
> A grounded-base bipolar has good high-frequency properties. I wonder if
this is
> the case for power MOSFETS. Probably.
It will still depend on the time to charge the gate capacitance. This
will go quicker in the cascode as the entire output of the lower
transistor is available for that task.
> Some MOSFETS (IRFP 460 for instance) have impressive capabilities:
500V, 20 A,
> but very large turn-on (150 ns) and turn-off times (180 ns), which
makes them
> unsuitable even for a 40-meter amplifier.
Those are a couple generations behind the current state of the art.
Better to use something with much less gate capacitance.
> Maybe they could be usable with this cascode circuit. I wonder if
> someone has already tried this.
I've never seen it...
73 & Good morning,
Marv WC6W
*
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