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 Wed, 05 May 2004 20:13:01 -0400, Ken Scharf
wrote:

Mosfets in the MRF5xx series (511, 521 for example) have been used
up to the 10 meter band with good results. A pair of them can give
at least 50w pep output. Depending on the input/output circuitry
used and the transistor they require 12-28v power supply. Layout
is somewhat critical.

THanks, very interesting. I wonder if anyone's tried paralleling up
half a dozen of 'em for more power?

--

The BBC: licenced at public expense to spread lies.

On Wed, 05 May 2004 20:13:01 -0400, Ken Scharf
wrote:

Mosfets in the MRF5xx series (511, 521 for example) have been used
up to the 10 meter band with good results. A pair of them can give
at least 50w pep output. Depending on the input/output circuitry
used and the transistor they require 12-28v power supply. Layout
is somewhat critical.

THanks, very interesting. I wonder if anyone's tried paralleling up
half a dozen of 'em for more power?

--

The BBC: licenced at public expense to spread lies.

On Wed, 05 May 2004 20:21:45 -0400, Ken Scharf
wrote:

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).

Nice. Got any KT88s?
--

The BBC: licenced at public expense to spread lies.

On 6 May 2004 06:29:13 -0700, (Steve
Kavanagh) wrote:

Paul:

There is an interesting two part article in QST May & June 1997
describing 300 W and 500 W output Class E (more efficient, non-linear)
amplifiers using a single IRFP440 (300 W) or IRFP450(500 W) at 7 MHz.
Since you are in the UK, there is a brief mention in Radcom (Technical
Topics) August 1997.

Ordinary garden-variety power FETs can also be used for HF linear
amplification using appropriate bias settings - though exactly how
linear they can be in practice I don't know.

Thanks, Steve. It's obviously doable. I haven't checked a Power FET
data sheet yet, but what's the betting they don't quote output
admittances for these devices? :-(
--

The BBC: licenced at public expense to spread lies.

On Wed, 05 May 2004 20:21:45 -0400, Ken Scharf
wrote:

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).

Nice. Got any KT88s?
--

The BBC: licenced at public expense to spread lies.

On 6 May 2004 06:29:13 -0700, (Steve
Kavanagh) wrote:

Paul:

There is an interesting two part article in QST May & June 1997
describing 300 W and 500 W output Class E (more efficient, non-linear)
amplifiers using a single IRFP440 (300 W) or IRFP450(500 W) at 7 MHz.
Since you are in the UK, there is a brief mention in Radcom (Technical
Topics) August 1997.

Ordinary garden-variety power FETs can also be used for HF linear
amplification using appropriate bias settings - though exactly how
linear they can be in practice I don't know.

Thanks, Steve. It's obviously doable. I haven't checked a Power FET
data sheet yet, but what's the betting they don't quote output
admittances for these devices? :-(
--

The BBC: licenced at public expense to spread lies.

On Thu, 6 May 2004 17:43:10 +0200, "Fred Bartoli"
r_AndThisToo wrote:


"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 ?


I think that most mosfet substrate diodes are now designed to have
soft recovery, so that they won't snap and make a horrible dv/dt. I
blew up a lot of early Motorola mosfets in an h-bridge motor driver...
had to switch to darlingtons, and only figured it out later.

Anything with a p-i-n structure has a chance of being a snap diode. I
think it needs a hyperbolic doping profile or something to work well.
1N4005-7 types work, but usually only after a brief forward bias, not
DC. Somebody told me that many varicaps snap, but I haven't verified
that. Specifically-designed PIN diodes (the kind used in RF switches
and attenuators) don't snap, as they are doped to have very long
recovery times.

We tested over 60 different TO-220 power diodes to find the best
high-voltage drift step-recovery part. Then we found something else,
much better and more repeatable, but that's still a secret.

John

On Thu, 6 May 2004 17:43:10 +0200, "Fred Bartoli"
r_AndThisToo wrote:


"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 ?


I think that most mosfet substrate diodes are now designed to have
soft recovery, so that they won't snap and make a horrible dv/dt. I
blew up a lot of early Motorola mosfets in an h-bridge motor driver...
had to switch to darlingtons, and only figured it out later.

Anything with a p-i-n structure has a chance of being a snap diode. I
think it needs a hyperbolic doping profile or something to work well.
1N4005-7 types work, but usually only after a brief forward bias, not
DC. Somebody told me that many varicaps snap, but I haven't verified
that. Specifically-designed PIN diodes (the kind used in RF switches
and attenuators) don't snap, as they are doped to have very long
recovery times.

We tested over 60 different TO-220 power diodes to find the best
high-voltage drift step-recovery part. Then we found something else,
much better and more repeatable, but that's still a secret.

John

THanks, very interesting. I wonder if anyone's tried paralleling up
half a dozen of 'em for more power?
==================================
Mind in-&output capacitances ,which add-on when parallelling.

Frank GM0CSZ / KN6WH