On 5 May 2004 16:41:11 -0700, (Tom Bruhns) wrote:


Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!

John

"John Larkin" a écrit dans
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On 5 May 2004 16:41:11 -0700, (Tom Bruhns) wrote:


Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!


If the drain load is resonated and you want some control over the resonant
frequency (i.e. you have external to the mosfet tuning Cs) I don't see much
discontinuities in inductors current. Of course when all the stuff is well
"wired" and that's an entirely different matter, isn't it Paul ? ;-)

John, can you suggest some refs that nicely snap ?

Thanks,
Fred.

"John Larkin" a écrit dans
le message news: ...
On 5 May 2004 16:41:11 -0700, (Tom Bruhns) wrote:


Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!


If the drain load is resonated and you want some control over the resonant
frequency (i.e. you have external to the mosfet tuning Cs) I don't see much
discontinuities in inductors current. Of course when all the stuff is well
"wired" and that's an entirely different matter, isn't it Paul ? ;-)

John, can you suggest some refs that nicely snap ?

Thanks,
Fred.

On 5 May 2004 16:41:11 -0700, (Tom Bruhns) wrote:


Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!

John

Paul Burridge wrote in message . ..

Okay, thanks all and keep it comin'. I had no idea there were power
FETs capable of UHF. I'll follow any links given and investigate.

Check out www.polyfet.com; SR706: 300 watts at 225MHz. SR705: 200
watts at 400MHz. LX401: 60W at 1000MHz. Also some very terse "ap
notes" and some somewhat less terse but very to the point technical
papers.

Paul Burridge wrote in message . ..
....
It'd be fun to see just how fast you can force current in and out of
an ordinary, GP power MOS FET gate to get it switching as fast as
possible, I reckon, before pressing ahead with the dedicated devices.

Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating. I
was seeing close to 100V in about 5nsec just before the
self-destruction as I recall. And the Siliconix V-mos transistors
were good for RF power back in that era.

If you're thinking driving it "digitally", that's probably the wrong
answer. Resonate the input and output capacitances, and life will be
much easier. Charging and discharging capacitance through a resistive
source is quite inefficient. Remember, too, that tuned triode valve
amplifiers are generally neutralized. Be VERY careful to not exceed
the gate-source voltage rating! Have fun playing, but expect some
"surprises."

Cheers,
Tom

Paul Burridge wrote in message . ..

Okay, thanks all and keep it comin'. I had no idea there were power
FETs capable of UHF. I'll follow any links given and investigate.

Check out www.polyfet.com; SR706: 300 watts at 225MHz. SR705: 200
watts at 400MHz. LX401: 60W at 1000MHz. Also some very terse "ap
notes" and some somewhat less terse but very to the point technical
papers.

On 5 May 2004 09:34:17 -0700, (Tom Bruhns) wrote:


Okay, thanks all and keep it comin'. I had no idea there were power
FETs capable of UHF. I'll follow any links given and investigate.

Do you want a linear amp (per the subject line) or a class C amp?
They're not the same critter.

Technically you're right, of course. I want to make an amp that
operates in Class C - yeah, I know that mode's the antithesis of
linear but over here in Britain, these bolt-on after market boosters -
though running in Class C., are knowns as "linears." I don't
understand it either.
Class C RF amps are basically switching on and off at high speed, so
it seemed like a job better suited to a power FET since power RF BJTs
are pretty expensive. Of course, I can now gather than the same
situation pertains with FETs as well.

There was an article in QEX just one issue ago about a P-P power
MOSFET amplifier for 7MHz.

It'd be fun to see just how fast you can force current in and out of
an ordinary, GP power MOS FET gate to get it switching as fast as
possible, I reckon, before pressing ahead with the dedicated devices.
--

The BBC: licenced at public expense to spread lies.

Hi Paul,

Is cost an issue or do you just want to see if it can be done with
MOSFETs? About 25 years ago I did a small amp (25 watts or so) above
100MHz with FETs. But I found that cost was much higher as if I'd done
it bipolar. When I looked at 100W my then pretty puny budget made the
decision for me to go bipolar.

Personally I haven't seen much happening in cost/performance and I'd
probably start out bipolar. Another cost saver is if you leave the
supply voltage level a wildcard even if this is for a mobile app. It's
no big deal these days to create whatever voltage the transistors like
best from whatever is there as a supply.

I am still a big fan of tubes, as long as they are popular enough and
thus cheap. Transistors are rather fickle and a tiny glitch in the
load connection can send them to the greener grass in a millisecond.
Tubes brush this off in stride.

For rock bottom bill of materials budget it has always paid off to
check what TV manufacturers use in the horizontal deflection finals.
That's as low cost as it gets. But I guess with the advent of big
plasma screens that may fizzle over the next 10 years.

Regards, Joerg
http://www.analogconsultants.com

Joerg wrote:
For rock bottom bill of materials budget it has always paid off to
check what TV manufacturers use in the horizontal deflection finals.
That's as low cost as it gets. But I guess with the advent of big
plasma screens that may fizzle over the next 10 years.

Horizontal deflection transistors generally have an FT way too low
to be usefull much higher in frequency than the 160 or 80 meter ham
bands. Sweep tubes are now rare and expensive bottles costing more
than 'common' 6146 types. Surplus 1625 tubes used to be only a few
bucks each, can be found for about $5-10 each at hamfests and fleabay
(Still cheaper than 6146's though). 807's cost a little more.
The best bargin in price vs power might be the 811A, or the 813.

BTW I have a bunch of 813's I'd be willing to sell. Someone make me
a good offer on a lot of 5 of them. (Used, but don't look too bad).