"flipper" wrote in message
...
On Tue, 4 May 2010 21:20:41 +0100, "ian field"
wrote:


"flipper" wrote in message
. ..
On Tue, 04 May 2010 09:11:47 -0700, John Larkin
wrote:

On Tue, 04 May 2010 01:03:43 -0500, flipper wrote:

On Mon, 3 May 2010 23:02:27 -0500, "Tim Williams"
wrote:

"flipper" wrote in message
news:1k4vt5pctqpqush7g57aappqgt1e4ooubd@4ax. com...
So you really don't think it matters whether one tries to drive the
gate through a 10 ohm or 100 megohm resistor?

I said cutoff frequency, which means voltage source. Do you have a 0
ohm
source at ~1GHz? No? Then you won't get to see fT. ;-)

Tim

I know we won't see it so I have no idea what the heck your tap dance
is for.

Gm/Co was a popular figure of merit for tubes used in video amps and
oscilloscopes. They took a hockey-stick turn upward when frame-grid
tubes were invented, just before semiconductors took over.

Mosfets have impressive Gm/Co values, at the cost of high input
capacitance.

John

I understand but that doesn't answer the question at hand because we
do not have unlimited gate drive capability. FOM, Rdson x Qg, isn't
the issue as the device is 'capable' of more bandwidth if we could
drive it but, because the 'driver' already exists, we do not have that
choice. At least not without buffering it with something and if we do
that we might as well use the jfet under the source version. But, in
either case, it's no longer 'plain Jane'.

For a given MOSFET technology FOM is roughly constant and I was asking
if the same kind of thing applied to our problem of trading off gm,
with source feedback, vs Ciss, because that is what the driving
impedance works into.

Unfortunately, Mouser doesn't let me search by Ciss, or gate charge
either, so it's not a simple task to find a low one but I seem to
remember that when I originally latched onto the STP2NK60Z it was
'pretty low'... but I wasn't taking gm into account.

As I said, it's probably moot for a (AA5) 'table radio', because we
don't need much bandwidth to begin with, but I was pondering it's
potential suitability as a more general replacement. I suspect we need
the more complex version if one wants 'full capability'.


Just a thought, but if the common source section of the cascode is boosted
by a bipolar, then a really low Idss FET can be used - like a 40673 or a
BF991, even with both gates tied together its only a few pF.

Sure, you can.

A 'typical' (if there is such a thing) tube in this power range looks
to be about 11 or 12 pF and the BF244 looks to be about 3 pF so I
wouldn't think that's a problem. The BF246 looks to be about 15pF so
it's not as good.

Being depletion MOSFETs, they should be even better.

Better than what in what way?

More similar in character to a grid.